Efficacy versus effectiveness of carotid endarterectomy

Several well-known clinical trials have demonstrated that the value of carotid endarterectomy depends on preoperative symptoms and the degree of carotid artery stenosis. The benefit of surgery also depends on how the results of these clinical trials (defining the efficacy of carotid endarterectomy) are applied to actual clinical practice (the effectiveness of the procedure), where surgical risks are greater.

The number of carotid endarterectomies needed to prevent one major stroke or death--that is, the number needed to treat (NNT).

Reciprocal of the difference between the 5-year cumulative incidence of major stroke or death with medical therapy and the 5-year cumulative incidence of major stroke or death with carotid endarterectomy.

Efficacy was calculated with data from the North American Symptomatic Carotid Endarterectomy Trials and the Asymptomatic Carotid Atherosclerosis Study. In calculating effectiveness, we accounted for increased surgical mortality rates reported in population-based studies.

For symptomatic patients, the NNT predicted by the effectiveness model differed little from that estimated by the efficacy model (10 versus 9 for severe carotid stenosis and 29 versus 23 for moderate carotid stenosis). However, the NNT predicted by the effectiveness model was substantially higher than that predicted by the efficacy model for patients with asymptomatic severe stenosis (63 versus 38).

In symptomatic patients, carotid endarterectomy is both efficacious and effective for severe and (to a lesser extent) moderate carotid stenosis. However, in asymptomatic patients, the benefits observed in published trials may overestimate those likely to be achieved in clinical practice.