Hyperventilation-induced nystagmus in patients with vestibular schwannoma

Objective: To analyze the nystagmus evoked by hyperventilation in patients with unilateral vestibular schwannoma and to use this information to predic t the effects of hyperventilation on individual ampullary nerves. Methods: Three-dimensional scleral search coil eye movement recording techniques wer e used to record the magnitude and time course of eye movements in six pati ents with unilateral vestibular schwannoma and hyperventilation-induced nys tagmus. The presenting complaints in five of these patients were vertigo or dysequilibrium, Results: The eye movement response to hyperventilation was a "recovery" nystagmus with slow-phase components corresponding to excitat ion of the affected vestibular nerve. Projection of the eye velocity vector into the plane of the semicircular canals revealed that fibers arising fro m the ampulla. of the horizontal canal were most affected by hyperventilati on with lesser activation of fibers to the superior canal and smaller, more variable responses from posterior canal fibers. Conclusions: The three-dim ensional characteristics of the nystagmus evoked by hyperventilation in pat ients with vestibular schwannoma provide insight into the vestibular end or gans affected by the tumor and the mechanism responsible for the nystagmus. This finding indicates that hyperventilation resulted in a transient incre ase in activity from these partially demyelinated axons.