Efferent suppression of oto-acoustic emissions in acoustic neuroma

Abstract

In a previous study1 we postulated that one possible mechanism of hearing loss in patients with acoustic neuroma may be loss of efferent control of active mechanical tuning leading to loss of selectivity in the early stages and then loss of sensitivity. As the vestibular nerve is the carrier of the efferent bundle and acoustic neuromas arise from the Schwann cells of the vestibular nerve, disruption of the efferent fibres may affect the control of cochlear mechanics. This study explored this hypothesis further by examining the contralateral sound activated efferent suppression ofoto-acoustic emissions in 13 patients with an acoustic neuromawith hearing better than 30 dB. In all patients the suppression was significantly reduced on the side of the tumour and in nine of the 13 patients the suppression was also reduced on the unaffected side. In all cases, the transient evoked oto-acoustic emissions were within the normal range of amplitudes. Loss of efferent suppression in these patients with lesions in the cerebello-pontine angle indicates the involvement of theefferent auditory system and further implies that in those patients with bilateral loss of suppression, that not only is the vestibular nerve efferent bundle affected but also the superior olivary complex on the tumour side which controls the contralateral cochlear mechanics.