CT demonstration of dorsal midbrain hemorrhage in traumatic fourth cranial nerve palsy.

A 21-year-old man who suffered a traumatic brain injury from a motor vehicle accident recovered brain function except for an isolated left fourth cranial nerve palsy. Brain CT showed a focal hemorrhage in the right dorsal midbrain, directly in the brainstem path of what would become the left fourth cranial nerve. Although there has been previous imaging documentation of midbrain and cisternal hemorrhage in patients with isolated post-traumatic fourth cranial nerve palsy, this is the first report to show a large midbrain hemorrhage on CT. The mechanism is likely to be concussive impact of the dorsal midbrain on the tentorium cerebelli.