Myasthenic abduction mystagmus in a patient with hyperthyroidism.

Intravenous edrophonium chloride promptly stopped a sustained high frequency abduction nystagmus, supporting a diagnosis of myasthenia gravis in a patient with hyperthyroidism. Myasthenia gravis should be considered in the differential diagnosis of any acquired, isolated, and sustained abduction nystagmus, and this possibility should be ruled out by intravenous edrophonium chloride.