Major variation in carotid bifurcation anatomy: a possible risk factor for plaque development?

BACKGROUND AND PURPOSE Why is carotid plaque often so strikingly asymmetrical within individuals, and why does the extent of disease vary so considerably between individuals with similar systemic risk factors? Variability of carotid bifurcation anatomy is a possible explanation. Flow models suggest that vessel anatomy, in particular vessel diameter and area ratios, affects plaque formation at arterial bifurcations. However, carotid bifurcation anatomy could only be a major risk factor for plaque formation if it was sufficiently variable. Since very few data exist on the extent of interindividual and intraindividual variability of bifurcation anatomy, we studied 5395 angiograms from the European Carotid Surgery Trial. METHODS To minimize changes in bifurcation anatomy secondary to atherosclerosis, we excluded vessels with >/=30% stenosis. We measured arterial diameters at disease-free points and calculated the following ratios: internal to common carotid (ICA/CCA), external to common carotid (ECA/CCA), external to internal carotid (ECA/ICA), and outflow/inflow area. For intraindividual asymmetry, we compared the ratios on both sides. RESULTS Each ratio varied markedly between individuals. The 95% ranges were as follows: ICA/CCA, 0.44 to 0.86; ECA/CCA, 0.34 to 0.80; ECA/ICA, 0.55 to 1.33; and outflow/inflow area, 0.38 to 1.28. The results were very similar in 407 bifurcations with no disease. Among the 755 patients with <30% stenosis bilaterally, side differences of >/=25% were present in 17% (95% CI, 15% to 20%) for the ICA/CCA ratio, 27% (95% CI, 24% to 30%) for the ECA/CCA ratio, 32% (95% CI, 28% to 35%) for the ECA/ICA ratio, and 42% (95% CI, 38% to 45%) for the outflow/inflow area ratio. CONCLUSIONS We found large interindividual differences in carotid bifurcation anatomy. For example, there was 4-fold variation of the ratio of outflow to inflow area. Intraindividual variation was also considerable. These data highlight the potential importance of anatomic variation as a risk factor for atheroma and provide a firm basis for flow modeling studies.