Hidden cochlear impairments

Abstract

Pure tone audiometry is a routine clinical examination used to identify hearing loss. A normal pure tone audiogram is usually taken as evidence of normal hearing. Auditory deficits detected in subjects with normal audiograms, such as poor sound discrimination and auditory perceptual disorders, are generally attributed to central problems. Does the pure tone audiogram truly reflect cochlear status? Recent evidence suggests that individuals with normal audiogram may still have reduced peripheral auditory responses but normal central responses, indicating that the pure tone audiometry may not detect some types of cochlear injuries. In the cochlea, the outer hair cells (OHCs), inner hair cells (IHCs), and the spiral ganglion neurons that synapse with IHCs are the 3 key cochlear components in transducing acoustical vibrations into the neural signals. This report reviews three types of cochlear damage identified in laboratory animals that may not lead to overt hearing loss. The first type of cochlear impairment, such as missing a certain proportion of IHCs without damage to OHCs, may reduce the cochlear output and elevate response threshold; however, the reduced peripheral auditory sensitivity may be restored along the auditory pathway via central gain enhancement. The second type of cochlear impairment, such as selective damage to the synapses of the high-threshold thin auditory nerve fibers (ANFs), reduces cochlear output at high stimulation levels with no effect on response threshold. In this case the reduced cochlear output may be compensated along the auditory pathway as well. The third type of cochlear impairment, such as missing a certain number of OHCs without damage to others, may not even affect cochlear function at all. These "hidden" cochlear impairments do not result in overt hearing loss, but they may increase the vulnerability of the cochlea to traumatic exposure and lead to disrupted central auditory processing.

Keywords: Hidden hearing loss; IHC damage; OHC damage; Synaptopathy.

Fig. 1

Behaviorally measured pure tone threshold shifts in the chinchillas as a function of carboplatin-induced IHC-loss, showing less than 5 dB of hearing loss until IHC-loss up to ∼80%. The vertical bars are standard errors (SEs). The figure is modified from Lobarinas et al. (2013).

Fig. 2

Noise-induced permanent threshold shift (PTS) in chinchillas increases linearly with the OHC-loss with ∼6 dB of PTS per 10% of OHC-loss. The figure is modified from Chen et al. (2008).

Fig. 3

Styrene-induced permanent CAP threshold shifts in rats as a function of OHC-loss. OHC-loss in the third row only induced a slight threshold shift; then the threshold shift increased linearly with OHC-loss with ∼10 dB of threshold elevation for each 10% of OHC-loss; and after the loss of the third and second rows the styrene-induced CAP threshold shift saturated at a level of ∼35 dB. The figure is modified from Chen et al. (2008).

Fig. 4

Styrene-induced cochlear injuries and functional losses. (A) Surface preparation from the mid-turn of the cochlea treated with styrene at a dose of 800 mg/kg; (B) Surface preparation from the mid-turn of the cochlea treated with styrene at a dose of 400 mg/kg; (C) CAP input/output functions at 12 kHz from the 2 rats shown in A and B. The figure is modified from Chen et al. (2008).