Mechanisms of Hearing Loss in Acoustic Neuroma: An Otoacoustic Emission Study

Abstract

Evoked otoacoustic emissions (EOAE) are active mechanical responses from the cochlea which provide information about the integrity of the preneural cochlear receptor mechanisms. It may be hypothesised, therefore, that if a hearing impairment is neural in origin, normal EOAEs may be obtained from the cochlea, which, although dissociated, is functioning normally. This study examined the status of the cochlea with EOAE in patients with cochlear (Meniere's disease) and neural (surgically proven acoustic neuroma) disease. In patients with presumed cochlear lesions, no emissions were present with mean hearing worse than 40 dB across a frequency range of 0.5 to 4 kHz. Similarly, an EOAE was not present in any of the 26 acoustic neuroma patients studied when the average (0.5 to 4 kHz) hearing was greater than 40 dB. We conclude that dissociation of the cochlea in patients with acoustic neuroma appears to be rare and, in fact, cochlear involvement occurs in most cases. Possible mechanisms responsible for the effect on the cochlea in this group include degenerative changes due to chronic partial obstruction of the blood supply by the tumour, biochemical alterations in the inner ear fluids, loss of efferent control of active mechanical tuning, and hair cell degeneration secondary to neuronal loss in the eighth nerve.

An update on unilateral sporadic small vestibular schwannoma

“…48,67 Considering the origin of the tumor, the most obvious hypothesis for hearing loss is the mechanical and/or neurotoxic effect that a growing tumor might have on its neighboring neurovascular structures. 57,81 The data, however, do not always show a linear correlation between the rate of tumor growth and deterioration of hearing. 38,39,82,96,104 …”

“…In general, there are 4 patterns of hearing loss: 21,27,37,38,51,57,71,72,81,99 1) hearing deterioration in "growing" tumors, 2) hearing deterioration in "nongrowing" tumors, 3) early hearing loss, and 4) sudden hearing loss regardless of tumor growth. Vestibular schwannomas are histologically benign, slow-growing tumors that originate from the Schwann cells of the inferior vestibular nerve in most cases.…”

“…81 This compromise of the vascular supply can be a result of an increase in intracanalicular pressure or due to direct compression caused by the laterally invading tumor. 7,51 Compression of labyrinthine vessels has been shown to cause degeneration in the organ of Corti and the spiral ganglion (cochlear dysfunction).…”

“…The factors that make the final decision a daunting task include variability in the data on the natural history of VS, 68,86,118 mechanism of hearing loss, 27,37,57,81,99 and treatment-specific hearing and facial function preservation rates. 96,98,101 These inconsistencies are a major hindrance to the practice of evidence-based medicine.…”

“…On the contrary, the proximity of the passing unmyelinated efferent olivocochlear tracts to the vestibular nerve suggests the etiology to be efferent auditory system dysfunction. 81 The efferent olivocochlear bundle comprises medial fibers that innervate the outer hair cells and lateral fibers that synapse with the dendrites of the inner hair cells. The functions assigned to the efferent system of the ear include noise protection, improvement of signal-to-noise ratio, selective attention, adaptation, and frequency selectivity (through regulation of the micromechanical properties of outer hair cells).…”