Myasthenic sustained gaze fatigue.

Sustained gaze fatigue was found in two patients whose diplopia was the first manifestation of myasthenia gravis. Although ocular excursions initially appeared full, a slow drift toward the primary position occurred during sustained extreme gaze. This motility pattern signifies pathologic extraocular muscle fatigue and strongly implicates myoneural junction disease. When myasthenia gravis is suspected, the presence of sustained gaze fatigue provides a tentative clinical diagnosis to be confirmed by pharmacologic testing.