How often does an incomplete circle of Willis predispose to cerebral ischemia during closure of carotid artery? Postmortem and clinical imaging studies.

PURPOSE: To evaluate the prevalence of anatomical variations in the circle of Willis predisposing to cerebral ischemia during intraoperative closure of a carotid artery. MATERIALS: Anatomy of the cerebral arteries of 92 deceased was assessed by angiography and permanent silicone casts. Cerebral ischemia during closure of a carotid artery with patent contralateral internal carotid artery (ICA) was considered possible in cases of simultaneous nonfunctioning anterior communicating artery (diameter <0.5 mm) and ipsilateral posterior communicating artery (PComA) (diameter <0.5 mm or fetal type posterior cerebral artery). In cases of contralateral ICA occlusion, cerebral ischemia was considered possible if ipsilateral PComA was nonfunctioning.
RESULTS: Cerebral ischemia during closure of the right or left carotid artery with patent contralateral ICA was estimated to be possible in 16 (17.4%) and 13 (14.1%) cases. In cases of occluded contralateral ICA, the corresponding numbers were 55 (59.8%) and 49 (53.3%). A review of magnetic resonance and catheter angiographies also identified other variants of the circle of Willis with increased risk.
CONCLUSIONS: Incomplete circle of Willis predisposes approximately one-sixth of individuals to cerebral ischemia during transient closure of carotid artery but the risk is more than threefold in case of contralateral ICA occlusion.