Hypoplasia of the proximal segment of the anterior cerebral artery.

Hypoplasia of the proximal (A1) segment of the anterior cerebral artery (ACA) has been examined in 50 human brains and on 8 selected carotid arteriograms. Hypoplasia has been found in 22% of the cases. Two types of this phenomenon exist, namely the mild and extreme hypoplasia. Mild hypoplasia has been noticed in 14% of the specimens. The hypoplastic vessel has ranged from 1.3 to 1.9 mm in diameter (average 1.6 mm) and it was from 0.6 to 0.9 mm smaller than the opposite A1 portion. Extremely hypoplastic proximal segment has been present in 8% of the cases. It has varied from 0.3 to 1.1 mm in size (average 0.9 mm) and has been more than 1 mm smaller than the opposite proximal segment. Both mild and extreme hypoplasia have been associated in 81.8% of the cases with the corresponding variations or malformations of the anterior cerebral, posterior cerebral, posterior communicating and basilar arteries.