The relative risk of stroke in heavy smokers (>40 cigarettes a day) is twice that of light smokers (<10 cigarettes a day). Stroke risk decreases significantly after 2 years cessation of smoking and is at the level of nonsmokers by 5 years after cessation [42].

People with blood pressure less than 120/80 mmHg have about half the lifetime risk of stroke, compared to those with hypertension [43].

A physicians’ health study showed that men who performed vigorous exercise had a lower stroke rate [44]. The Harvard Alumni Study showed similar findings in males [45]. A nurses’ health study confirmed these findings for females [46].

The Multiple Risk Factor Intervention Trial (MRFIT) comprised more than 350,000 men showing an increasing relative risk of death with progressively higher levels of total cholesterol [47]. On the other hand, epidemiologic data for the association between triglyceride levels and HDL with ischemic stroke have been inconsistent [48].

A robust degree of epidemiologic data along with randomized trials suggest that diets low in sodium and rich in fruits and vegetables reduce stroke risk [48].

Diabetics typically have elevated cardiovascular risk factors, such as dyslipidemia, hypertension, obesity, and the metabolic syndrome which lead to increased susceptibility to atherosclerosis. It is an independent risk factor for stroke that more than doubles the risk and roughly 20% of diabetics will die of stroke [48].

D’Agostino et al. [56] collected data prospectively for 1835 patients undergoing coronary artery surgery; 1279 of these patients underwent screening carotid duplex examinations preoperatively. Two and a half percent of all patients developed an operative stroke. A multivariate analysis identified the following clinical predictors for postoperative stroke: advanced age, female sex, prior stroke or transient ischemic attack, atherosclerotic disease involving the ascending aorta, peripheral vascular disease, prior vascular operation, and smoking. The study suggested that preoperative carotid duplex should be considered for patients with a history of neurologic events or peripheral vascular disease; however, the majority of strokes in their study were caused by embolic phenomenon from an atherosclerotic aorta or from the heart. Reed et al. [57] noted that patients with a previous stroke have a sixfold increased risk of postcoronary artery bypass grafting (CABG) stroke. Li et al. [58] retrospectively reviewed 4325 patients who underwent CABG and/or valve replacement with a postoperative stroke incidence of 1.8%. Among stroke patients, only 5.3% of strokes were attributed to ICA stenosis. Interestingly, for patients who had >70% ICA stenosis, the postoperative stroke rate was 15% if they underwent both CABG and CEA, while it was 0% if no CEA was done. A 2011 meta-analysis calculated the perioperative stroke rate for asymptomatic unilateral carotid stenosis to be only 2% [59]. An observational study of 878 patients in 2011 showed a similar perioperative stroke rate after CABG of 3% in both patients with and without severe carotid stenosis (≥75%) [60]. These latter studies demonstrate that (1) the postoperative stroke rate for CABG patients is low and (2) in patients diagnosed preoperatively with a high-grade ICA stenosis, combined treatment is unlikely to reduce overall rate of operative stroke.

The real question is whether a high-prevalence population can be reliably identified. Marek et al. [61] performed screening carotid duplex in 188 patients who presented with intermittent claudication and no cerebrovascular symptoms. 20% were found to have a stenosis of 50–79%, 1.6% had stenosis of 80–99%, and 2.7% had evidence of carotid occlusion. The authors concluded that the subset of patients aged >65 years, the presence of carotid bruit, and an ABI of <0.7 had a 45% incidence of >50% ICA stenosis (OR = 5.42).

Turnipseed et al. [62] performed preoperative carotid duplex imaging in 330 patients who underwent coronary artery bypass (170) and peripheral vascular surgery (160). Patients with peripheral arterial disease had a higher incidence of carotid bruits compared to those with coronary artery disease (44% vs. 16%). In those patients who had a carotid bruit, there was 54% incidence of significant carotid artery disease. Patients with PAD had a 52% incidence of significant carotid disease compared with 11.7% of patients who underwent coronary artery bypass. This was not a true screening study however since 43% of the patients with PAD had symptoms suggestive of cerebrovascular disease.

Barnes et al. [63] prospectively screened 449 asymptomatic patients with carotid duplex before coronary or peripheral arterial reconstruction. They noted that the prevalence of carotid artery disease was significantly higher in patients who had PAD (28%) than in those patients presenting with coronary artery heart disease (15%). Additionally, patients who had asymptomatic carotid artery disease had an increased risk of neurologic events (15%) compared with patients without carotid artery disease (0.8%) during a 2-year follow-up. Moreover, there was an increased risk of perioperative and late death (10.6% and 9.2%, respectively) in patients who had asymptomatic carotid disease, compared with patients who did not (0.3% and 0.8%, respectively, p < 0.001).

Ahn et al. [64] reviewed the duplex scans of 78 patients who underwent carotid screening solely because of PAD and found that 14% had stenosis >50%, although they did not correlate the severity of the PAD to the presence of carotid stenoses. Their analysis showed that the risk factors of male sex, age > 68 years, hypertension, and previous cardiovascular surgery strongly correlated with carotid stenosis. They concluded that routine carotid duplex screening is indicated in older patients (age > 68 years) who have peripheral vascular disease.

Fowl et al. [65] screened two patient groups in a Veterans Hospital setting for the presence of asymptomatic carotid stenosis. The first group of 152 patients had no history of PAD, and the second group consisted of 116 patients with PAD. Duplex screening revealed a 6.5% incidence of >50% carotid stenosis in the first group, compared with 12% in the second group (p = 0.058). They recommended carotid surveillance in asymptomatic patients who had multiple atherosclerotic risk factors.

Gentile et al. [66] reviewed retrospective data on 225 patients who underwent infrainguinal revascularization procedures with no previous carotid surgery and found >50% stenosis in 28.4% of subjects. Furthermore, there was a statistically significant association between the presence of either a carotid bruit or the presence of rest pain with >50% carotid stenosis. Among the subset of patients who had carotid bruit, 58% were found to have asymptomatic carotid stenosis of more than 50%.

Virgilio et al. [67] prospectively screened patients with lower extremity atherosclerosis and found that 20% of asymptomatic male patients had a carotid stenosis of more than 50%.

Razzouk et al. performed a cross-sectional analysis of 3.67 million self-referred individuals that demonstrated a high prevalence of any carotid stenosis in subjects with PAD compared to those without PAD (18.8% vs. 3.3%, P < 0.0001) using a Life Line Screening^® database [68]. The analysis showed that PAD was associated with greater odds of any carotid stenosis (OR 2.91, 95% CI 3.22–3.34). Both symptomatic (OR 3.66, 95% CI 3.58–3.75) and asymptotic PAD (OR 2.91, 95% CI 2.84–2.98) were associated with carotid stenosis. Increased severity of PAD based on ABIs was associated with greater odds of carotid stenosis (OR 2.32, 3.61, 4.19, 5.14, and 7.59 for ABI categories 0.81–0.90, 0.71–0.80, 0.61–0.70, 0.41–0.60, and ≤0.40, respectively). This study suggests that prevalence and severity of carotid stenosis was significantly increased in patients with PAD regardless of their symptomatic status and was proportional to the severity of their PAD as measured by their ABI. They concluded that due to the high prevalence of carotid stenosis in asymptomatic patients with PAD, screening may be indicated in this population.

The rationale behind postoperative carotid imaging is twofold. First, recurrent stenosis may be identified, and second, disease progression in the non-operated ICA may be monitored regularly. Naylor et al. [69] followed 219 patients after carotid endarterectomy and specifically monitored their contralateral (non-operated, asymptomatic) ICA . One hundred fifty-one patients had regular duplex ultrasounds in the postoperative period. Cumulative freedom from stroke in the non-operated hemisphere was 99%, 96%, and 86% at 1, 5, and 10 years, respectively, giving a mean incidence of stroke of 1 percent per annum. Only one stroke was preceded by a transient ischemic event, and no stroke was associated with >70% stenosis of ICA. Ten patients (7%) with initially mild or moderate disease of the non-operated ICA progressed to severe stenosis during follow-up, only three became symptomatic, and, in each case, the onset of symptoms preceded recognition of disease progression. The long-term risk of stroke in the non-operated ICA territory was small. The authors concluded that none of the observed strokes could have been prevented by postoperative surveillance following CEA.

AbuRahma [70] performed a similar study evaluating arteries contralateral to a CEA in 534 patients. Serial duplex ultrasound was performed 1 month postoperatively and thereafter every 6 months. Overall, carotid artery stenosis progressed in 36% of patients at mean follow-up of 41 months. Progression of stenosis was noted in 3% of patients with baseline normal carotid arteries. Carotid artery stenosis progressed in 36% of patients with less than 50% stenosis versus 47% of patients with 50–79% ICA stenosis. Late neurologic events referable to carotid artery stenosis were infrequent (6.7% in the entire series), including 2.4% strokes and 4.3% TIAs. Contralateral CEA was performed in 15% of patients. They concluded that duplex ultrasound should be performed every 6–13 months, if the stenosis is between 50 and 69%, and every 12–24 months if stenosis is less than 50%.

Ballotta followed asymptomatic contralateral ICAs of 599 patients who had undergone CEA for symptomatic or asymptomatic severe carotid disease [71]. They performed duplex at 1 month and then every 6 months for a mean follow-up of 4.1 years. Disease progressed in 34% of patients with mild stenosis (30–49%) versus 47.9% of patients with moderate stenosis (50–69%). The median time to progression was 29.8 months for mild and 18.5 months for moderate stenosis. The rate of late neurologic events referable to contralateral ICA was 3.2% for the entire series and 4.8% for patients with a 30% or greater ICA stenosis. The authors suggested duplex surveillance every 6 months in patients with >50% stenosis.

Cervical radiation resulted in direct damage to the carotid artery causing intimal hyperplasia, medial necrosis and fibrosis, and acceleration of atherosclerosis. Mahlmann et al. [72] included previous neck irradiation as an indication for the use of carotid artery screening in asymptomatic patients. They determined the 3-year risk of high-grade carotid stenosis after neck irradiation was 37.5%. A recent prospective study utilizing CE-MRI determined that >50% stenosis was present in 23% of patients 3 years after neck irradiation for nasopharyngeal cancer with a matched control group of nonirradiated nasopharyngeal cancer patients having only a 2% incidence [73]. Mortiz et al. [74] and Carmody et al. [75] have shown that head and neck radiotherapy correlate with an increase in CAS ranging from an increase incidence of 12–30%. This relationship has been well established in the literature. Steele et al. [76] recently performed a prospective study of 40 patients receiving high-dose cervical radiotherapy. All the patients were screened with bilateral carotid arterial duplex ultrasonography. The results revealed that 40% had significant CAS defined as >50% stenosis, 15% had unilateral complete occlusion, 15% had significant bilateral CAS, and 7.5% sustained a stroke. The authors concluded that the CAS in patients with prior cervical radiation therapy is clinically significant and warrants aggressive screening as part of routine postradiation care.

Most vascular surgeons, however, agree that prior radiation increases the complication rate of carotid surgery, and the extent of disease is across the entire field of prior irradiation in the neck. Recognizing this increased complexity of intervention, one must certainly scrutinize the wisdom of intervening on asymptomatic disease in the previously irradiated neck.