Brian Chinnock
PART 1: DISSECTION
Internal carotid artery (ICA) or vertebral artery (VA) dissection results from an intimal tear. Following the tear, blood under pressure enters the wall of the artery within the tunica media, forming an intramural hematoma. The hematoma, while contained within the tunica media, may press toward either the intima causing vessel stenosis (eFig. 91.1A), or toward the adventitia (eFig. 91.1B) causing aneurysmal dilation. Neurologic sequelae may result from either decreased blood flow, or more often from distal embolization of clots formed in this thrombogenic environment, causing CVA or TIA. In the intracranial portion of the artery, dissection may result in subarachnoid hemorrhage.
eFIGURE 91.1 A: Intramural hematoma producing vessel stenosis. B: Intramural hematoma causing aneurysmal dilation without vessel stenosis.
The extracranial ICA and VA are much more vulnerable to dissection than are the intracranial segments, probably due to increased mobility. Thus, the origin of the ICA dissection is typically just distal to the carotid bulb. VA dissection most commonly begins at its exit point from the transverse foramen between the first and second cervical vertebrae.
Together, ICA and VA dissections cause about 20% of strokes in patients younger than 45 years of age (1). ICA dissection is twice as common as VA dissection.
Spontaneous (atraumatic) dissections are thought to be the result of structural defects and connective tissue disorders, such as Ehlers–Danlos syndrome type IV, osteogenesis imperfecta, Marfan syndrome, and autosomal dominant polycystic kidney disease (2). Family history is important because 5% of patients with a spontaneous ICA or VA dissection will have a family member with an arterial dissection. Risk factors for the development of carotid and VA stenosis, such as hypertension, hypercholesterolemia, diabetes, or tobacco use, have not been found to be risk factors for dissection.
Spontaneous dissection of the ICA or VA is often associated with commonplace events that cause sudden neck movement or sustained positioning of the neck in a position that may predispose to vessel damage. Some of the reported events include forceful flexion–extension movements of the neck (coughing, sneezing, roller-coaster riding), prolonged neck extension (painting a ceiling or “beauty-parlor” syndrome where the neck is extended over the sink), or prolonged lateral bending (propping a phone between the neck and shoulder) (3–5). Chiropractic treatment is more commonly associated with VA than ICA dissection (6), probably because of the rapid rotation of C1 on C2 stretching the VA as it exits the foramen.
CLINICAL PRESENTATION
The median ages of patients presenting with extracranial ICA or VA dissection are 44 and 41 years, respectively (7). Intracranial dissection of either artery reportedly occurs in younger patients, 20 to 30 years of age (3). The clinical effects from arterial dissection generally result from the local effects of the dissection causing pain, and the distal embolization from the dissection site causing ischemic symptoms.
ICA Dissection
Table 91.1 illustrates the differences in the clinical presentation of ICA versus VA dissection. Headache is the most prominent initial complaint of the patient with ICA dissection. The headache is typically frontotemporal and unilateral. The onset is usually gradual, but “thunderclap” headaches have been described. Facial, eye, or ear pain, without headache, is described in 10% of patients (7). The unilateral neck pain is typically over the area of the carotid artery itself and has also been mistaken for a peritonsillar abscess. Oculosympathetic palsy (ipsilateral miosis and ptosis) is classically associated with ICA dissection, but only half of patients will have it. Oculosympathetic palsy can be differentiated from Horner syndrome in that there is no facial anhidrosis, because facial sweat glands are innervated by the sympathetic plexus surrounding the external, not internal, carotid artery. The focal cerebral and retinal ischemic symptoms of extracranial ICA dissection begin a median of four days after the onset of pain and are similar to those described for ICA-territory strokes (see Chapter 151) (7). In intracranial ICA dissection, the neurologic symptoms begin only a few hours after the onset of pain (8). The lower cranial nerves are commonly affected, particularly the hypoglossal nerve, and is thought to be caused by either compression or stretching of the nerve by the enlarged ICA or by interruption of nutrient vessels of the nerves (3–5).
TABLE 91.1
ICA Versus VA Dissection
Vertebral Artery Dissection
As with ICA dissection, headache is the most common presenting complaint. The headache is almost always occipital, may involve the frontal area and may be intermittent. Neck pain is more commonly noted in VA dissection than in ICA dissection and is more commonly described as posterior. Unilateral arm pain may be noted when the dissection occurs within the cervical spine and compresses a cervical nerve root most commonly at the C5 to C6 level (4,9).
The focal ischemic neurologic symptoms are typically those of vertebrobasilar insufficiency, as described in Chapter 155. The classic finding is involvement of the posterolateral medulla and cerebellum, termed Wallenberg syndrome. The thalamus and cerebral hemispheres may also be affected, although less commonly than the cerebellum and brainstem. Importantly, neck pain has been described as beginning a median of about 2 weeks before the development of neurologic findings, but headache may precede neurologic symptoms by a matter of hours (10).
DIFFERENTIAL DIAGNOSIS
The main differential diagnoses for carotid and VA dissection before the development of focal neurologic symptoms are meningitis, subarachnoid hemorrhage, subdural or epidural hematoma, migraine or tension headache, and cervical strain, fracture, or radiculopathy. Once neurologic symptoms appear, the differential is similar to that of ischemic stroke. One of the main ways to differentiate ICA or VA dissection from these other disorders is the history of head or neck pain, followed hours to days later by ICA or VA-territory neurologic symptoms. The ideal situation is to diagnose and treat dissection before the development of neurologic symptoms. This is why it is important to consider ICA and VA dissection in the differential for headache and neck pain.
ED EVALUATION
When ICA or VA dissection is suspected by history and physical examination findings, the most important consideration is the imaging modality. The gold standard by which other radiologic tests are measured is four-vessel contrast digital subtraction angiography (DSA). The disadvantages of this procedure include its invasiveness and the need to have access to a specialist to perform the procedure. The most commonly used alternative radiologic tests are duplex ultrasound (DUS), magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), and computed tomography angiography (CTA).
Duplex Ultrasound
Extracranial DUS can identify intimal defects and aneurysmal dilation. In addition, color flow DUS is useful for identifying the true and false lumens seen in dissection. The true lumen typically has antegrade flow, and the false lumen has high-resistance flow that is forward, reversed, or bidirectional. DUS is limited by its inability to visualize intracranial dissection.
Two prospective studies have compared DUS to the gold standard of DSA or MRA in the evaluation of ICA dissection. The sensitivity of Doppler to detect flow abnormalities was 93% to 96%. Standard DUS had a sensitivity of 72% to 79%, and color flow scanning had a sensitivity of 82% (11,12). The sensitivity of DUS has improved in the last decade owing to technologic improvements.
The use of DUS in the diagnosis of VA dissection is less clear because it is technically more difficult. Most VA dissections show abnormal flow patterns, but no finding is characteristic. Small case series have shown promise, but it is not currently the study of choice for suspected VA dissection.
Magnetic Resonance Imaging/Magnetic Resonance Angiography
MRI allows visualization of the intramural hematoma itself, as well as aneurysmal dilation. Contrast-enhanced MRA shows luminal irregularities and stenosis. The combination of MRI with MRA demonstrates a sensitivity of 84% to 100% and specificity of 95% to 100% when compared against DSA in small studies (13). One study that examined ICA dissection demonstrated a better sensitivity of the MRI/MRA combination than with DSA (14). DSA is unable to detect an abnormality if there is no luminal irregularity (eFig. 91.1B). MRI can also show cerebral parenchymal abnormalities in the affected vascular area, including acute ischemic lesions with diffusion-weighted MRI.
Computed Tomography Angiography
Multislice CTA gives excellent visualization of vessels because of the contrast difference between the enhanced lumen and surrounding structures. Postprocessing of the data in multiplanar formats can also provide three-dimensional images that closely approximate DSA but also show the surrounding soft tissues. Two studies examining CTA showed sensitivities near 100% for the diagnosis of ICA and VA dissection (15).
MRI/MRA VERSUS CT/CTA
Studies comparing MRI/MRA, CTA, and conventional angiography have shown similar high sensitivities and specificities (16). Small studies have reported that CT/CTA has better ability to visualize specific anatomic features of the dissection, such as pseudoaneurysm, intimal flaps, and high-grade stenoses than MRI/MRA. However, MRI/MRA has the advantage of being able to detect ischemic lesions. The decision of which test is more appropriate should be guided by local protocol and resources. Whether CT or MRI is chosen, it is important to include both the standard and angiographic components.
KEY TESTING
• For suspected ICA or VA dissection, perform MRI/MRA or CT/CTA
ED MANAGEMENT
The treatment for an extracranial ICA or VA dissection is anticoagulation with heparin followed by coumadin. The goal is to reduce the risk of distal embolization, the most common cause of neurologic insult. Anticoagulation is contraindicated if there is intracranial dissection because of its theoretical risk of hemorrhagic transformation, or pseudoaneurysm. Despite the wide acceptance of anticoagulation, there is no controlled study comparing anticoagulation to placebo in the treatment of dissection. Antiplatelet agents such as aspirin, and, less commonly, ticlopidine and clopidogrel have been described as treatment options. There are no randomized trials comparing anticoagulant with antiplatelet drugs, and nonrandomized studies did not demonstrate any difference. Antiplatelet agents are often considered as first-line treatment in patients who are poor candidates for anticoagulation and in those patients who have a dissection with no ischemic symptoms. Thrombolytics have been used safely in one small study, but in 9 of 11 treated patients the vessel remained occluded (17). Blood pressure control to a normotensive level has a theoretical benefit in preventing extension of the dissection, but no studies have examined this treatment.
Surgery or endovascular treatment for ICA or VA dissection is used for those not responding to conservative measures. This includes patients with neurologic deterioration or ischemic insult despite adequate anticoagulation, or a persistent or worsening aneurysm after 6 months of anticoagulation. Intradural VA dissections are treated aggressively with endovascular therapy, as the rate of subarachnoid hemorrhage is high (15).
CRITICAL INTERVENTIONS
• Anticoagulation is the most common treatment option for extracranial ICA or VA dissection.
DISPOSITION
Patients with ICA or VA dissection should be admitted to the hospital to monitor for further progression of neurologic injury and to establish anticoagulation. Either neurology, neurosurgery, or vascular surgery should be consulted, depending on institutional arrangements.
The prognosis for neurologic recovery is good. Approximately 70% of extracranial ICA dissection patients have full neurologic recovery, 14% have a mild deficit, and 16% have a moderate to severe deficit or death (5). One long-term follow-up study of extracranial VA dissection patients showed 88% with good or excellent recovery and 12% with moderate or severe disability (18). Intracranial dissection patients and bilateral extracranial dissection patients have significantly higher morbidity and mortality rates.
Common Pitfalls
• Failure to consider the diagnosis in a patient with headache or neck pain associated with minor trauma, particularly if neurologic symptoms are present.
• Failure to consider the diagnosis in a relatively young patient with stroke symptoms.
• Failure to perform a CT or MRI in addition to the contrast angiography component of each of these examinations, as some dissections will not be detectable on angiography
PART 2: STENOSIS
Stenosis or occlusion of the ICA or VA is most commonly caused by atherosclerosis. There is a 2% to 5% annual ipsilateral stroke risk for people with ICA stenosis of >50%. Although less is known about VA stenosis, symptomatic patients with >50% stenosis have a 1-year stroke risk of as high as 22% (19). ICA stenosis is responsible for approximately 30% of all anterior circulation strokes and transient ischemic attacks (TIAs). Cerebral ischemia may result directly from decreased blood flow, or more commonly, from distal embolization from the plaque.
The risk factors are those of atherosclerosis, and include hypertension, tobacco use, hypercholesterolemia, and obesity. Other less common causes of stenosis include fibromuscular dysplasia, radiation, and arteritis (i.e., Takayasu arteritis).
The most common location for extracranial ICA stenosis is at the carotid bulb, and for extracranial VA stenosis, at its origin from the aorta or brachiocephalic trunk.
ICA kinking and VA occlusion resulting from rotation (bow hunter syndrome) deserve special mention as they are causes of vessel occlusion without stenosis or dissection (20). ICA tortuosity with kinking is a common incidental finding on ultrasound or angiography. In some patients, the site of kinking can be caused by atherosclerosis and stenosis, and this is a source of potential embolization. It may also cause reversible cerebral ischemia during head or neck rotation.
The VA may also be occluded because of head or neck rotation. Os odontoideum, a condition in which the dens is hypoplastic, absent, or incompletely fused to the body of C2 with subsequent atlantoaxial instability and bony impingement on the VA during movement, is described as a cause. Other causes include impingement by nerve fibers of the stellate ganglion, or lateral cervical disk herniation.
CLINICAL PRESENTATION
The patient with symptomatic ICA or VA stenosis will usually present to the ED with symptoms related to stroke or TIA of the affected vascular area (see Chapter 155). Table 91.2 contrasts the neurologic findings associated with ICA versus VA stenosis, although there can be considerable overlap between the two.
TABLE 91.2
ICA- Versus VA-Territory Stroke or TIA
DIFFERENTIAL DIAGNOSIS
As noted previously, TIA and stroke are the most common events bringing the patient with ICA or VA stenosis to the ED.
ED EVALUATION
In addition to evaluating the patients for history and physical examination findings of stroke, imaging can be an important part of the evaluation for ICA or VA stenosis.
Internal Carotid Artery Stenosis
The degree of extracranial ICA stenosis can be used to predict which patients are at the highest risk for stroke and will get the most benefit from surgery. The North American Symptomatic Carotid Endarterectomy Trial (NASCET) demonstrated that in patients with 70% to 99% stenosis demonstrated on DUS, carotid endarterectomy is superior to medical therapy in preventing stroke (21). A systematic review of trials done between 1994 and 2001 compared imaging modalities to the gold standard of DSA in detecting 70% to 99% stenosis and gave the following pooled test characteristics: DUS, sensitivity 86% and specificity 87%; MRI/MRA, 95% and 90%, respectively; and the combination of MRA + DUS, 95% and 95%, respectively (22). More recently, CTA has shown good agreement with DSA, and may be especially helpful in evaluating the ICA in a patient with complete obstruction, as CTA can better show a small residual lumen, which may influence the decision of medical versus procedural management (23). DUS is highly operator dependent, and it is often used as the sole test in determining degree of stenosis at centers with expertise.
Vertebral Artery Stenosis
The degree of VA stenosis that is most clinically important is not well defined, although 50% to 99% is the range usually cited. A systematic literature review comparing DUS, MRI, MRA, and CTA to the reference standard of DSA demonstrated little high quality data directly comparing these modalities. However, using this data, MRA and CTA had the best sensitivity for detection of VA stenosis (24).
KEY TESTING
• Extracranial ICA stenosis is most commonly evaluated with DUS. MRA or CTA may be used if there is continued diagnostic uncertainty.
• Extracranial VA stenosis is evaluated with MRA or CTA.
ED MANAGEMENT
Acute treatment of stroke caused by ICA or VA stenosis is described in detail in Chapter 155 and includes prevention of hyperglycemia and hyperthermia and monitoring of volume status and blood pressure. An important intervention is to start the patient on antiplatelet therapy after ruling out hemorrhagic stroke with a CT scan. Aspirin, at a dose of 160 to 300 mg, has been shown to significantly decrease death and disability in acute ischemic stroke. Clopidogrel and ticlopidine are alternatives for patients unable to take aspirin.
Patients with 70% to 99% symptomatic ICA stenosis as demonstrated on ultrasound are likely to benefit from carotid endarterectomy, but in patients with 50% to 69% stenosis, only certain subgroups, such as those with large hemispheric TIAs, may benefit from surgery (21). The CREST study compared carotid stenting to endarterectomy in an RCT of 2,502 patients and found no difference in combined stroke, MI, or death rate in a 2.5-year follow-up (25). Current practice is to stabilize the symptomatic patient medically prior to either procedure, but future studies may challenge this practice. Patients with 100% occlusion are treated medically.
The optimal treatment strategies for extracranial VA stenosis are more poorly understood. No RCT has shown an advantage of medical management, surgery, or endovascular stenting. Case series have demonstrated drug-eluting stent placement to have a low rate of periprocedural complications and restenosis rates comparable to non-drug-eluting stenting. Surgical options include endarterectomy or reconstruction (often done by transposing the VA to the common carotid artery). Surgery is recommended for patients with symptomatic ICA kinking. Correction of the process that causes extrinsic VA compression during neck rotation (i.e., atlantoaxial stability or cervical disk protrusion) is also necessary to prevent further cerebral ischemia.
CRITICAL INTERVENTIONS
• Aspirin (160 to 325 mg) is recommended for patients with symptomatic ICA or VA stenosis, or clopidogrel or ticlopidine for those patients who cannot take aspirin.
• Obtain imaging studies of patients with stroke or TIA to determine whether they may be candidates for carotid stenting or endarterectomy.
DISPOSITION
Stroke resulting from significant ICA or VA stenosis mandates admission to the hospital to monitor for deterioration, cerebral edema, or hemorrhagic transformation. Cardiac monitoring, swallowing studies, and aggressive rehabilitation are also commonly needed.
Patients with TIAs that are not crescendo or recurrent and are unlikely to have an embolic source may be candidates for outpatient care with urgent imaging and rapid follow-up. However, 5% of patients presenting to an ED with a TIA will have a stroke within 2 days after presentation. Consultation should involve the patient’s primary care doctor and/or a neurologist. Patients who are found to have significant ICA or VA stenosis as an acute cause of TIA or stroke are candidates for inpatient evaluation for endarterectomy or angioplasty.
Common Pitfalls
• Failure to recognize the difference between ICA and VA territory neurologic symptoms. A normal carotid DUS study is irrelevant when the symptomatology suggests VA stenosis.
• Failure to consider ICA kinking or “bow hunter syndrome” in the patient with stroke or TIA symptoms with head or neck rotation.
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