Anomalies of the P1 segment of the posterior cerebral artery: early bifurcation or duplication, fenestration, common trunk with the superior cerebellar artery.

In a series of anatomical dissections on 100 fixed human brains, 3% of anomalies of the precommunicating segment of the posterior cerebral artery (P1) were found, among which a case of duplication of the P1 segment. This finding is very unusual and it is much rarer than the many other anatomical patterns reported in the circle of Willis. It is to be considered a very early bifurcation, as reported at the middle cerebral artery level. Another two unusual anatomical patterns were found. One was a large fenestration of the P1 segment, which is rather frequent in the vertebrobasilar system. The other was a bilateral common trunk between the posterior cerebral artery and the superior cerebellar artery, which represents a rare anatomical variation. The existence of such "anomalies" can be explained by the embryological development of the region. Their pathogenic and neurosurgical implications are discussed in the light of the literature.