♦ They should be suspected in trauma patients presenting with focal neurological symptoms or Horner’s sign (due to damage of the sympathetic plexus surrounding the carotid wall). However, many cases occur in the absence of obvious precipitating injury. In fact, in these “spontaneous” cases, it is not uncommon to identify minor trauma or predisposing activities (such as chiropractic maneuvers, roller-coaster rides, certain sports, or other factors associated with sudden or prolonged hyperextension or extension/torsion of the neck). Abnormalities of connective tissue, particularly elastic fibers, have been documented in ultrastructural analysis of skin biopsies of patients with spontaneous dissections.³ Conditions disrupting the integrity of the connective tissue in the arterial wall predispose to dissection; these include fibromuscular dysplasia (discussed later in the chapter), Marfan syndrome, Ehlers-Danlos type III, and alpha-1 antitrypsin deficiency, among others.^4–6 Familial cases have also been reported.^7,8

♦ Most dissections affect the extracranial portions of the internal carotid and vertebral arteries (Figures 8-1 and 8-2). Carotid dissections most often occur 1.5 to 2 cm above the carotid bifurcation (different from atherosclerosis, which characteristically affects the carotid bulb) and usually end at the skull base, before the artery penetrates the petrous bone. Vertebral artery dissections typically affect the V3 segment, originating at the C1–C2 level as the artery leaves the transverse foramen of the axis and makes its turn to enter the intracranial compartment.^2,10

♦ MRI and MRA of the neck are the best imaging modalities for the diagnosis of cervical artery dissections (Figure 8-3).^11–14

♦ CT angiograms are also useful for the noninvasive detection of arterial dissections (Figure 8-4), particularly in the emergency setting.

♦ The angiographic signs of cervical artery dissections are listed in Table 8-1. These changes can be seen on MRA, CT angiography (CTA), or catheter digital subtraction angiography.

♦ Angiography of the intracranial vessels allows identification of dissections extending into the intracranial compartment and intracranial pseudo-aneurysms (see Figure 8-2). Some consider the presence of these findings a relative contraindication for anticoagulation (used by many practitioners to prevent embolic infarctions after dissections, although never formally studied for this particular indication). However, subarachnoid hemorrhage typically occurs at the time of formation of dissecting aneurysms. The risk of this complication subsequently appears to be very low.¹⁵

♦ MRI of the neck should include thin cuts of fat-suppressed T1-weighted sequence to allow the identification of the intramural hematoma (see Figure 8-3). It appears as a hyperintense signal, which may be eccentric (as a crescent) or concentric (as a doughnut), and widens the external diameter of the vessel. This sequence may also permit visualization of an intimal flap (generally seen as a thin, curvilinear, hypointense signal change between the true and a false lumen). Flow void may be normal, narrowed (eccentrically or concentrically), or absent. However, loss of flow void in these cases does not necessarily represent vessel occlusion, because very slow flow may cause signal loss in a patent vessel.

♦ Ultrasound may be useful for the diagnosis and monitoring of extracranial carotid artery dissection. It is reliable to demonstrate the narrowing or occlusion of the vessel lumen (except when its location is too high). Intramural thrombus and intimal flap may occasionally be recognized by their increased echogenicity (but the sensitivity of ultrasound to detect these abnormalities is low).^17,18 When stenosis or occlusion was seen on ultrasound upon presentation, this simple test can be used for subsequent monitoring.

♦ In cases of vertebral artery dissection the pattern of acute multiple brain infarctions also predominates (Figure 8-5), affecting the terminal branches of the basilar artery. More than a third of patients present signs of ischemia in the cerebellar border-zone distribution. Pontine ischemia is less common after vertebral artery dissections than with atherosclerosis of this vessel, and isolated small thalamic infarctions appear to be distinctly uncommon with both mechanisms of large-vessel vertebrobasilar disease.²⁰

♦ In cases of arterial occlusion, recanalization is possible within the first few weeks, and it may be more common in the vertebral arteries.²¹ Thus, follow-up imaging is indicated in these patients. We typically recommend repeating imaging of the neck 8 to 12 weeks after a documented dissection with compromise of the vessel lumen.

♦ Recurrence of dissections are possible, but the risk is low,^25–27 particularly after the first two months.^28,29

♦ The diagnosis of thoracic aortic dissection can be reliably established by noninvasive angiography (MRA or CTA) (Figure 8-6) and even by ultrasound (Figure 8-7).

♦ The stroke pattern varies according to the mechanism of infarction. Aortic embolism may result in bilateral infarctions (Figures 8-6 and 8-7) and may involve anterior and posterior circulation territories. Extension of dissection may also provoke brain ischemia from artery-to-artery-embolism, but in this case the infarctions will be confined to the territory of the affected cervical vessel. If the dissection causes severe narrowing or occlusion of the cervical vessels, hemodynamic infarctions in watershed distributions may occur.

♦ Fibromuscular dysplasia (FMD) is a systemic arteriopathy involving the craniocervical, splanchnic, and renal vessels. Pathological findings consist of rings of fibrous tissue (from dysplastic smooth muscle) in the tunica media alternating with areas of medial thickening and disrupted elastic lamina. As a result, the lumen has narrower and wider areas along the affected segments, leading to the characteristic angiographic appearance of a “string of beads” (Figure 8-8).

♦ FMD predominantly affects the distal extracranial segments of the vertebral and carotid arteries. Intracranial involvement is observed less frequently (Figure 8-8, D), mostly in the petrous and cavernous segments of the internal carotid arteries.

♦ Spontaneous dissections are more common in these patients, and this mechanism constitutes the most solidly proven association with an increased risk of stroke.^6,32 Recurrent dissections may occur more commonly in patients with FMD.³³

♦ Arterial dolichoectasia, a dilatative angiopathy with predilection for intracranial vessels, may affect the carotid and vertebrobasilar systems. Its etiology is unclear, but it has been reported to be associated with traditional vascular risk factors (older age, male sex, hypertension, and history of myocardial infarction), although not with carotid atherosclerosis.³⁴ Intracranial artery dolichoectasia is also associated with enlargement of the descending thoracic aorta, suggesting that the vasculopathy is not restricted to intracranial vessels.³⁵

♦ It can produce symptoms from compression of adjacent structures, ischemia (typically in distributions of penetrating arteries or, in the case of vertebrobasilar dolichoectasia [Figure 8-9] in the territory of the posterior cerebral arteries), or rupture with intraparenchymal or subarachnoid hemorrhage.^36–40 It has also been associated with increased incidence of pathologically proved small vessel disease.⁴¹ Risk of recurrent ischemic infarctions is elevated.⁴⁰

♦ The term “moyamoya” (which translates from Japanese as “hazy puff of smoke”) identifies an angiographic pattern of collateralization that occurs following progressive, severe narrowing (often culminating in occlusion) of the supraclinoid segments of the internal carotid arteries and proximal branches of the circle of Willis (Figure 8-10).

♦ Moyamoya disease is a nonatherosclerotic, noninflammatory vasculopathy that occurs predominantly in patients of Japanese ancestry,^42,43 in whom it was initially described. However, it may also affect other ethnic groups.^44–49 The etiopathogenesis of moyamoya disease is unknown; occurrence of familial cases argues for a genetic contribution.

♦ Japanese cases present in childhood with ischemic infarctions and in adulthood with brain hemorrhages. In Occidental patients, ischemia predominates and, although the natural history of the condition may be more benign than in Asian cases,⁴⁴ the risk of recurrent ischemic strokes is high.⁴⁷

♦ Angiographic findings of moyamoya can also be seen in patients with advanced intracranial atherosclerosis, radiation-induced vasculopathy, sickle cell disease, meningitis, systemic vasculitis, and cocaine use.^50–53 Although these cases may be classified as moyamoya syndrome or moyamoya phenomenon, they should not be confused with cases of moyamoya disease.

♦ The diagnosis is made angiographically. However, brain imaging may suggest the diagnosis by disclosing multiple flow voids in the basal ganglia on CT scan or T1-weighted sequence, appearing as dots that enhance after contrast is administered. Sulci appear bright on FLAIR because of engorgement of pial vessels. Areas of ischemic infarction may be multiple and typically distributed in deep or watershed regions. DWI is helpful in distinguishing acute versus chronic lesions. Hemorrhages tend to be ganglionic or intraventricular (Figure 8-10).

♦ Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an autosomal dominant disorder caused by a mutation in the NOTCH3 gene. It is characterized by migraines, recurrent strokes, psychiatric symptoms, and rapidly progressive dementia.⁵⁵ The disease process affects primarily the vascular smooth muscle cells with predilection for the arterioles of the brain.

♦ Although the diagnosis requires genetic confirmation, brain imaging is extremely valuable to evaluate this diagnosis. Brain MRI shows multiple subcortical infarctions with diffuse leukoencephalopathy (Figure 8-11), findings that progress over time. Predominant involvement of the anterior temporal lobes is characteristic, although lesions are also common in the frontal-parietal white matter and external capsule.^56–58 Lesions tend to become confluent over time, particularly in the temporal poles.⁵⁷

♦ Intracerebral hemorrhages can also occur, although they are very infrequent.⁵⁹ Microhemorrhages on T2^*/GRE sequence are probably more common, especially when patients are also hypertensive and diabetic.⁶⁰

♦ Lesions are more common in the parieto-occipital regions (including cortex normally perfused by the middle and posterior cerebral arteries) but may involve the temporal lobes and basal ganglia (Figure 8-12, A and B). They are most often multiple, and their size is variable.

♦ Initially they show an inflammatory appearance with gyriform swelling and compression of sulci. Subcortical white matter may be involved or spared. On MRI, these acute lesions are bright on DWI (Figure 8-12, C) but often normal on apparent diffusion coefficient (ADC; distinguishing them from true infarctions),⁶³ hyperintense on T2-weighted imaging and FLAIR, and frequently enhance with contrast. GRE rarely shows evidence of microhemorrhage.⁶⁴

♦ Multivoxel MR spectroscopy (MRS) is enormously valuable to substantiate the diagnosis of MELAS. Acute lesions have a characteristic lactate “doublet” peak resonating at 1.3 ppm when spectra is acquired with short echo time (TE; 35 msec; Figure 8-12, E) and inversion of the lactate peak when a long TE (136–144 msec) is used.⁶⁶ MRS abnormalities may precede changes on DWI.⁶⁷

♦ This form of angiopathy, often referred to as Call-Fleming syndrome,⁷⁰ is caused by transient, reversible vasoconstriction of the arteries of the circle of Willis and their branches.⁷¹

♦ Brain imaging typically demonstrates small, multifocal areas of infarction in the posterior head regions or watershed distributions (Figure 8-13, A and B). Sparing of the calcarine cortex and medial occipital lobes differentiates these infarctions from those produced by embolism.⁷² Perfusion scans may reveal large areas of hypoperfusion. Serial brain imaging may show additional areas of ischemia before the vasculopathy subsides.

♦ Angiography indicates the diagnosis by disclosing multifocal areas of arterial narrowing and dilatation during the acute phase (Figure 8-13, C and D) with subsequent resolution (over days or weeks). Large and medium-sized arteries are most often affected. Noninvasive angiographic studies (MRA, CTA) may be useful, but conventional catheter angiography remains the best method to certify the diagnosis. Transcranial Doppler may be used to monitor disease progression.