Ocular neuromyotonia of the right third cranial nerve

Ocular neuromyotonia is a rare disorder of the oculomotor nerves caused by chronic damage to the nerve, usually secondary to previous irradiation or longstanding compression. We present the case of a 40-year-old man who had received proton beam therapy for a right cavernous sinus chondrosarcoma 15 years earlier. He developed intermittent episodes of self-limited horizontal diplopia, which occurred over the course of 1 year. At its peak, the deviation reached 20^Δ in primary gaze and 25^Δ in right gaze. Signs included right upper lid retraction and right medial rectus muscle absence of relaxation after sustained left gaze, with mild deficit of elevation and depression but no pupillary involvement. Full recovery occurred after he closed his eyes. Ocular neuromyotonia of the right oculomotor nerve was diagnosed after recurrence of primary disease relapse was ruled out. Symptoms remitted completely under oxcarbazepine.