Asymptomatic carotid artery stenosis treated with medical therapy alone: temporal trends and implications for risk assessment and the design of future studies.

BACKGROUND: The rate of adverse clinical outcomes among patients with asymptomatic carotid stenosis receiving medical therapy alone can be used to guide clinical decision-making and to inform future research. We aimed to investigate temporal changes in the incidence rate of clinical outcomes among patients with asymptomatic carotid stenosis receiving medical therapy alone and to explore the implications of these changes for the design of future comparative studies. SUMMARY: We searched MEDLINE, the Cochrane Central Register of Controlled Trials, US Food and Drug Administration documents, and reference lists of included studies (last search: December 31, 2012). We selected prospective cohort studies of medical therapy for asymptomatic carotid artery stenosis and we extracted information on study characteristics, risk of bias, and outcomes. We performed meta-analyses to estimate summary incidence rates, meta-regressions to assess trends over time, and simulations to explore sample size requirements for the design of future studies comparing new treatments against medical therapy. The main outcomes of interest were ipsilateral stroke, any stroke, cardiovascular death, death, and myocardial infarction. We identified 41 studies of medical therapy for patients with asymptomatic carotid stenosis (last recruitment year: 1978-2009). The summary incidence rate of ipsilateral carotid territory stroke (25 studies) was 1.7 per 100 person-years. This incidence rate was significantly lower in recent studies (last recruitment year from 2000 onwards) as compared to studies that ended recruitment earlier (1.0 vs. 2.3 events per 100 person-years; p < 0.001). The incidence rates of any territory stroke (17 studies), cardiovascular death (6 studies), death (13 studies), and myocardial infarction (5 studies) were 2.7, 4.1, 4.6, and 1.8 per 100 person-years, respectively. Simulations showed that future studies would need to enroll large numbers of patients with a relatively high incidence rate under medical therapy, and evaluate interventions with large effect sizes, to have adequate power to reliably detect treatment effects. KEY MESSAGES: Improved prognosis under medical therapy alone has narrowed the potential range of risk reduction attainable with new treatments for asymptomatic carotid stenosis. Future comparative studies will need to enroll large numbers of patients to assess treatment effectiveness.