Natural History of Cerebrovascular Occlusive Disease
Ruth L. Bush
Peter H. Lin
Eric K. Peden
Alan B. Lumsden
This chapter discusses the natural history of extracranial carotid, vertebral, and aortic arch branch vessel disease. Each of these specific disease locations may result in a cerebrovascular accident (CVA), which is the third most common cause of death in the United States, accounting for greater than 160,000 deaths annually. Management of all CVAs results in an estimated expenditure of $45 billion per year and is responsible for more than 1 million hospital discharges each year. The incidence of stroke is about 2 per 1,000 population, but concurrent risk factors such as age, gender, and ethnicity significantly contribute to increases in rates. The morbidity caused by a CVA may be more disabling than that encountered with other arterial ischemia, including myocardial infarction (MI). Neurologic sequelae related to CVA, including aphasia, paralysis, blindness, and weakness, severely limit a patient’s ability to carry out routine daily activity and invariably contribute to this immense burden on the health care system.
Vascular compromise involving the upper extremities and/or carotid artery distribution was first described and published in 1944 by Martorell and Fabre. Shimizu and Sano described surgical therapy of two common carotid lesions in 1951, and soon after that they introduced prosthetic bypass (see below) and extra-anatomic procedures for high-risk patients. Percutaneous endovascular interventions for atherosclerotic lesions, both stenoses and occlusions, are now viable options for the arch vessels, as well as vertebral and carotid arteries. Credentialing and reporting standards do not exist for endovascular procedures in these areas. Furthermore, clinical trials are currently under way to assess carotid angioplasty and stenting compared to carotid endarterectomy. Though appealing because it is minimally invasive, percutaneous therapy warrants further investigation at the time of this writing.
Etiology of Ischemic Stroke and Transient Ischemic Attacks
Stroke, or focal cerebral ischemic disease, is defined as a loss of neurologic function for more than 24 hours. The term completed stroke refers to the fact that the severity of the neurologic deficit has reached its peak and has shown no signs of getting worse. Transient ischemic attacks (TIA) are neurologic deficits that last for less than 24 hours, although most resolve within minutes rather than hours. In the United States, the prevalence of TIA in men aged 65 to 69 years is 2.7%, and it is 1.6% in women. These figures increase with age to 3.6% for men and 4.1% for women between 75 and 79 years.
About 80% of all strokes are caused by ischemic etiologies, while the remaining 20% are caused by hemorrhagic disease. Patients with ischemic neurologic deficits can be further classified into anterior or hemispheric symptoms and posterior or vertebrobasilar symptoms. This is because hemispheric symptoms are frequently caused by emboli from the carotid circulation, and vertebrobasilar symptoms originate from either flow-limiting or embolic lesions of the aortic arch vessels, the vertebral arteries, or the basilar artery. The predominant causes of strokes and TIAs arise from the occlusive lesions of the extracranial carotid artery. These lesions include internal carotid artery thrombosis, flow-related ischemic events, and cerebral embolization.
Carotid artery thrombosis represents a terminal event in a severely diseased artery. The clinical sequelae of carotid thrombosis depend on a number of factors, including the status of the circle of Willis and the amount of collateralization that has formed, as well as the chronicity and extent of the thrombosis. Once the internal carotid artery (ICA) has thrombosed, the column of thrombus usually propagates up to the ophthalmic artery, and if collateral flow is sufficient, the event may be clinically silent. However, in some circumstances, the thrombus may occasionally extend beyond the ophthalmic artery and propagate into the circle of Willis, resulting in a hemispheric event with neurologic deficits ranging from a TIA to a severe stroke.
A very small proportion of strokes (<4%) are secondary to isolated cerebral hypoperfusion. Patients susceptible to this type of stroke include those with a critical ICA stenosis, poor collateralization via the circle of Willis, and secondary triggers, such as hypotension following an acute cardiac event. Flow-related ischemic events usually occur in the presence of both a hemodynamically significant stenosis and transient decreases in cerebral perfusion. This is a rare event, due to multiple collateral pathways form the circle of Willis, the contralateral carotid artery (unless it is also severely stenosed), and external-to-internal carotid artery connections.
The majority (more than 50%) of ischemic strokes are due to cholesterol or platelet-fibrin emboli from carotid stenosis into territories supplied by either the middle cerebral artery and/or the anterior cerebral artery. These embolic events may result in transient or permanent neurologic deficit, or they may be silent. Hollenhorst plaques, observed during ophthalmologic examinations, are secondary to emboli lodging in and obstructing retinal branches of the ophthalmic artery, which is the first branch of the ICA.
Multiple other causes of ischemic stroke exist but are beyond the scope of this discussion. These include cardiac emboli, paradoxical emboli, and hematologic causes, such as hypercoagulable state or malignancy, vascular arthritides, fibromuscular dysplasia, carotid dissection, trauma, and radiation arteritis.
Clinical Presentation, Diagnosis, and Treatment
Extracranial Carotid Disease
A careful history and complete neurologic examination, which should localize the area of cerebral ischemia responsible for the neurologic deficit, are the most important tools in the diagnosis of carotid artery disease. The neurologic examination should be accompanied by a complete physical examination. A high percentage of patients may have concomitant vascular occlusive disease in either the coronary or peripheral arteries. Other risk factors for stroke and atherosclerosis should be elucidated from the patient, such as an acute arrhythmia, hypertension, diabetes, smoking history, and so on. The diagnosis of carotid bifurcation disease is facilitated by the relatively superficial location of the carotid artery, which renders it accessible to auscultation and palpation. The cervical carotid pulse is usually normal in patients with carotid bifurcation disease, because the common carotid artery is the only palpable vessel in the neck and is rarely significantly diseased. Carotid bifurcation bruits may be heard just anterior to the sternocleidomastoid muscle near the angle of the mandible. Bruits do not become audible until the stenosis is severe enough to reduce the luminal diameter by at least 50%. Conversely, bruits may be absent in extremely severe lesions because of the extreme reduction of flow across the stenosis.
Noninvasive carotid imaging modalities provide accurate information regarding the nature and severity of the carotid artery lesion. Furthermore, color duplex is the most accessible and cost-effective screening technique for diagnosing carotid stenosis. Color-flow duplex scanning uses real-time B-mode ultrasound and colorenhanced pulsed Doppler flow measurements to determine the extent of the carotid stenosis with reliable sensitivity and specificity. Real-time B-mode imaging permits localization of the disease and determination of the presence or absence of calcification within the plaque. Determination of the extent of stenosis is based largely on velocity criteria. As the stenosis increasingly obliterates the lumen of the vessel, the velocity of blood must increase in the area of the stenosis so that the total volume of flow remains constant within the vessel. Thus, the velocity is correlated with the extent of carotid artery stenosis. The ICA velocity profile is one of a low-resistance artery characterized by a significant period of antegrade carotid blood flow during diastole. In contrast, the external carotid artery reflects a signal typically found in a high-resistance artery, in which little blood flow occurs during diastole. Standard color-flow duplex scans cannot assess the cerebral arterial circulation beyond the first several centimeters of the ICA. A transcranial Doppler has been developed to evaluate the middle cerebral artery and other intracranial vessels, using a low-frequency Doppler signal to penetrate the thin bone of the temporal and occipital regions.
The accuracy of a carotid duplex scan largely depends on the technician who performs the study, as well as the type of scanner that is used. Ultrasound criteria vary between units, and each vascular laboratory should validate the technical skills of the ultrasonographer before duplex imaging is used as the sole diagnostic study. The duplex scan findings should also be compared to a second imaging modality to determine the sensitivity and specificity of noninvasive imaging at a single institution. Many surgeons now perform carotid surgery on the basis of duplex ultrasound alone. However, corroborative magnetic resonance (MR) angiography or diagnostic angiography may be required in patients who have one or more of the following:
Gross calcification causing severe acoustic shadowing
Inability to image proximal or distal limits of plaque
Damped inflow waveform suggestive of proximal common carotid disease
High-resistance ICA waveform suggestive of distal severe disease
Some surgeons may perform a corroborative study in patients with a duplex diagnosis of ICA occlusion, as ultrasound may fail to see a very near-total occlusion or string sign. One of the immense advantages of duplex is that it can be brought down into a single visit outpatient clinic.
MR imaging and MR angiography have been evaluated as an alternative imaging modality for the carotid arteries. MR imaging is more sensitive than computed tomography (CT) scanning for the detection of an acute stroke, as it can detect a stroke immediately after the infarction occurs, whereas CT scanning cannot. MR angiography, which is a rapidly evolving technique, permits evaluation of both the extracranial and intracranial cerebral circulations. The precision of MR angiography in determining the extent of stenosis, although improving, remains inferior to that achieved by conventional contrast angiography. Nonetheless, MR angiography will likely play an increasingly important role in the diagnostic evaluation of patients with cerebrovascular disease.
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