Variations in Carotid Bifurcation Anatomy: Differences within and between individuals
Carotid plaque formation is thought to depend on haemodynamic factors related to vessel anatomy. Side differences in bifurcation anatomy could explain the frequent asymmetry in the extent of carotid plaque within individuals. Differences between individuals could account for variation in the tendency to develop carotid atheroma. However, there are very few data on the extent of variation in bifurcation anatomy. We studied the inter- and intra-individual variability of bifurcation anatomy in 5395 angiograms from the 3007 patients in the European Carotid Surgery Trial. To minimise changes in bifurcation anatomy secondary to atherosclerosis, we excluded angiograms of vessels with ≥30% NASCET stenosis. We measured the arterial diameters at disease free points and calculated the following ratios: internal to common carotid (ICA/CCA), external to common carotid (ECA/CCA) and external to internal carotid (ECA/ICA). We calculated the 95% ranges to describe the variability of the ratios. To ascertain intra-individual differences we compared the ratios on the symptomatic side with those on the contralateral side in individual patients. Among the 2201 bifurcations with <30% stenosis there was considerable inter-individual variation in the ratios. The 95% ranges were: ICA/CCA (0.44–0.86); ECA/CCA (0.34–0.80) and ECA/ICA (0.55–1.33). Among the 767 patients with <30% stenosis bilaterally we found considerable intra-individual asymmetry in the vessel diameter ratios. Side differences of ≥25% were present in 17% (95% CI 15–20) of angiograms for the ICA/CCA ratio, 27% (24–30) for the ECA/CCA ratio and 32% (28–35) for the ECA/ICA ratio. There are large inter-individual differences in carotid bifurcation anatomy. For example, the normal range of the ECA diameter varies from half that of the ICA to a third more than the ICA. Intra-individual variability is equally considerable. This variability has not been documented previously. It may partly explain why the extent of carotid atheroma is often highly asymmetrical within individuals and why some individuals are more prone to develop carotid disease than others. Prospective studies are required to confirm this.