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Hear Res. 2016 Feb;332:29-38. doi: 10.1016/j.heares.2015.11.005. Epub 2015 Nov 30.

The middle ear muscle reflex in the diagnosis of cochlear neuropathy.

Author information

1
Eaton-Peabody Laboratories, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA; Department of Otology and Laryngology, Harvard Medical School, Boston, MA 02115, USA. Electronic address: Michelle_Valero@meei.harvard.edu.
2
Eaton-Peabody Laboratories, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA; Department of Otology and Laryngology, Harvard Medical School, Boston, MA 02115, USA.

Abstract

Cochlear neuropathy, i.e. the loss of auditory nerve fibers (ANFs) without loss of hair cells, may cause hearing deficits without affecting threshold sensitivity, particularly if the subset of ANFs with high thresholds and low spontaneous rates (SRs) is preferentially lost, as appears to be the case in both aging and noise-damaged cochleas. Because low-SR fibers may also be important drivers of the medial olivocochlear reflex (MOCR) and middle-ear muscle reflex (MEMR), these reflexes might be sensitive metrics of cochlear neuropathy. To test this hypothesis, we measured reflex strength and reflex threshold in mice with noise-induced neuropathy, as documented by confocal analysis of immunostained cochlear whole-mounts. To assay the MOCR, we measured contra-noise modulation of ipsilateral distortion-product otoacoustic emissions (DPOAEs) before and after the administration of curare to block the MEMR or curare + strychnine to also block the MOCR. The modulation of DPOAEs was 1) dominated by the MEMR in anesthetized mice, with a smaller contribution from the MOCR, and 2) significantly attenuated in neuropathic mice, but only when the MEMR was intact. We then measured MEMR growth functions by monitoring contra-noise induced changes in the wideband reflectance of chirps presented to the ipsilateral ear. We found 1) that the changes in wideband reflectance were mediated by the MEMR alone, and 2) that MEMR threshold was elevated and its maximum amplitude was attenuated in neuropathic mice. These data suggest that the MEMR may be valuable in the early detection of cochlear neuropathy.

KEYWORDS:

Acoustic overexposure; Cochlear neuropathy; Hidden hearing loss; Medial olivocochlear reflex; Middle-ear muscle reflex; Wideband reflectance

PMID:
26657094
PMCID:
PMC5244259
DOI:
10.1016/j.heares.2015.11.005
[Indexed for MEDLINE]
Free PMC Article

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