Isolated Unilateral Trochlear Nerve Palsy Due to Ipsilateral Dorsal Midbrain Infarction

A 67-year-old Japanese man with essential hypertension and diabetes mellitus abruptly developed isolated right-sided trochlear nerve palsy without pain. Four days later, cranial magnetic resonance imaging demonstrated a tiny ischaemic lesion in the caudal and dorsal midbrain on the right side, which might involve the trochlear fascicles after its decussation. The trochlear nerve palsy resolved within 4 days while treated with intravenous anti-platelet agent. This is a first reported case of isolated unilateral trochlear nerve palsy due to ipsilateral midbrain infarction.