Handbook of Neurosurgery 7th Ed

8. Developmental anomalies

8.1. Arachnoid cysts

8.1.1. Arachnoid cysts, intracranial

Key concepts:

• a congenital abnormality, most common in middle fossa, cerebellopontine angle (CPA), suprasellar region, and posterior fossa

• usually an incidental finding

• imaging often shows remodeling of bone; imaging characteristics exactly mimic CSF on CT or MRI in most cases

• recommendation for incidentally discovered arachnoid cyst in adults: a single follow-up imaging study in 6-8 months is usually adequate to rule-out any increase in size. Subsequent studies only if concerning symptoms develop

AKA leptomeningeal cysts, distinct from posttraumatic leptomeningeal cysts (AKA growing skull fractures, see page 892), and unrelated to infection. Arachnoid cysts (AC) are congenital lesions that arise during development from splitting of arachnoid membrane (thus they are technically intra-arachnoid cysts) and contain fluid that is usually identical to CSF. They do not communicate with the ventricles or subarachnoid space. May be unloculated or may have septations. Typically lined with meningothelial cells positive for epithelial membrane antigen (EMA) and negative for carcinoembryonic antigen (CEA). AC may also occur in the spinal canal.

“Temporal lobe agenesis syndrome” is a label that had been used to describe the findings with middle cranial fossa ACs. This term is now obsolete since brain volumes on each side are actually the same¹, bone expansion and shift of brain matter account for the parenchyma that appears to be replaced by the AC.

Two types of histological findings²:

1. “simple arachnoid cysts”: arachnoid lining with cells that appear to be capable of active CSF secretion. Middle fossa cysts seem to be exclusively of this type

2. cysts with more complex lining which may also contain neuroglia, ependyma, and other tissue types

EPIDEMIOLOGY OF INTRACRANIAL ARACHNOID CYSTS

Incidence: 5 per 1000 in autopsy series. Comprise ≈ 1% of intracranial masses.

Male:female ratio is 4:1. More common on the left side.

Bilateral arachnoid cysts may occur in Hurler syndrome (a mucopolysaccharidosis).

Table 8-1 Distribution of arachnoid cysts⁶

Location	%
sylvian fissure	49%
CPA	11%
supracollicular	10%
vermian	9%
sellar & suprasellar	9%
interhemispheric	5%
cerebral convexity	4%
clival	3%

DISTRIBUTION

Almost all occur in relation to an arachnoid cistern (exception: intrasellar, the only one that is extradural), see Table 8-1.

Epidermoid cysts in the cerebellopontine angle (CPA) may mimic an arachnoid cyst, but are high signal on DWI MRI (see page 690).

For the differential diagnosis of midline posterior fossa arachnoid cysts, (see page 240).

PRESENTATION

Most ACs are asymptomatic. Those that become symptomatic usually do so in early childhood³. The presentation varies with location of the cyst, and oftentimes appear mild considering the large size of some.

Typical presentations are shown in Table 8-2³ and include:

1. symptoms of intracranial hypertension (elevated ICP): H/A, N/V, lethargy

2. seizures

3. sudden deterioration:

A. due to hemorrhage (into cyst or subdural compartment): middle fossa cysts are notorious for hemorrhage due to tearing of bridging veins. Some sports organizations do not allow participation in contact sports for these patients

B. due to rupture of the cyst

4. as a focal protrusion of the skull

5. with focal signs/symptoms of a space occupying lesion

6. incidental finding discovered during evaluation for unrelated condition

7. suprasellar cysts may additionally present with⁴:

A. hydrocephalus (probably due to compression of the third ventricle)

B. endocrine symptoms: occurs in up to 60%. Includes precocious puberty

C. head bobbing (the so-called “bobble-head doll syndrome”⁵): considered suggestive of suprasellar cysts, but occurs in as few as 10%

D. visual impairment

Table 8-2 Typical presentations of arachnoid cysts

Middle fossa cysts	Suprasellar cysts with hydrocephalus	Diffuse supra- or infratentorial cysts with hydrocephalus
seizures headache hemiparesis	intracranial hypertension craniomegaly developmental delay visual loss precocious puberty bobble-head doll syndrome	intracranial hypertension craniomegaly developmental delay

Figure 8-1 CT Classification of Sylvian fissure cysts⁷

EVALUATION

Routine evaluation with CT or MRI is usually satisfactory. Further evaluation with CSF contrast or flow studies (cisternograms, ventriculograms…) are only occasionally necessary for the diagnosis of midline suprasellar and posterior fossa lesions³ (for Differential diagnosis, see Intracranial cysts, page 1217). See Figure 8-1 for classification for middle fossa cysts.

CT SCAN

Smooth bordered non-calcified extraparenchymal cystic mass with density similar to CSF and no enhancement with IV contrast. Expansion of nearby bone by remodelling is usually seen, confirming their chronic nature. Often associated with ventriculomegaly (in 64% of supratentorial and 80% of infratentorial cysts).

Convexity or middle fossa cysts exert mass effect on adjacent brain and may compress ipsilateral lateral ventricle and cause midline shift. Suprasellar, quadrigeminal plate, and midline posterior-fossa cysts may compress the third and fourth ventricle and cause hydrocephalus by obstructing the foramina of Monro or the Sylvian aqueduct.

MRI

Better than CT in differentiating the CSF contained in arachnoid cysts from the fluid of neoplastic cysts. May also show cyst walls.

CISTERNOGRAMS AND/OR VENTRICULOGRAMS

Using either iodinated contrast or radionuclide tracers. Variable rate of opacification has resulted in difficulty correlating results with operative findings. Some cysts are actually diverticula, and may fill with radiotracer or contrast.

TREATMENT

Many (but not all) authors recommend not treating arachnoid cysts that do not cause mass effect or symptoms, regardless of their size and location. For incidentally discovered arachnoid cyst in an adult not considered for surgery: a single follow-up imaging study in 6-8 months is usually adequate to rule-out any changes (since they may grow in size). Subsequent studies may be done if concerning symptoms develop. Pediatric patients may need to be followed until adulthood.

Surgical treatment options are summarized in Table 8-3.

Table 8-3 Surgical treatment options for arachnoid cysts

Procedure	Advantages	Disadvantages
drainage by needle aspiration or burr hole evacuation	• simple • quick	• high rate of recurrence of cyst and neurologic deficit
craniotomy, excising cyst wall and fenestrating it into basal cisterns	• permits direct inspection of cyst (may help with diagnosis) • loculated cysts (rare) treated more effectively • avoids permanent shunt (in some cases) • allows visualization of bridging vessels (small advantage)	• subsequent scarring may block fenestration allowing reaccumulation of cyst • flow through subarachnoid space may be deficient; many patients develop shunt dependency post-op • significant morbidity and mortality (may be due to abrupt decompression)
endoscopic cyst fenestration through a burr hole8	• as above	• as above
shunting of cyst into peritoneum or into vascular system	• definitive treatment • low morbidity/mortality • low rate of recurrence	• patient becomes “shunt dependent” • risk of infection of foreign body (shunt)

CYST SHUNTING

Probably the best overall treatment. For shunting into peritoneum, use a low pressure valve. If there is concurrent ventriculomegaly, one may simultaneously place a ventricular shunt (e.g. through a “Y” connector). Ultrasound, ventriculoscope, or image guidance may assist in locating suprasellar cysts. Shunting of middle fossa ACs may also be accomplished through the lateral ventricle, thus shunting both compartments⁹.

✖ NB: in running the shunt tubing from the middle fossa, it should be routed behind the ear (do not tunnel caudally in front of ear to avoid injury to facial nerve).

SUPRASELLAR CYSTS

Treatments proposed include:

1. transcallosal cystectomy¹⁰

2. percutaneous ventriculo-cystostomy: procedure of choice of Pierre-Kahn et al.⁴. Performed via a paramedian coronal burr hole through the lateral ventricle and foramen of Monro (may be facilitated by using a ventriculoscope⁸)

3. subfrontal approach (for fenestration or removal): dangerous and ineffective⁴

✖ ventricular drainage: ineffective (actually promotes cyst enlargement)

OUTCOME

Even following successful treatment a portion of the cyst may remain due to the remodeling of the bone and chronic shift of brain contents. Hydrocephalus may develop following treatment. Endocrinopathies tend to persist even after successful treatment of suprasellar cysts.

8.1.2. Arachnoid cysts, spinal

Almost always dorsal, most common in thoracic spine. With a ventral cyst, consider a neurenteric cyst (see below). Most are actually extradural and these are sometimes referred to as arachnoid diverticula - these may be associated with kyphoscoliosis in juveniles or with spinal dysraphism. Intradural arachnoid cysts may be congenital or may follow infection or trauma.

Usually asymptomatic, even if large.

TREATMENT

When indicated, treatment options include:

1. percutaneous procedures: may be done under MRI¹¹ or CT guidance. CT guidance usually requires use of intrathecal contrast to delineate the cyst

A. needle aspiration

B. needle fenestration¹¹

2. open surgical resection or fenestration

8.2. Intracranial lipomas

Intracranial and intraspinal lipomas are felt to be of maldevelopmental origin^{12 (p 706)} and may arise from failure of involution of the primitive meninges¹³.

Epidemiology of intracranial lipomas

Incidence: 8 in 10,000 autopsies. Usually found in or near the midsagittal plane, particularly over the corpus callosum (lipomas in this region are frequently associated with agenesis of the corpus callosum, see page 246). The tuber cinereum and quadrigeminal plate are less frequently affected¹⁴. Rarely, the CP angle or cerebellar vermis may be involved. May occur in isolation, but also has been described in association with a number of congenital anomalies, including: trisomy 21, Pai’s syndrome, frontal encephalocele, facial anomalies…. Other midline abnormalities may also be found: agenesis of the corpus callosum, myelomeningocele, and spina bifida¹³.

Evaluation

May be diagnosed by CT, MRI (study of choice), and by ultrasound in infants.

CT: Low density, may have peripheral calcification (difficult to appreciate on MRI)¹³. Differential diagnosis on CT: primarily between dermoid cyst, teratoma¹⁵ and germinoma¹³.

MRI: characteristic finding is a midline lesion with signal characteristics of fat (high intensity on T1WI, low intensity on T2WI).

Presentation

Often discovered incidentally. Large lipomas may be associated with seizures, hypothalamic dysfunction, or hydrocephalus (possibly from compression of the aqueduct). Associated findings that may or may not be directly related: mental retardation, behavioral disorders and headache.

Treatment

Direct surgical approach is seldom necessary for intracranial lipomas¹⁵. Shunting may be required for cases where hydrocephalus results from obstruction of CSF circulation¹⁵.

8.3. Hypothalamic hamartomas

Key concepts:

• rare, non-neoplastic congenital malformation, usually occurs in tuber cinereum

• may be parahypothalamic (pedunculated) or intrahypothalamic (sessile)

• presentation: precocious puberty, seizures (usually starting with gelastic seizures (brief unprovoked laughter)), developmental delay

• treatment: GnRH analogs for precocious puberty. Latero-basal craniotomy for pedunculated lesions, transcallosal interforniceal approach for intrahypothalamic lesions, option of endoscopic approach for lesions ≤ 1.5 cm dia, stereotactic radiosurgery may be an alternative

Hypothalamic hamartomas^A (HH) AKA diencephalic hamartomas or hamartomas of the tuber cinereum. Rare, non-neoplastic congenital malformations arising from inferior hypothalamus or tuber cinereum (floor of the third ventricle between the infundibular stalk and the mammillary bodies). May occur as part of Pallister-Hall syndrome (genetics: AD inherited defect in GL¹³ gene resulting in abnormally short GL¹³ protein which participates in normal shaping of many organs).

A. hamartoma: an abnormal conglomeration of cells normally found in the same area

Clinical findings

1. specific types of seizures:

A. gelastic seizures (brief episodes of unprovoked laughter¹⁶) are the most characteristic type and are the earliest seizure manifestation. Present in up to 92% of patients¹⁷. They are resistant to medical management and can lead to cognitive and behavioral deficits¹⁸. Not pathognomonic. A neocortical origin has been described¹⁹

B. epileptic encephalopathy: gelastic fits gradually increase in frequency and other seizure types accrue: complex partial seizures, drop attacks, tonic seizures, tonic-clonic seizures, and secondarily generalized seizures. This phase is associated with marked deterioration of cognitive and behavioral abilities. Develops in 52% by a mean age of 7 years¹⁷

2. precocious puberty: believed to be due to release of gonadotropin-releasing hormone (GnRH) found within hamartoma cells²⁰. HH are the most common CNS tumor to cause precocious puberty, other causes include: other CNS tumors (astrocytoma, ependymoma, pineal tumors (see page 692), optic/hypothalamic gliomas (especially in NFT patients)), CNS XRT, hydrocephalus, CNS inflammation, septo-optic dysplasia (see page 247), and chronic hypothyroidism

3. developmental delay: primarily in patients with seizure disorder (severity correlates with duration of seizures). 46% of patients have borderline intellectual function (mental retardation)

4. behavioral disturbances²¹: aggressive behavior, rage attacks…

Imaging

MRI: nonenhancing, isointense on T1WI, slightly hyperintense or isointense on T2WI²².

Pathology

Two subtypes of hypothalamic hamartomas^{17, 22}:

1. pedunculated or parahypothalamic: narrower base attached to the floor of the hypothalamus (not arising within hypothalamus). No distortion of 3rd ventricle. Generally associated with precocious puberty more than seizures

2. intrathalamic or sessile: within hypothalamus (distorting the 3rd ventricle) or broad attachment to hypothalamus. More often associated with seizures. 66% have developmental delay, 50% have precocious puberty

Microscopic pathology: Clusters of disorganized small neurons surrounded by large pyramidal like neurons in an astrocyte-rich neuropil²³ (in contrast to the usual ganglion cells surrounded by oligodendrocytes found in the hypothalamus).

Treatment

Precocious puberty usually responds well to GnRH analogs²⁴.

Indications for surgery:

1. precocious puberty that fails to respond to medical therapy (GnRH analogs)

2. seizures that cannot be adequately controlled medically. Post-op seizure control is related to completeness of resection

3. neurologic deficit from mass effect of the tumor

Options:

1. surgical resection

A. pedunculated lesions: approaches include²⁵ subtemporal, subfrontal, pterional, orbitozygomatic (most commonly recommended). Risks: cranial neuropathy, CVA²⁵

B. sessile lesions with intraventricular component: transcallosal anterior interforniceal approach^26-28. Risks: memory impairment (forniceal injury), endocrine disturbances, weight gain^{26, 28}

C. neuroendoscopic approach: considered for HH ≤ 1.5 cm diameter²⁹. Risks: 25% incidence of thalamic cerebrovascular injury

2. stereotactic radiosurgery: especially for small sessile lesions, subtotal resection, or patients refusing or not candidates for surgery. In small series, 3-year outcome showed improvement similar to surgical resection with less neurologic and endocrinologic morbidity^{30, 31}

8.4. Neurenteric cysts

No uniformly accepted nomenclature. Working definition: CNS cyst lined by endot-helium primarily resembling that of the GI tract, or less often, respiratory tract. Congenital. Not true neoplasms. Most common alternate term: enterogenous cyst. Less common terms include: teratomatous cyst, intestinoma, archenteric cyst³², enterogene cyst, and endodermal cyst. Usually affect the upper thoracic and lower cervical spine³³. Associated developmental vertebral anomalies (e.g. diastematomyelia) are common³⁴. Rarely intracranial (see below). Spinal neurenteric cysts (NEC) may have a fistulous or fibrous connection to the GI tract (through a spinal dysraphism) and some call these endodermal sinus cysts. Occurs as a result of persistence of the neurenteric canal (temporary duct between the notochord and the primitive gut (amniotic and yolk sacs) formed during week 3 of embryogenesis).

Intracranial neurenteric cysts: Rare, most common in p-fossa. Initially, may be difficult to rule-out metastasis from an extremely well-differentiated primary adenocarcinoma of unknown origin (absence of progressive disease suggests NEC). Locations:

1. posterior fossa

A. cerebellopontine angle (CPA)³²: usually intradural, extraaxial (case report of extradural lesion with bone destruction³⁵)

B. in midline anterior to brainstem³³

C. cisterna magna³⁶

2. supratentorial: only 15 case reports as of 2004³⁷. Locations: suprasellar³⁸ (possible confusion with Rathke’s cleft cyst), frontal lobe intraparenchymal³⁷, quadrigeminal plate region, dural-based extra-axial. Source of endoderm is controversial since the primitive foregut extends cranially only to the midbrain³⁹. Theory: colloid cysts, Rathke cleft cysts, and supratentorial NECs may all arise from remnants of Seesel’s pouch, a transient endodermally derived diverticulum of the cranial end of the embryonic foregut⁴⁰

Clinical

Most commonly present during the first decade of life³⁴. Pain or myelopathy from the intraspinal mass are the most common presentations in older children and adults. Neonates and young children may present with cardiorespiratory compromise from an intrathoracic mass, or with cervical spinal cord compression³⁴. Meningitis may occur from the fistulous tract, especially in newborns and infants.

Imaging

Intracranial NEC:

• CT: usually low density, nonenhancing⁴¹

• T1WI MRI: isointense or slightly hyperintense to CSF (may be hyperintense if there are blood products). T2WI isointense to CSF⁴¹. Nonenhancing

Histology

Most are simple cysts lined by cuboidal-columnar epithelium and mucin secreting goblet cells. Less common types of epithelium described include: stratified squamous and pseudostratified columnar, and ciliated epithelial cells. Mesodermal components may be present, including smooth muscle and adipose tissue, and some have called these teratomatous cysts^{42, 43} which is not to be confused with teratomas which are true germinal cell neoplasms. May be histologically identical to colloid cysts.

Treatment

Spinal NEC: Surgical removal usually reverses the symptoms. Recurrence is uncommon with complete removal of cyst wall.

Intracranial NEC: Capsule adherent to brainstem may prevent complete resection, which predisposes to delayed recurrence. Apparently successful treatment by evacuation of contents and marsupialization has been reported (5 cases, mean follow-up: 5 yrs⁴⁴). Incomplete removal requires long-term follow-up. Hydrocephalus is shunted if indicated.

8.5. Craniofacial development

8.5.1. Normal development

FONTANELLES

Anterior fontanelle: the largest fontanelle. Diamond shaped, 4 cm (AP) x 2.5 cm (transverse) at birth. Normally closes by age 2.5 yrs.

Posterior fontanelle: triangular. Normally closes by age 2-3 mos.

Sphenoid and mastoid fontanelles: small, irregular. Normally, former closes by age 2-3 mos, latter by age 1 yr.

CRANIAL VAULT

Growth: largely determined by growth of brain; 90% of adult head size is achieved by age 1 yr; 95% by age 6 yrs. Growth essentially ceases at age 7 yrs. By end of 2nd yr, bones have interlocked at sutures and further growth occurs by accretion and absorption.

Skull is unilaminar at birth. Diplöe appear by 4th yr and reach a maximum by age 35 yrs (when diploic veins form).

Mastoid process: formation commences by age 2 yrs, air cell formation occurs during 6th yr.

8.5.2. Craniosynostosis

Originally called craniostenosis. Incidence: ≈ 0.6/1000 live births.

Primarily a prenatal deformity, postnatal craniosynostosis (CSO) occurs uncommonly (postnatal causes consist primarily of positional alterations which may not represent true synostosis). CSO is rarely associated with hydrocephalus (HCP)⁴⁵. The assertion that CSO may follow CSF shunting for HCP is unproven (see page 328). Other causes for failure of normal skull growth include lack of brain growth due to any of the causes of arrested development of the cerebral hemispheres (lissencephaly, micropolygyria, some cases of hydranencephaly…).

Treatment is usually surgical. In most instances, the indication for surgery is for cosmesis and to prevent the severe psychological effects of having a disfiguring deformity. However, with multiple CSO, brain growth may be impeded by the unyielding skull. Also, ICP may be pathologically elevated, and although this is more common in multiple CSO⁴⁶, elevated ICP occurs in ≈ 11% of cases with a single stenotic suture. Coronal synostosis can cause amblyopia. Most cases of single suture involvement can be treated with linear excision of the suture. Involvement of multiple sutures or the skull base usually requires the combined efforts of a neurosurgeon and craniofacial surgeon, and may need to be staged in some cases. Risks of surgery include: blood loss, seizures, stroke.

DIAGNOSIS

Many cases of “synostosis” are really due to positional flattening (e.g. “lazy lamb-doid”, see below). If this is suspected, instruct parents to keep head off of flattened area and recheck patient in 6-8 weeks: if it was positional, it should be improved, if it was CSO then it usually declares itself. The diagnosis of CSO may be aided by:

1. palpation of a bony prominence over the suspected synostotic suture (exception: lambdoidal synostosis, see below)

2. gentle firm pressure with the thumbs fails to cause relative movement of the bones on either side of the suture

3. plain skull x-rays:

A. lack of normal lucency in center of suture. Some cases with normal x-ray appearance of the suture (even on CT) may be due to focal bony spicule formation⁴⁷

B. beaten copper calvaria (see page 231), sutural diastasis and erosion of the sella may be seen in cases of increased ICP⁴⁸

4. CT scan:

A. helps demonstrate cranial contour

B. may show thickening and/or ridging at the site of synostosis

C. will demonstrate hydrocephalus if present

D. may show expansion of the frontal subarachnoid space⁴⁹

E. three-dimensional CT may help better visualize abnormalities

5. in questionable cases, a technetium bone scan can be performed⁵⁰:

• there is little isotope uptake by any of the cranial sutures in the first weeks of life

• in prematurely closing sutures, increased activity compared to the other (normal) sutures will be demonstrated

• in completely closed sutures, no uptake will be demonstrated

6. MRI: usually reserved for cases with associated intracranial abnormalities. Often not as helpful as CT

7. measurements, such as occipito-frontal-circumference may not be abnormal even in the face of a deformed skull shape

Increased ICP

Evidence of increased ICP in the newborn with craniosynostosis include:

1. radiographic signs (on plain skull x-ray or CT, see above)

2. failure of calvarial growth (unlike the non-synostotic skull where increased ICP causes macrocrania in the newborn, here it is the synostosis that causes the increased ICP and lack of skull growth)

3. papilledema

4. developmental delay

TYPES OF CRANIOSYNOSTOSIS

SAGITTAL SYNOSTOSIS

The most common CSO affecting a single suture; 80% male. Results in dolichocephaly or scaphocephaly (boat shaped skull) with frontal bossing, prominent occiput, palpable keel-like sagittal ridge. OFC remains close to normal, but the biparietal diameter is markedly reduced.

Surgical treatment

Skin incision may be longitudinal or transverse. A linear “strip” craniectomy is performed, excising the sagittal suture from the coronal to the lambdoid suture, preferably within the first 3-6 months of life. The width of the strip should be at least 3 cm, no proof exists that interposing artificial substances (e.g. silastic sheeting over the exposed edges of the parietal bone) retards the recurrence of synostosis. Great care is taken to avoid dural laceration with potential injury to the underlying superior sagittal sinus. The child is followed and reoperated if fusion recurs before 6 months age. After ≈ 1 yr age, more extensive cranial remodelling is usually required.

CORONAL SYNOSTOSIS

Accounts for 18% of CSO, more common in females. In Crouzon’s syndrome this is accompanied by abnormalities of sphenoid, orbital and facial bones (hypoplasia of mid-face), and in Apert’s syndrome is accompanied by syndactyly⁵¹. Unilateral coronal CSO → plagiocephaly with forehead on affected side flattened or concave above eye (normal side falsely appears to bulge abnormally), supra-orbital margin higher than normal side (on skull x-ray → harlequin eye sign). The orbit rotates out on the abnormal side, and can produce amblyopia. Without treatment, flattened cheeks develop and the nose deviates to the normal side (root of nose tends to rotate towards deformity).

Bilateral coronal CSO (usually in craniofacial dysmorphism with multiple suture CSO, e.g. Apert’s) → brachycephaly with broad, flattened forehead (acrocephaly). When combined with premature closure of frontosphenoidal and frontoethmoidal sutures, results in foreshortened anterior fossa with maxillary hypoplasia, shallow orbits, progressive proptosis.

Surgical treatment

Simple strip craniectomy of the involved suture has been used, often with excellent cosmetic result. However, some argument that this may not be adequate has been presented. Therefore, a more current recommendation is to do frontal craniotomy (uni- or bilateral) with lateral canthal advancement by taking off orbital bar.

METOPIC SYNOSTOSIS

At birth, the frontal bone consists of two halves separated by the frontal or metopic suture. Abnormal closure results in a pointed forehead with a midline ridge (trigonocephaly). Many of these have a 19p chromosome abnormality and are retarded.

LAMBDOID SYNOSTOSIS

Epidemiology

Long considered a clinical rarity with a reported incidence range 1-9% of CSO⁵², recent reports suggest a higher incidence of 10-20%⁵³ which may be due to an actual increased incidence, or simply to increased awareness or changing diagnostic criteria. More common in males (male:female = 4:1), and the right side is involved in 70% of cases. Usually presents between 3-18 months of age, but may be seen as early as 1-2 months of age.

Controversy exists regarding the actual criteria for this condition, and some authors differentiate between those cases which appear to have a primary abnormality of the lambdoid suture from those which may be due to positional flattening, the so-called “lazy lambdoid”. Others do not make this distinction, and sometimes refer to the condition as occipital plagiocephaly to avoid the need to implicate abnormalities of the lambdoid suture.

Positional flattening (or molding) may be produced by:

1. decreased mobility: patients who constantly lie supine with the head to the same side, e.g. cerebral palsy, mental retardation, prematurity, chronic illness

2. abnormal postures: congenital torticollis⁵⁴, congenital disorders of the cervical spine

3. intentional positioning: trend since 1992 to place newborns in a supine sleeping position to reduce the risk of sudden infant death syndrome (SIDS)⁵⁵, sometimes with a foam wedge to tilt the child to one side to reduce the risk of aspiration

4. intrauterine etiologies⁵⁶: intrauterine crowding (e.g. from multiparous births or large fetal size), uterine anomalies

Clinical findings

Flattening of the occiput. May be unilateral or bilateral. If unilateral, it is sometimes termed lambdoid plagiocephaly which when severe also produces bulging of the ipsilateral forehead resulting in a “rhomboid” skull with the ipsilateral ear located anterior and inferior to the contralateral ear. The contralateral orbit and forehead may also be flattened. This may be confused with hemifacial microsomia or with plagiocephaly seen in unilateral coronal craniosynostosis. Bilateral lambdoid synostosis produces brachycephaly with both ears displaced anteriorly and inferiorly⁵². Unlike the palpable ridge of sagittal or coronal synostosis, an indentation may be palpated along the synostotic lambdoid suture (although a perisutural ridge may be found in some).

Diagnostic evaluation

The physical exam is the most important aspect of diagnosis. Skull x-ray may help differentiate (see below). If the skull x-ray is equivocal, prevent the infant from laying on the affected side for several weeks. A bone scan should be obtained if no improvement occurs (see below). In definite cases of synostosis, and for some cases of refractory positional flattening (which usually corrects with time, but may take up to 2 years) surgical treatment may be indicated.

Skull x-ray: Shows a sclerotic margin along one edge of the lambdoid suture in 70% of cases. Local “beaten copper cranium” (BCC) occasionally may be seen due to indentations in the bone from underlying gyri which may be due to locally increased ICP. BCC produces a characteristic mottled appearance of the bone with lucencies of varying depth having round and poorly marginated edges. BCC correlates with generalized ↑ ICP only when it is seen with sellar erosion and sutural diastasis⁴⁸.

CT scan: Bone windows may show eroded or thinned inner table in the occipital region in 15-20% of cases⁵³, > 95% are on the side of the involvement. The suture may appear closed. Brain windows show parenchymal brain abnormalities in < 2%: heterotopias, hydrocephalus, agenesis of the corpus callosum; but ≈ 70% will have significant expansion of the frontal subarachnoid space (may be seen in synostosis of other sutures, see above).

Bone scan: Isotope uptake in the lambdoid suture increases during the first year, with a peak at 3 months of age⁵⁷ (following the usual inactivity of the first weeks of life). The findings with synostosis are those typical for CSO (see page 229).

Treatment

Early surgical treatment is indicated in cases with severe craniofacial disfigurement or those with evidence of increased ICP. Otherwise, children may be managed non-surgically for 3-6 months. The majority of cases will remain static or will improve with time and simple nonsurgical intervention. Approximately 15% will continue to develop a significant cosmetic deformity.

Nonsurgical management⁵⁸:

Although improvement can usually be attained, some degree of permanent disfigurement is frequent.

Repositioning will be effective in ≈ 85% of cases. Patients are placed on the unaffected side or on the abdomen. Infants with occipital flattening from torticollis should have aggressive physical therapy and resolution should be observed within 3-6 months.

More severe involvement may be treated with a trial of molding helmets⁵⁹ (however, no controlled study has proven the efficacy).

Surgical treatment:

Required in only ≈ 20% of cases. The ideal age for surgery is between 6 and 18 months. The patient is positioned prone on a well-padded cerebellar headrest (the face should be lifted and gently massaged every ≈ 30 minutes by the anesthesiologist to prevent pressure injuries).

Surgical options range from simple unilateral craniectomy of the suture to elaborate reconstruction by a craniofacial team.

Linear craniectomy extends from the sagittal suture to the asterion is often adequate for patients ≤ 12 weeks of age without severe disfigurement. Great care is taken to avoid dural laceration near the asterion which is in the region of the transverse sinus. The excised suture demonstrates an internal ridge. Better results are obtained with earlier surgery, more radical surgery may be necessary after the age of 6 months.

Average blood loss for uncomplicated cases is 100-200 ml and therefore transfusion is often required.

MULTIPLE SYNOSTOSES

Fusion of many or all cranial sutures → oxycephaly (tower skull with undeveloped sinuses and shallow orbits). These patients have elevated ICP.

CRANIOFACIAL DYSMORPHIC SYNDROMES

Over 50 syndromes have been described, Table 8-4, shows a few selected ones.

A number of craniosynostosis syndromes are due to mutations in the FGFR (fibro-blast growth factor receptor) genes. FGFR generelated craniosynostosis syndromes include some classic syndromes (Apert, Crouzon, Pfeiffer…) as well as several newer entities (Beare-Stevenson, Muenke, Jackson-Weiss syndromes). All exhibit autosomal dominant inheritance.

8.5.3. Encephalocele

Cranium bifidum is a defect in the fusion of the cranial bone, it occurs in the mid-line, and is most common in the occipital region. If meninges and CSF herniate through the defect, it is called a meningocele. If meninges and cerebral tissue protrude, it is called an encephalocele.

Encephalocele AKA cephalocele is an extension of intracranial structures outside of the normal confines of the skull. One case was seen for every five cases of spinal myelomeningoceles⁶¹. A nasal polypoid mass in a newbornshould be considered an encephalocele until proven otherwise. See also Differential diagnosis,page 1229.

CLASSIFICATION

System based on Suwanwela and Suwanwela⁶²:

1. occipital: often involves vascular structures

2. cranial vault: comprises ≈ 80% of encephaloceles in Western hemisphere

A. interfrontal

B. anterior fontanelle

C. interparietal: often involves vascular structures

D. temporal

E. posterior fontanelle

3. fronto-ethmoidal: AKA sincipital; 15% of encephaloceles; external opening into face in one of the following 3 regions:

A. nasofrontal: external defect in the nasion

B. naso-ethmoidal: defect between nasal bone and nasal cartilage

C. naso-orbital: defect in the anteroinferior portion of medial orbital wall

4. basal: 1.5% of encephaloceles; (see below)

A. transethmoidal: protrudes into nasal cavity through defect in cribriform plate

B. spheno-ethmoidal: protrudes into posterior nasal cavity

C. transsphenoidal: protrudes into sphenoid sinus or nasopharynx through patent craniopharyngeal canal (foramen cecum)

D. fronto-sphenoidal or spheno-orbital: protrudes into orbit through superior orbital fissure

5. posterior fossa: usually contains cerebellar tissue and ventricular component

BASAL ENCEPHALOCELE

The only group that does not produce a visible soft tissue mass. May present as CSF leak or recurrent meningitis. May be associated with other craniofacial deformities, including: cleft lip, bifid nose, opticnerve dysplasia, coloboma and microphthalmia, hypothalamic-pituitary dysfunction.

Iniencephaly is characterized by defects around the foramen magnum, rachischisis and retrocollis. Most are stillborn, some survive up to age 17.

ETIOLOGY

Two main theories:

1. arrested closure of normal confining tissue allows herniation through persistent defect

2. early outgrowth of neural tissue prevents normal closure of cranial coverings

TREATMENT

Occipital encephalocele

Surgical excision of the sac and its contents with water-tight dural closure. It must be kept in mind that vascular structures are often included in the sac. Hydrocephalus is often present and may need to be treated separately.

Basal encephalocele

Caution: a transnasal approach to a basal encephalocele (even for biopsy alone) may be fraught with intracranial hemorrhage, meningitis, or persistent CSF leak. Usually a combined intracranial approach (with amputation of the extracranial mass) and trans-nasal approach is used.

OUTCOME

Occipital encephalocele

The prognosis is better in occipital meningocele than in encephalocele. The prognosis is worse if a significant amount cerebral tissue is present in the sac, if the ventricles extend into the sac, or if there is hydrocephalus. Less than ≈ 5% of infants with encephalocele develop normally.

8.6. Chiari malformation

The term “Chiari malformation” (after pathologist, Hans Chiari) is preferred for type 1 malformations, with the commonly used term “Arnold-Chiari malformation” reserved for type 2 malformation.

The Chiari malformations consists of four types of hindbrain abnormalities, probably unrelated to each other. The majority are types 1 or 2 (see Table 8-5), a very limited number of cases comprise the remaining types.

Table 8-5 Comparisons of Chiari type 1 and 2 anomalies (adapted⁶³)

Finding	Chiari type 1 (see below)	Chiari type 2 (see page 238)
caudal dislocation of medulla	unusual	yes
caudal dislocation into cervical canal	tonsils	inferior vermis, medulla, 4th ventricle
spina bifida (myelomeningocele)	may be present	rarely absent
hydrocephalus	may be absent	rarely absent
medullary “kink”	absent	present in 55%
course of upper cervical nerves	usually normal	usually cephalad
usual age of presentation	young adult	infancy
usual presentation	cervical pain, suboccipital H/A	progressive hydrocephalus, respiratory distress

TYPE 1 CHIARI MALFORMATION

Key concepts:

• a heterogeneous entity with the common feature of impaired CSF circulation through the foramen magnum

• cerebellar tonsillar herniation on MRI: criteria vary, > 5 mm below the foramen magnum is often cited, but is neither essential nor diagnostic of the condition

• treatment, when indicated, is surgical, but aspects of what that surgery should entail are controversial (enlargement of foramen magnum is usually involved)

• associated with syringomyelia in 30-70% which almost always improves with treatment of the Chiari malformation

AKA primary cerebellar ectopia⁶⁴, AKA adult Chiari malformation (since it tends to be diagnosed in the 2nd or 3rd decade of life). A heterogeneous group of conditions, with the underlying commonality of disruption of normal CSF flow through the foramen magnum (FM). Some cases are congenital, but others are acquired.

Classically described as a rare abnormality restricted to caudal displacement of cerebellum with tonsillar herniation below the foramen magnum (see MRI below for criteria) and “peg-like elongation of tonsils”. Unlike Chiari type 2, the medulla is not caudally displaced (some authors disagree on this point⁶⁵), the brainstem is not involved, lower cranial nerves are not elongated, and upper cervical nerves do not course cephalad. Syringomyelia^A of the spinal cord is present in 30-70%⁶⁶. Hydrocephalus occurs in 7-9% of patients with Chiari type 1 malformation and syringomyelia⁶⁶.

A. true hydromyelia probably doesn’t occur. CSF flow has not been documented in man, and it is generally not possible to find communication between the syrinx and the central canal in Chiari 1 patients

Cerebellar tonsil descent below FM with impaction, while common, is no longer a sine qua non of diagnosis.

Etiology: may be associated with

1. a small posterior fossa

A. underdevelopment of the occipital bone

B. low lying tentorium (the roof of the p-fossa)

C. thickened or elevated occipital bone (the floor of the p-fossa)

D. space occupying lesion in p-fossa: arachnoid cyst (retrocerebellar or supracerebellar⁶⁷), tumor (e.g. FM meningioma or cerebellar astrocytoma), hypervascular dura

2. has been described with just about anything that takes up intracranial space

A. chronic subdural hematomas

B. hydrocephalus

3. following lumboperitoneal shunt (see page 317) or multiple (traumatic) LPs⁶⁸: acquired Chiari 1 malformation (usually asymptomatic)

4. arachnoid web or scar or fibrosis around brainstem and tonsils near FM

5. abnormalities of the upper cervical spine

A. hypermobility of the craniovertebral junction

B. Klippel-Feil syndrome

C. occipitalization of the atlas

D. anterior indentation at foramen magnum: e.g. basilar invagination or retroversion of the odontoid process

6. Ehlers-Danlos syndrome

7. craniosynostosis: especially cases involving all sutures

8. retained rhomboid roof: rare

EPIDEMIOLOGY

Average age at presentation is 41 years (range: 12-73 yrs). Slight female preponderance (female:male = 1.3:1). Average duration of symptoms clearly related to Chiari malformation is 3.1 yrs (range: 1 month-20 yrs); if nonspecific complaints, e.g. H/A, are included, this becomes 7.3 years⁶⁹. This latency is probably lower in the MRI era.

CLINICAL

Patients with Chiari type 1 malformation may present due to any or all of the following:

1. compression of brain stem at the level of the foramen magnum

2. hydrocephalus

3. syringomyelia

4. isolation of the intracranial pressure compartment from the spinal compartment causing transient elevations of intracranial pressure

5. 15-30% of patients with adult Chiari malformation are asymptomatic⁷⁰

SYMPTOMS

The most common symptom is pain (69%), especially headache which is usually felt in the suboccipital region (see Table 8-6). H/A are often brought on by neck extension or valsalva maneuver. Weakness is also prominent, especially unilateral grasp. Lhermitte’s sign may also occur. Lower extremity involvement usually consists of bilateral spasticity.

Table 8-6 Presenting symptoms in Chiari 1 malformation (71 cases⁶⁵)

Symptom	%
pain	69%
H/A	34%
neck (suboccipital, cervical)	13%
girdle	11%
arm	8%
leg	3%
weakness (1 or more limbs)	56%
numbness (1 or more limbs)	52%
loss of temperature sensation	40%
painless burns	15%
unsteadiness	40%
diplopia	13%
dysphasia	8%
tinnitus	7%
vomiting	5%
dysarthria	4%
miscellaneous
dizziness	3%
deafness	3%
fainting	3%
facial numbness	3%
hiccough	1%
facial hyperhidrosis	1%

Table 8-7 Presenting signs in Chiari I malformation (127 patients⁶⁹)

Sign	%
hyperactive lower extremity reflexes	52%
nystagmus*	47%
gait disturbance	43%
hand atrophy	35%
upper extremity weakness	33%
“cape” sensory loss	31%
cerebellar signs	27%
hyperactive upper extremity reflexes	26%
lower cranial nerve dysfunction	26%
Babinski sign	24%
lower extremity weakness	17%
dysesthesia	17%
fasciculation	11%
Horner’s sign	6%

* classically: downbeat nystagmus on vertical movement, and rotatory nystagmus on horizontal movement; also includes oscillopsia⁷²

SIGNS

See Table 8-7. Three main patterns of clustering of signs⁶⁵:

1. foramen magnum compression syndrome (22%): ataxia, corticospinal and sensory deficits, cerebellar signs, lower cranial nerve palsies. 37% have severe H/A

2. central cord syndrome (65%): dissociated sensory loss (loss of pain & temperature sensation with preserved touch & JPS), occasional segmental weakness, and long tract signs (syringomyelic syndrome⁷¹). 11% have lower cranial nerve palsies

3. cerebellar syndrome (11%): truncal and limb ataxia, nystagmus, dysarthria

Downbeat nystagmus is considered a characteristic of this condition. 10% will have a normal neurologic exam with occipital H/A as their only complaint. Some patients may present primarily with spasticity.

NATURAL HISTORY

The natural history is not known with certainty (only 2 reports on “natural history”). A patient may remain stable for years, with intermittent periods of deterioration. Rarely, spontaneous improvement may occur (debated).

EVALUATION

Plain x-rays

Of 70 skull x-rays, only 36% were abnormal (26% showed basilar impression, 7% platybasia, and 1 patient each with Paget’s and concave clivus); in 60 C-spine x-rays, 35% were abnormal (including assimilation of atlas, widened canal, cervical fusions, agenesis of posterior arch of atlas).

MRI

Diagnostic test of choice. Easily shows many of the classic abnormalities described earlier, including tonsillar herniation, as well as hydrosyringomyelia which occurs in 20-30% of cases. Also demonstrates ventral brain stem compression when present. Other findings include: hydrocephalus, empty sella.

Tonsillar herniation: Criteria for the descent of the tonsillar tips below the foramen magnum (FM) to diagnose Chiari type 1 malformation have gone through a number of reconsiderations.

Σ	Tonsillar herniation identified radiographically is of limited prognostic value in diagnosing Chiari I malformation, and requires clinical correlation.

Initially, > 5 mm was defined as clearly pathologic⁷³ (with 3-5 mm being borderline). Barkovich⁷⁴ found tonsillar positions as shown in Table 8-8, and Table 8-9 shows the effect of utilizing 2 vs. 3 mm as the lowest normal position.

Table 8-8 Location of cerebellar tonsils below foramen magnum⁷⁴

Group	Mean*	Range
normal	1 mm above	8 mm above to 5 mm below
Chiari I	13 mm below	3-29 mm below

* based on measurements in 200 normals and 25 Chiari I patients taken in relation to the lower part of the foramen magnum

Table 8-9 Criteria for Chiari I⁷⁴

Criteria for lowest extent of tonsils accepted as normal	Sensitivity for Chiari I	Specificity for Chiari I
2 mm below FM	100%	98.5%
3 mm below FM	96%	99.5%

The tonsils normally ascend with age⁷⁵ as shown in Table 8-10.

Patients with syringohydromyelia without hindbrain herniation that responded to p-fossa decompression have been described⁷⁶ (so-called “Chiari zero malformation”). Conversely, 14% of patients with tonsillar herniation > 5 mm are asymptomatic⁷⁷ (average extent of ectopia in this group was 11.4 ± 4.86 mm).

Potentially more significant than the absolute tonsillar descent is the amount of compression of the brainstem at the FM, best appreciated on axial T2WI MRI though the FM. Complete obliteration of CSF signal and compression of the brainstem at the FM by impacted tonsils is a common significant finding.

Cine MRI: AKA CSF flow study. May demonstrate blockage of CSF flow at FM. Not widely available. Accuracy is not high, therefore usually does not alter management.

Table 8-10 Tonsillar position relative to FM at various ages⁷⁵

Age (years)	Normal (mm)*	2 S.D.† (mm)
0-9	–1.5	–6
10-19	–0.4	–5
20-29	–1.1
30-39	0.0	–4
40-49	0.1
50-59	0.2
60-69	0.2
70-79	0.6
80-89	1.3	–3

* negative number indicates distance below FM

^† S.D. = standard deviation. Descent > 2 S.D. beyond normal is suggested as a criteria for tonsillar ectopia

Myelography

Only 6% false negative. Must run dye all the way up to the foramen magnum.

CT

CT has difficulty evaluating the foramen magnum region due to bony artifact. When combined with intrathecal iodinated contrast (myelogram), reliability improves. Findings: tonsillar descent and/or ventricular dilatation.

TREATMENT

Indications for surgery

Since patients respond best when operated on within 2 years of the onset of symptoms (see Operative results below), early surgery is recommended for symptomatic patients. Asymptomatic patients may be followed and operated upon if and when they become symptomatic. Patients who have been symptomatic and stable for years may be considered for observation, with surgery indicated for signs of deterioration.

Surgical techniques

The most frequently performed operation is posterior fossa decompression (suboccipital craniectomy), with or without other procedures (usually combined with dural patch grafting and cervical laminectomy of C1, sometimes to C2 or C3). Options for grafts: same incision (pericranium), separate incision (e.g. or fascia lata), and allograft (avoided by many authors because of dissatisfaction with ability to provide water-tight closure and because of infectious risks.

Goals of surgery: decompress the brain stem and reestablish normal flow of CSF at the craniocervical junction.

The patient is positioned prone on chest rolls with the head in a Mayfield head-holder or in a horseshoe headrest. Flex the neck to open the interspace between the occiput and posterior arch of C1. The shoulders are retracted inferiorly with adhesive tape. If a fascia lata graft is to be taken, elevate one thigh on a sandbag. A midline incision from inion to ≈ C2 spinous process is made. The removal of bone above the foramen magnum should be ≈ 3 cm high by ≈ 3 cm wide (keep the posterior-fossa part of these operations small, the main thrust is to open the foramen magnum to decompress the tonsils and an upper cervical laminectomy; the compression is not in the p-fossa). Excessive removal of occipital bone may allow the cerebellar hemispheres to herniate through the opening, and create additional problems. If a pericranial graft is to be taken, it should be harvested at this time to reduce the amount of blood entering the subsequent dural opening⁷⁸.

Pericranial graft can be procured without extending the incision about the inion using the technique of Dr. Robert Ojemann⁷⁸ with subgaleal dissection and using a monopolar cautery with a bent tip to incise the periosteum and then a Penfield #1 dissector to free it from the bone surface.

Open the dura in a “Y” shaped incision, and excise the triangular top flap. CAUTION: the transverse sinuses are usually abnormally low in Chiari malformations. Suture the patch graft to provide more room for the contents (tonsils + medulla).

An option that is sometimes used in pediatrics is to not initially open the dura but to lyse constricting bands over the dura at the foramen magnum and then and use intraoperative ultrasound to determine if there is adequate room for CSF flow, the dura is then opened only if there is not.

Historical procedures that have been appended to the above: plugging the obex (with muscle or teflon), drainage of syrinx if present (fenestration, usually through dorsal root entry zone, with or without stent or shunt), 4th ventricular shunting, terminal ventriculostomy, and opening foramen of Magendie if obstructed (see reference for illustrations⁷¹). Current recommendations are that these or other additional procedures beyond dural patch grafting are usually not warranted.

Some authors repeatedly admonish not to attempt to remove adhesions binding the tonsils together (to avoid injuring vital structures, including PICAs). Others recommend cautiously separating the tonsils and even shrinking them down with bipolar cautery.

In cases with ventral brain-stem compression, some authors advocate performing a transoral clivus-odontoid resection as they feel these patients may potentially deteriorate with posterior fossa decompression alone⁷⁹. Since this deterioration was reversible with odontoidectomy, it may be reasonable to perform this procedure on patients who show signs of deterioration or progression of basilar impression on serial MRIs after posterior fossa decompression.

OPERATIVE FINDINGS

See Table 8-11. Tonsillar herniation is present in all cases (by definition); the most common position being at C1 (62%). Fibrous adhesions between dura, arachnoid and tonsils with occlusion of foramina of Luschka and Magendie in 41%. The tonsils separated easily in 40%.

Table 8-11 Operative findings in Chiari I (71 patients⁶⁵)

Finding	%
tonsillar descent	100%
below foramen magnum	4%
C1	62%
C2	25%
C3	3%
unspecified level	6%
adhesions	41%
syringomyelia	32%
dural band (at foramen magnum or C1 arch)	30%
vascular abnormalities*	20%
skeletal abnormalities
inverted foramen magnum	10%
keel of bone	3%
C1 arch atresia	3%
occipitalization of C1 arch	1%
cervicomedullary “hump”	12%

* vascular abnormalities: PICA dilated or abnormal course in 8 patients (PICA often descends to lower margin of tonsils⁷¹); large dural venous lakes in 3

SURGICAL COMPLICATIONS

After suboccipital craniectomy plus C1-3 laminectomy in 71 patients, with dural patch grafting in 69, one death due to sleep apnea occurred 36 hrs post-op. Respiratory depression was the most common post-op complication (in 10 patients), usually within 5 days, mostly at night. Close respiratory monitoring is therefore recommended⁶⁵. Other risks of the procedure include: CSF leak, herniation of cerebellar hemispheres, vascular injuries (to PICA…).

OPERATIVE RESULTS

See Table 8-12. Patients with pre-op complaints of pain generally respond well to surgery. Weakness is less responsive to surgery, especially when muscle atrophy is present⁷⁹. Sensation may improve when the posterior columns are unaffected and the deficit is due to spinothalamic involvement alone.

Rhoton feels that the main benefit of operation is to arrest progression.

The most favorable results occurred in patients with cerebellar syndrome (87% showing improvement, no late deterioration). Factors that correlate with a worse outcome are the presence of atrophy, ataxia, scoliosis, and symptoms lasting longer than 2 years⁷⁹.

Table 8-12 Long-term follow-up after surgery for Chiari I malformation (69 patients, 4 years mean F/U⁶⁵)

early improvement of pre-op symptoms	82%
percent of above that relapsed*	21%
early improvement of pre-op signs	70%
no change from pre-op status	16%
worse than pre-op	0

* these patients deteriorated to pre-op status (none deteriorated further) within 2-3 years of surgery; relapse occurred in 30% with foramen magnum compression syndrome, and in 21% with central cord syndrome

TYPE 2 (ARNOLD)-CHIARI MALFORMATION

Key concepts:

• usually associated with myelomeningocele, often accompanied by hydrocephalus

• pathology includes: caudally displaced cervicomedullary junction, small posterior fossa, tectal beaking. Is probably not due to tethering

• major clinical findings: swallowing difficulties, apnea, stridor, opisthotonos, downbeat nystagmus

• when symptomatic: always check the shunt first! Then, consider surgical decompression (which cannot correct intrinsic brainstem abnormalities)

Usually associated with myelomeningocele (MM), or rarely spina bifida occulta.

PATHOPHYSIOLOGY

Probably does not result from tethering of the cord by the associated MM. More likely due to primary dysgenesis of the brainstem with multiple other developmental anomalies⁸⁰.

Major findings

Caudally dislocated cervicomedullary junction, pons, 4th ventricle and medulla. Cerebellar tonsils located at or below the foramen magnum. Replacement of normal cervicomedullary junction flexure with a “kink-like deformity”.

Other possible associated findings:

1. beaking of tectum

2. absence of the septum pellucidum with enlarged interthalamic adhesion: absence of the septum pellucidum is thought to be due to necrosis with resorption secondary to hydrocephalus, and not a congenital absence^{81 (p 178)}

3. poorly myelinated cerebellar folia

4. hydrocephalus: present in most

5. heterotopias

6. hypoplasia of falx

7. microgyria

8. degeneration of lower cranial nerve nuclei

9. bony abnormalities:

A. of cervicomedullary junction

B. assimilation of atlas

C. platybasia

D. basilar impression

E. Klippel-Feil deformity: see page 253

10. hydromyelia

11. craniolacunia of the skull (see below)

PRESENTATION

Findings are due to brain stem and lower cranial nerve dysfunction. Onset is rare in adulthood. The presentation of neonates differs substantially from older children, and neonates were more likely to develop rapid neurological deterioration with profound brain stem dysfunction over a period of several days than were older children in whom symptoms were more insidious and rarely as severe⁸².

Findings include^{82, 83}:

1. swallowing difficulties (neurogenic dysphagia) (69%)⁸⁴. Manifests as poor feeding, cyanosis during feeding, nasal regurgitation, prolonged feeding time, or pooling of oral secretions. Gag reflex often decreased. More severe in neonates

2. apneic spells (58%): due to impaired ventilatory drive. More common in neonates

3. stridor (56%): more common in neonates, usually worse on inspiration (abductor and occasionally adductor vocal cord paralysis seen on laryngoscopy) due to 10th nerve paresis; usually transient, but may progress to respiratory arrest

4. aspiration (40%)

5. arm weakness (27%) that may progress to quadriparesis⁸⁵

6. opisthotonos (18%)

7. nystagmus: especially downbeat nystagmus

8. weak or absent cry

9. facial weakness

DIAGNOSTIC EVALUATION

Skull films

May demonstrate cephalofacial disproportion from congenital HCP. Craniolacunia (AKA lückenschädel) in 85% (round defects in the skull with sharp borders, separated by irregularly branching bands of bone; not due to increased ICP). Low lying internal occipital protuberance (foreshortened posterior fossa). Enlarged foramen magnum in 70%; elongation of upper cervical lamina⁶³.

CT and/or MRI findings

• primary findings

A. “Z” bend deformity of medulla*

B. cerebellar peg

C. tectal fusion (“tectal beaking”)

D. enlarged massa intermedia (interthalamic adhesion)*

E. elongation/cervicalization of medulla

F. low attachment of tentorium

• associated findings

A. hydrocephalus

B. syringomyelia in the area of the cervicomedullary junction (reported incidence in pre MRI era⁷⁹ ranges from 48-88%)

C. trapped fourth ventricle

D. cerebellomedullary compression

E. agenesis/dysgenesis of corpus callosum*

* items with an asterisk are best appreciated on MRI

Laryngoscopy

Performed in patients with stridor to rule out croup or other upper respiratory tract infection.

TREATMENT

• insert CSF shunt for hydrocephalus (or check function of existing shunt)

• if neurogenic dysphagia, stridor, or apneic spells occur, expeditious posterior fossa decompression is recommended (see below) (required in 18.7% of MM patients⁸³); before recommending decompression, always make sure the patient has a functioning shunt!

Surgical decompression

NB: it has been argued that part of the explanation for the poor operative results in infants is that many of the neurological findings may be due in part to intrinsic (uncorrectable) abnormalities which surgical decompression cannot improve^{86, 87}. A dissenting view is that the histologic lesions are due to chronic brain stem compression and concomitant ischemia, and that expeditious brain stem decompression should be carried out when any of the following critical warning signs develop: neurogenic dysphagia, stridor, apneic spells⁸².

Surgical technique:

Decompression of cerebellar tonsils, usually with dural graft to decompress dura. Patients is placed prone, with the neck flexed. A suboccipital craniectomy is combined with a cervical laminectomy which must be carried down to the bottom of the tonsillar tip⁸⁵. A thick constricting dural band is usually found between the C1 arch and foramen magnum. The dura is opened in a “Y” shaped incision. Caution when opening the dura above the level of the foramen magnum in infants as they have a well developed occipital sinus and may have large dural lakes⁸³. DO NOT attempt to dissect tonsils from underlying medulla. In cases with a significant syringomyelic cavity, a syringo-subarachnoid shunt is placed⁸².

Tracheostomy (usually temporary) is recommended if stridor and abductor laryngeal palsy were present pre-op. Close post-op respiratory monitoring is needed for obstruction and reduced ventilatory drive (mechanical ventilation is indicated for hypoxia or hypercarbia).

OUTCOME

68% had complete or near complete resolution of symptoms, 12% had mild to moderate residual deficits, and 20% had no improvement (in general, neonates fared worse than older children)⁸².

Respiratory arrest is the most common cause of mortality (8 of 17 patients who died), with the rest due to meningitis/ventriculitis (6 patients), aspiration (2 patients), and biliary atresia (1 patient)⁸³.

In follow-up ranging 7 mos-6 yrs, 37.8% mortality in operated patients.

Pre-op status and the rapidity of neurologic deterioration were the most important prognosticators. Mortality rate is 71% in infants having cardiopulmonary arrest, vocal cord paralysis or arm weakness within 2 weeks of presentation; compared to 23% mortality in patients with a more gradual deterioration. Bilateral vocal cord paralysis was a particularly poor prognosticator for response to surgery⁸².

OTHER CHIARI MALFORMATIONS

CHIARI TYPE 3

Rare. The most severe form. Displacement of posterior fossa structures, with cerebellum herniated through foramen magnum into cervical canal, often with a high cervical or suboccipital encephalomeningocele. Usually incompatible with life.

CHIARI TYPE 4

Cerebellar hypoplasia without cerebellar herniation.

8.7. Dandy Walker malformation

Definition: an enlarged posterior fossa with complete or partial agenesis of the cerebellar vermis and cystic dilatation of the fourth ventricle which is distorted and encased in a membrane. The anomaly was first described by Dandy & Blackfan in 1914, and was named Dandy Walker malformation forty years later by Benda to acknowledge Taggart and Walker’s contributions in 1942⁸⁸.

Differential diagnosis

Disorders with posterior fossa CSF collections include¹⁴⁸:

1. Dandy Walker malformation (DWM)

2. Dandy Walker variant (DWV): vermian hypoplasia and cystic dilatation of the fourth ventricle, without enlargement of the posterior fossa

3. persistent Blake’s pouch cyst (BPC): tetraventricular hydrocephalus, communicating 4th ventricle and posterior fossa cyst, with or without hypoplasia of both the cerebellar vermis and the medial aspects of the cerebellar hemispheres

4. retrocerebellar arachnoid cyst: anteriorly displaces the 4th ventricle and cerebellum, which can produce significant mass effect

5. Joubert’s syndrome: absence or underdevelopment of the cerebellar vermis

6. mega cisterna magna: enlarged posterior fossa secondary to an enlarged cisterna magna with a normal vermis and fourth ventricle)

Differentiating features: DWM and DWV are difficult to distinguish, and may represent a continuum of developmental anomalies that are grouped together as Dandy Walker complex⁸⁹.

Retrocerebellar arachnoid cysts and BPCs may mimic DWM, but these do not have vermian agenesis and the cyst does not open into the 4th ventricle. The position of the choroid plexus of the fourth ventricle is normal in arachnoid cysts, absent in Dandy Walker malformations, and displaced into the superior cyst wall in BPC. An intrathecal enhanced CT scan (performed after instilling iodinated contrast into a ventricular catheter) would identify a mega cisterna magna which communicates with the ventricles, while DWM and most but not all arachnoid cysts do not.

Pathophysiology

The etiology of DWM is unknown. Multiple unsatisfactory theories have been abandoned. DWM is likely due to dysembryogenesis, secondary to insults of varying severity to the cerebellum and 4th ventricle. This results in agenesis of the cerebellar vermis with a large posterior fossa cyst communicating with an enlarged 4th ventricle^{88, 89}.

Hydrocephalus occurs in 70-90% of cases, and Dandy Walker malformation is present in 2-4% of all cases of hydrocephalus.

Risk factors and epidemiology

Gestational exposure to rubella, CMV, toxoplasmosis, warfarin, alcohol, and isotretinoin are thought to be predisposing factors. Autosomal recessive inheritance has been identified in a few cases, but a genetic basis is lacking in most. Incidence: 1 per 25,000-35,000 live births⁸⁸. Male:female=1:3.

Associated abnormalities

CNS abnormalities include agenesis of the corpus callosum in 17%⁹⁰, and occipital encephalocele in 7%. Other findings include heterotopias, spina bifida, syringomyelia, microcephaly, dermoid cysts, porencephaly, and Klippel-Feil deformity. Most have an enlarged posterior fossa with elevation of the torcular herophili. Atresia of the foramina of Magendie and Luschka may occur⁹¹.

Systemic abnormalities include⁹⁰: facial abnormalities (e.g. angiomas, cleft palates, macroglossia, facial dysmorphia), ocular abnormalities (e.g. coloboma, retinal dysgenesis, microphthalmia), and cardiovascular anomalies (e.g. septal defects, patent ductus arteriosus, aortic coarctation, dextrocardia). Note: be aware of the possibility of a cardiac abnormality when considering surgery on these patients.

Treatment

Early decompression of ventriculomegaly is recommended to achieve maximum cognitive development. In the absence of hydrocephalus, DWM may be followed. When treatment is necessary, the posterior fossa cyst must be shunted. Shunting the lateral ventricles alone is contraindicated because of the risk of upward herniation⁹². However, it is important to confirm patency of the cerebral aqueduct, otherwise the supratentorial ventricles need to be shunted concomitantly. Varying reports exist regarding rates of associated aqueductal stenosis, although it is widely believed to be rare.

Another option once used commonly is excision of the obstructing membrane. This has fallen out of favor due to its associated risks of morbidity and mortality. However, it remains an option for patients with frequent shunt malfunctions.

Newer treatments include endoscopic third ventriculostomy in cases where the aqueduct is patent, however further study is necessary^{93, 94}.

Prognosis

Prognosis ranges widely as there are various levels of severity of the malformation. Some pediatric neurosurgical literature quotes 12-50% mortality rates, although this is improving with modern shunting techniques. Only 50% have normal IQ. Ataxia, spasticity, and poor fine motor control are common. Seizures occur in 15%.

8.8. Aqueductal stenosis

Aqueductal stenosis (AqS) produces what is sometimes called triventricular hydrocephalus, characterized by a normal sized 4th ventricle and enlarged third and lateral ventricles on MRI or CT. Most cases occur in children, however some present for the first time in adulthood.

ETIOLOGIES

1. a congenital malformation: may be associated with Chiari malformation or neurofibromatosis

2. acquired

A. due to inflammation (following hemorrhage or infection, e.g. syphilis, T.B.)

B. neoplasm: especially brainstem astrocytomas (including tectal gliomas, see page 608), lipomas

C. quadrigeminal plate arachnoid cysts

IN INFANCY

AqS is a frequent cause of congenital hydrocephalus (HCP) (up to 70% of cases⁶⁰), but occasionally may be the result of HCP. Patients with congenital AqS usually have HCP at birth or develop it within ≈ 2-3 mos. Congenital AqS may be due to an X-linked recessive gene⁶¹. Four types of congenital AqS described by Russell (summarized⁹⁵):

1. forking: multiple channels (often narrowed) with normal epithelial lining that do not meet, separated by normal nervous tissue. Usually associated with other congenital abnormalities (spina bifida, myelomeningocele)

2. periaqueductal gliosis: luminal narrowing due to subependymal astrocytic proliferation

3. true stenosis: aqueduct histologically normal

4. septum

IN ADULTHOOD

AqS may be an overlooked cause of “normal pressure hydrocephalus” in the adult⁹⁶. It is unknown why some cases of AqS would remain occult, and manifest only in adult-hood. In one series of 55 cases⁹⁷, 35% had duration of symptoms < 1 year, 47% for 1-5 years; the longest was 40 yrs. Although most follow this longstanding benign course, there are reports of elevated ICP and sudden death.

Symptoms

See Table 8-13. Headache was the most common symptom, and had characteristics of H/A associated with elevated ICP. Visual changes were next, and usually consisted of blurring or loss of acuity. Endocrine changes included menstrual irregularities, hypothyroidism, and hirsutism.

Signs

Papilledema was the most common finding (53%). Visual fields were normal in 78%, the remainder having reduced peripheral vision, increased blind spots, quadrantic or hemianopic field cuts, or scotomata. Intellectual impairment was present in at least 36%. Other signs included: ataxia (29%), “pyramidal tract signs” in 44% (mild hemi- or paraparesis (22%), spasticity (22%), or Babinski’s (20%)), anosmia (9%).

EVALUATION

MRI is the test of choice. MRI will show the absence of the normal flow void in the Sylvian aqueduct. Contrast should be given to rule-out tumor.

Table 8-13 Symptoms of aqueductal stenosis presenting in adulthood (55 patients > 16 years age⁹⁷)

Symptom	No.	%
H/A	32	58%
visual disturbances	22	40%
mental deterioration	17	31%
gait disturbance	16	29%
frequent falling	13	24%
endocrine disturbance	10	18%
nausea/vomiting	9	16%
seizures	8	15%
incontinence	7	13%
vertigo	6	11%
LE weakness	4	7%
hemiparesis or hemianesthesia	4	7%
diplopia	3	5%
dysarthria	1
deafness	1

TREATMENT (OF NON-TUMORAL AQS)

Although treatments of the primary lesion have been attempted (e.g. lysis of aqueductal septum), this has fallen into disfavor with the improved efficacy of CSF shunting. CSF is usually shunted to the peritoneum or the vascular system, however shunting to subarachnoid space is also feasible (once obstruction at the level of the arachnoid granulations has been ruled out). A Torkildsen shunt may work in adult cases⁹⁵, however pediatric patients with obstructive hydrocephalus may not have an adequately developed subarachnoid space for this to function properly.

Follow-up of at least two years to rule-out tumor is recommended.

8.9. Neural tube defects

CLASSIFICATION

Various classification systems exist, this one is adapted from Lemire⁹⁸.

1. neurulation defects: non-closure of the neural tube results in open lesions

A. craniorachischisis: total dysraphism. Many die as spontaneous abortion

B. anencephaly: AKA exencephaly. Due to failure of fusion of the anterior neuropore. Neither cranial vault nor scalp covers the partially destroyed brain. Uniformly fatal. Risk of recurrence in future pregnancies: 3%

C. meningomyelocele: most common in lumbar region

1. myelomeningocele (MM): see page 248

2. myelocele

2. postneurulation defects: produces skin-covered (AKA closed) lesions (some may also be considered “migration abnormalities”, see below)

A. cranial

1. microcephaly: see below

2. hydranencephaly: loss of most of cerebral hemispheres, replaced by CSF (see below). Must R/O maximal hydrocephalus (see below)

3. holoprosencephaly: see below

4. lissencephaly: see below

5. porencephaly: see below to distinguish from schizencephaly

6. agenesis of corpus callosum: see below

7. cerebellar hypoplasia/Dandy Walker syndrome: see page 240

8. macroencephaly AKA megalencephaly: see below

B. spinal

1. diastematomyelia, diplomyelia: see Split cord malformation, page 256

2. hydromyelia/syringomyelia: see page 510

Migration abnormalities

A slightly different classification scheme defines the following as abnormalities of neuronal migration (some are considered postneurulation defects, see above):

1. lissencephaly: The most severe neuronal migration abnormality. Maldevelopment of cerebral convolutions (probably an arrest of cortical development at an early fetal age). Infants are severely retarded and usually don’t survive > 2 yrs

A. agyria: completely smooth surface

B. pachygyria: few broad & flat gyri with shallow sulci

C. polymicrogyria: small gyri with shallow sulci. May be difficult to diagnose by CT/MRI, and may be confused with pachygyria

2. heterotopia: abnormal foci of (nonenhancing) gray matter which may be located anywhere from the subcortical white matter to (most commonly) the subependymal lining of the ventricles. May manifest as nodules or as a band of cortex. An early migration defect that results from arrest of radial migration. Almost always presents with seizures

3. cortical dysplasia: a cleft that does not communicate with the ventricle. Heterotopias are common. A migration abnormality not quite as severe as schizencephaly

4. schizencephaly:

A. sine qua non: cleft that communicates with the ventricle (communication may be confirmed with CT cisternogram if necessary)

B. cleft lined with cortical grey matter (often abnormal, may have polymicrogyria). This distinguishes it from porencephaly, a cystic lesion lined with connective or glial tissue that may communicate with the ventricular system, often caused by vascular infarcts or following intracerebral hemorrhage or penetrating trauma (including repeated ventricular punctures)

C. two forms:

1. open lipped: large cleft to ventricle. Very severe forms may mimic hydranencephaly (see below)

2. close lipped (walls fused): look for a dimple in the lateral wall of the lateral ventricle immediately under the cortical cleft (the appearance of which may mimic an enlarged sulcus)

D. may be unilateral or bilateral

E. pia and arachnoid fuse

F. there may be an “abnormal” vein that represents a cortical vein that now looks medullary because it follows the cortex into the cleft)

G. absence of septum pellucidum in 80-90%

H. presentation may range from seizures to hemiparesis depending on size and location

HYDRANENCEPHALY

A post-neurulation defect. Total or near-total absence of the cerebrum (small bands of cerebrum may be consistent with the diagnosis⁹⁹), with intact cranial vault and meninges, the intracranial cavity being filled with CSF. There is usually progressive macrocrania, but head size may be normal (especially at birth), and, occasionally, microcephaly may occur. Facial dysmorphism is rare.

May be due to a variety of causes, the most commonly cited is bilateral ICA infarcts (which results in absence of brain tissue supplied by the anterior and middle cerebral arteries with preservation in the distribution of the PCA). May also be due to infection (congenital or neonatal herpes, toxoplasmosis, equine virus).

Less affected infants may appear normal at birth, but are often hyperirritable and retain primitive reflexes (Moro, grasp, and stepping reflex) beyond 6 mo. They rarely progress beyond spontaneous vowel production and social smiling. Seizures are common.

Differentiation from hydrocephalus: Progressive enlargement of CSF spaces may occur which can mimic severe (“maximal”) hydrocephalus (HCP). It is critical to differentiate the two since true HCP may be treated by shunting which may produce some re-expansion of the cortical mantle. Many means to distinguish hydranencephaly and HCP have been described, including:

1. EEG: shows no cortical activity in hydranencephaly (maximal HCP typically produces an abnormal EEG, but background activity will be present throughout the brain⁹⁹) and is one of the best ways to differentiate the two

2. CT^{99, 100}, MRI or ultrasound: majority of intracranial space is occupied by CSF. Usually do not see frontal lobes or frontal horns of lateral ventricles (there may be remnants of temporal, occipital or subfrontal cortex). A structure consisting of brainstem nodule (rounded thalamic masses, hypothalamus) and medial occipital lobes sitting on the tentorium occupies a midline position surrounded by CSF. Posterior fossa structures are grossly intact. The falx is usually intact (unlike alobar holoprosencephaly), and is not thickened, but may be displaced laterally. In HCP, some cortical mantle is usually identifiable

3. transillumination of the skull: in a darkened room, a bright light is placed against the surface of the skull. To transilluminate, the patient must be < 9 mos old and the cortical mantle under the light source must be < 1 cm thick^{61 (p 215)}, can also occur if fluid displaces the cortex inward (e.g. subdural effusions). Too insensitive to be very helpful

4. angiography: in “classic” cases resulting from bilateral ICA occlusion, no flow through supraclinoid carotids and a normal posterior circulation is expected

Treatment: Shunting may be performed to control head size, but unlike the case with maximal hydrocephalus, there is no restitution of the cerebral mantle.

HOLOPROSENCEPHALY

AKA arhinencephaly. Failure of the telencephalic vesicle to cleave into two cerebral hemispheres. The degree of cleavage failure ranges from the severe alobar (single ventricle, no interhemispheric fissure) to semilobar and lobar (less severe malformations). The olfactory bulbs are usually small and the cingulate gyrus remains fused. Median faciocerebral dysplasia is common, and the degree of severity parallels the extent of the cleavage failure (see Table 8-14). 80% are associated with trisomy (primarily trisomy 13, and to a lesser extent trisomy 18). Survival beyond infancy is uncommon, most survivors are severely retarded, a minority are able to function in society. Some develop shunt dependent hydrocephalus. The risk of holoprosencephaly is increased in subsequent pregnancies of the same couple.

Table 8-14 The five facies of severe holoprosencephaly¹⁰¹

Type of face	Facial features	Cranium and brain findings
cyclopia	single eye or partially divided eye in single orbit; arhinia with proboscis	microcephaly; alobar holoprosencephaly
ethmocephaly	extreme orbital hypotelorism; separate orbits; arhinia with proboscis	microcephaly; alobar holoprosencephaly
cebocephaly	orbital hypotelorism; proboscis-like nose; no median cleft lip	microcephaly; usually has alobar holoprosencephaly
with median cleft lip	orbital hypotelorism; flat nose	microcephaly; sometimes has trigonocephaly; usually has alobar holoprosencephaly
with median philtrum-premaxilla anlage	orbital hypotelorism; bilateral lateral cleft lip with median process representing philtrumpremaxillary anlage; flat nose	microcephaly; sometimes has trigonocephaly; semilobar or lobar holoprosencephaly

MICROCEPHALY

Definition: head circumference more than 2 standard deviations below the mean for sex and gestational age. Terms that are sometimes used synonymously: microcrania, microcephalus. Not a single entity, many of the conditions in Table 8-14 may be associated with microcephaly. It may also result from maternal cocaine abuse¹⁰². It is important to differentiate microcephaly from a small skull resulting from craniosynostosis in which surgical treatment may provide opportunity for improved cerebral development.

MACROENCEPHALY^{60 (PP 109)}

AKA macrencephaly, AKA megalencephaly (not to be confused with macrocephaly, which is enlargement of the skull (see page 1206)). Not a single pathologic entity. An enlarged brain which may be due to: hypertrophy of gray matter alone, gray and white matter, presence of additional structures (glial overgrowth, diffuse gliomas, heterotopias, metabolic storage diseases…). May be seen in neurocutaneous syndromes (especially neurofibromatosis).

Brains may weigh up to 1600-2850 grams. IQ may be normal, but developmental delay, retardation, spasticity and hypotonia may occur. Head circumference is 4-7 cm above mean. The usual signs of hydrocephalus (frontal bossing, bulging fontanelle, “setting sun” sign, scalp vein engorgement) are absent. Imaging studies (CT or MRI) show normal sized ventricles and can be used to rule out extra-axial fluid collections.

RISK FACTORS

1. early administration of folic acid^103-105 (0.4 mg/d if no history of neural tube defects) reduces the incidence of neural tube defects (NTDs) (confirm that vitamin B₁₂ levels are normal, see page 1187)

2. folate antagonists (e.g. carbamazepine) doubles the incidence of MM

3. mothers with 5, 10-methylenetetrahydrofolate reductase (MTHFR) gene poly-morphism → reduced levels of tissue folate¹⁰⁶

4. use of valproic acid (Depakene®) during pregnancy is associated with a 1-2% risk of NTD¹⁰⁷

5. maternal heat exposure in the form of hottubs, saunas or fever (but not electric blankets) in the first trimester was associated with an increased risk of NTDs¹⁰⁸

6. obesity (before and during pregnancy) increases the risk of NTD^{109, 110}

7. maternal cocaine abuse may increase the risk of microcephaly, disorders of neuronal migration, neuronal differentiation and myelination¹⁰²

PRENATAL DETECTION OF NEURAL TUBE DEFECTS

Serum alpha-fetoprotein (AFP)

(See Alpha-fetoprotein on page 721 for background). A high maternal serum AFP (≥ 2 multiples of the median for the appropriate week of gestation) between 15-20 weeks gestation carries a relative risk of 224 for neural tube defects, and an abnormal value (high or low) was associated with 34% of all major congenital defects¹¹¹. The sensitivity of maternal serum AFP for spina bifida was 91% (10 of 11 cases), it was 100% for 9 cases of anencephaly. However, other series show a lower sensitivity. Closed lumbosacral spine defects, accounting for ≈ 20% of spina bifida patients¹¹², will probably be missed by serum AFP screening, and may also be missed on ultrasound. Since maternal serum AFP rises during normal pregnancy, an overestimate of gestational age may cause an elevated AFP to be interpreted as normal, and an underestimate may cause a normal level to be interpreted as elevated¹¹³.

Ultrasound

Prenatal ultrasound will detect 90-95% of cases of spina bifida, and thus in cases of elevated AFP, it can help differentiate NTDs from non-neurologic causes of elevated AFP (e.g. omphalocele), and can help to more accurately estimate gestational age.

Amniocentesis

For pregnancies subsequent to a MM, if prenatal ultrasound does not show spinal dysraphism, then amniocentesis is recommended (even if abortion is not considered, it may allow for optimal post-partum care if MM is diagnosed). Amniotic fluid AFP levels are elevated with open neural tube defects, with a peak between weeks 13-15 of pregnancy. Amniocentesis also carries a ≈ 6% risk of fetal loss in this population.

8.9.1. Agenesis of the corpus callosum

A failure of commissuration occurring ≈ 2 weeks after conception. Results in expansion of the third ventricle and separation of the lateral ventricles (which develop dilated occipital horns and atria, and concave medial borders).

The corpus callosum (CC) forms from rostrum (genu) to splenium¹¹⁴, in agenesis there may be an anterior portion with absence of the posterior segment (the converse occurs infrequently). Absence of the anterior CC with presence of some posterior CC is indicative of some form of holoprosencephaly.

Incidence

1 in 2,000-3,000 neuroradiological examinations.

Associated neuropathologic findings¹⁴

• porencephaly

• microgyria

• interhemispheric lipomas and lipomas of the corpus callosum (see page 225)

• arhinencephaly

• optic atrophy

• colobomas

• hypoplasia of the limbic system

• bundles of Probst: aborted beginnings of corpus callosum, bulge into lateral ventricles

• loss of horizontal orientation of cingulate gyrus

• schizencephaly (see page 243)

• anterior and hippocampal commissures may be totally or partially absent¹¹⁵

• hydrocephalus

• cysts in the region of the corpus callosum

• spina bifida with or without myelomeningocele

• absence of the septum pellucidum: see page 247

Possible presentation

• hydrocephalus

• microcephaly

• seizures (rare)

• precocious puberty

• disconnection syndrome: more likely with acquired CC defect than in congenital

May be an incidental finding, and by itself may have no clinical significance. How-ever, may be occur as part of a more complex clinical syndrome or chromosomal abnormality (e.g. Aicardi syndrome: agenesis of CC, seizures, retardation, patches of retinal pigmentation).

8.10. Absence of the septum pellucidum

Etiologies^{81 (p 178)}

1. holoprosencephaly: see page 244

2. schizencephaly: see page 243

3. agenesis of the corpus callosum: see page 246

4. Chiari type 2 malformation: see page 238

5. basal encephalocele

6. porencephaly/hydranencephaly

7. may occur in severe hydrocephalus: thought to be due to necrosis with resorption

8. septo-optic dysplasia: see below

Septo-optic dysplasia^{81 (p 175-8), 116}

AKA de Morsier syndrome. Incomplete early morphogenesis of anterior midline structures produces hypoplasia of the optic nerves and possibly optic chiasm (affected patients are blind) and pituitary infundibulum. The septum pellucidum is absent in about half the cases. About half the cases also have schizencephaly (see page 243).

Presentation may be due to secondary hypopituitarism manifesting as dwarfism, isolated growth hormone deficiency, or panhypopituitarism. Occasionally hypersecretion of growth hormone, corticotropin or prolactin may occur, and precocious puberty may occur. Most patients are of normal intelligence although retardation may occur. Septo-optic dysplasia may be a less severe form of holoprosencephaly, and occasionally may occur as part of this anomaly (with its attendant poorer prognosis for function or survival, see page 244). The ventricles may be normal or dilated. May be seen by the neurosurgeon because of concerns of possible hydrocephalus.

8.11. Spinal dysraphism (spina bifida)

DEFINITIONS⁶¹

spina bifida occulta

Congenital absence of a spinous process and variable amounts of lamina. No visible exposure of meninges or neural tissue (see below).

The following two entities are grouped together under the term spina bifida aperta (aperta from the Latin for “open”) or spina bifida cystica.

meningocele	Congenital defect in vertebral arches with cystic distension of meninges, but no abnormality of neural tissue. One third have some neurologic deficit.
myelomeningocele	Congenital defect in vertebral arches with cystic dilatation of meninges and structural or functional abnormality of spinal cord or cauda equina (see below).

SPINA BIFIDA OCCULTA (SBO)

Reported prevalence range of SBO: 5-30% of North Americans (5-10% is probably more realistic). The defect may be palpable, and there may be overlying cutaneous manifestations (see cutaneous stigmata of dysraphism in Table 8-17, page 255).

Often an incidental finding, usually of no clinical importance when it occurs alone. Numerous reviews have shown no statistical association of SBO with nonspecific LBP^{117, 118}. An increased incidence of disc herniation was shown in one study¹¹⁹.

SBO may occasionally be associated with diastematomyelia, tethered cord, lipoma, or dermoid tumor. When symptomatic from one of these associated conditions, the presentation is usually that of tethered cord (gait disturbance, leg weakness and atrophy, urinary disturbance, foot deformities…, see Tethered cord syndrome, page 254).

MYELOMENINGOCELE

EMBRYOLOGY

The anterior neuropore closes at gestation day 25. The caudal neuropore closes at day 28.

EPIDEMIOLOGY/GENETICS

Incidence of spina bifida with meningocele or myelomeningocele (MM) is 1-2/1000 live births (0.1-0.2%). Risk increases to 2-3% if there is one previous birth with MM, and 6-8% after two affected children. The risk is also increased in families where close relatives (e.g. siblings) have given birth to MM children, especially when on the mother’s side of the family. Incidence may increase in times of war, famine or economic disasters, but it may be gradually declining overall¹²⁰. Transmission follows non-Mendelian genetics, and is probably multifactorial. Prenatal folate (in the form of folic acid) lowers the incidence of MM (see page 245).

Hydrocephalus in myelomeningocele

Hydrocephalus (HCP) develops in 65-85% of patients with MM, and 5-10% of MM patients have clinically overt HCP at birth¹²¹. Over 80% of MM patients who will develop HCP do so before age 6 mos. Most MM patients will have an associated Chiari type 2 malformation (see Type 2 (Arnold)-Chiari malformation, page 238). Closure of the MM defect may convert a latent HCP to active HCP by eliminating a route of egress of CSF.

Latex allergy in myelomeningocele

Up to 73% of MM patients are allergic to proteins present in latex (the milky sap from the rubber tree Hevea brasiliensis), found only in naturally occurring rubber products (and which are not present in synthetics such as: silicone, vinyl, plastic, neoprene, nitrile…). The allergy is thought to arise from early and frequent exposure to latex products during medical care for these patients, and there is a suggestion that latex-free surgery on these infants may reduce the risk of the development of latex allergy¹²².

PRENATAL DIAGNOSIS

See Prenatal detection of neural tube defects on page 245.

MANAGEMENT

Intrauterine closure of MM defect

Controversial. Does reduce incidence of Chiari II defect, but it has not been determined if this is clinically significant. Argued whether this reduces incidence of hydrocephalus. Does not improve distal neurologic function.

ADMISSION

1. assessment and management of lesion:

A. measure size of defect

B. assess whether lesion is ruptured or unruptured

1. ruptured: start antibiotics (e.g. nafcillin and gentamicin; D/C 6 hrs after MM closure, or continue if shunt anticipated in next 5 or 6 days)

2. unruptured: no antibiotics necessary

C. cover lesion with telfa, then sponges soaked in lactated ringers or normal saline (form a sterile gauze ring around the lesion if it is cystic and protruding) to prevent desiccation

D. Trendelenburg position, patient on stomach (keeps pressure off lesion)

E. perform surgical closure within 36 hrs unless there is a contraindication to surgery (simultaneous shunt is not usually done except if overt hydrocephalus (HCP) at birth): see Timing of MM closure below

2. neurological assessment and management:

A. items related to spinal lesion

1. watch for spontaneous movement of the LEs (good spontaneous movement correlates with better later functional outcome¹²³)

2. assess lowest level of neurologic function (see Table 8-15) by checking response of LEs to painful stimulus: although some infants will have a clear demarcation between normal and abnormal levels, at least 50% show some mixture of normal, reflex, and autonomous activity (arising from uninhibited anterior horn motor neurons)¹²³

a. differentiating reflex movement from voluntary may be difficult. In general, voluntary movement is not stereotyped with repetitive stimulus and reflex movement usually only persists as long as the noxious stimulus is applied

Table 8-15 Findings in various levels of MM lesion¹²⁴

Paralysis below	Findings
T12	complete paralysis of all muscles in LEs
L1	weak to moderate hip flexion, palpable contraction in sartorius
L2	strong hip flexion and moderate hip adduction
L3	normal hip adduction & almost normal knee extension
L4	normal hip adduction, knee extension & dorsiflexion/inversion of foot; some hip abduction in flexion
L5	normal adduction, flexion & lateral rotation of hip; moderate abduction; normal knee extension, moderate flexion; normal foot dorsiflexion; hip extension absent; • produces dorsiflexed foot and flexed thigh
S1	normal hip flexion & abduction/adduction, moderate extension and lateral rotation; strong knee flexion & inversion/eversion of foot; moderate plantarflexion of foot; extension of all toes, but flexion only of terminal phalanx of great toe; normal medial & lateral hip rotation; complete paralysis of foot intrinsic (except abductor and flexor hallucis brevis); • produces clawing of toes and flattening of sole of foot
S2	difficult to detect abnormality clinically; • with growth this produces clawing of the toes due to weakness of intrinsic muscles of sole of foot (innervated by S3)

B. items related to the commonly associated Chiari type 2 malformation:

1. measure OFC: risk of developing hydrocephalus (see above). Use OFC graphs (see page 312), and also look for abnormal rate of growth (e.g. > 1 cm/day)

2. head U/S within ≈ 24 hrs

3. check for inspiratory stridor, apneic episodes

3. ancillary assessment and management:

A. evaluation by neonatologist to assess for other abnormalities, especially those that may preclude surgery (e.g. pulmonary immaturity). There is an average incidence of 2-2.5 additional anomalies in MM patients

B. bladder: start patient on regular urinary catheterizations, obtain urological consultation (non-emergent)

C. AP & lat spine films: assess scoliosis (baseline)

D. orthopedic consultation for severe kyphotic or scoliotic spine deformities and for hip or knee deformities

SURGICAL CONSIDERATIONS

TIMING OF MM CLOSURE

Early closure of MM defect is not associated with improvement of neurologic function, but evidence supports lower infection rate with early closure. MM should be closed within 24 hrs whether or not membrane is intact (after ≈ 36 hrs the back lesion is colonized and there is increased risk of postoperative infection).

Simultaneous MM defect closure and VP shunting

In patients without hydrocephalus, most surgeons wait at least ≈ 3 days after MM repair before shunting. In MM patients with clinically overt HCP at birth (ventriculomegaly with enlarged OFC and/or symptoms), MM repair and shunting may be performed in the same sitting without increased incidence of infection, and with shorter hospitalization^{125, 126}. It may also reduce the risk of MM repair breakdown previously seen during the interval before shunting. Patient is positioned prone, head turned to right (to expose the right occiput), right knee and thigh flexed to expose right flank (consider using left flank to prevent confusion with appendectomy scar later in life).

TECHNIQUE

Key concepts of surgical treatment:

• critical goals: 1) free placode from dura (to avoid tethering), 2) water-tight dural closure, 3) skin closure (can be accomplished in essentially all cases). Closure does not restore any neurologic function

• timing goal: surgical closure with latex-free setup ideally ≤ 36 hours after birth

• helpful tips: start at normal dura, open as wide as the defect, trim placode if necessary to close dura, undermine skin to achieve closure (avoid trapping skin -> dermoid tumor)

• post-op CSF leak usually means a shunt is required

General principles¹²⁷: keep the exposed neural tissue moist; prevent desiccation. Use latex-free environment (may reduce development of latex allergy, and reduces maternal antibodies may also cross placenta). Do not allow scrubs or chemical antimicrobials to contact neural placode. Do not use monopolar cautery. At every point during the closure, avoid placing tension on the neural placode.

Multiple layer closure is advocated, 5 layers should be attempted, although occasionally only 2 or so layers may be closed. There is no evidence that multiple layer closure either improves neurologic function or prevents later tethering, but there is a suggestion that when tethering does occur, it may be easier to release when a previous multilayered closure was performed. Silastic does not prevent adherence in series with long follow-up (> 6 yrs), and may even render untethering procedures more difficult.

Begin by dividing the abnormal epithelial covering from the normal skin. The piaarachnoid may be separated from the neural tissue. The placode is folded into a tube and the piaarachnoid is then approximated around it with 7-0 suture (absorbable suture, e.g. PDS, may make future re-operation easier). It often helps to start with normal dura above, and then work down. The dura can then be isolated around the periphery and followed deep to the spinal canal superiorly. The dura is then also formed into a tube and approximated in a water-tight closure. If the dura cannot be closed, the placode may be judiciously trimmed. The filum terminale should be divided if it can be located. The skin is then mobilized and closed. Dermoid tumors may result from retained skin during the closure, but alternatively dermoids may also be present congenitally¹²⁸.

If there is a kyphotic deformity, it is repaired at the same sitting as the MM defect closure. The kyphotic bone is rongeured, and 2-0 Vicryl is used to suture the adjacent bones. Some surgeons use a brace post-op, some do not.

POST-OP MANAGEMENT OF MM REPAIR

1. keep patient off all incisions

2. bladder catheterization regimen

3. daily OFC measurements

4. avoid narcotics (midbrain malformation renders these patient more sensitive to respiratory depression from narcotics)

5. if not shunted

A. regular head U/S (twice weekly to weekly)

B. keep patient flat to ↓ CSF pressure on incision

6. if a kyphectomy was done, use of a brace is optional (surgeon preference)

LATE PROBLEMS/ISSUES

Include:

1. hydrocephalus: may mimic ≈ anything listed below. ALWAYS RULE OUT SHUNT MALFUNCTION when a MM patient deteriorates

2. syringomyelia (and/or syringobulbia): see page 510

3. tethered cord (see Tethered cord syndrome, page 254): as many as 70% of MM patients have a tethered cord radiographically (some quote 10-20%), but only a minority are symptomatic. Unfortunately there is no good test to check for symptomatic retethering (SSEPs may deteriorate¹²⁹, myelography may help)

A. scoliosis: early untethering of cord may improve scoliosis (see Scoliosis in tethered cord, page 255)

B. symptomatic tethering may manifest as delayed neurological deterioration¹³⁰

4. dermoid tumor at the MM site¹³¹: incidence ≈ 16% (see page 729)

5. medullary compression at foramen magnum (symptomatic Chiari II malformation, see page 238)

6. use of growth hormone to increase stature is controversial

OUTCOME

Without any treatment, only 14-30% of MM infants survive infancy; these usually represent the least severely involved; 70% will have normal IQ’s. 50% are ambulatory.

With modern treatment, ≈ 85% of MM infants survive. The most common cause of early mortality are complications from the Chiari malformation (respiratory arrest, aspiration…), where late mortality is usually due to shunt malfunction. 80% will have normal IQ. Mental retardation is most closely linked to shunt infection. 40-85% are ambulatory with bracing, however, most choose to use wheelchairs for ease. 3-10% have normal urinary continence, but most may be able to remain dry with intermittent catheterization.

LIPOMYELOSCHISIS

Dorsal spinal dysraphism with lipoma. Six forms are described¹³², the following 3 are clinically important as possible causes of progressive neurologic dysfunction via tethering (see Tethered cord syndrome, page 254) and/or compression:

1. (intra)dural lipoma

2. lipomyelomeningocele (see below)

3. fibrolipoma of the filum terminale

LIPOMYELOMENINGOCELE

A subcutaneous lipoma that passes through a midline defect in the lumbodorsal fascia, vertebral neural arch, and dura, and merges with an abnormally low tethered cord¹³². These may be terminal, dorsal, or transitional (between the two).

The intradural fatty tumor may also be known as lipoma of the cauda equina. In addition to being abnormally low, the conus medullaris is split in the midline dorsally usually at the same level as the bifid spine, and this dorsal myeloschisis may extend superiorly under intact spinal arches¹³³. There is a thick fibrovascular band that joins the lamina of the most cephalic vertebrae with the bifid lamina. This band constricts the meningocele sac and neural tissue, causing a kink in the superior surface of the meningocele. Asymptomatic lipomas of the filum terminale occur in 0.2-4%^{134, 135} of MRIs.

The dura is dehiscent at the level of the dorsal myeloschisis, and reflects onto the placode. The lipoma passes through this dehiscence to become attached to the dorsal surface of the placode, and may continue cephalad under intact arches with the possibility of extension into the central canal superiorly to levels without dorsal myeloschisis. The lipoma is distinct from the normal epidural fat which is looser and more areolar. The subarachnoid space typically bulges to the side contralateral to the lipoma. These lipomas account for 20% of covered lumbosacral masses.

PRESENTATION

In a pediatric series, 56% presented with a back mass, 32% with bladder problems, and 10% because of foot deformities, paralysis or leg pain¹³⁶.

PHYSICAL EXAMINATION

Almost all patients have cutaneous stigmata of the associated spina bifida: fatty subcutaneous pads (located over the midline and usually extends asymmetrically to one side) with or without dimples, port-wine stains, abnormal hair, dermal sinus opening, or skin appendages¹³⁷. Clubbing of the feet (talipes equinovarus) may occur.

The neurologic exam may be normal in up to 50% of patients (most presenting with skin lesion only). The most common neurologic abnormality was sensory loss in the sacral dermatomes.

EVALUATION

Plain LS spine x-rays will show spina bifida in most cases (present in almost all by definition, but some may have segmentation anomalies instead such as butterfly vertebra (see page 138)). Abnormalities of fusion and sacral defects may also be seen.

The abnormally low conus can be demonstrated on myelogram/CT or on MRI. MRI also demonstrates the lipomatous mass (high signal on T1WI, low signal on T2WI).

All patients should have pre-op urological evaluation to document any deficit.

TREATMENT

Since symptoms are due to (1) tethering of the spinal cord, especially during growth spurts, and (2) compression due to progressive deposition of fat, especially during periods of rapid weight gain; the goals of surgery are to release the tethering and reduce the bulk of fatty tumor. Simple cosmetic treatment of the subcutaneous fat pad does not prevent neurologic deficit, and may make later definitive repair more difficult or impossible.

Surgical treatment is indicated when the patient reaches 2 months of age, or at the time of diagnosis if the patient presents later in life. Adjuncts to surgical treatment include evoked potential monitoring and laser. Overall, with surgery, 19% will improve, 75% will be unchanged, and 6% will worsen. Foot deformities often progress regardless.

Surgical technique (modified¹³³)

1. mobilize the subcutaneous mass, it funnels down through the deep fascia

2. open last intact vertebral arch (work from normal dura)

3. identify the fibrovascular band that crosses the most cephalic widely bifid lamina

4. sectioning the fibrovascular band frees the dural tube and releases the sharp kink in the superior surface of the meningocele

5. taking care to preserve dorsal nerve roots, the dura is incised anterior to the duralipoma junction

6. similar procedure is carried out with arachnoid membrane

7. dural/arachnoid incisions are continued around entire extent of tethered conus

8. cord and placode are untethered (monitoring techniques described in Tethered cord syndrome on page 254 are an option)

9. subtotal removal of lipoma: lipoma is then trimmed as completely as possible, intentionally leaving some fat behind to avoid injury to dorsal surface of placode. Superior extension along dorsal surface of cord or into central canal is debulked as much as is safely possible

10. the placode is reformed into a closed neural tube

11. close the pial margins

12. the dura is closed (primarily if possible, or using fascia lata graft if too much tension is placed on folded placode)

DERMAL SINUS

A tract beginning at the skin surface, lined with epithelium. Usually located at either end of neural tube: cephalic or caudal. Most common location is lumbosacral. Probably results from failure of the cutaneous ectoderm to separate from the neuro-ectoderm at the time of closure of the neural groove⁶¹.

SPINAL DERMAL SINUS

May appear as a dimple or as a sinus, with or without hairs, usually very close to midline, with an opening of only 1-2 mm. Surrounding skin may be normal, pigmented (“port wine” discoloration), or distorted by an underlying mass.

The sinus may terminate superficially, may connect with the coccyx, or may traverse between normal vertebrae or through bifid spines to the dural tube. It may widen at any point along its path to form a cyst; called an epidermoid cyst if lined with stratified squamous epithelium and containing only keratin from desquamated epithelium, or called a dermoid cyst if also lined with dermis (containing skin appendages, such as hair follicles and sebaceous glands) and also containing sebum and hair.

Although innocuous in appearance, they are a potential pathway for intradural infection which may result in meningitis (sometimes recurrent) and/or intrathecal abscess. Less serious, a local infection may occur. The lining dermis contains normal skin appendages which may result in hair, sebum, desquamated epithelium and cholesterol, within the tract. As a result, the contents of the sinus tract are irritating and can cause a sterile (chemical) meningitis with possible delayed arachnoiditis if it enters the dural space.

Incidence of a presumed sacral sinus (a dimple whose bottom could not be seen on skin retraction): 1.2% of neonates¹³⁸.

Dermal sinuses are similar but distinct from pilonidal cysts which may also be congenital (although some authors say they are acquired), contain hair, are located superficial to the postsacral fascia, and may become infected.

If the tract expands intrathecally to form a cyst, the mass may present as a tethered cord or as an intradural tumor. Bladder dysfunction is usually the first manifestation.

The tract from a spinal dermal sinus always courses cephalad as it dives inward from the surface. An occipital sinus may penetrate the skull and can communicate with dermoid cysts as deep as the cerebellum or fourth ventricle.

EVALUATION

These tracts are NOT to be probed or injected with contrast as this can precipitate infection or sterile meningitis.

Exam is directed towards detecting abnormalities in sphincter function (anal and urinary), lumbosacral reflexes, and lower extremity sensation and function.

Radiologic evaluation

When seen at birth, ultrasound is the best means to evaluate for spina bifida and a possible mass inside the canal.

If seen initially following birth, an MRI should be obtained. Sagittal images may demonstrate the tract and its point of attachment. MRI also optimally demonstrates masses (lipomas, epidermoids…) within the canal.

Plain x-rays and CT are unable to demonstrate the fine tract which may exist between the skin and the dura.

Plain x-rays must be done when embarking on surgery as part of operative planning, as preparation for the possibility of a complete laminectomy.

TREATMENT

Sinuses above the lumbosacral region should be surgically removed. More caudally located sinuses are slightly controversial. Although ≈ 25% of presumed sacral sinuses seen at birth will regress to a deep dimple on follow-up (time not specified), it is recommended that all dermal sinuses should be surgically explored and fully excised prior to the development of neurologic deficit or signs of infection. The results following intradural infection are never as good as when undertaken prior to infection. Surgery within the week of diagnosis is appropriate. Sinuses that terminate on the tip of the coccyx rarely penetrate the dura, and may not need to be treated unless local infection occurs.

Surgical technique

An ellipse is cut around the opening, and the sinus is followed deep until the termination of the tract is encountered. Careful insertion of a lacrimal duct probe under direct vision may facilitate excision without violating the tract. If the tracts penetrates the spine, laminectomy must be performed and the tract followed to its full extent (even if necessary to extend the laminectomy to T12). An extradural cyst may be present. If the tract enters the dura, it usually does so in the midline, and in these cases the dura should be opened and inspected. Extreme care is taken to prevent spilling the contents into the subdural space.

CRANIAL DERMAL SINUS

Stalk begins with a dimple in the occipital or nasal region. Cutaneous stigmata of hemangioma, subcutaneous dermoid cyst, or abnormal hair formation may occur. Occipital sinuses extend caudally, and if they enter the skull, they do so caudal to the torcular herophili. Presentation may include recurrent bacterial (usually S. aureus) or aseptic meningitis. Evaluation should include MRI to look for intracranial extension and associated anomalies, including an intracranial dermoid cyst.

Treatment

When operating on a cranial dermal sinus, use a sagittally based incision to permit deep exploration. The tract must be followed completely. Be prepared to enter the posterior fossa.

8.11. Klippel-Feil syndrome

Congenital fusion of two or more cervical vertebrae. Ranges from fusion of only the bodies (congenital block vertebrae) to fusion of the entire vertebrae (including posterior elements). Results from failure of normal segmentation of cervical somites between 3-8 weeks gestation. Involved vertebral bodies are often flattened and associated disc spaces are absent or hypoplastic. Hemivertebrae may also occur. Neural foramina are smaller than normal and oval. Cervical stenosis is rare. Complete absence of the posterior elements with an enlarged foramen magnum and fixed hyperextension posture is called iniencephalyand is rare. Incidence of Klippel-Feil is unknown due to its rarity and the fact that it is frequently asymptomatic.

Classic clinical triad (all 3 are present in < 50%): low posterior hairline, shortened neck (brevicollis), and limitation of neck motion (may not be evident if < 3 vertebrae are fused, if fusion is limited only to the lower cervical levels¹³⁹, or if hypermobility of non-fused segments compensates). Limitation of movement is more common in rotation than flexion-extension or lateral bending.

May occur in conjunction with other congenital cervical spine anomalies such as basilar impression and atlanto-occipital fusion. Other clinical associations include scoliosis in 60%, facial asymmetry, torticollis, webbing of the neck (called pterygium colli when severe), Sprengel’s deformity in 25-35% (raised scapula due to failure of the scapula to properly descend from its region of formation high in the neck to its normal position about the same time as the Klippel-Feil lesion occurs), synkinesis (mirror motions, primarily of hands but occasionally arms also) and less commonly facial nerve palsy, ptosis, cleft or high arched palate. Systemic congenital abnormalities may also occur including: genitourinary (the most frequent being unilateral absence of a kidney), cardiopulmonary, CNS, and in ≈ 30% deafness¹⁴⁰(due to defective development of the osseous inner ear).

No symptoms have ever been directly attributed to the fused vertebrae, however symptoms may occur from nonfused segments (less common in short-segment fusions) which may be hypermobile possibly leading to instability or degenerative arthritic changes.

TREATMENT

Usually directed at detecting and managing the associated systemic anomalies. Patients should have cardiac evaluation (EKG), CXR, and a renal ultrasound. Serial examinations with lateral flexion-extension lateral C-spine x-rays to monitor for instability. Occasionally, judicious fusion of an unstable nonfused segment may be needed at the risk of further loss of mobility. Also see page 981, for recommendations regarding athletic competition.

8.12. Tethered cord syndrome

Abnormally low conus medullaris. Usually associated with a short, thickened filum terminale, or with an intradural lipoma (other lesions, e.g. lipoma extending through dura, or diastematomyelia are considered as separate entities). Most common in myelomeningocele (MM). Diagnosis must be made clinically in MM, as almost all of these patients will have tethering radiographically.

Table 8-16 Presenting signs and symptoms of tethered cord^{141 (p 1331-2)}

Finding	%
cutaneous findings	54%
hypertrichosis	22%
sub-Q lipoma (no intraspinal extension)	15%
miscellaneous (hemangiomatous discoloration, dermal sinus, multiple manifestations)	17%
gait difficulty with LE weakness	93%
visible muscle atrophy, short limb, or ankle deformity	63%
sensory deficit	70%
bladder dysfunction	40%
bladder dysfunction as only deficit	4%
pain in back, leg, or foot arches	37%
scoliosis or kyphosis*	29%
posterior spina bifida (lumbar or sacral)	98%

* high incidence of scoliosis and kyphosis due to inclusion of series by Hoffman

PRESENTATION

Presenting signs and symptoms in patients with tethered cord are shown in Table 8-16.

MYELOMENINGOCELE PATIENTS

If a MM patient has increasing scoliosis, increasing spasticity, worsening gait (in those previously ambulatory), or deteriorating urodynamics¹⁴²:

• always make sure that there is a working shunt with normal ICP

• if painful, should be considered tethered cord until proven otherwise

• if painless, should be considered syringomyelia until proven otherwise

• may be due to brainstem compression (symptomatic Chiari II malformation, see page 238) requiring posterior fossa decompression

Scoliosis in tethered cord

Progressive scoliosis may be seen in conjunction with tethered cord; early untethering of the cord may result in improvement of scoliosis, however, untethering must be done when the scoliosis is mild. When cases of ≤ 10° scoliosis were untethered, 68% had neurologic improvement and the remaining 32% were stabilized, whereas when scoliosis is severe (≥ 50°) ≈ 16% deteriorated.

TETHERED CORD IN ADULTS

Although most cases of tethered cord present in childhood, cases of adult tethered cord have been reported (≈ 50 published cases as of 1982). For comparison of adult and childhood forms, see Table 8-17.

Table 8-17 Comparison of childhood and adult tethered cord syndrome¹⁴³

(from J Neurosurg, D. Pang and J.E. Wilberger, Vol. 57, pp. 40, 1982, with permission)

Finding	Childhood tethered cord	Adult tethered cord
pain	uncommon; usually in back and legs, not peri-anal nor perineal	present in 86%, often peri-anal & perineal; diffuse & bilateral; occasionally shock-like
foot deformities	common early; usually progressive cavovarus deformity (club foot)	not seen
progressive spinal deformity	common; usually progressive scoliosis	uncommon (< 5%)
motor deficits	common; usually gait abnormalities & regression of gait training	usually presents as leg weakness
urological symptoms	common; usually continuous urinary dribbling, delayed toilet training, recurrent UTIs, enuresis	common; usually urinary frequency, urgency, sensation of incomplete emptying, stress incontinence, overflow incontinence
trophic ulcerations	relatively common in LEs	rare
cutaneous stigmata of dysraphism	present in 80-100% (tuft of hair, dimple, capillary angioma (naevus flammeus)	present in < 50%
aggravating factors	growth spurts	trauma, maneuvers associated with stretching conus, lumbar spondylosis, disc herniation, spinal stenosis

EVALUATION

Radiographically: low conus medullaris (below L2) and thickened filum terminale (definition of thickened filum: normal diameter < 1 mm; diameters > 2 mm are pathological). NB: apparent filum diameter on CT-myelogram may vary with concentration of contrast material.

It is difficult to differentiate a tethered cord from a congenitally low lying conus (filum diameter is generally normal in the latter).

Pre-op evaluation

Pre-operative cystometrogram is strongly recommended, especially if the patient seems continent (postoperative changes in bladder function are not uncommon, possibly due to stretching of the lower fibers of the cauda equina).

TREATMENT

If the only abnormality is a thickened, shortened filum, then a limited lumbosacral laminectomy may suffice, with division of the filum once identified.

If a lipoma is found, it may be removed with the filum if it separates easily from neural tissues.

Distinguishing features of the filum terminale intraoperatively

The filum is differentiated from nerve roots by presence of characteristic squiggly vessel on surface of filum. Also, under the microscope, the filum has a distinctively whiter appearance than the nerve roots, and ligamentous-like strands can be seen running through it. NB: intra-op electrical stimulation and recording of anal sphincter EMG are more definitive.

OUTCOME

In MM, it is usually impossible to permanently untether a cord, however, in a growing MM child, it may be that after 2-4 untetherings that the child will be finished growing and tethering may cease to be a problem. Cases that are untethered early in childhood may recur later, especially during the adolescent growth-spurt. Incidence of post-op CSF leak: 15%.

Adult form: surgical release is usually good for pain relief. However, it is poor for return of bladder function.

8.13. Split cord malformation

There is no uniformly accepted nomenclature for malformations characterized by duplicate or split spinal cords. Pang et al.¹⁴⁴ have proposed the following.

The term split cord malformation (SCM) should be used for all double spinal cords, all of which appear to have a common embryologic etiology.

Type I SCM

Defined as two hemicords, each with its own central canal and surrounding pia, each within a separate dural tube separated by a dural-sheathed rigid osseocartilaginous (bony) median septum. This has often (but not consistently) been referred to as diastematomyelia. There are abnormalities of the spine at the level of the split (absent disc, dorsal hypertrophic bone where the median “spike” attaches)¹⁴⁵. Two-thirds have overlying skin abnormalities including: nevi, hypertrichosis (tuft of hair), lipomas, dimples or hemangiomas. These patients often have and an orthopedic foot deformity (neurogenic high arches).

Treatment: symptoms are most commonly due to tethering of the cord; and are usually improved by untethering. In addition to untethering, the bony septum must be removed and the dura reconstituted as a single tube (these spines are often very distorted and rotated, therefore start at normal anatomy and work towards defect). ✖ DO NOT cut the tethered filum until after the median septum is removed to avoid having the cord retract up against septum.

Type II SCM

Consists of two hemicords within a single dural tube, separated by a nonrigid fibrous median septum. This has sometimes been referred to as diplomyelia. Each hemicord has nerve roots arising from it. There is usually no spine abnormality at the level of the split, but there is usually spina bifida occulta in the lumbosacral region.

Treatment: consists of untethering the cord at the level of the spina bifida occulta, and occasionally at the level of the split¹⁴⁵.

8.14. Lumbosacral nerve root anomalies

Congenital anomalies of nerve roots are rare. Should be considered in cases of failed back surgery for herniated disc.

Classification system of Cannon et al.¹⁴⁶.

Type 1 anomalies: include conjoined nerve root (2 nerve roots arise from a common dural sheath). They separate at various distances from the thecal sac, and exit through the same or separate neural foramina. Neurosurgeons need to be aware of this anomaly to avoid inadvertent injury e.g. during surgery for herniated disc

Type 2 anomalies: 2 nerve roots exit through one foramen. Variants¹⁴⁷:

A. leaves an unoccupied neural foramen

B. all foramina occupied, but one foramen has 2 nerve roots

Type 3 anomalies: adjacent nerve roots are connected by an anastomosis

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