Anatomical Considerations

Dissection of the carotid arteries is generally thought to arise from an intimal tear. The tear allows blood under arterial pressure to enter the wall of the artery, creating a so-called false lumen. Subintimal dissection of blood tends to result in stenosis of the anatomical arterial lumen (▶ Fig. 14.1a ). Subsequent activation of platelets at the site of intimal disruption leads to local thrombus formation, with potential extension into the vessel lumen and a high incidence of thromboembolic phenomena (▶ Fig. 14.1b ). Dissection into the subadventitial layers causes aneurysmal dilation of the artery wall (▶ Fig. 14.1c ). Combinations of stenosis and aneurysm formation are often observed in the same patient. Although the aneurysms associated with dissection may be referred to as pseudoaneurysms, they are not true pseudoaneurysms, because their walls are composed of blood vessel elements (i.e., media and adventitia). The term dissecting aneurysm is more appropriate and will be used in this chapter.

Intimal tears can be difficult to identify, but the absence of any communication between the false arterial lumen and the true arterial lumen in carefully studied patients suggests that at least some spontaneous dissections are caused by a primary intramural hematoma without a subsequent intimal tear (▶ Fig. 14.1d ). Primary intramural hemorrhage has been reported as the cause of cervical arterial dissection in up to 25% of patients with spontaneous ICA dissection and in up to 75% of those with vertebral artery dissection.

Clinical Evaluation

The most common presenting symptoms for patients with spontaneous ICA dissection include headache and focal cerebral ischemia ( 1 , 2 in algorithm ). The classic triad of unilateral headache and/or neck pain, cerebral ischemia, and Horner′s syndrome is present in only 30% of patients. Unilateral headache and focal cerebral ischemia are the most common presentation. Cerebral ischemia is reported as the major presenting symptom in 70 to 90% of patients and is much more commonly the result of artery-to-artery embolism secondary to thrombus formation at the site of dissection rather than the result of hemodynamic compromise caused by dissection-related stenosis or occlusion ( 1 –6 in algorithm ).

Major stroke is usually preceded by warning signs, such as transient ischemic attacks (TIAs) and amaurosis fugax. However, approximately 20% of patients present with an ischemic stroke without any prior warning. Major intracranial branch occlusions are observed on angiography in 15 to 30% of cases of ischemic stroke after dissection.

Imaging

Digital subtraction angiography (DSA) is regarded as the gold standard for the radiographic diagnosis of cervical arterial dissections. However, noninvasive evaluation methods such as magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), and CTA have emerged as reliable alternatives.

The most common finding on CTA/DSA in carotid dissection is an eccentric smooth-tapered stenosis or a flame-shaped occlusion (▶ Fig. 14.2a,b ). Other angiographic findings include an intimal flap and an associated false lumen, stenosis with intimal thrombosis, and dissecting aneurysms (▶ Fig. 14.2c ). An intimal flap and a double lumen, which are pathognomonic features of dissection, are found in less than 10% of cases.

MRI and MRA are both highly sensitive for diagnosis of carotid dissection. Compared with DSA, the sensitivity of MRI and MRA is reported to be 87 to 99%. In addition, diffusion-weighted MRI provides an accurate and detailed assessment of ischemic brain injury. Imaging evidence of ischemic cerebral injury is present in more than 50% of patients with carotid dissection.

CTA is another alternative diagnostic modality. The comparison of CTA with MRI or MRA has shown the two modalities to be equally accurate in the diagnosis of carotid dissection. The most reliable indicator of a dissection on CTA is an eccentric arterial lumen combined with mural thickening. Other CTA findings of dissection include stenosis, occlusion, and dissecting aneurysms. Carotid occlusions usually have a tapered shape similar to that seen on DSA (▶ Fig. 14.2a ). The primary advantage of CTA in the evaluation of patients with dissection is the relative ease and speed of this imaging modality, as well as the fact that it is readily available at all designated stroke center hospitals ( 4-6 in algorithm ).

Duplex ultrasonography is the least invasive imaging technique, but it is the most limited in usefulness. Factors that curtail the accuracy of ultrasonography include operator-dependent precision and limitations in delineating atherosclerotic lesions from those of a dissecting nature. In addition, spontaneous dissections often begin well above the bifurcation beyond the typical region of screening. Ultrasonography may be the most useful for follow-up imaging and to guide the duration of treatment in patients with known dissection.

The initial workup of a patient with suspected carotid artery dissection should include a basic head CT scan to rule out subarachnoid, intracranial, and intraventricular hemorrhage ( 1 in algorithm ). The choice of further diagnostic evaluation is typically dictated by the clinical presentation of the patient. The workup of patients presenting with acute focal neurological deficits should follow established acute stroke protocols that include level 1 stroke evaluation ( 2 in algorithm ) and the administration of intravenous tissue plasminogen activator (t-PA) in patients who meet the appropriate criteria ( 3 in algorithm ). At the author′s institution, CTA and CT perfusion studies are obtained as part of the acute stroke workup ( 4 in algorithm ), and patients with major intracranial vessel occlusion ( 5 in algorithm ) or major cerebral perfusion deficits ( 6 in algorithm ) are referred, as appropriate, to the endovascular team.