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Critical Cap Thickness and Rupture in Symptomatic Carotid Plaques: The Oxford Plaque Study

Background and Purpose— Advances in carotid plaque imaging could allow quantification of fibrous cap thickness in vivo. While a cap thickness <65 &mgr;m is the accepted definition of rupture-prone plaque in the coronary circulation, the threshold value for carotid plaques is unknown. Methods— We made detailed histological assessments of 526 carotid plaques from consecutive patients undergoing endarterectomy for symptomatic carotid stenosis. The thickness of the fibrous cap at the thinnest and most representative part was measured. Results— Cap thickness could be measured reliably in 428 (81%) plaques. In the ruptured plaques (n=257), the median representative cap thickness was 300 &mgr;m (IQR 200 to 500 &mgr;m) and the median minimum cap thickness was 150 &mgr;m (80 to 210 &mgr;m; mean=181 &mgr;m), which is much greater than the mean cap thickness of 23 &mgr;m at the point of rupture that has been reported for coronary plaques. For nonruptured plaques, the median cap thickness values were 500 &mgr;m (300 to 700 &mgr;m) and 250 &mgr;m (180 to 400 &mgr;m), respectively. The optimum cut-offs for discriminating between ruptured and nonruptured plaques were a minimum cap thickness <200 &mgr;m (OR 5.00, 3.26 to 7.65, P<0.001), a representative cap thickness <500 &mgr;m (OR 3.38, 2.25 to 5.08, P<0.001), or a combination of both (OR 5.11, 3.19 to 8.19, P<0.001). Minimum and representative cap thickness were only modestly correlated (r2=0.30) and were both independently associated with cap rupture. Conclusions— Critical cap thickness is greater in carotid plaques than coronary plaques. Minimum and representative cap thicknesses were both independently associated with cap rupture. A combination of minimum cap thickness <200 &mgr;m and a representative cap thickness <500 &mgr;m identified ruptured plaques most reliably. Prospective imaging studies are required to establish whether these cut points predict clinical events in patients with asymptomatic carotid stenosis.