Drop attacks secondary to superior canal dehiscence syndrome.

Two patients with unprovoked drop attacks were found to have dehiscence of the superior semicircular canal on CT of the temporal bone. Both had conductive hearing loss, preservation of stapedius reflex, and abnormal vestibular evoked myogenic potentials. Neither had sound- or pressure-induced nystagmus. Repair of the dehiscence in one case stopped the drop attacks, supporting a causal relationship between the dehiscence and the drop attacks.