Critical Cap Thickness and Rupture in Symptomatic Carotid Plaques

Background and Purpose— Advances in carotid plaque imaging could allow quantification of fibrous cap thickness in vivo. While a cap thickness <65 μm 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 μm (IQR 200 to 500 μm) and the median minimum cap thickness was 150 μm (80 to 210 μm; mean=181 μm), which is much greater than the mean cap thickness of 23 μm at the point of rupture that has been reported for coronary plaques. For nonruptured plaques, the median cap thickness values were 500 μm (300 to 700 μm) and 250 μm (180 to 400 μm), respectively. The optimum cut-offs for discriminating between ruptured and nonruptured plaques were a minimum cap thickness <200 μm (OR 5.00, 3.26 to 7.65, P <0.001), a representative cap thickness <500 μm (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 (r 2 =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 μm and a representative cap thickness <500 μm 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.