Voluntary nystagmus masquerading as Tullio's Phenomenon.

Nystagmoid eye movements elicited by humming loudly was initially misdiagnosed as Tullio's phenomenon (abnormal acoustical stimulation of the vestibular apparatus owing to labyrinthine fistula). Further workup lead to a final diagnosis of a form of voluntary nystagmus characterized in some reports as "hysterical mystagmus". Some of the characteristics of this example of voluntary nystagmus differed from generally accepted descriptions.