Loudness recruitment in abrupt drop sensorineural hearing impairment.

Monaural bifrequency loudness balance tests were performed on 11 adults with abrupt drop sensorineural hearing impairment. To study frequencies on the slope, 2-octave Bekesy threshold measurements were made beginning at a frequency exhibiting normal hearing nearest to initiation of the slope (F1). Loudness was matched (Test 1) between F1 and F2 (a frequency nearest to termination of the slope), (Test 2) between F1 and F1 + F2/2 (a frequency halfway between F1 and F2), and (Test 3) between F1 and 2F1 (a frequency an octave above F1). Our data indicated that a combination of asymptotic and straight-line recruitment was demonstrated in Test 1, pure asymptotic recruitment in Test 2, and straight-line recruitment in Test 3. It was speculated that diffuse hair cell lesions such as in frequency areas in the vicinity of 2F1 in abrupt drop sensorineural hearing impairment or endolymphatic hydrops may show straight-line recruitment, and discrete outer hair cell lesions such as frequency regions around F1 + F2/2 may demonstrate pure asymptotic recruitment. Our results were discussed in relation to histopathological findings of human and animal subjects with abrupt drop hearing loss reported by other investigators. Data appeared to show a relationship between the types of loudness recruitment and the hair cell damage.