Ocular neuromyotonia: clinical features, physiological mechanisms, and response to therapy

Ocular neuromyotonia (ONM) is a rare disorder characterized by episodic diplopia, occurring either spontaneously or following sustained eccentric gaze. Most patients have had prior radiation therapy to the sellar and parasellar region. ONM is thought to reflect impaired muscle relaxation due to inappropriate discharges from oculomotor, trochlear, or abducens neurons or axons with unstable cell membranes. Patients with ONM often benefit from membrane stabilizing agents such as carbamazepine. Here we describe a 71-year-old man, with no history of radiation therapy, who for 18 months had had transient episodic diplopia that occurred after down gaze. Clinical examination indicated ONM in muscles supplied by the right oculomotor nerve. Binocular scleral search coil eye movement recordings revealed a defect not only of muscle relaxation but also of maximal muscle contraction. The patient was treated with carbamazepine 200 mg per day with complete resolution of his symptoms. ONM may be more common than previously recognized, and patients with unexplained transient episodic diplopia should be specifically tested for diplopia and ocular misalignment following sustained eccentric gaze.