Isolated trochlear nerve palsy due to a contusion at the trochlear nerve exit zone.

A 52-year-old man struck his occiput. A few hours later, he became aware of vertical diplopia. Brain CT disclosed no intracranial hemorrhage; however, MRI demonstrated a lesion at the right side of the surface of the midbrain tegmentum ( fig. 1 a–c). Isolated right trochlear nerve palsy was revealed by Hess screen test ( fig. 1 d). His symptom disappeared during a 1-month follow-up. Trochlear nerve palsy following head trauma is not rare, but detection of contusional lesion by neuroimaging has been possible in a few cases. It is speculated that traumatic force caused the trochlear nerve exit zone to impact on the tentorium in this patient.