HypothesisThe goal of the investigation was to determine if vector analysis of nystagmus in a patient with the Tullio phenomenon could determine the source of the nystagmus.BackgroundThe Tullio phenomenon consists of the combination of vertigo and abnormal eye and/or head movements provoked by sound. Dehiscence of the superior semicircular canal can be found in certain patients with the Tullio phenomenon.MethodsThe patient was tested with pure tones ranging from 250 to 3000 Hz at 95dB HL. The time course of the three-dimensional vector of eye movement, including torsion and vertical and horizontal displacement angles was determined by individual stop-frame analysis of digitized video.ResultsTorsion amplitude varied from 1 to 7 degrees; vertical amplitude varied from 1 to 5 degrees; and horizontal amplitude varied less than 1.5 degrees. The maximal response occurred on stimulation of the right ear with a 1250-Hz 95-dB HL tone. This elicited a reliable counterclockwise torsional and down-beating fast phase nystagmus as seen from the examiner's point of view. Comparison of the nystagmus with known canal vectors identified the right superior semicircular canal as the source of stimulation. High-resolution computed tomography scan of the temporal bone showed a definite right superior canal dehiscence.ConclusionThe origin of nystagmus from the Tullio phenomenon can be identified by calculating the three-dimensional vector of the observed nystagmus. We show that vector analysis of the observed eye movement can be used to infer the source of nystagmus in these patients. The development of real-time, three-dimensional vector analysis of nystagmus is desirable.