Anatomical basis for the lateral approach to the fourth ventricle.

This study focused on the posterior inferior cerebellar artery bifurcation and branching patterns in the fissures around the fourth ventricle. The vertebral arteries in 25 unfixed human cerebellum were cannulated and injected with polyester colored resin. The suboccipital surface of the cerebellum was exposed and the cisterna magna main landmarks localized. The average distance was 12.6 mm between the tonsillovermian notches and 21.8 mm between the inferior tips of the tonsils. The mean vertical distance between these horizontal planes was 14.5 mm. The posterior inferior cerebellar artery was found in the cerebellomedullary fissure in 42 of 50 cerebellar hemispheres, in seven cases the artery was absent and in one it was hypoplastic. The mean outer diameter was 1.8 mm and the average length was 27.9 mm. The posterior inferior cerebellar artery presented four bifurcation point patterns: superomedial, superolateral, inferomedial, and inferolateral. These patterns were characterized into subtypes based on the courses of the vermian and tonsillohemispheric branches. The perforating and choroidal branches originating in these segments were also studied. The mean number of perforating branches per hemisphere was 5.1. The range of the length was 2-10 mm and the range of the outer diameter was 0.1-0.3 mm. An average of 4.6 choroidal arteries originated from the tonsillomedullary and telovelotonsillar segments, a mean of 4 arose from both vermian and tonsillohemispheric branches. This information will facilitate surgical planning in approaching the fourth ventricle as well as the interpretation of cerebellar infarcts in the posterior inferior cerebellar artery area.