Dorsal midbrain syndrome from giant aneurysm of the posterior fossa: report of two cases.

Two patients with giant posterior fossa aneurysms compressing the upper ventral brainstem presented principally with findings referable to the dorsal midbrain (upgaze paralysis and lid retraction). We hypothesize that the signs were produced by kinking of the upper brainstem, thus compressing the dorsal midbrain against the free edge of the tentorium of the cerebellum.