Lumbar Puncture
Indications
Contraindications
Preparation
Procedure
Complications
Analysis of Results
Procedure Notes
Electrophysiologic Studies
Electroencephalography
Evaluation of Suspected Epilepsy
Classification of Seizure Disorders
Assessment & Prognosis of Seizures
Management of Status Epilepticus
Diagnosis of Neurologic Disorders
Evaluation of Altered Consciousness
Evoked Potentials
Types of Evoked Potentials
Indications for Use
Electromyography & Nerve Conduction Studies
Electromyography
Nerve Conduction Studies
F-Response Studies
Repetitive Nerve Stimulation
Description
Normal Response
Response in Disorders of Neuromuscular Transmission
Cranial Imaging Studies
Computed Tomography
Description
Indications for Use
Magnetic Resonance Imaging
Description
Indications for Use & Comparison with CT Scan
Contraindications
Diffusion-Weighted Magnetic Resonance Imaging
Diffusion Tensor Magnetic Resonance Imaging
Perfusion-Weighted Magnetic Resonance Imaging
Positron Emission Tomography
Single-Photon Emission Computed Tomography
Functional Magnetic Resonance Imaging
Magnetic Resonance Spectroscopy
Arteriography
Description
Indications for Use
Magnetic Resonance Angiography
CT Angiography
Spinal Imaging Studies
Plain X-Rays
Myelography
Computed Tomography
Magnetic Resonance Imaging
Ultrasonography
Biopsies
Brain Biopsy
Muscle Biopsy
Nerve Biopsy
Artery Biopsy
References
LUMBAR PUNCTURE
INDICATIONS
Lumbar puncture is indicated for the following purposes:
1. Diagnosis of meningitis and other infective or inflammatory disorders, subarachnoid hemorrhage, hepatic encephalopathy, meningeal malignancies, paraneoplastic disorders, or suspected abnormalities of intracranial pressure.
2. Assessment of the response to therapy in meningitis and other infective or inflammatory disorders.
3. Administration of intrathecal medications or radiologic contrast media.
4. Rarely, to reduce cerebrospinal fluid (CSF) pressure.
CONTRAINDICATIONS
1. Suspected intracranial mass lesion. In this situation, performing a lumbar puncture can hasten incipient transtentorial herniation.
2. Local infection overlying the site of puncture. Under this circumstance, cervical or cisternal puncture should be performed instead.
3. Coagulopathy. Clotting-factor deficiencies and thrombocytopenia (platelet count below 50,000/μL or rapidly falling) should be corrected before lumbar puncture is undertaken to reduce the risk of hemorrhage.
4. Suspected spinal cord mass lesion. In the case of complete spinal block, only a small quantity of CSF should be removed because fluid removal can produce a pressure differential above and below the block, which can increase the degree of spinal cord compression.
PREPARATION
A. Personnel
With a cooperative patient, lumbar puncture can generally be performed by a single person. An assistant can be helpful in positioning the patient and handling CSF samples, especially if the patient is uncooperative or frightened.
B. Equipment and Supplies
The following items, which are usually included in preassem-bled lumbar puncture trays, are required. All must be sterile.
1. Gloves.
2. Iodine-containing solution for sterilizing the skin.
3. Sponges.
4. Drapes.
5. Lidocaine (1%).
6. Syringe (5 mL).
7. Needles (22- and 25-gauge).
8. Spinal needles (preferably 22-gauge) with stylets.
9. Three-way stopcock.
10. Manometer.
11. Collection tubes.
12. Adhesive bandage.
C. Positioning
Lumbar puncture is usually performed with the patient in the lateral decubitus position (Figure 2-1), lying at the edge of the bed and facing away from the person performing the procedure. The patient’s lumbar spine should be maximally flexed to open the intervertebral spaces. The spine should be parallel to the surface of the bed, and the hips and shoulders should be aligned in the vertical plane.
Figure 2-1. Lateral decubitus position for lumbar puncture.
Occasionally it is desirable to perform lumbar puncture with the patient seated. In this case, the patient is seated on the side of the bed, bent over a pillow that rests on a bedside table, while the physician reaches over the bed from the opposite side to perform the procedure.
D. Site of Puncture
The usual practice is to enter the L3-L4 or L4-L5 vertebral interspace, because the spinal cord (conus medullaris) terminates at approximately the L1-L2 level in adults. Thus the procedure is performed without danger of puncturing the cord. The L3-L4 interspace is located at the level of the posterior iliac crests.
PROCEDURE
1. If a comparison between blood and CSF glucose levels is planned, venous blood is drawn for glucose determination. Ideally, blood and CSF glucose levels should be measured in samples obtained simultaneously after the patient has fasted for at least 4 hours.
2. The necessary equipment and supplies are placed within easy reach.
3. Sterile gloves are worn by the person performing the procedure.
4. A wide area surrounding the interspace to be entered is sterilized, using iodine-containing solution applied to sponges; the solution is then wiped off with clean sponges.
5. The area surrounding the sterile field may be draped.
6. The skin overlying the puncture site is anesthetized using lidocaine, a 5-mL syringe, and a 25-gauge needle. A 22-gauge needle is then substituted to anesthetize the underlying tissues.
7. With the stylet in place, the spinal needle is inserted at the midpoint of the chosen interspace. The needle should be parallel to the surface of the bed and angled slightly cephalad, or toward the umbilicus. The bevel of the needle should face upward, toward the face of the person performing the procedure.
8. The needle is advanced slowly until a pop, from penetration of the ligamentum flavum, is felt. The stylet is withdrawn to determine whether the CSF space has been entered, which is indicated by flow of CSF through the needle. If no CSF appears, the stylet is replaced and the needle advanced a short distance; this is continued until CSF is obtained. If at some point the needle cannot be advanced, it is likely that bone has been encountered. The needle is withdrawn partway, maintained parallel to the surface of the bed, and advanced again at a slightly different angle.
9. When CSF is obtained, the stylet is reinserted. The patient is asked to straighten his or her legs, and the stopcock and manometer are attached to the needle. The stopcock is turned to allow CSF to enter the manometer to measure the opening pressure. The pressure should fluctuate with the phases of respiration.
10. The stopcock is turned to allow the CSF to be collected, and the appearance (clarity and color) of the fluid is noted. The amount obtained and the number of tubes required varies, depending on the tests to be performed. Typically, 1 to 2 mL is collected in each of five tubes for cell count, glucose and protein determination, Venereal Disease Research Laboratory (VDRL) test for syphilis, Gram stain, and cultures. Additional specimens may be collected for other tests, such as cryptococcal antigen, other fungal and bacterial antibody studies, polymerase chain reaction for herpes simplex virus and other viruses, oligoclonal bands, glutamine, and cytologic study. If the CSF appears to contain blood, additional fluid should be obtained so that the cell count can be repeated on the specimen in the last tube collected. Cytologic studies, if desired, require at least 10 mL of CSF.
11. The stopcock and manometer are replaced to record a closing pressure.
12. The needle is withdrawn and an adhesive bandage is applied over the puncture site.
13. It has been customary to have the patient lie prone or supine for 1 or 2 hours after the procedure to reduce the risk of post–lumbar puncture headache. Current evidence suggests this is unnecessary.
COMPLICATIONS
A. Unsuccessful Tap
A variety of conditions, including marked obesity, degenerative disease of the spine, previous spinal surgery, recent lumbar puncture, and dehydration, can make it difficult to perform lumbar puncture in the conventional manner. When puncture in the lateral decubitus position is impossible, the procedure should be attempted with the patient in a sitting position. If the tap is again unsuccessful, alternative methods include lumbar puncture by an oblique approach or guided by fluoroscopy; lateral cervical puncture; or cisternal puncture. These procedures should be undertaken by a neurologist, neurosurgeon, or neuroradiologist experienced in performing them.
B. Arterial or Venous Puncture
If the needle enters a blood vessel rather than the spinal subarachnoid space, the needle should be withdrawn and a new needle should be used to attempt the tap at a different level. Patients who have coagulopathy or are receiving aspirin or anticoagulants should be observed with particular care for signs of spinal cord compression (Chapter 9) from spinal subdural or epidural hematoma.
C. Post–Lumbar Puncture Headache
A mild headache, worse in the upright position but relieved by recumbency, is not uncommon after lumbar puncture and will usually resolve spontaneously over hours to days. The frequency of this complication is directly related to the size of the spinal needle, but not to the volume of fluid removed. Vigorous hydration or keeping the patient in bed for 1 or 2 hours after the procedure does not reduce the likelihood of headache. The headache usually responds to nonsteroidal anti-inflammatory drugs or caffeine (Chapter 6). Severe and protracted headache can be treated by an autologous blood clot patch, which should be applied by experienced personnel. The use of an atraumatic spinal needle has been shown to reduce the incidence of post–lumbar puncture headache.
ANALYSIS OF RESULTS
A. Appearance
The clarity and color of the CSF should be observed as it leaves the spinal needle, and any changes in the appearance of fluid during the course of the procedure should be noted. CSF is normally clear and colorless. It may appear cloudy or turbid with white blood cell counts that exceed approximately 200/μL, but counts as low as about 50/μL can be detected by holding the tube up to direct sunlight and observing the light-scattering (Tyndall) effect of suspended cells. Color can be imparted to the CSF by hemoglobin (pink), bilirubin (yellow), or, rarely, melanin (black).
B. Pressure
With the patient in the lateral decubitus position, CSF pressure in the lumbar region does not normally exceed 180 to 200 mm water in adults. In children, the 90th percentile for opening pressure is 280 mm water. When lumbar puncture is performed with patients in the sitting position, patients should assume a lateral decubitus posture before CSF pressure is measured. Increased CSF pressure may result from obesity, agitation, or increased intra-abdominal pressure related to position; the latter factor may be eliminated by having the patient extend the legs and straighten the back once the CSF space has been entered and before the opening pressure is recorded. Pathologic conditions associated with the increased CSF pressure include intracranial mass lesions, meningoencephalitis, subarachnoid hemorrhage, and pseudotumor cerebri.
C. Microscopic Examination
This may be performed either by the person who performed the lumbar puncture or by a technician at the clinical laboratory; it always includes a cell count and differential. Gram stain for bacteria, acid-fast stain for mycobacteria, and cytologic examination for tumor cells may also be indicated. The CSF normally contains up to five mononuclear leukocytes (lymphocytes or monocytes) per microliter, no polymorphonuclear cells, and no erythrocytes. Erythrocytes may be present, however, if the lumbar puncture is traumatic (see next section). Normal CSF is sterile, so that in the absence of central nervous system (CNS) infection, no organisms should be observed with the various stains listed above.
D. Bloody CSF
If the lumbar puncture yields bloody CSF, it is crucial to distinguish between CNS hemorrhage and a traumatic tap. The fluid should be watched as it leaves the spinal needle to determine whether the blood clears, which suggests a traumatic tap. This can be established with greater accuracy by comparing cell counts in the first and last tubes of CSF obtained; a marked decrease in the number of red cells supports a traumatic cause.
The specimen should also be centrifuged promptly and the supernatant examined. With a traumatic lumbar puncture, the supernatant is colorless. In contrast, after CNS hemorrhage, enzymatic degradation of hemoglobin to bilirubin in situ renders the supernatant yellow (xanthochromic). Xanthochromia may be subtle, however. Visual inspection requires comparison with a colorless standard (a tube of water) and is best assessed by spectrophotometric quantitation of bilirubin.
The time course of changes in CSF color after subarachnoid hemorrhage is outlined in Table 2-1. Blood in the CSF after a traumatic lumbar puncture usually clears within 24 hours; blood is usually present after subarachnoid hemorrhage for at least 6 days. In addition, blood related to traumatic puncture does not clot, whereas clotting may occur with subarachnoid hemorrhage. Crenation (shriveling) of red blood cells is of no diagnostic value. In addition to breakdown of hemoglobin from red blood cells, other causes of CSF xanthochromia include jaundice with serum bilirubin levels above 4 to 6 mg/dL, CSF protein concentrations greater than 150 mg/dL, and, rarely, the presence of carotene pigments.
Table 2-1. Pigmentation of the CSF after Subarachnoid Hemorrhage.
White blood cells seen in the CSF early after subarachnoid hemorrhage or with traumatic lumbar puncture result from leakage of circulating whole blood. If the hematocrit and peripheral white blood cell count are within normal limits, there is approximately one white blood cell for each 1,000 red blood cells. If the peripheral white cell count is elevated, a proportionate increase in this ratio should be expected. In addition, every 1,000 red blood cells present in the CSF will increase the CSF protein concentration by approximately 1 mg/dL.
PROCEDURE NOTES
Whenever a lumbar puncture is performed, a note describing the procedure should be recorded in the patient’s chart. This note should provide the following information:
1. Date and time performed.
2. Name of person or persons performing the procedure.
3. Indication.
4. Position of patient.
5. Anesthetic used.
6. Interspace entered.
7. Opening pressure.
8. Appearance of CSF, including changes in appearance during the course of the procedure.
9. Amount of fluid removed.
10. Closing pressure.
11. Tests ordered; for example: Tube #1 (1 mL), cell count; tube #2 (1 mL), glucose and protein levels; tube #3 (1 mL), microbiologic stains; tube #4 (1 mL), bacterial, fungal, and mycobacterial cultures.
12. Results of any studies, such as microbiologic stains, performed by the operator.
13. Complications, if any.
ELECTROPHYSIOLOGIC STUDIES
ELECTROENCEPHALOGRAPHY
The electrical activity of the brain can be recorded noninvasively from electrodes placed on the scalp. Electroencephalography (EEG) is easy to perform, is relatively inexpensive, and is helpful in several different clinical contexts.
EVALUATION OF SUSPECTED EPILEPSY
EEG is useful in evaluating patients with suspected epilepsy. The presence of electrographic seizure activity (abnormal, rhythmic electrocerebral activity of abrupt onset and termination and showing an evolving pattern) during a behavioral disturbance that could represent a seizure, but about which there is clinical uncertainty, establishes the diagnosis beyond doubt. Because seizures occur unpredictably, it is often not possible to obtain an EEG during a seizure. Despite that, the EEG findings may be abnormal interictally (at times when the patient is not experiencing clinical attacks) and are therefore still useful for diagnostic purposes. The interictal presence of epileptiform activity (abnormal paroxysmal activity containing some spike discharges) is of particular help in this regard. Such activity is occasionally encountered in patients who have never had a seizure, but its prevalence is greater in patients with epilepsy than in normal subjects. Epileptiform activity in the EEG of a patient with an episodic behavioral disturbance that could be a manifestation of seizures on clinical grounds markedly increases the likelihood that the attacks are indeed epileptic, thus providing support for the clinical diagnosis.
CLASSIFICATION OF SEIZURE DISORDERS
In known epileptic patients, the EEG findings may help in classifying the seizure disorder and thus in selecting appropriate anticonvulsant medication. For example, in patients with the typical absences of petit mal epilepsy (Chapter 12), the EEG is characterized both ictally and interictally by episodic generalized spike-and-wave activity (Figure 12-3). In contrast, in patients with episodes of impaired external awareness caused by complex partial seizures, the EEG may be normal or show focal epileptiform discharges interictally. During the seizures, there may be abnormal rhythmic activity of variable frequency with a localized or generalized distribution, or, in some instances, there may be no electrographic correlates. The presence of a focal or lateralized epileptogenic source is of particular importance if surgical treatment is under consideration.
ASSESSMENT & PROGNOSIS OF SEIZURES
The EEG findings may provide a guide to prognosis and have been used to follow the course of seizure disorders. A normal EEG implies a more favorable prognosis for seizure control, whereas an abnormal background or profuse epileptiform activity implies a poor prognosis. The EEG findings do not, however, provide a reliable guide to the subsequent development of seizures in patients with head injuries, stroke, or brain tumors. Some physicians have used the EEG findings to determine whether anticonvulsant medication can be discontinued in patients who have been free of seizures for several years. Although patients are more likely to be weaned successfully if the EEG is normal, the findings provide only a general guide, and patients can have further seizures, despite a normal EEG, after withdrawal of anticonvulsant medication. Conversely, they may have no further seizures despite a continuing EEG disturbance.
MANAGEMENT OF STATUS EPILEPTICUS
The EEG is of little help in managing tonic–clonic status epilepticus unless patients have received neuromuscular blocking agents and are in a coma induced by medication. In this case, the electrophysiologic findings are useful in indicating the level of anesthesia and determining whether the seizures are continuing. The status itself is characterized by repeated electrographic seizures or continuous epileptiform (spike-and-wave) activity. Nonconvulsive status may follow control of convulsive status. In patients with nonconvulsive status epilepticus, the EEG findings provide the only means of making the diagnosis with confidence and in distinguishing the two main types. In absence status epilepticus, continuous spike-and-wave activity is seen, whereas repetitive electrographic seizures are found in complex partial status.
DIAGNOSIS OF NEUROLOGIC DISORDERS
Certain neurologic disorders produce characteristic but nonspecific abnormalities in the EEG. Their presence is helpful in suggesting, establishing, or supporting the diagnosis. In patients presenting with an acute disturbance of cerebral function, for example, the presence of repetitive slow-wave complexes over one or both temporal lobes suggests a diagnosis of herpes simplex encephalitis. Similarly, the presence of periodic complexes in a patient with an acute dementing disorder suggests a diagnosis of Creutzfeldt-Jakob disease or subacute sclerosing panencephalitis.
EVALUATION OF ALTERED CONSCIOUSNESS
The EEG tends to become slower as consciousness is depressed, but the findings depend at least in part on the etiology of the clinical disorder. Findings such as the presence of electrographic seizure activity can suggest diagnostic possibilities (eg, nonconvulsive status epilepticus) that might otherwise be overlooked. Serial records permit the prognosis and course of the disorder to be followed. The EEG response to external stimulation is an important diagnostic and prognostic guide: Electrocerebral responsiveness implies a lighter level of coma. Electrocerebral silence in a technically adequate record implies neocortical death in the absence of hypothermia or drug overdose. In some patients who appear to be comatose, consciousness is, in fact, preserved. Although there is quadriplegia and a supra-nuclear paralysis of the facial and bulbar muscles, the EEG is usually normal and helps in indicating the diagnosis of locked-in syndrome.
EVOKED POTENTIALS
The spinal or cerebral potentials evoked by noninvasive stimulation of specific afferent pathways are an important means of monitoring the functional integrity of these pathways. They do not, however, indicate the nature of any lesion that may involve these pathways. The responses are very small compared with the background EEG activity (noise), which has no relationship to the time of stimulation. The responses to a number of stimuli are therefore recorded and averaged with a computer to eliminate the random noise.
TYPES OF EVOKED POTENTIALS
A. Visual
Monocular visual stimulation with a checkerboard pattern is used to elicit visual evoked potentials, which are recorded from the midoccipital region of the scalp. The most clinically relevant component is the P100 response, a positive peak with a latency of approximately 100 ms. The presence and latency of the response are noted. Although its amplitude can also be measured, alterations in amplitude are far less helpful in recognizing pathology.
B. Auditory
Monaural stimulation with repetitive clicks is used to elicit brainstem auditory evoked potentials, which are recorded at the vertex of the scalp. A series of potentials are evoked in the first 10 ms after the auditory stimulus; these represent the sequential activation of various structures in the subcortical auditory pathway. For clinical purposes, attention is directed at the presence, latency, and interpeak intervals of the first five positive potentials recorded at the vertex.
C. Somatosensory
Electrical stimulation of a peripheral nerve is used to elicit the somatosensory evoked potentials, which are recorded over the scalp and spine. The configuration and latency of the responses depend on the nerve that is stimulated.
INDICATIONS FOR USE
Evoked potential studies are useful in several clinical contexts.
A. Detection of Lesions in Multiple Sclerosis
Evoked potentials have been used to detect and localize lesions in the CNS. This is particularly important in multiple sclerosis, where the diagnosis depends on detecting lesions in several regions of the CNS. When patients present with clinical evidence of a lesion at only one site, electrophysiologic recognition of abnormalities in other locations helps to establish the diagnosis. When patients with suspected multiple sclerosis present with ill-defined complaints, electrophysiologic abnormalities in the appropriate afferent pathways are helpful in indicating the organic basis of the symptoms. Although noninvasive imaging studies such as magnetic resonance imaging (MRI) are more useful for detecting lesions, they complement evoked potential studies rather than substitute for them. Evoked potential studies monitor the functional status rather than anatomic integrity of the afferent pathways and can sometimes reveal abnormalities that are not detected by MRI (and the reverse also holds true). Their cost is also considerably lower than that of MRI. In patients with established multiple sclerosis, the evoked potential findings are sometimes used to follow the course of the disorder or monitor the response to novel forms of treatment, but their value in this regard is unclear.
B. Detection of Lesions in Other CNS Disorders
Evoked potential abnormalities are encountered in disorders other than multiple sclerosis; multimodal evoked potential abnormalities may be encountered in certain spinocerebellar degenerations, familial spastic paraplegia, Lyme disease, acquired immunodeficiency syndrome (AIDS), neurosyphilis, and vitamin E or B12 deficiency. The diagnostic value of electrophysiologic abnormalities therefore depends on the context in which they are found. Although the findings may permit lesions to be localized within broad areas of the CNS, precise localization may not be possible because the generators of many of the recorded components are unknown.
C. Assessment and Prognosis After CNS Trauma or Hypoxia
Evoked potentials studies can provide information of prognostic relevance. In posttraumatic or postanoxic coma, for example, the bilateral absence of cortically generated components of the somatosensory evoked potential implies that cognition will not be recovered; the prognosis is more optimistic when cortical responses are present on one or both sides. Such studies may be particularly useful in patients with suspected brain death. Somatosensory evoked potentials have also been used to determine the completeness of a traumatic spinal cord lesion; the presence or early return of a response after stimulation of a nerve below the level of the cord injury indicates that the lesion is incomplete and thus suggests a better prognosis than otherwise.
D. Intraoperative Monitoring
Evoked potentials are also used to monitor the functional integrity of certain neural structures during operative procedures in an attempt to permit the early recognition of any dysfunction and thereby minimize damage. When the dysfunction relates to a surgical maneuver, it may be possible to prevent or diminish any permanent neurologic deficit by reversing the maneuver.
E. Evaluation of Visual or Auditory Acuity
Visual and auditory acuity may be evaluated through evoked potential studies in patients who are unable to cooperate with behavioral testing because of age or abnormal mental state.
ELECTROMYOGRAPHY & NERVE CONDUCTION STUDIES
ELECTROMYOGRAPHY
The electrical activity within a discrete region of an accessible muscle can be recorded by inserting a needle electrode into it. The pattern of electrical activity in muscle (electromyogram, or EMG) both at rest and during activity has been characterized, and abnormalities have been correlated with disorders at different levels of the motor unit.
A. Activity at Rest
Relaxed muscle normally shows no spontaneous electrical activity except in the end-plate region where neuromuscular junctions are located, but various types of abnormal activity occur spontaneously in diseased muscle. Fibrillation potentials and positive sharp waves (which reflect muscle fiber irritability) are typically found in denervated muscle; they are not invariably present, however. They are sometimes also found in myopathic disorders, especially inflammatory disorders such as polymyositis. Although fasciculation potentials, which reflect the spontaneous activation of individual motor units, are occasionally encountered in normal muscle, they are characteristic of neuropathic disorders, especially those with primary involvement of anterior horn cells (eg, amyotrophic lateral sclerosis). Myotonic discharges (high-frequency discharges of potentials from muscle fibers that wax and wane in amplitude and frequency) are found most commonly in disorders such as myotonic dystrophy or myotonia congenita and occasionally in polymyositis or other, rarer disorders. Other types of abnormal spontaneous activity also occur.
B. Activity During Voluntary Muscle Contraction
A slight voluntary contraction of a muscle activates a small number of motor units. The potentials generated by the muscle fibers of individual units within the detection range of the needle electrode can be recorded. Normal motor-unit potentials have clearly defined limits of duration, amplitude, configuration, and firing rates. These limits depend, in part, on the muscle under study, and the number of units activated for a specified degree of voluntary activity is known within broad limits. In many myopathic disorders, there is an increased incidence of small, short-duration, polyphasic motor units in affected muscles, and an excessive number of units may be activated for a specified degree of voluntary activity. There is a loss of motor units in neuropathic disorders, so that the number of units activated during a maximal contraction is reduced, and units fire at a faster rate than normal. In addition, the configuration and dimensions of the potentials may be abnormal, depending on the acuteness of the neuropathic process and on whether reinnervation is occurring. Variations in the configuration and size of individual motor-unit potentials are characteristic of disorders of neuromuscular transmission.
C. Clinical Utility
Lesions can involve the neural or muscle component of the motor unit, or the neuromuscular junction. When the neural component is affected, the pathologic process can be either at the level of the anterior horn cells or at some point along the length of the axon as it traverses a nerve root, limb plexus, and peripheral nerve before branching into its terminal arborizations. Electromyography can detect disorders of the motor units and can indicate the site of the underlying lesion. The technique also permits neuromuscular disorders to be recognized when clinical examination is unrewarding because the disease is still at a mild stage, or because poor cooperation on the part of the patient or the presence of other symptoms such as pain makes clinical evaluation difficult. Note that the electro-myographic findings do not, of themselves, permit an etio-logic diagnosis to be reached, and the electrophysiologic findings must be correlated with the clinical findings and the results of other laboratory studies.
The electromyographic findings may provide a guide to prognosis. For example, in patients with an acute disorder of a peripheral or cranial nerve (eg, a pressure palsy of the radial nerve or a Bell palsy), electromyographic evidence of denervation implies a poorer prognosis for recovery than when denervation has not occurred.
In contrast to needle electromyography, the clinical utility of surface-recorded electromyography is not established.
NERVE CONDUCTION STUDIES
A. Motor Nerve Conduction Studies
These studies are performed by recording the electrical response of a muscle to stimulation of its motor nerve at two or more points along its course. This permits conduction velocity to be determined in the fastest-conducting motor fibers between the points of stimulation.
B. Sensory Nerve Conduction Studies
These are performed by analogous means, by determining the conduction velocity and amplitude of action potentials in sensory fibers when these fibers are stimulated at one point and their responses are recorded at another point along the course of the nerve.
C. Indications for Use
Nerve conduction studies provide a means of confirming the presence and extent of peripheral nerve damage. Such studies are particularly helpful when clinical examination is difficult (eg, in children). Nerve conduction studies are particularly helpful in the following contexts.
1. Determining whether sensory symptoms are caused by a lesion proximal or distal to the dorsal root ganglia (in the latter case, sensory conduction studies of the involved fibers will be abnormal) and whether neuromuscular dysfunction relates to peripheral nerve disease.
2. Detecting subclinical involvement of other peripheral nerves in patients who present with a mononeuropathy.
3. Determining the site of a focal lesion and providing a guide to prognosis in patients with a mononeuropathy.
4. Distinguishing between a polyneuropathy and a mononeuropathy multiplex. This distinction may not be possible clinically, but it is important because the causes of these conditions differ.
5. Clarifying the extent to which the disabilities experienced by patients with polyneuropathy relate to superimposed compressive focal neuropathies, which are common complications.
6. Following the progression of peripheral nerve disorders and their response to treatment.
7. Indicating the predominant pathologic change in peripheral nerve disorders. In demyelinating neuropathies, conduction velocity is often markedly slowed and conduction block may occur; in axonal neuropathies, conduction velocity is usually normal or slowed only mildly, sensory nerve action potentials are small or absent, and electromyography shows evidence of denervation in affected muscles.
8. Detecting hereditary disorders of the peripheral nerves at a subclinical stage in genetic and epidemiologic studies.
F-RESPONSE STUDIES
When a stimulus is applied to a motor nerve, its impulses travel antidromically (toward the spinal cord) as well as orthodromically (toward the nerve terminals) and lead to the discharge of a few anterior horn cells. This produces a small motor response that occurs considerably later than the direct muscle response elicited by nerve stimulation. The F wave so elicited is sometimes abnormal in patients with lesions of the proximal portions of the peripheral nervous system, such as the nerve roots. These studies may be helpful in detecting abnormalities when conventional nerve conduction studies are normal.
REPETITIVE NERVE STIMULATION
DESCRIPTION
The size of the electrical response of a muscle to supra-maximal electrical stimulation of its motor nerve depends on a number of factors but correlates with the number of activated muscle fibers. Neuromuscular transmission can be tested by recording (with surface electrodes) the response of a muscle to supramaximal stimulation of its motor nerve either repetitively or by single shocks or trains of shocks at selected intervals after a maximal voluntary contraction.
NORMAL RESPONSE
In normal subjects, there is little or no change in the size of the compound muscle action potential after repetitive stimulation of a motor nerve at 10 Hz or less or with a single stimulus or a train of stimuli delivered at intervals after a voluntary muscle contraction of about 10 seconds. This lack of change is the case even though preceding activity in the junctional region influences the amount of acetylcholine released and thus the size of the end-plate potentials elicited by the stimuli. Although the amount of acetylcholine released is increased briefly after maximal voluntary activity and is then reduced, more acetylcholine is normally released than is required to bring the motor end-plate potentials to the threshold for generating muscle-fiber action potentials.
RESPONSE IN DISORDERS OF NEUROMUSCULAR TRANSMISSION
A. Myasthenia Gravis
In myasthenia gravis, depletion of postsynaptic acetylcholine receptors at the neuromuscular junction makes it impossible to compensate for the reduced release of acetylcholine that follows repetitive firing of the motor neuron. Accordingly, repetitive stimulation, particularly between 2 and 5 Hz, may lead to a depression of neuromuscular transmission, with a decremental response in the size of the compound muscle action potential recorded from an affected muscle. Similarly, an electrical stimulus of the motor nerve immediately after a 10-second period of maximal voluntary activity may elicit a muscle response that is slightly larger than before, indicating that more muscle fibers are responding. This postactivation facilitation of neuromuscular transmission is followed by a longer-lasting period of depression that is maximal from 2 to 4 minutes after the conditioning period and lasts up to 10 minutes or so. During this period, the compound muscle action potential is reduced in size.
Decrementing responses to repetitive stimulation at 2 to 5 Hz can also occur in congenital myasthenic syndromes.
B. Myasthenic Syndrome and Botulism
In Lambert-Eaton myasthenic syndrome, in which there is defective release of acetylcholine at the neuromuscular junction, the compound muscle action potential elicited by a single stimulus is generally very small. With repetitive stimulation at rates of up to 10 Hz, the first few responses may decline in size, but subsequent responses increase, and their amplitude is eventually several times larger than the initial response. Patients with botulism exhibit a similar response to repetitive stimulation, but the findings are somewhat more variable, and not all muscles are affected. Incremental responses in Lambert-Eaton syndrome and botulism are more conspicuous with high rates of stimulation and may result from the facilitation of acetylcholine release by the progressive accumulation of calcium in the motor nerve terminal.
CRANIAL IMAGING STUDIES
COMPUTED TOMOGRAPHY
DESCRIPTION
Computed tomographic (CT) scanning is a noninvasive computer-assisted radiologic means of examining anatomic structures (Figure 2-2). It permits the detection of structural intracranial abnormalities with precision, speed, and facility. It is thus of particular use in evaluating patients with progressive neurologic disorders or focal neurologic deficits in whom a structural lesion is suspected, patients with dementia or increased intracranial pressure, and patients with suspected stroke or with head injuries. Intravenous administration of an iodinated contrast agent improves the ability of CT scan to detect and define lesions, such as tumors and abscesses, associated with a disturbance of the blood–brain barrier. Because the contrast agents may have an adverse effect on the kidneys, they should be used with discrimination. Other adverse effects of the contrast agents in common use are pain, nausea, thermal sensations, and anaphylactoid reactions that include bronchospasm and death. Contrast-enhanced scans may provide more information than that obtained by unenhanced scans in patients with known or suspected primary or secondary brain tumors, arteriovenous malformations (AVMs), cerebral abscesses, chronic isodense subdural hematomas, infarcts, or hydrocephalus.
Figure 2-2. Contrast-enhanced CT brain scans from a 62-year-old man, showing the normal anatomy. Images are at the level of the lateral ventricles (left) and midbrain (right) (same patient as in Figure 2-3).
INDICATIONS FOR USE
A. Stroke
CT scan is particularly helpful in evaluating strokes because it can distinguish infarction from intracranial hemorrhage; it is particularly sensitive in detecting intracerebral hematomas (Figure 13-18), and the location of such lesions may provide a guide to their cause. Moreover, the CT scan occasionally demonstrates a nonvascular cause of the patient’s clinical deficit, such as a tumor or abscess.
B. Tumor
CT scans can indicate the site of a brain tumor, the extent of any surrounding edema, whether the lesion is cystic or solid, and whether it has displaced midline or other normal anatomic structures. It also demonstrates any hemorrhagic component.
C. Trauma
The CT scan is an important means of evaluating patients after head injury, in particular for detecting traumatic intracranial (epidural, subdural, subarachnoid, or intracerebral) hemorrhage and bony injuries. It also provides a more precise delineation of associated fractures than do plain x-rays.
D. Dementia
In patients with dementia, CT scan may indicate the presence of a tumor or hydrocephalus (enlarged ventricles), with or without accompanying cerebral atrophy. The occurrence of hydrocephalus without cerebral atrophy in demented patients suggests normal pressure or communicating hydrocephalus. Cerebral atrophy can occur in demented or normal elderly subjects.
E. Subarachnoid Hemorrhage
In patients with subarachnoid hemorrhage, the CT scan generally indicates the presence of blood in the subarachnoid space and may even suggest the source of the bleeding (Figure 6-5). If the CT scan findings are normal despite clinical findings suggestive of subarachnoid hemorrhage, the CSF should be examined to exclude hemorrhage or meningitis. CT angiography (see later) may demonstrate an underlying vascular malformation or aneurysm.
MAGNETIC RESONANCE IMAGING
DESCRIPTION
Magnetic resonance imaging (MRI) is an imaging procedure that involves no ionizing radiation. The patient lies within a large magnet that aligns some of the protons in the body along the magnet’s axis. The protons resonate when stimulated with radiofrequency energy, producing a tiny echo that is strong enough to be detected. The position and intensity of these radiofrequency emissions are recorded and mapped by a computer. The signal intensity depends on the concentration of mobile hydrogen nuclei (or nuclear-spin density) of the tissues. Spin–lattice (T1) and spin–spin (T2) relaxation times are mainly responsible for the relative differences in signal intensity of the various soft tissues; these parameters are sensitive to the state of water in biologic tissues. Pulse sequences with varying dependence on T1 and T2 selectively alter the contrast between soft tissues (Figure 2-3).
Figure 2-3. Brain MR images from a 62-year-old man, showing the normal anatomy. A-B: gadolinium-enhanced T1-weighted (CSF dark) images; C-D: T2-weighted (CSF white) images. Images are at the level of the lateral ventricles (A and C) and midbrain (B and D).
Figure 2-3. (Continued) A midsagittal T1-weighted image is shown in (E). Brain images are from the same patient as in Figure 2-2.
The soft-tissue contrast available with MRI makes it more sensitive than CT scanning in detecting certain structural lesions. MRI provides better contrast than does CT scans between the gray and white matter of the brain. It is superior for visualizing abnormalities in the posterior fossa and spinal cord and for detecting lesions associated with multiple sclerosis or those that cause seizures. In addition to its greater sensitivity, it is also free of bony artifact and permits multiplane (axial, sagittal, and coronal) imaging with no need to manipulate the position of the patient. Because there are no known hazardous effects, MRI studies can be repeated in a serial manner if necessary. Occasional patients cannot tolerate the procedure because of claustrophobia, but sedation usually alleviates this problem.
Gadopentetate dimeglumine (gadolinium-DPTA) is stable, well-tolerated intravenously, and an effective enhancing MRI agent that is useful in identifying small tumors that, because of their similar relaxation times to normal cerebral tissue, may be missed on unenhanced MRI. It also helps to separate tumor from surrounding edema, identify leptomeningeal disease, and provide information about the blood–brain barrier. Gadolinium has been associated with nephrogenic systemic fibrosis in patients with renal insufficiency, so it should be used judiciously in this setting.
INDICATIONS FOR USE & COMPARISON WITH CT SCAN
A. Stroke
Within a few hours of vascular occlusion, it may be possible to detect and localize cerebral infarcts by MRI. Breakdown in the blood–brain barrier (which occurs several hours after onset of cerebral ischemia) permits the intravascular content to be extravasated into the extracellular space. This can be detected by T2-weighted imaging and fluid-attenuated inversion-recovery (FLAIR) sequences. Diffusion-weighted MRI also has an important role in the early assessment of stroke, as is discussed in a later section. CT scans, on the other hand, may be unrevealing for up to 48 hours. After that period, there is less advantage to MRI over CT scanning except for the ability of the former to detect smaller lesions and its superior imaging of the posterior fossa.
Nevertheless, CT scanning without contrast is usually the preferred initial study in patients with acute stroke to determine whether hemorrhage has occurred. Intracranial hemorrhage is not easily detected by MRI within the first 36 hours, and CT scan is more reliable for this purpose. Hematomas of more than 2 to 3 days’ duration, however, are better visualized by MRI. Although MRI is very effective in detecting and localizing vascular malformations, angiography is still necessary to define their anatomic features and plan effective treatment. In cases of unexplained hematoma, a follow-up MRI obtained 3 months later may reveal the underlying cause, which is sometimes unmasked as the hematoma resolves.
B. Tumor
Both CT scans and MRI are very useful in detecting brain tumors, but the absence of bone artifacts makes MRI superior for visualizing tumors at the vertex or in the posterior fossa and for detecting acoustic neuromas. Secondary effects of tumors, such as cerebral herniation, can be seen with either MRI or CT scan, but MRI provides more anatomic information. Neither technique, however, permits the nature of the underlying tumor to be determined with any certainty. Pituitary tumors are often visualized more easily by MRI than CT scan because of the absence of bone or dental metal artifacts.
C. Trauma
In the acute phase after head injury, CT scan is preferable to MRI because it requires less time, is superior for detecting intracranial hemorrhage, and may reveal bony injuries. Similarly, spinal MRI should not be used in the initial evaluation of patients with spinal injuries because nondis-placed fractures are often not visualized. For follow-up purposes, however, MRI is helpful for detecting parenchymal pathology of the brain or spinal cord.
D. Dementia
In patients with dementia, either CT scan or MRI can help in demonstrating treatable structural causes, but MRI appears to be more sensitive in demonstrating abnormal white matter signal and associated atrophy.
E. Multiple Sclerosis
In patients with multiple sclerosis, it is often possible to detect lesions in the cerebral white matter or the cervical cord by MRI, even though such lesions may not be visualized on CT scans. The demyelinating lesions detected by MRI may have signal characteristics resembling those of ischemic changes, however, and clinical correlation is therefore always necessary. Gadolinium-enhanced MRI permits lesions of different ages to be distinguished. This ability facilitates the diagnosis of multiple sclerosis: The presence of lesions of different ages suggests a multiphasic disease, whereas lesions of similar age suggest a monophasic disorder, such as acute disseminated encephalomyelitis.
F. Infections
MRI is very sensitive in detecting white matter edema and probably permits earlier recognition of focal areas of cerebritis and abscess formation than is possible with CT scan.
CONTRAINDICATIONS
Contraindications to MRI include the presence of intracranial clips, metallic foreign bodies in the eye or elsewhere, pacemakers, cochlear implants, and conditions requiring close monitoring of patients. Furthermore, it can be difficult to image patients with claustrophobia, extreme obesity, uncontrolled movement disorders, or respiratory disorders that require assisted ventilation or carry any risk of apnea. Advances in MRI-compatible mechanical ventilators and monitoring equipment now allow even critically ill patients to be scanned safely.
DIFFUSION-WEIGHTED MAGNETIC RESONANCE IMAGING
This technique, in which contrast within the image is based on the microscopic motion of water protons in tissue, provides information that is not available on standard MRI. It is particularly important in the assessment of stroke because it can discriminate cytotoxic edema (which occurs in strokes) from vasogenic edema (found with other types of cerebral lesion) and thus reveals cerebral ischemia early and with high specificity. Diffusion-weighted MRI permits reliable identification of acute cerebral ischemia during the first few hours after onset, before it is detectable on standard MRI. This is important because it reveals infarcts early enough for treatment with thrombolytic agents. However, because diffusion-weighted imaging will be positive in the setting of cytotoxic edema of any cause (eg, brain abscess, highly cellular tumors), clinical correlation is always required. When more than one infarct is found on routine MRI, diffusion-weighted imaging permits the discrimination of acute from older infarcts by the relative increase in signal intensity of the former.
DIFFUSION TENSOR MAGNETIC RESONANCE IMAGING
This technique allows determination of the diffusion of water in tissue in order to produce neural tract images. Such tractography is assuming increasing importance as a clinical and investigative tool, such as in distinguishing different types of dementia, determining the severity and extent of cerebral involvement after head injury, aiding in the precise localization of brain tumors, and planning of surgical procedures. The technique allows the detection of white matter changes that might not be seen on conventional MRI.
PERFUSION-WEIGHTED MAGNETIC RESONANCE IMAGING
Perfusion-weighted imaging measures relative blood flow through the brain by either an injected contrast medium (eg, gadolinium) or an endogenous technique (in which the patient’s own blood provides the contrast). It allows cerebral blood-flow abnormalities to be recognized and can confirm the early reperfusion of tissues after treatment. Cerebral ischemia may be detected very soon after clinical onset. Comparison of the findings from diffusion-weighted and perfusion-weighted MRI may have a prognostic role and is currently under study. The distinction of reversible from irreversible ischemic damage is important in this regard. Perfusion-weighted imaging also contributes in distinguishing between various types of brain tumors such as gliomas and metastases.
POSITRON EMISSION TOMOGRAPHY
Positron emission tomography (PET) is an imaging technique that uses positron-emitting radiopharmaceuticals, such as 18F-fluoro-2-deoxy-D-glucose or 18F-L-dopa, to map brain biochemistry and physiology. PET thus complements other imaging methods that provide primarily anatomic information, such as CT scan and MRI, and may demonstrate functional brain abnormalities before structural abnormalities are detectable. Although its availability is currently limited, PET has proved useful in several clinical settings. When patients with medically refractory epilepsy are being considered for surgical treatment, PET can help identify focal areas of hypometabolism in the temporal lobe as likely sites of the origin of seizures. PET can also be useful in the differential diagnosis of dementia, because common dementing disorders such as Alzheimer disease and fronto-temporal dementia exhibit different patterns of abnormal cerebral metabolism. PET can help distinguish between clinically similar movement disorders, such as Parkinson disease and progressive supranuclear palsy, and can provide confirmatory evidence of early Huntington disease. PET may also be of value in grading gliomas, selecting tumor biopsy sites, and distinguishing recurrent tumors from radiation-induced brain necrosis. It has been an important tool with which to investigate the functional involvement of different cerebral areas in behavioral and cognitive tasks.
The major problems associated with PET are its expense, the requirement that radioactive isotopes are produced near the site of imaging, and the exposure of subjects to radiation.
SINGLE-PHOTON EMISSION COMPUTED TOMOGRAPHY
Single-photon emission computed tomography (SPECT) involves the administration intravenously or by inhalation of chemicals containing isotopes that emit single photons in order to image the brain. SPECT has been used, in particular, for perfusion studies, the investigation of receptor distribution, and the detection of areas of increased metabolism such as occurs with seizures. At present the technique is more of academic interest than of clinical relevance, but it is considerably cheaper than PET, and the isotopes in use do not have to be produced near the site of imaging.
FUNCTIONAL MAGNETIC RESONANCE IMAGING
Functional MRI (fMRI) involves the intravenous administration of contrast material that lowers signal intensity on MRI in relation to blood flow as the material passes through the cerebral vasculature. Studies are performed with the subject at rest and then after an activation procedure so that the change in signal intensity reflects the effect of the activation procedure on local cerebral blood flow (Figure 2-4). An alternative approach involves pulse sequences that show changes in signal intensity from alterations in the oxygen concentration of venous blood (blood oxygen level–dependent [BOLD]-fMRI), which correlate with focal cerebral activity.
Figure 2-4. A functional MR brain image obtained from a patient during rapid finger tapping of the left hand. An increase in relative blood flow in the region of the right motor strip is imaged (arrow) and superimposed on a T1-weighted MR scan. (Reproduced from Waxman SG. Correlative Neuroanatomy. 23rd ed. Norwalk, CT: Appleton & Lange; 1996.)
MAGNETIC RESONANCE SPECTROSCOPY
Magnetic resonance spectroscopy is a tool available in some centers; it provides information about the chemical composition of tissue. Proton magnetic resonance spectroscopy (1H-MRS) may be used to determine levels of N-acetylaspartate (exclusive to neurons) or choline, creatinine, and lactate (glia and neurons). Measurements of brain concentrations of these metabolites may be useful in detecting specific tissue loss in diseases such as Alzheimer disease or hypoxic–ischemic encephalopathy, or to classify brain tumors or lateralize temporal lobe epilepsy. Phosphorus magnetic resonance spectroscopy (31P-MRS) may be useful in the evaluation of metabolic muscle diseases.
ARTERIOGRAPHY
DESCRIPTION
The intracranial circulation is visualized most satisfactorily by arteriography, a technique in which the major vessels to the head are opacified and radiographed. A catheter is introduced into the femoral or brachial artery and passed into one of the major cervical vessels. A radiopaque contrast material is then injected through the catheter, allowing the vessel (or its origin) to be visualized. Access to the cranial vessels with a catheter also allows for the delivery of certain therapies. The technique, generally performed after noninvasive imaging by CT scanning or MRI, has a definite (approximately 1%) morbidity and mortality associated with it and involves considerable exposure to radiation. It is contraindicated in patients who are allergic to the contrast medium. Stroke may result as a complication of arteriography. Moreover, at the conclusion of the procedure, bleeding may occur at the puncture site, and occlusion of the catheterized artery (usually the femoral artery) may lead to distal ischemic complications. The puncture site and the distal circulation must therefore be monitored with these complications in mind.
INDICATIONS FOR USE
The major indications for cerebral arteriography include the following:
1. Diagnosis of intracranial aneurysms, arteriovenous malformations (AVMs), or fistulas. Although these lesions can be visualized by CT scan or MRI, their detailed anatomy and the vessels that feed, drain, or are otherwise implicated in them cannot reliably be defined by these other means. Moreover, arteriography is required for interventional procedures such as embolization, the injection of occlusive polymers, or the placement of detachable balloons or coils to treat certain vascular anomalies.
2. Detection and definition of the underlying lesion in patients with subarachnoid hemorrhage who are considered good operative candidates (Chapter 6).
3. Detection and management of vasospasm after subarachnoid hemorrhage.
4. Emergency embolectomy in the setting of ischemic stroke due to large-vessel occlusion. In addition, arteriography can define vascular lesions in patients with transient cerebral ischemic attacks or strokes if surgical treatment such as carotid endarterectomy is being considered.
5. Evaluation of small vessels, such as when a vasculitis is under consideration.
6. Diagnosis of cerebral venous sinus thrombosis.
7. Evaluation of space-occupying intracranial lesions, particularly when CT scanning or MRI is unavailable. There may be displacement of the normal vasculature, and in some tumors neovasculature may produce a blush or stain on the angiogram. Meningiomas can be recognized by their blood supply from the external carotid circulation. Presurgical embolization of certain tumors is often performed to reduce their blood supply and decrease the risk of major bleeding during resection.
MAGNETIC RESONANCE ANGIOGRAPHY
Several imaging techniques that have been used to visualize blood vessels by MRI depend on certain physical properties of flowing blood, thereby allowing visualization of vasculature without the use of intravenous contrast. These properties include the rate at which blood is supplied to the imaged area, its velocity and relaxation time, and the absence of turbulent flow. Magnetic resonance (MR) angiography is a noninvasive technique that has a reduced cost and reduced risks compared with conventional angiography. It has been most useful in visualizing the carotid arteries and proximal portions of the intracranial circulation, where flow is relatively fast. The images are used to screen for stenosis or occlusion of vessels and for large atheromatous lesions. It has particular utility in screening for venous sinus occlusion. Resolution is inferior to that of conventional angiography and, in vessels with slow flow, it may be difficult to recognize occlusive disease. Moreover, intracranial MR angiograms may be marred by irregular or discontinuous signal intensity in vessels close to the skull base. Although current techniques allow visualization of AVMs and aneurysms greater than 3 mm in diameter, conventional angiography remains the “gold standard” in this context. Finally, MR angiography may reveal dissection of major vessels: Narrowing is produced by the dissection, and cross-sectional images reveal the false lumen as a crescent of abnormal signal intensity next to the vascular flow void.
CT ANGIOGRAPHY
Spiral CT angiography is a minimally invasive procedure that requires the CT scanner to be capable of rapidly acquiring numerous thin, overlapping sections after intravenous injection of a bolus of contrast material. It can be performed within minutes and is less likely to be affected by patient movement than MR angiography. A wide range of vessels can be imaged with the technique.
CT angiography of the carotid bifurcation is being used increasingly in patients with suspected disease of the carotid arteries. It can also be used for intracranial imaging and can detect stenotic or aneurysmal lesions. However, sensitivity is reduced for aneurysms less than 3 mm, and the method cannot adequately define aneurysmal morphology in the preoperative evaluation of patients. It is sensitive in visualizing the anatomy in the circle of Willis, the vasculature of the anterior and posterior circulations, and intracranial vasoocclusive lesions, but it may not reveal plaque ulceration. It is a reliable alternative to MR angiography, but both techniques are less sensitive than conventional angiography in this regard.
In patients with acute stroke, CT angiography provides important information complementary to conventional CT scan studies, revealing the site and length of vascular occlusion and the contrast-enhanced arteries distal to the occlusion as a reflection of collateral blood flow. CT perfusion, in which the relative blood flow to an area of the brain is measured as iodinated contrast passes through over time, can provide additional information regarding the proportion of ischemic to infarcted tissue in this setting.
SPINAL IMAGING STUDIES
PLAIN X-RAYS
Plain x-rays of the spine can reveal congenital, traumatic, degenerative, or neoplastic bony abnormalities, or narrowing (stenosis) of the spinal canal. Degenerative changes become increasingly common with advancing age, and their clinical relevance depends on the context in which they are found.
MYELOGRAPHY
Injecting radiopaque contrast medium into the subarachnoid space permits visualization of part or all of the spinal subarachnoid system. The cord and nerve roots, which are silhouetted by the contrast material in the subarachnoid space, are visualized indirectly. The procedure is invasive and carries the risks of headache, low back pain, confusion, arachnoiditis, inadvertent intravenous injection of contrast material, and vasovagal reactions. Rarely, traumatic herniated intervertebral disks have occurred because of poor technique, as has damage to nerve roots.
Water-soluble agents (eg, iohexol) have now replaced oil-based formulations (eg, iophendylate) as the preferred contrast medium. The dye is absorbed from the CSF and excreted by the kidneys, with approximately 75% eliminated over the first 24 hours. Unlike iophendylate, iohexol does not produce significant arachnoiditis, but tonic–clonic seizures have been reported in some instances when contrast entered the intracranial cavity. Other complications include headaches, nausea and vomiting, and CSF leak. Contrast myelography may be followed by a CT scan of the spine while the medium is still in place. This shows the soft tissue structures in or about the spinal cord and provides information complementary to that obtained by the myelogram (see below).
Myelography has largely been replaced by MRI and CT scanning in the investigation of patients with suspected spinal cord or nerve root compression or structural anomalies in the foramen magnum region. Nevertheless, it is still sometimes performed, particularly in patients with spinal hardware precluding useful MRI studies. It is helpful in detecting both extradural and intradural lesions and in providing evidence of an intramedullary abnormality.
COMPUTED TOMOGRAPHY
CT scanning after myelography has become a routine procedure but is particularly helpful when the myelogram either fails to reveal any abnormality or provides poor visualization of the area of interest. The myelogram may be normal, for example, when there is a laterally placed disk protrusion; in such circumstances, a contrast-enhanced CT scan may reveal the lesion. It is also useful in visualizing more fully the area above or below an almost complete block in the subarachnoid space and in providing further information in patients with cord tumors.
CT scan is most helpful in defining the bony anatomy of the spine. It is performed routinely after trauma to exclude cervical spine fractures in cases in which spinal injury cannot be excluded on clinical grounds. CT scanning may show osteophytic narrowing of neural foramina or the spinal canal in patients with cervical spondylosis and may show spinal stenosis or disk protrusions in patients with neurogenic claudication. In patients with neurologic deficits, however, MRI is generally preferred because it provides more useful information about the spinal canal, neural foramina, and spinal cord.
MAGNETIC RESONANCE IMAGING
Spinal MRI is the simplest and best method for visualizing the spinal canal and its contents. In most instances, the information obtained previously by myelography is now obtained more simply by MRI of the spine. Imaging of the spinal canal by MRI is direct and noninvasive.
Spinal MRI is indicated in the urgent evaluation of patients with suspected spinal cord compression. It permits differentiation of solid from cystic intramedullary lesions. In syringomyelia, MRI is the preferred imaging method for visualizing cord cavitation and detecting any associated abnormalities at the craniocervical junction. Congenital abnormalities associated with spinal dysraphism are also easily visualized by MRI. In patients with degenerative disk disease, MRI is an important means of detecting cord or root compression (Figure 2-5). However, abnormal MRI findings in the lumbar and cervical spine are common in asymptomatic subjects, especially in middle or later life, and care must therefore be exercised in attributing symptoms such as back pain to anatomic abnormalities that may be coincidental. When a spinal AVM is suspected but MRI is unrevealing, a myelogram is sometimes helpful, but spinal angiography is often undertaken without proceeding to myelography.
Figure 2-5. Spinal MRI showing disk herniation at the L3-L4 level (arrows).
ULTRASONOGRAPHY
In B-mode ultrasonography, echoes reflected from anatomic structures are plotted on an oscilloscope screen in two dimensions. The resulting brightness at each point reflects the density of the imaged structure. The technique has been used to image the carotid artery and its bifurcation in the neck, permitting evaluation of the extent of extracranial vascular disease. Blood flowing within an artery does not reflect sound, and the lumen of the vessels therefore appears black. The arterial wall can be seen, however, and atherosclerotic lesions can be detected. Note that with severe stenosis or complete occlusion of the internal carotid artery, it may not be possible to visualize the carotid artery bifurcation.
The velocity of blood flow through an artery can be measured by Doppler ultrasonography. Sound waves within a certain frequency range are reflected off red blood cells, and the frequency of the echo provides a guide to the velocity of the flow. Any shift in frequency is proportional to the velocity of the red cells and the angle of the beam of sound waves. When the arterial lumen is narrowed, the velocity of flow increases; increased frequencies are therefore recorded by Doppler ultrasonography. Spectral analysis of Doppler frequencies is also used to evaluate the anatomic status of the carotid artery.
Transcranial Doppler studies can be used to detect intracranial arterial lesions or vasospasm (eg, after subarachnoid hemorrhage) and to assess the hemodynamic consequences of extracranial disease of the carotid arteries.
Duplex instruments perform a combination of both B-mode imaging and Doppler ultrasonography, thereby providing information at the same time about both structure and the hemodynamics of the circulation in a color-coded format. The technique is commonly used to evaluate patients with suspected atheromatous lesions of the carotid artery in the neck to determine whether carotid endarterectomy is indicated. Depending on the quality of the study, a CT angiogram may be necessary to confirm the extent and severity of disease.
BIOPSIES
BRAIN BIOPSY
Biopsy of brain tissue can be useful in certain cases when less invasive methods, such as imaging studies, fail to provide a diagnosis. Brain lesions most amenable to biopsy are those that can be localized by imaging studies; are situated in superficial, surgically accessible sites; and do not involve critical brain regions, such as the brainstem or the areas of cerebral cortex involved in language or motor function. Cerebral disorders that can be diagnosed by biopsy include primary and metastatic brain tumors, inflammatory conditions such as vasculitis or sarcoidosis, infectious disorders such as brain abscess, and certain degenerative diseases such as Creutzfeldt-Jakob disease, although MRI has largely supplanted biopsy in the diagnosis of this disorder.
MUSCLE BIOPSY
Histopathologic examination of a biopsy specimen of a weak muscle can indicate whether the underlying weakness is neurogenic or myopathic in origin. In neurogenic disorders, atrophied fibers occur in groups, with adjacent groups of larger uninvolved fibers. In myopathies, atrophy occurs in a random pattern; the nuclei of muscle cells may be centrally situated, rather than in their normal peripheral location; and fibrosis or fatty infiltration may also be found. Examination of a muscle biopsy specimen may also permit certain inflammatory diseases of muscle, such as polymyositis, to be recognized and treated.
In some patients with a suspected myopathy, although the electromyographic findings are normal, examination of a muscle biopsy specimen reveals the nature of the underlying disorder. Conversely, electromyographic abnormalities suggestive of a myopathy are sometimes found in patients in whom the histologic or histochemical studies fail to establish a diagnosis of myopathy. The two approaches are therefore complementary.
NERVE BIOPSY
It is not necessary to undertake nerve biopsy to establish a diagnosis of peripheral neuropathy. The nature of any neuropathologic abnormalities can be important, however, in suggesting the underlying disorder of peripheral nerves. In particular, evidence may be found of metabolic storage disease (eg, Fabry disease, Tangier disease), infection (eg, leprosy), inflammatory change, vasculitis, or neoplastic involvement. The findings are not always of diagnostic relevance, however, and nerve biopsy itself can be performed only on accessible nerves. It is rarely undertaken on more than a single occasion.
ARTERY BIOPSY
In patients with suspected giant cell arteritis, temporal artery biopsy may help to confirm the diagnosis, but the pathologic abnormalities are usually patchy in distribution. Therefore, a normal study should not exclude the diagnosis or lead to withdrawal of treatment.
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Ultrasonography
de Groot E, van Leuven SI, Duivenvoorden R, et al. Measurement of carotid intima-media thickness to assess progression and regression of atherosclerosis. Nat Clin Pract Cardiovasc Med. 2008;5:280-288.
Goessens BM, Visseren FL, Kappelle LJ, Algra A, van der Graaf Y. Asymptomatic carotid artery stenosis and the risk of new vascular events in patients with manifest arterial disease: the SMART study. Stroke. 2007;38:1470-1475.
Norris JW, Halliday A. Is ultrasound sufficient for vascular imaging prior to carotid endarterectomy? Stroke. 2004;35: 370-371.
U-King-Im JM, Hollingworth W, Trivedi RA, et al. Cost-effectiveness of diagnostic strategies prior to carotid endarterectomy. Ann Neurol. 2005;58:506-515.
Biopsy
Lacomis D. Clinical utility of peripheral nerve biopsy. Curr Neurol Neurosci Rep. 2005;5:41-47.
Schuette AJ, Taub JS, Hadjipanayis CG, Olson JJ. Open biopsy in patients with acute progressive neurologic decline and absence of mass lesion. Neurology. 2010;75:419-424.