Carotid Echo Doppler: Diagnostic Tool or Impact on Indication and Treatment?
The grade of stenosis due to plaque encroaching the lumen constitutes the most commonly utilized criterion to identify subgroups of patients at high risk for stroke. 2 Cumulative clinical evidence reported by different trials suggests that more severe stenosis correlates with a greater severity of the clinical event. 3 In addition, the risk of stroke is increased in patients bearing symptomatic stenosis. 4 For stenosis less than 50% the annual stroke risk is 1%. Conversely, stenosis greater than 50% has an annual risk of around 3%. As reported in the NASCET study 3 in patients who have already suffered a cerebrovascular event, the annual stroke risk is 13% in the presence of a carotid stenosis greater than 70%. Although the value of angiography is
recognized for more advanced disease, its reliability for predicting the benefit of surgery for lowergrade narrowing remains less clear, particularly for those with 30-69% stenosis. 2
The natural history of asymptomatic stenosis is different. Interestingly, in ACAS there was no association between the stroke rate and the severity of stenosis, although the number of patients with 80-99% stenosis was only 88. Barnett et al 4 reported a 5-year rate of ipsilateral stroke of 15.7% in individuals with 50-69% symptomatic stenosis treated with endarterectomy, compared with 22.2% for those treated medically ( p = 0.04). For those with < 50% stenosis, the rate was lower for those treated surgically (14.9% vs 18.7%, p = 0.16). Reanalysis of the ECST study to allow comparison with NASCET showed a similar modest reduction in 5-year absolute risk in the 50-69% group treated surgically (5.7%, 95% confidence interval (CI) 0-11.6%). 5 In this reanalysis, surgery had no effect on outcome in symptomatic patients with 30-49% stenosis. Additional randomization and longer follow-up showed that the stroke-free life expectancy within an 8-year follow-up period was shorter for surgery patients with 30-49% stenosis (6.16 vs 6.63 years for the non-surgical group) and for surgery patients with 50-69% stenosis (5.93 vs 6.14 years for the non-surgical group). 6 The ECST found very little 3-year risk of ipsilateral ischemic stroke for symptomatic individuals with 0-29% stenosis, even in the absence of surgery. 7
The risk of stroke from plaques causing low-grade narrowing in asymptomatic individuals is even less well understood. The Asymptomatic Carotid Atherosclerosis Study (ACAS) showed a reduction in the aggregate risk for stroke and perioperative stroke or death over 5 years to be 53% (95% CI 22-72%) for patients with 60% or more carotid narrowing treated surgically compared with those treated medically. 8 This study included only asymptomatic individuals with carotid narrowing > 59%. The identification of asymptomatic individuals with low-grade narrowing who would benefit from surgical management would necessitate a highly specific method for stratifying risk not achievable by angiography, considering the high prevalence of low-grade disease. Furthermore, the prevalence of low-grade carotid stenosis is very high. The Cardiovascular Health Study detected carotid stenosis in 75% of men and 62% of women over 64 years of age by ultrasound, although prevalence of stenosis above 49% was only 7% in men and 5% in women. 9 Therefore, although the risk of stroke with < 50% carotid stenosis is low, the attributable risk for stroke resulting from < 50% carotid stenosis may be significant as a result of the high prevalence of this finding.