Sex differences in carotid bifurcation anatomy and the distribution of atherosclerotic plaque

Background and Purpose-Plaque formation at arterial bifurcations depends on vessel anatomy, particularly the relative sizes of the branches, and the ratio of the outflow to inflow area. The facts that carotid plaque is more common in men and that carotid bruits in the absence of stenosis are more frequent in women raise the possibility that there are sex differences in carotid bifurcation anatomy. We studied 5395 angiograms from the European Carotid Surgery Trial. Methods-To minimize secondary changes we excluded angiograms with greater than or equal to 50% stenosis and also studied vessels with no disease. We measured arterial diameters at disease-free points and calculated the following ratios: internal/common (ICA/CCA); external/common (ECA/CCA); internal/external (ICA/ECA) carotid arteries; carotid bulb/CCA; and outflow/inflow area. We related these to sex and also studied the distribution of plaque in the whole trial population. Results-Among 2930 angiograms with < 50% stenosis, the mean ICA/CCA ratio, ICA/ECA ratio, and outflow/inflow area ratio were larger in women than in men (all P <0.0001). The findings were similar in 622 bifurcations without atheroma. There were also differences in the distribution of plaque, with men more likely to have the maximum stenosis distal to the carotid bulb (odds ratio, 2.29; 95% CI, 1.33 to 4.01; P=0.001) and women more likely to have stenosis of the ECA (odds ratio, 1.54; 95% CI, 1.30 to 1.85; P <0.0001). Conclusions-Sex differences in carotid bifurcation anatomy are not limited to absolute vessel size. In addition, the outflow to inflow area ratio is bigger in women, and relative to the CCA and EGA, women have larger ICAs than men. Irrespective of whether these differences are congenital or acquired, they may partly explain the sex differences that we found in the distribution of plaque and the sex differences in the prevalence of carotid atheroma in the general population.