Paradoxical Effects of Contralateral White Noise on Evoked Otoacoustic Emissions in Ears with Acoustic Neuroma

Abstract

A contralateral suppression effect on evoked otoacoustic emissions (EOAEs) is usually present in normally hearing subjects and in patients with sensorineural hearing loss, while it is absent or reduced in ears to which the vestibular nerve has been cut and in ears with acoustic neuroma (AN). To date, a paradoxical effect, that is an increase in EOAE amplitude during contralateral stimulation, has been described in one ear with sensorineural hearing loss of unknown aetiology and in three ears with AN (two in the present paper). Evidence has been provided that the contralateral suppression effect on EOAEs is accomplished largely, if not entirely, via the medial olivocochlear bundle (OCB). According to clinical data the absence or the reduced amount of contralateral suppression effect on EAOEs may be attributed to a totally, or partially, damaged or malfunctioning medial OCB. The way in which a contralateral noise may increase EOAE amplitude is more difficult to explain. One attractive hypothesis is that this paradoxical effect is a result of some pathological adaptive process in the medial OCB.