Carotid disease may manifest as only an asymptomatic bruit. It can also present, though, with amaurosis fugax, a transient ischemic attack (TIA), or a cerebrovascular accident. Amaurosis fugax is a temporary monocular blindness caused by a TIA of the retina. It is an indication of an evolving arterial thrombus in the internal carotid artery, which is the main blood supply to the optic nerve and retina through the ophthalmic artery. The symptoms of amaurosis fugax, which have been described as a shade descending over one eye, often last less than 10 minutes, and are ipsilateral to the evolving and symptomatic vascular disease.

Other manifestations of symptomatic carotid artery disease include episodes of paresthesias, clumsiness, or difficulties with speech that resolve spontaneously over a short period.

Carotid artery disease is a manifestation of generalized arteriosclerosis. The reported prevalence of asymptomatic carotid stenosis varies with differences in demographic factors, screening methods, and cut-off points used to identify stenosis. In a meta-analysis of 40 studies reviewing the prevalence of carotid stenosis in asymptomatic patients, the pooled prevalence of moderate (≥50%) stenosis was 4.2%, and the pooled prevalence of severe (≥70%) stenosis was 1.7%. Both advanced age and male gender were found to be associated with increased prevalence of disease. Symptomatic carotid stenosis may be defined as focal neurologic symptoms that are sudden in onset and referable to the appropriate carotid artery distribution, including one or more TIAs or nondisabling strokes. A population-based study in Rochester, Minnesota, found the incidence rate of TIAs from the years 1985 to 1989 to
be 68 per 100,000. Another population-based study in Italy found the incidence rate of TIAs between the years 2007 and 2009 to be 52 per 100,000. Cerebrovascular disease is the fourth leading cause of death in the United States, after heart disease, cancer, and chronic lower respiratory disease.

The risk factors for the development of carotid artery bifurcation disease are similar to those causing atherosclerotic occlusive disease in other vascular beds, namely, increasing age, male gender, hypertension, tobacco smoking, diabetes mellitus, homocysteinemia, and hyperlipidemia. In carotid artery disease, atherosclerotic plaques develop at the lateral aspect of the bifurcation of the carotid artery. Thrombus may become superimposed on the atheroma; this most likely occurs where the plaque narrows the lumen to the greatest degree. The mechanism of resultant TIA or stroke may be embolism of the thrombotic material or low flow due to the stenosis with inadequate collateral compensation. Severe carotid stenosis is a strong predictor for stroke, and a prior history of neurologic symptoms such as TIA or stroke is a predictor for recurrent ipsilateral stroke.

Approximately 15% of all strokes are preceded by a TIA. Meta-analyses have shown that the short-term risk of stroke following a TIA is approximately 3% to 10% at 2 days and 9% to 17% at 90 days. Mortality within the first year following a TIA is approximately 12%. The incidence and fatality rates associated with stroke have been declining over the past several decades, which has mainly been attributed to cardiovascular risk control interventions.

Diabetes is a major risk factor for stroke. When matched for associated risk factors, symptomatic cerebrovascular disease is more common in patients who have diabetes mellitus than in those who do not have diabetes. The relative risk conferred is greatest in patients younger than 65 years of age; in a population-based study in the Greater Cincinnati/Northern Kentucky region in 2005, risk ratio for ischemic strokes in Blacks younger than 65 years of age with diabetes was 5.2, compared with a risk ratio of 12.0 for Whites younger than 65 years of age with diabetes. Additionally, mortality and the severity of stroke are greater in patients with diabetes.

In a study from 1992 to 1995, 732 CEAs were performed in diabetic and nondiabetic patients. Patients with diabetes were younger at presentation than nondiabetic patients and were more likely to have history of coronary artery disease. This may contribute to the higher incidence of postoperative cardiac morbidity in diabetic patients. However, diabetes mellitus by itself is not a risk factor for postoperative cardiac morbidity in patients who undergo carotid surgery.

Diabetes mellitus and obesity are considered independent risk factors for neurocognitive decline after CEA.

Several randomized controlled trials have evaluated the efficacy of CEA for patients with asymptomatic high-grade stenosis. A meta-analysis of these trials found that CEA confers a small absolute risk reduction for stroke compared to medical management. In the two largest trials, the asymptomatic carotid atherosclerosis study (ACAS) and the European asymptomatic carotid surgery trial (ACST-1), the risk reduction was about 3% over 3 years, corresponding to a number needed to treat of approximately 33 to prevent one stroke at 3 years. The role of CAS in asymptomatic patients is also unclear. No randomized controlled trials have been done to evaluate CEA versus CAS in asymptomatic patients. However, a meta-analysis of studies in both symptomatic and asymptomatic patients suggests that in asymptomatic patients, the periprocedural (30-day) rate of stroke or death is higher with CAS than with CEA, although no difference has been shown past 30 days. Thus, recommendations for the treatment of asymptomatic carotid stenosis remain somewhat controversial. Medical management, including treatment of hypertension, hyperlipidemia, diabetes, and smoking coupled with the use of antiplatelet agents, should be considered. Prophylactic CEA may be undertaken in highly selected patients with asymptomatic severe stenosis (60% to 99% according to various guidelines) while taking into account the patient’s comorbidities and overall fitness for surgery. The usefulness of CEA versus CAS remains unclear, although guidelines from the Society of Vascular Surgery state that prophylactic CAS should not be undertaken.

Symptomatic carotid disease refers to focal neurologic symptoms, which are acute and referable to the ipsilateral carotid distribution. Optimal medical therapy is recommended in all patients with symptomatic disease. Several large randomized controlled trials have evaluated the efficacy of CEA in symptomatic patients. The North American Symptomatic Carotid Endarterectomy Trial (NASCET) demonstrated that CEA is superior to medical therapy alone in symptomatic patients with >70% stenosis, with a 9% risk of stroke in the CEA group compared to 26% in the medical group, or an absolute risk reduction of 17% over 2 years. These results were largely confirmed by the European Carotid Surgery Trial (ECST). Current guidelines state that revascularization should be undertaken in patients with moderate to severe symptomatic stenosis as long as the perioperative morbidity and mortality risk is estimated to be <6% for the surgeon or center. Many trials have been done in recent years comparing CEA to CAS for symptomatic patients. A meta-analysis of seven trials suggests that long-term outcomes are similar for CEA and CAS groups. However, CEA is associated with a greater risk of periprocedural myocardial infarction, whereas CAS is associated with a greater increase in periprocedural death/stroke. Thus, the decision between CEA and CAS for symptomatic patients depends on a combination of patient-specific factors, including the gender, surgical/anatomic considerations, and the presence of significant comorbidities that would increase surgical risk.

The common carotid arteries originate in the thorax. The right common carotid artery originates at the bifurcation of the brachiocephalic trunk, and the left originates from the aortic arch. In the neck, the common carotid arteries travel within the carotid sheath. At the level of the thyroid cartilage, each common carotid artery bifurcates into internal and external carotid arteries.

Branches of the external carotid artery include the superior thyroid, lingual, facial, ascending pharyngeal, occipital, and posterior auricular arteries.

The internal carotid artery passes through the neck without branching to enter the middle cranial fossa. It enters the middle cranial fossa through the carotid canal of the temporal bone, adjacent to the sphenoid bone. It supplies the hypophysis cerebri, the orbit, and the major portion of the supratentorial region of the brain. The cerebral arteries are derived from the internal carotid and vertebral arteries. The anastomosis they form at the base of the brain is known as the circle of Willis. The two anterior cerebral arteries form the circle anteriorly. They are connected through the anterior communicating artery. The two posterior cerebral arteries form the circle posteriorly, which then ends at the junction of the basilar artery. The posterior cerebral arteries are connected to the internal carotid arteries by the two posterior communicating arteries (Fig. 18.1). The middle cerebral artery primarily supplies the lateral surface of each cerebral hemisphere. The anterior cerebral and posterior cerebral arteries supply the medial and inferior surfaces of the cerebral hemisphere.

Autoregulation is assumed to be lost in underperfused areas of the brain. Vascular regions subjected to chronic hypoperfusion and relative ischemia are maximally vasodilated and unresponsive to factors that induce vasoconstriction in normally reactive vascular beds. Therefore, in patients with carotid artery disease, blood flow through ischemic regions is passive and dependent on systemic blood pressure. For this reason, hypotension is best avoided in the period before restoration of unobstructed CBF.

CEA involves the removal of atheromatous plaque from the vessel lumen through a fairly standardized surgical procedure. This involves occluding the common, external, and internal carotid arteries; isolating the diseased segment; opening the vessel wall; and removing
the plaque. The vessel is then closed. If the remaining intima is too thin, the vessel is closed with a vein graft or a synthetic (Dacron) patch. The use of a shunt during the period of carotid cross-clamping depends on whether evidence of cerebral ischemia becomes apparent with cross-clamping of the carotid artery. Variations in shunt usage exist because there is little evidence that one therapy is superior to the others. Placement of a shunt allows hemispheric CBF to be maintained during cross-clamping and may be especially advantageous when the endarterectomy is expected to be complex and require a long time.

On the other hand, shunt usage is not without its problems. It may make the surgery technically more difficult. Technical problems with shunts include plaque or air embolism, kinking of the shunt, shunt occlusion on the side of the vessel wall, and injury to the distal internal carotid artery. The use of a shunt does not guarantee adequate CBF nor prevent thromboembolic strokes.

Another intervention to treat carotid stenosis is percutaneous transluminal angioplasty and stenting by interventional radiologists. Advantages to endovascular techniques include avoidance of surgical incision; minimal anesthetic requirements; avoidance of minor complications associated with endarterectomy, including cranial nerve injury and wound infections; shorter duration of induced carotid occlusion by the balloon catheter than with surgical clamping of the carotid artery; and lower hospital costs for shorter hospital and intensive care unit stays.

Clinical trials comparing CEA with angioplasty/stenting have been done. Overall, endovascular treatment is associated with an increased risk of periprocedural stroke or death compared with CEA. This excess risk appears to be limited to older patients. Long-term outcomes do not appear to differ between patients treated with CAS versus CEA.

Normal CBF is approximately 50 mL/100 g/min for the entire brain. Blood flow is approximately four times higher in gray matter than it is in white matter, with the flows being 80 and 20 mL/100 g/min, respectively.

Assuming a normal temperature and hematocrit, 100 mL of blood contains approximately 20 mL of oxygen. A normal global CBF of 50 mL/100 g/min delivers oxygen to the brain at a rate of 10 mL/100 g/min. This is in excess of the high metabolic requirements of the brain for oxygen (3 to 5 mL/100 g/min), which affords a relative margin of safety. The CBF at which ischemia becomes apparent in the EEG is approximately 20 mL/100 g/min. Changes on EEG may be delayed for up to 150 seconds following the onset of ischemia. It is impossible to define the specific changes that represent irreversible ischemia. In the setting of an isoelectric EEG, oxygen delivery to the brain may be adequate to keep the neurons alive but may provide insufficient energy for them to function.

The development of cerebral infarction depends on both the degree and duration of ischemia. Jones et al. showed in an animal model of reversible ischemia that with CBF of 18 to 23 mL/100 g/min, animals would recover from impaired neurologic function when blood flow
was returned to normal levels, regardless of the duration of the ischemic period. Infarction development at lower flows depends on both the degree of regional CBF reduction and the duration of ischemia. Neurons that are nonfunctional but will recover fully with restoration of adequate flow are said to be in an ischemic penumbra. Neuronal destruction occurs with CBF less than 10 mL/100 g/min.

Cerebral autoregulation is the tendency of the brain to match cerebral metabolic oxygen requirements with oxygen delivery in spite of variations in blood pressure. In normotensive individuals, CBF is constant between mean arterial pressures of 50 and 150 mm Hg. What this means is that cerebrovascular resistance increases, through vasoconstriction, as mean arterial pressure increases from 50 to 150 mm Hg. At pressures greater than 150 mm Hg, the cerebral vasculature is maximally vasoconstricted and CBF increases with increasing pressure. At pressures less than 50 mm Hg, cerebral vessels are maximally vasodilated, so that CBF decreases as mean arterial pressure falls.

In hypertensive patients, the upper and lower limits of the autoregulatory curve are shifted to the right, to higher pressures (Fig. 18.2). What this means is that a mean arterial pressure of 60 mm Hg, which would be well-tolerated in a normotensive individual, may actually be below the lower limit of autoregulation in the hypertensive individual, resulting in cerebral hypoperfusion. Conversely, hypertensive patients tolerate marked increases in mean arterial pressure much better than their normotensive counterparts. In treated hypertensive patients, the limits of autoregulation are shifted with time toward normal.

Hypercarbia results in cerebral vasodilation and hypocarbia in cerebral vasoconstriction. CBF changes approximately 4% for each millimeter of mercury increase or decrease in arterial PaCO₂ for partial pressures of arterial carbon dioxide between 20 and 80 mm Hg.

The principal determinants of CBF are nerve cell activity, cerebral perfusion pressure, PaCO₂, the pH of the extracellular fluid in the brain, PaO₂, and neurogenic influences.

Luxury perfusion is blood flow that is in excess of metabolic need (increased CBF relative to cerebral metabolic rate for oxygen). It is most frequently observed in tissues surrounding tumors or areas of infarction. It has also been described in tissues that have been manipulated during surgery.

Intracerebral steal is a paradoxical response to carbon dioxide in which hypercapnia decreases the blood flow in an ischemic area. It is the consequence of the vasodilatory effect of carbon dioxide on the normally perfused arterioles at the periphery of an ischemic lesion. Because chronically ischemic vascular beds are maximally vasodilated, they cannot dilate further in response to hypercapnia.

Inverse steal is the effect of hypocapnia producing increased blood flow to ischemic regions of the brain. Vasoconstriction occurs in adjacent, normal arterioles, thereby causing a local increase in perfusion pressure and augmenting collateral flow to the ischemic, unreactive, maximally vasodilated area of the brain.