Extracranial Large Artery Atherothrombosis

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Chapter 5 Extracranial Large Artery Atherothrombosis

Sebastian Koch

Extracranial atherothrombotic disease is an important cause of stroke, and the management of asymptomatic occlusive disease poses additional challenges and opportunities for stroke prevention. Depending on the population studied, approximately 20% of strokes are attributed to large vessel extracranial disease.1 The prevalence of asymptomatic carotid stenosis greater than 50% stenosis is approximately 3% in the general population.2–4 There is a higher prevalence of extracranial atherosclerotic disease in whites compared with African Americans and other ethnic groups.5,6

The workup of stroke patients requires a careful examination of the extravascular tree to determine vessel pathology and stroke etiology. This investigation must include the carotid bifurcation and vertebral artery origin, which are sites characteristically affected by atherosclerosis. Other forms of extracranial vasculopathy such as arterial dissections typically spare these regions but tend to occur in the distal cervical segments of the carotid and vertebral arteries. Inflammatory extracranial vasculopathies are rare; the most common, Takayasu arteritis, typically affects the origins of the large extracranial vessels as they exit the aortic arch. Understanding how various arterial segments are preferentially affected by different vascular pathologies is often helpful in making an accurate diagnosis of stroke etiology.

Diagnostic imaging of the extracranial cerebrovasculature includes ultrasonography, magnetic resonance (MR), computed tomography (CT), and catheter cerebral angiography. In patients with stroke or transient ischemic attacks (TIA), it is essential to determine the degree of internal carotid artery (ICA) stenosis because this is the most important predictor of stroke recurrence. Symptomatic patients with 50% to 69% and greater than 70% stenosis and asymptomatic patients with stenosis exceeding 60% must be reliably identified because arterial revascularization is typically considered at these cutoff points.

ULTRASONOGRAPHY

Ultrasound remains a powerful screening and diagnostic tool in the examination of extracranial carotid and vertebral vessels. It has excellent diagnostic accuracy for carotid stenosis and correlation with the gold standard cerebral catheter angiography. The advantages of ultrasonography include its ease of performance and direct visualization of plaque composition and contour.

Determining the exact degree of stenosis remains the single most important goal of extracranial ultrasonography. However, plaque composition is an additional determinant of future stroke risk, particularly in the setting of symptomatic stenosis. Carotid plaques that are hypoechoic (i.e., have a dark intraplaque area), or ulcerated are of particular concern.7–11 These hypoechoic areas are likely to represent intraplaque hemorrhage, and these plaques should be considered unstable, thus having an increased thromboembolic potential. The presence of calcifications, in contrast, imparts relative plaque stability, and such plaques are less likely to be symptomatic12 (Figure 5-1).

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Figure 5-1 (A) Heavily calcified plaque at the internal carotid artery (ICA) origin. The arrows outline an acoustic shadow created by the plaque on the near wall of the artery (top wall on the image shown). The shadow prevents further morphological characterization of the plaque as well as velocity determination within the plaque. This may potentially interfere with determining the true degree of stenosis with ultrasonography and should be recognized as a potential limitation of the study. (B) Hypoechoic plaque. The arrows point to the dark outlines of a circumferential echolucent plaque. Echolucencies are associated with plaque instability. (C) Ulcerated plaque: power Doppler of an ulcerated plaque. The image on the left shows the depth of the ulcer measured at 2.5 mm.

Ultrasonography is technician-dependent and requires considerable skill and expertise. It is typically used as an initial screening tool, and thereby sensitivity is optimized at the expense of specificity-that is, it will tend to overestimate the degree of stenosis resulting in some false-positive results. Therefore a confirmatory test is necessary to improve the overall accuracy of the diagnostic approach.13 Ultrasound is less reliable in identifying carotid stenosis in moderate ranges (between 50% and 69%)14,15 and carotid occlusion.13

Because ultrasonography uses an increase in flow velocities as the main diagnostic criterion, the findings of ultrasonography in the setting of abnormal collateral flow patterns must be carefully interpreted. This situation often arises during insonation of a moderately stenotic plaque in patients with contralateral occlusion or high-grade stenosis. In these cases, ultrasound may overestimate the degree of stenosis of the moderately stenotic plaque,16,17 and the findings on ultrasonography must be cautiously reviewed and confirmed with other imaging modalities.

It is also important to realize that ultrasonography only visualizes the proximal portion of the carotid artery, and pathology involving the distal cervical segment may be unrecognized.

MR AND CT ANGIOGRAPHY

In general clinical practice, the findings on ultrasonography are often confirmed with an alternate imaging technique. MR and CT angiography have essentially replaced catheter angiography as the confirmatory test. The advantages MR and CT angiography are that they are noninvasive and readily available. The sensitivity and specificity for both techniques exceeds 80% to 90%.13,15,18,19 For the majority of patients, this approach (i.e., ultrasonography and CT/MR angiography combined) will correctly identify the true degree of stenosis. However, it continues to be controversial if noninvasive techniques are reliable enough to make decisions about surgical intervention, particularly if the degree of stenosis is in the moderate (50%–69%) range.15

CT angiography has recently experienced a revival with the advent of multislice detectors and development of sophisticated postimaging software. Advantages of CT over MR angiography lie in its acquisition speed. Even though few data are available with these newer techniques, they are likely to improve further the diagnostic accuracy of CT angiography.19

MR angiography is widely used at present but has some inherent limitations. There is a tendency for overestimation of the degree of stenosis because of sampling error. Therefore MR angiography may be more valuable as a screening tool rather than to confirm that stenosis is actually present.20 The specificity and sensitivity can be improved with gadolinium administration.20,21

Diagnostic accuracy of noninvasive imaging is generally improved if results of ultrasonography and MR or CT angiography are concordant.13

CATHETER CEREBRAL ANGIOGRAPHY

Catheter cerebral angiography remains the gold standard in determining the degree of stenosis and identifying surgical candidates. However, even in recent clinical trials, there has been a trend in favor of noninvasive testing, particularly in asymptomatic patients, given the risks of catheter angiography.22 The risk of stroke may be as high as 1.2% in asymptomatic patients; however, other reports have shown a reduced risk of serious complications in general practice.23,24

Less well recognized are subclinical infarcts detected by diffusion-weighted MR imaging after diagnostic cerebral angiography. Such lesions are present in up to 20% of patients and are the result of silent microembolism,25 but might produce subtle neuropsychiatric manifestations that go undetected.

Case Vignette

A 75-year-old hypertensive diabetic woman developed right-sided weakness. Extracranial ultrasonography showed a right ICA occlusion and left ICA high-grade stenosis meeting diagnostic criteria for stenosis exceeding 70% of the luminal diameter. MR angiography of the neck showed a high-grade left internal stenosis and confirmed the right internal carotid occlusion. On cerebral angiography, the left carotid stenosis was carefully measured to be 60% (Figure 5-2). The external carotid artery was occluded.

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Figure 5-2 (A) Ultrasound shows elevation of flow velocities across the plaque with a peak systolic velocity of 279.9 cm/sec. This is suggestive of a high-grade stenosis exceeding 70%. Peak systolic flow velocities are the most reliable and reproducible diagnostic criteria in the determination of the degree of stenosis. (B) A high-grade stenosis is also apparent on magnetic resonance angiography. (C) Cerebral angiography measures the degree of stenosis at 60%.

♦ The results of ultrasonography must be carefully interpreted in situations in which increased blood flow from collateralization is expected, for example, in this case because of occlusion of the contralateral ICA and possibly the ipsilateral external carotid artery.
♦ Noninvasive tests may overestimate the degree of stenosis in cases of moderate (50%–69%) narrowing.

CLINICAL FEATURES OF CAROTID ATHEROTHROMBOSIS

Case Vignette

A 76-year-old man with a prior history of hypertension developed three episodes of sudden transient left hand and forearm numbness lasting 10 to 15 minutes over the previous week. His neurological examination showed no abnormalities. A harsh right carotid bruit was heard on neck auscultation. He underwent an emergent carotid ultrasound, which revealed a hypoechoic plaque at the right internal carotid origin causing high-grade stenosis of the lumen. MR angiography of the neck performed later confirmed these findings (Figure 5-3). He was admitted to a local hospital and placed on antithrombotic therapy. The following morning, he developed left-sided facial droop, dysarthria, and left-arm paralysis. He underwent an emergent endarterectomy and recovered without residual deficits.

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Figure 5-3 (A and B) Longitudinal and axial ultrasound images of a hypoechoic plaque indicated by arrows. ICA, internal carotid artery. (C) Magnetic resonance angiography image confirms a high-grade stenosis.

This case illustrates the clinical hallmarks of large arterial atherothrombosis:

♦ High association with TIA compared with other stroke mechanisms
♦ Repetitive stereotypical symptoms within the same vascular distribution that are unlikely to occur with cardiac embolism
♦ Presence of a bruit, even though the sensitivity and specificity of a carotid bruit for high-grade stenosis is only approximately 60%26
♦ High risk of stroke and hyperacute stroke recurrence after initial presentation. Large arterial disease is the stroke subtype associated with the highest incidence of early stroke recurrence27
♦ Failure of medical therapy to prevent recurrent symptoms and need for early surgical intervention (In fact the benefit of endarterectomy diminishes greatly if performed more than 2 weeks after symptom onset.28)

Case Vignette

A 55-year-old smoker underwent a routine ocular examination and was found to have a Hollenhorst plaque in her right eye. She denied any visual symptoms. Carotid ultrasound revealed an ulcerated plaque at the origin of the right ICA causing no more than 30% to 40% stenosis (Figure 5-4). She was placed on aspirin, and her vascular risk factors were treated intensively.

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Figure 5-4 Ultrasound image of a low-grade bifurcation stenosis. ICA, internal carotid artery.

MECHANISMS OF INFARCTION

In extracranial large vessel disease the brain is affected by important pathophysiological events taking place at significant distances. These processes include thromboembolism and the downstream effects of decreased perfusion with increasing degrees of stenosis. These two mechanisms are embolic and hemodynamic.

Atherothrombotic Embolism

Artery-to-artery embolism is the most common mechanism of symptoms in large-artery disease. Emboli generated from an unstable carotid plaque may lead to a wide spectrum of symptoms from benign to disabling. These include asymptomatic retinal emboli, transient monocular blindness, retinal arterial infarction, hemispheric TIA, and small or large territorial strokes. Emboli are likely to consist of varying combinations of platelets, cholesterol particles and fibrin.

♦ Cholesterol emboli are often asymptomatic.
♦ Asymptomatic retinal emboli are surprisingly common and have a prevalence of approximately 1% in a general population aged 50 and older.29
♦ Retinal emboli identify a subgroup of patients with asymptomatic atherosclerotic disease who will require aggressive management of vascular risk factors.

Case Vignette

A 68 year-old man with history of hypertension, dyslipidemia, and diabetes developed sudden onset of left arm and leg numbness. On examination, he had diminished left-arm sensation and sensory extinction. Brain MRI showed an acute infarct in the right parietal lobe on diffusion-weighted imaging sequence. Fluid-attenuated inversion recovery (FLAIR) showed a prior infarct in right middle cerebral territory (Figure 5-5). Carotid ultrasound and MR angiography revealed high-grade stenosis of the right ICA artery at its origin.

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Figure 5-5 (A) Diffusion-weighted imaging (DWI) showing an acute right parietal infarction. (B) Review of fluid-attenuated inversion recovery images show prior ischemic insult; DW imaging does not demonstrate this infarction to be acute.

♦ Carotid atherosclerotic disease frequently causes partial territorial infarctions as a consequence of artery-to-artery embolism.
♦ Subclinical infarction may be seen in the vascular distribution of the stenotic vessel, suggesting prior asymptomatic embolism.
♦ Infarcts related to atherothrombosis are generally smaller than those associated with cardiac embolism, which tend to generate large emboli and complete territorial infarctions.

Even though the degree of carotid stenosis remains the single most important factor in predicting stroke risk in symptomatic patients, certain situations warrant a more careful assessment of carotid disease, as illustrated in the next case vignette.

Case Vignette

A 56-year-old man with hypertension and dyslipidemia had a history of two previous strokes in the territory of the left middle cerebral artery. He presented with new onset of right-arm weakness and mild speech difficulties. Brain MRI showed a small acute ischemic stroke in the left frontal lobe. He underwent a careful investigation of his vascular tree and cardiac evaluations including MR angiography of the brain, transthoracic and transesophageal echocardiograms, and Holter monitoring of the cardiac rhythm. All tests failed to reveal the mechanism of his infarction.

A carotid ultrasound showed plaque formation in the left carotid bulb, which produced no alteration of Doppler flow pattern and was estimated to cause no more than 50% stenosis of the luminal diameter (Figure 5-6). Subsequently, he underwent a catheter cerebral angiogram, which confirmed the presence of an ulcerated left carotid bulb plaque, causing no significant stenosis. Given the repetitive nature of his symptoms restricted to a single vascular distribution, it was felt that his symptoms were most likely explained by repeated thromboembolism from the proximal ICA plaque. Consequently, he was treated with left carotid endarterectomy. A hemorrhagic ulcerated plaque was found at the time of the operation. He recovered well from surgery and has been symptom-free for the following 2 years.

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Figure 5-6 (A) Ultrasound of proximal internal carotid artery shows plaque formation in the carotid bulb. The arrows outline the wall of the artery. Velocity measurements do not suggest a high-grade stenosis. (B) Cerebral angiography shows an ulcerated plaque. The arterial wall of the bulb filled with a sizable atheroma is outlined by the arrows, not causing significant luminal stenosis.

♦ A sizable plaque can develop in the carotid bulb without causing stenosis, as measured by current imaging modalities.
♦ In the coronary circulation, the term “plaque burden” refers to the volume of plaque deposition rather than the degree of stenosis as the major determinant of symptoms.30,31 The anatomy of the carotid bulb shows significant variation between individuals. This may lead to a significant atheroma deposition in some individuals who have large bulbous dilatation of the proximal ICA. In these instances, surgical treatment may need to be considered, even though no clear high-grade stenosis is identified.
♦ Plaque composition assessed by noninvasive means may not predict well the findings on the pathological sample obtained at the time of endarterectomy.32,33

Alternatively, high-grade stenosis may not require further revascularization, as illustrated in the following case.

Case Vignette

A 68-year-old man with hypertension was seen at a stroke referral center for a past history of left-sided cortical stroke, which occurred more than 1 year earlier. On follow-up Duplex examination, he was found to have a severely narrowed right ICA with a dampened flow pattern (Figure 5-7). MR angiography suggested occlusion of the right cervical carotid artery. Catheter cerebral angiogram demonstrated a proximal high-grade stenosis with a severely narrowed and collapsed ICA distal to the stenosis. He was deemed to have a functional occlusion and treated with medical therapy.

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Figure 5-7 Extracranial (A) and intracranial (B) cerebral angiogram of a left common carotid artery injection in a patient with near occlusion. The narrowed remaining internal carotid artery is demonstrated by the arrows and contributes little to filling of the ipsilateral middle cerebral artery (top arrow, panel B).

♦ The diameter of the carotid artery distal to a high-grade stenosis is variable. It may assume near-normal diameter or be significantly narrowed.
♦ Narrowing of the distal carotid artery is also called “string sign” or near occlusion. It is produced by the delayed arrival of contrast distally in the presence of collateral flow to the affected hemisphere.34
♦ Contrary to previous reports suggesting that stroke risk is high in this patient population, recent data suggest that the stroke risk is low on medical therapy, and revascularization may not be necessary.34
♦ The low stroke risk is explained by the reduced blood flow across the tight stenosis, which may fail to dislodge emboli.34,35
♦ The case also demonstrates the difficulties of noninvasive tests in diagnosing complete occlusions and near occlusions in some instances.18

Hemodynamic Infarction

Cerebral infarction may also occur as a result of perfusion failure, and the prevalence of hemodynamic mechanisms may have been underestimated.36,37 Such types of infarctions are typically associated with large arterial vessel occlusion or high-grade stenosis. Situations that may lead to changes in cerebral blood flow and perfusion pressure are often precipitating factors. This type of infarction tends to occur in watershed and border-zone areas, that is, the distal irrigation fields between the anterior and middle cerebral artery (anterior watershed), posterior and middle cerebral artery (posterior watershed), and the medullary penetrating and lenticulostriate arteries of the middle cerebral artery along the lateral and cephalad border of the lateral ventricle (internal border zone).38

Case Vignette

An 82-year-old man with history of hypertension and smoking had a documented right carotid occlusion. He presented to the consultation after awakening with left-sided weakness. A careful review of recent symptoms revealed that over the previous 2 to 3 weeks, he had experienced several episodes of lightheadedness and left-sided heaviness after getting up from the bed or a chair; each episode resolved within a few minutes. On several of those occasions, his left arm would shake uncontrollably for several seconds. The day prior to his stroke he had fallen on his left hip. His examination showed left hemiparesis. He had a large hematoma over his left hip. His hematocrit had decreased from 39% (his baseline) to 32%. Brain MRI showed infarction in the deep internal middle cerebral artery watershed distribution, which included the distal irrigation fields of the lenticulostriate and cortical penetrating medullary branches (Figure 5-8). Carotid ultrasound, MR angiography, and CT angiography confirmed occlusion of the ipsilateral ICA.

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Figure 5-8 (A) Diffusion-weighted image of an internal border-zone infarct. The linear stroke pattern is reminiscent of beads on a string. (B) Ultrasound power Doppler shows absence of flow in the right internal carotid artery. ECA, external carotid artery; ICA, internal carotid artery. (C) Postimaging reconstruction of CT angiogram CT delineating the extracerebral and intracerebral vasculature and right internal carotid occlusion (arrow). (D) Axial CT angiography showing absence of contrast in the right internal carotid artery (arrow) consistent with occlusion. The views are used in the postimaging processing to obtain the reconstruction shown in panel C. (E) Magnetic resonance angiography confirms absent distal right internal carotid flow.

♦ Hemodynamic symptoms in patients with large artery occlusion (or critical stenosis) may be precipitated when coexistent conditions or situations further compromise cerebral perfusion (i.e., orthostatic changes, acute blood loss, nocturnal blood pressure dipping). Another common precipitant factor is the introduction of new antihypertensive treatment.
♦ Hemodynamic compromise may be manifested with limb-shaking TIA,39 a rare presentation of cerebral ischemia with positive symptoms rather than negative symptoms (i.e., loss of function).
♦ The characteristic pattern of infarction in cases of hemodynamic insufficiency affects the internal border-zone distribution; the resulting infarctions often resemble “beads on a string.”
♦ Other symptoms characteristically associated with watershed infarctions include the following:

♦ transcortical motor aphasia-infarction in the anterior watershed area of the dominant hemisphere
♦ transcortical sensory aphasia-infarction of posterior watershed area of the dominant hemisphere
♦ “man-in-the-barrel” distribution of weakness (bi-brachial weakness with preserved leg strength seen with bilateral anterior watershed infarcts).40

TREATMENT

Symptomatic Carotid Disease

Treatment of atherosclerotic disease comprises careful considerations of medical and surgical therapy. Surgical intervention has to be the primary consideration in symptomatic disease exceeding 70% stenosis because of the high risk of stroke recurrence on medical therapy.23,41 The risk of recurrent stroke on medical therapy is 26% in 2 years and is reduced to 9% with endarterectomy.23,41 The risk of subsequent stroke is greater with increasing degrees of stenosis.

Surgical therapy must be instituted rapidly because the risk of early reinfarction is high. In fact, the benefit of carotid endarterectomy in stroke prevention is significantly reduced if performed more than 2 weeks from symptom onset.28 In symptomatic carotid disease with 50% to 69% stenosis, the risk of recurrent infarction is lower and the benefit of surgical intervention is more modest.42 Other features such as patient sex, age, and plaque characteristics (ulcerations, intraplaque hemorrhage) may need to be taken into consideration to identify patients at increased risk for stroke recurrence.

Asymptomatic Carotid Artery Stenosis

The management of asymptomatic carotid stenosis continues to be a challenge and remains controversial. The risk of first stroke on medical treatment in patients with greater than 60% asymptomatic carotid stenosis is low, approximately 2% annually, and with endarterectomy, it may be reduced to 1%.22,43 Aggressive management of vascular risk factors with newer antithrombotic, antihypertensive, and lipid-lowering agents may have reduced this risk of stroke with medical therapy even further in recent years.22,44,45

Carefully selected asymptomatic patients with greater than 60% stenosis benefit from endarterectomy compared with medical therapy.22,43 The benefits, however, are moderate, were not found for women, and are only evident with a low surgical complications rate of approximately 2%.22,43

Identifying a subgroup of asymptomatic patients that may be at higher risk of stroke would limit intervention to those patients likely to benefit the most and maximize the risk–benefit ratio of surgical intervention. Careful consideration must be given to age, sex, and life expectancy, because the benefits of surgery only become apparent over time. Additional factors that might identify populations at risk for first stroke are not entirely defined but may include presence of asymptomatic microemboli on transcranial Doppler,44,46 asymptomatic infarction in the distribution of the stenosis,47 plaque characteristics (ulcerations and intraplaque hemorrhage),7–11 and impaired vasomotor reactivity.48

Case Vignette

An asymptomatic 65-year-old diabetic and hypertensive active man was found to have a carotid bruit. An initial carotid ultrasound suggested a 50% to 60% carotid stenosis. He was followed with serial carotid ultrasonography, and 1 year later, the ultrasound performed in the same laboratory showed progression of the stenosis to 80% to 90%, caused by a larger hypoechoic, ulcerated plaque. This was subsequently confirmed by MR angiography of the neck, and MR angiography of the brain disclosed poor compensatory collateral flow in the affected hemisphere. Brain MRI showed several cortical FLAIR hyperintensities in the ipsilateral hemisphere, indicating subclinical infarctions. He had several microembolic signals on transcranial Doppler and impaired vasomotor reactivity (Figure 5-9). He was counseled on his future stroke risk as well as risks and benefits of medical versus surgical therapy. His stroke risk was felt to exceed the annual average of 2% generally applied to asymptomatic patients with greater than 60% carotid stenosis.

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Figure 5-9 (A) Fluid-attenuated inversion recovery image showing cortical hyperintensities suggestive of previous subclinical ischemic injury. (B) Transcranial Doppler microembolic signal detected in the middle cerebral artery (arrow). The absence or presence of microembolic signals distal to an asymptomatic stenosis may have important prognostic implications. (C) Vasomotor reactivity study assessing response to inhalation of CO2 (started at first arrow) leading to an increase in flow velocities and hyperventilation (hypocapnia) causing a decrease in flow velocities in the normally functioning middle cerebral artery depicted by the black line. The green line shows the flow in the contralateral middle cerebral artery, which exhibits no significant response to CO2.

This case illustrates:

♦ The need to select asymptomatic patients carefully for carotid revascularization
♦ Consideration of clinical factors

♦ relatively young age
♦ lack of medical comorbidities

♦ Consideration of ancillary factors that might influence risk of first stroke

♦ Progression of disease,49,50 easily assessed with serial ultrasonography (ideally performed in the same laboratory and using the same equipment)
♦ Presence of microemboli signals
♦ Impaired cerebral vasoreactivity
♦ Plaque characteristics (i.e., hypoechoic areas, ulcers)
♦ Collateral flow pattern
♦ Radiological evidence of silent embolic infarctions

Carotid Revascularization

New options for revascularization are currently emerging. Carotid endarterectomy is a proven and effective technique with a low complication rate. It will be difficult to improve further on the safety record and durability of carotid endarterectomy. There are, however, situations that increase the perioperative complication rate of carotid endarterectomy. Such scenarios include contralateral high-grade stenosis or occlusion, radiation induced carotid stenosis, inaccessible high cervical carotid bifurcation, restenosis after endarterectomy, and medical comorbidities, particularly poor cardiopulmonary status.51

Carotid artery stenting is a treatment option in those settings; however, it is increasingly applied for the treatment of carotid stenosis in general practice. A clear benefit of carotid stenting over carotid endarterectomy has not yet been demonstrated,51–53 and carotid endarterectomy should continue to be considered the gold standard intervention for the treatment of carotid stenosis.

Case Vignette

A 55-year-old man developed an episode of right-arm numbness and weakness. The patient had a prior history of neck radiation for lymphoma. Carotid ultrasonography revealed high-grade left carotid stenosis, which was later confirmed angiographically. The prior history of radiation was considered a relative indication for stent placement rather than endarterectomy (Figure 5-10).

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Figure 5-10 (A) Catheter angiogram showing a long segment of narrowing extending from the proximal to the distal internal carotid artery consistent with radiation induced arteriopathy outlined by arrows. (B and C) Ultrasound B-mode image of carotid stent axial view (B) and longitudinal view (C) identified by arrows. (D) Appearance of stent on cerebral angiography (arrows).

♦ Accelerated atherosclerosis may occur after radiation therapy in the absence of other vascular risk factors.
♦ Radiation-induced lesions tend to involve segments of the carotid artery typically spared by atherosclerosis (i.e., longitudinally extensive segments involved portions of the cervical carotid artery relatively distant from the bulb).
♦ Radiation-induced vasculopathy is considered a relative indication for stenting because of the suboptimal results achieved by endarterectomy.

VERTEBRAL ARTERY ATHEROSCLEROSIS

Extracranial vertebral atherosclerotic disease remains an underrecognized cause of posterior circulation infarction. Segments typically affected by atherosclerosis are the vertebral artery origin and the proximal few centimeters just distal to the origin. The remaining extracranial segments of the vertebral artery are spared by atherosclerosis. Intracranially, the segments preceding and following origin of the posterior inferior cerebellar artery and the vertebrobasilar junction are typically affected by atherosclerosis.

As in carotid disease, the important mechanisms of infarction are artery-to-artery embolism and hemodynamic mechanisms.54 Hemodynamic mechanisms are further dependent on the patency of the contralateral vertebral artery, which may either be affected by atherosclerosis or congenitally hypoplastic.

Vertebral arteries are often incompletely investigated during the evaluation of posterior circulation stroke patients. During ultrasonography, the vertebral artery is often only insonated in its transforaminal segment, and flow is reported as antegrade. Pathology of the vertebral origin is often undetected. The MR and CT imaging protocol for angiography of the neck may also fail to include the vertebral origins unless otherwise specified.

The vertebral artery origin may be assessed with ultrasound and CT and MR angiography. However, only a few studies have reported on the diagnostic accuracy of these imaging modalities. A sensitivity of approximately 70% and specificity exceeding 90% has been reported for the diagnosis of proximal vertebral artery disease with ultrasonography compared with the gold standard of cerebral angiography.55,56 A similar sensitivity and specificity has been reported for CT and MR angiography for stenosis of the vertebral artery origin.57–59

Case Vignette

A 65-year-old hypertensive man developed recurrent symptoms of vertigo, diplopia, and hemibody numbness. Initial vascular evaluation with ultrasound reported antegrade flow bilaterally in both vertebral arteries insonated in the midcervical area. MR angiography of the neck showed normal vertebral arteries but did not include their most proximal segments. The patient’s symptoms persisted despite several changes in antithrombotic regimen. Repeated complete vertebral ultrasonography suggested a high-grade vertebral artery-origin stenosis. This finding was later confirmed by catheter angiography. A stent was placed at the vertebral artery origin, and the patient had no further symptoms (Figure 5-11).

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Figure 5-11 (A) B-mode image showing the proximal vertebral artery including a plaque at the origin (arrow). VAO, vertebral artery origin. (B) Catheter angiography confirms the proximal vertebral artery stenosis. (C) Angiographic result after vertebral origin stent.

♦ Repetitive stereotypical symptoms should be considered highly suspicious of fixed stenosis of a large vessel and demand examination of the entire arterial tree.
♦ This case demonstrates the diagnostic difficulties encountered when attempting to examine the vertebral artery by noninvasive methods.
♦ The best management for proximal vertebral artery occlusive disease is not known. Medical treatment, angioplasty and stent placement, or vertebral transposition are treatment options that may be considered.

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