Investigating individual subjects and screening populations for asymptomatic carotid stenosis can be harmful

Abstract

Objectives - Trials suggesting that carotid endarterectomy in individual subjects with asymptomatic carotid stenosis reduces stroke risk have led to calls for screening. This study aimed to determine which groups might be harmed and which might benefit from a screening programme, and also to identify which individual subjects identified as positive for severe asymptomatic stenosis by carotid ultrasound are appropriate to put forward for further tests or procedures. Methods - A probability model was used to estimate the outcomes of three screening strategies: carotid ultrasound followed by catheter angiography, or by magnetic resonance angiography (MRA), or ultrasound alone, followed by carotid endarterectomy if severe stenosis is detected. Information from the current literature was used to estimate sensitivity and specificity of ultrasound and MRA, risks of angiography and endarterectomy, and risk reduction after surgery for severe stenosis. For each strategy over a range of possible prevalences of severe asymptomatic stenosis, overall benefit to harm ratio was calculated, and number of strokes or deaths prevented or caused per 10,000 subjects screened. Results - At the prevalence of carotid stenosis found in the general population (< 1%) screening will cause more strokes than it prevents, even using the most optimistic published figures. Only at prevalences of over 20% are significant benefits seen, and then only in centres with high test sensitivity and specificity and very low angiographic and surgical risk. Groups with such a high prevalence have not yet been reliably identified. Screening individual subjects from high prevalence groups would have limited public health impact, with at best about 100 strokes prevented for every 10,000 screened at 20% prevalence. Conclusions - Investigating asymptomatic individual subjects for carotid stenosis may be harmful except in high prevalence groups. There is insufficient information about which these groups are, and at present screening cannot be recommended. Acting on a positive carotid ultrasound test in individual subjects without well defined risk factors for severe stenosis is unjustified and potentially dangerous.