Introduction

The presence of adequate cerebral circulation is important to maintain cerebral perfusion and brain function in patients with severe carotid artery disease (Powers, 1991; Klijn et al., 1997; Caplan and Hennerici, 1998; Derdeyn et al., 1999). The level of cerebral perfusion is influenced by many factors such as degree of ipsilateral stenosis, degree of contralateral stenosis, capacity of posterior circulation, the anatomical variants and completeness of the circle of Willis, the remaining vasomotor reactivity, which on itself is influenced by underlying pathology such as atherosclerosis or inflammatory processes, and capacity to reroute blood flow, for instance, via leptomeningeal anastomoses. Although Liebeskind correctly mentioned that the understanding of the cerebral circulation is improved by determining the extra- and intra-cranial collateral capacity (Liebeskind, 2003), the large number of confounding factors, mentioned above, makes it virtually impossible to predict which patients with carotid artery disease are at (high) risk for recurrent symptoms caused by hemodynamical factors. It has been shown that in patients with symptomatic severe internal carotid artery (ICA) stenosis carotid endarterectomy reduces the risk of recurrent stroke by removal of the atheromatous plaque (European Carotid Surgery Trialists' Collaborative Group, 1991; NASCET, 1991; Barnett et al., 2000). Although in patients with occlusive disease of the ICA, the cause of stroke is primarily thromboembolic, the presence of hemodynamic impairment is also recognized as additional risk factor (Caplan and Hennerici, 1998; Grubb et al., 1998).