[A case of solitary and unilateral trochlear nerve palsy due to a blunt head impact].

A case of solitary and unilateral trochlear nerve palsy following a minor, blunt head impact is reported. A 72-year-old man fell down backwards from the top of a staircase having 13 steps and hit his right parietal head and shoulder. Routine neurological examination revealed no remarkable findings, however, the subject complained of double vision when he focused downwards. The computed tomography four hours after the initial impact showed a localized small high-density lesion on the left quadrigminal cistern. The magnetic resonance imaging also indicated subarachnoid bleeding expressed by the low-intensity area in T2 weighted images in the same place. There was no evidence of any contusion within the midbrain. Hess chart test clearly showed solitary left superior oblique muscle palsy. The manner of the injury was thought to be quite typical, since he had hit his right parietal area up-side-down when falling, that is, the accelerated force acted as an impact against the firm floor. This impact caused the midbrain concussion against the cerebellar tentorial notch resulting in a localized subarachnoid hemorrhage which caused the unilateral trochlear nerve palsy.