Pathophysiology of hearing impairment in acoustic neuroma with profound deafness: analysis by evoked otoacoustic emission and promontory stimulation test.

Evoked otoacoustic emissions (e-OAEs) were examined and promontory stimulus test (PST) done prior to tumor removal in 15 cases of surgically proven unilateral acoustic neuroma (AN) with more than 80 dB in pure-tone hearing level. The e-OAEs were elicited by tone-bursts at 1 kHz and 2 kHz, and an interaural difference in pseudothreshold of more than 10 dB was defined as a significant impairment of the cochlear function. In PST, electrical stimulation was applied by a burst mode at 6 stimulus frequencies between 50 Hz and 1600 Hz. The number of stimulus frequencies with a positive response was defined as the PST score. The results obtained were as follows; i) Mean interaural difference in e-OAE pseudothreshold was 16.0 dB at 1 kHz and 8.0 dB at 2 kHz, ii) Positive PST was found in only 40% of the subjects and the mean PST score was 0.93, iii). When the pathophysiology of hearing impairment was inferred from the combination of the findings in e-OAEs and PST, it was found that 33.3% of the subjects had pure cochlear impairment, 13.3% pure retrocochlear impairment and 46.7% cochlear-retrocochlear impairment. It is concluded that analysis of both e-OAEs and PST could clarify the pathophysiology of hearing impairment in AN with profound deafness, which cannot be determined by conventional psychoacoustic tests.