Histopathology of the inner ear in unoperated acoustic neuroma.

Although hearing loss is the most common presenting symptom in patients with acoustic neuroma, the pathophysiology of hearing loss associated with acoustic neuroma is unknown. Although primary dysfunction of the auditory nerve is intuitively logical, available histopathologic and clinical data suggest that although neural degeneration is common, it alone does not adequately account for hearing loss in many cases. The purpose of this study was to evaluate 11 cases of unoperated unilateral acoustic neuromas. Temporal bones were identified by means of a search mechanism provided by the National Temporal Bone, Hearing, and Balance Pathology Resource Registry and were prepared for light microscopy by standard techniques. Quantification of spiral ganglion cells, hair cells, stria vascularis, and spiral ligament was accomplished for each specimen. In addition, the maximum diameter and volume of each tumor were calculated from histopathologic sections. Increasing tumor size did predict a reduced spiral ganglion count. However, although there was a tendency for decreasing spiral ganglion cell count and for increasing tumor size to predict a higher pure tone average and lower speech discrimination score, these correlations did not reach statistical significance. In tumor ears in which the speech discrimination score was 50% or less, there was always significant degeneration of other structures of the inner ear in addition to neurons, including hair cells, the stria vascularis, and the spiral ligament. Endolymphatic hydrops and eosinophilic precipitate in the perilymphatic spaces were found in 2 of 3 such cases. It is concluded that acoustic neuromas appear to cause hearing loss, not only by causing degeneration of the auditory nerve, but also by inducing degenerative changes in the inner ear. It is hypothesized that the proteinaceous material seen histologically may represent the products of up-regulated genes in acoustic neuroma, some of which may interfere with normal cochlear function.