Ocular myasthenia presenting as superior oblique weakness.

We diagnosed ocular myasthenia in a 39-year-old man whose presenting symptom was vertical diplopia. Unrecognized weakness of the right superior oblique and secondary overaction of the right inferior oblique caused a right hypertropia, which was worse on left gaze. One week later left blepharoptosis, right medial rectus weakness, and bilateral obicularis oculi weakness were found on ocular examination. All findings were partially reversed by the injection of Tensilon. Superior oblique muscle palsy simulating a fourth cranial nerve palsy is infrequently reported in patients with ocular myasthenia. Pseudofourth nerve palsy is another sign of myasthenia gravis.