Alternating episodes of vestibular nerve excitation and failure.

Recurrent episodes of oscillopsia, rotational vertigo, and postural imbalance were elicited and modulated by changing the horizontal head positions of a patient with an arachnoid cyst in the right cerebellopontine angle that distorted the vestibulocochlear nerve. Oculomotor analysis revealed two different types of attacks depending on the particular head position: 1) episodes of vestibular hypofunction (minutes to several hours) with normal head position and 2) paroxysmal vestibular excitation (seconds) with head rotation to the left. The most likely cause is a transition from conduction block to ectopic discharges, which occurs when various peripheral nerves are compressed. One week after resection of the cyst and decompression of the eighth cranial nerve the patient was symptom free, and the electronystagmogram was normal.