Transient Abnormalities in Masking Tuning Curve in Early Progressive Hearing Loss Mouse Model.

Marion Souchal,Paul Avan,Sirley Alves Da Silva Carvalho,Fabrice Giraudet,Luciana Macedo De Resende,Christelle Blavignac,Ludimila Labanca

doi:10.1155/2018/6280969

Abstract

Damage to cochlear outer hair cells (OHCs) usually affects frequency selectivity in proportion to hearing threshold increase. However, the current clinical heuristics that attributes poor hearing performance despite near-normal auditory sensitivity to auditory neuropathy or “hidden” synaptopathy overlooks possible underlying OHC impairment. Here, we document the part played by OHCs in influencing suprathreshold auditory performance in the presence of noise in a mouse model of progressive hair cell degeneration, the CD1 strain, at postnatal day 18–30 stages when high-frequency auditory thresholds remained near-normal. Nonetheless, total loss of high-frequency distortion product otoacoustic emissions pointed to nonfunctioning basal OHCs. This “discordant profile” came with a huge low-frequency shift of masking tuning curves that plot the level of interfering sound necessary to mask the response to a probe tone, against interfering frequency. Histology revealed intense OHC hair bundle abnormalities in the basal cochlea uncharacteristically associated with OHC survival and preserved coupling with the tectorial membrane. This pattern dismisses the superficial diagnosis of “hidden” neuropathy while underpinning a disorganization of cochlear frequency mapping with optimistic high-frequency auditory thresholds perhaps because responses to high frequencies are apically shifted. The audiometric advantage of frequency transposition is offset by enhanced masking by low-frequency sounds, a finding essential for guiding rehabilitation.

Highlights

Sensorineural hearing losses (SNHL) stem from a wide spectrum of diseases affecting the sensory receptors, outer hair cells (OHCs), inner hair cells (IHCs), or auditory neurons [1,2,3,4,5]
Even though it cannot provide any fine-grained reflection of the mechanism of SNHL and may not delineate individual needs, it works in a large majority of cases because it is the damage to OHCs that usually accounts for the hearing impairment, frequency selectivity that, in simple cases, is affected in proportion to the increase in hearing thresholds [6]
The Masking Tuning Curve (MTC) plotted in these cases at 32 kHz (Figure 3(c), red lines) contrast with the MTCs built in animals with a good correspondence between Auditory brainstem response CM (ABR) and Distortion product otoacoustic emission ENT (DPOAE) thresholds (Figure 3(c), black lines) and with all MTCs built at 10 kHz, a frequency at which the animals kept normal thresholds between P18 and P25 (Figure 3(b))

Summary

Introduction

Sensorineural hearing losses (SNHL) stem from a wide spectrum of diseases affecting the sensory receptors, outer hair cells (OHCs), inner hair cells (IHCs), or auditory neurons (afferent and even efferent fibers) [1,2,3,4,5]. Pure-tone audiometry is the routine clinical audiological test used for measuring hearing sensitivity, and the audiometric classification of hearing impairments is the main basis upon which audiologists determine their rehabilitation choice Even though it cannot provide any fine-grained reflection of the mechanism of SNHL and may not delineate individual needs, it works in a large majority of cases because it is the damage to OHCs that usually accounts for the hearing impairment, frequency selectivity that, in simple cases, is affected in proportion to the increase in hearing thresholds [6]. Recent papers stress the importance of OHC function as a determinant of speech-in-noise performance, highlighted by its decrease with decreased OAEs in a sample of subjects with audiometric thresholds within the normal range [15, 16] All these studies warn against considering that hidden hearing loss excludes OHCs as a potential contributor. The discordant profile, already observed at P18 (3 cases) occurred more frequently at P21 (10 cases) but almost vanished at P25 (one case, with most MTCs showing a blunt and elevated tip at the probe frequency)

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