Vertebral flow void and lateral medullary syndrome.

that supplied both frontal lobes. The circle of Willis as an anatomic structure can be defined as a network of blood vessels affording communication between the carotid system and the basilar system, encompassing an anterior and two posterior communicating arteries and proximal portions of the anterior and posterior cerebral arteries. However, this description is still not fully satisfying because part of the middle cerebral artery and the top of the basilar artery also contribute to this communicating circle. It was our deliberate choice to list the anomaly among anomalies of the circle of Willis to avoid confusing the reader, who will find virtually all other previously reported variations of the postcommunicating ACA in articles about anomalies of the circle of Willis, as is illustrated in several important papers considering large populations. Moreover, De Smet illustrates the ambiguous use of the term "circle of Willis" even by Willis himself when De Smet refers to "the circle depicted in Willis' Cerebri Anatome" showing "the ACAs proceeding medially and coming together in a confluence, a possible but most unlikely anomaly...." Furthermore, the statement that the unique anomaly we described could in fact have first been reported by Willis himself in 1664 is extremely speculative, as is also illustrated by the use of the conditional tense by De Smet in his letter. Even if, as he suggests, the draftsman who contributed to Willis' 1664 paper included "the unknown anterior communicating artery" in the precommunicating part of the right ACA, the anomaly in our patient still differs from this would-be variant because of the existence of a plain communicating ACA.