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      Hearing Loss in Adults

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      New England Journal of Medicine
      New England Journal of Medicine (NEJM/MMS)

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          Cochlear synaptopathy in acquired sensorineural hearing loss: Manifestations and mechanisms.

          Common causes of hearing loss in humans - exposure to loud noise or ototoxic drugs and aging - often damage sensory hair cells, reflected as elevated thresholds on the clinical audiogram. Recent studies in animal models suggest, however, that well before this overt hearing loss can be seen, a more insidious, but likely more common, process is taking place that permanently interrupts synaptic communication between sensory inner hair cells and subsets of cochlear nerve fibers. The silencing of affected neurons alters auditory information processing, whether accompanied by threshold elevations or not, and is a likely contributor to a variety of perceptual abnormalities, including speech-in-noise difficulties, tinnitus and hyperacusis. Work described here will review structural and functional manifestations of this cochlear synaptopathy and will consider possible mechanisms underlying its appearance and progression in ears with and without traditional 'hearing loss' arising from several common causes in humans.
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            Acceleration of age-related hearing loss by early noise exposure: evidence of a misspent youth.

            Age-related and noise-induced hearing losses in humans are multifactorial, with contributions from, and potential interactions among, numerous variables that can shape final outcome. A recent retrospective clinical study suggests an age-noise interaction that exacerbates age-related hearing loss in previously noise-damaged ears (Gates et al., 2000). Here, we address the issue in an animal model by comparing noise-induced and age-related hearing loss (NIHL; AHL) in groups of CBA/CaJ mice exposed identically (8-16 kHz noise band at 100 dB sound pressure level for 2 h) but at different ages (4-124 weeks) and held with unexposed cohorts for different postexposure times (2-96 weeks). When evaluated 2 weeks after exposure, maximum threshold shifts in young-exposed animals (4-8 weeks) were 40-50 dB; older-exposed animals (> or =16 weeks) showed essentially no shift at the same postexposure time. However, when held for long postexposure times, animals with previous exposure demonstrated AHL and histopathology fundamentally unlike unexposed, aging animals or old-exposed animals held for 2 weeks only. Specifically, they showed substantial, ongoing deterioration of cochlear neural responses, without additional change in preneural responses, and corresponding histologic evidence of primary neural degeneration throughout the cochlea. This was true particularly for young-exposed animals; however, delayed neuropathy was observed in all noise-exposed animals held 96 weeks after exposure, even those that showed no NIHL 2 weeks after exposure. Data suggest that pathologic but sublethal changes initiated by early noise exposure render the inner ears significantly more vulnerable to aging.
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              Toward a Differential Diagnosis of Hidden Hearing Loss in Humans

              Recent work suggests that hair cells are not the most vulnerable elements in the inner ear; rather, it is the synapses between hair cells and cochlear nerve terminals that degenerate first in the aging or noise-exposed ear. This primary neural degeneration does not affect hearing thresholds, but likely contributes to problems understanding speech in difficult listening environments, and may be important in the generation of tinnitus and/or hyperacusis. To look for signs of cochlear synaptopathy in humans, we recruited college students and divided them into low-risk and high-risk groups based on self-report of noise exposure and use of hearing protection. Cochlear function was assessed by otoacoustic emissions and click-evoked electrocochleography; hearing was assessed by behavioral audiometry and word recognition with or without noise or time compression and reverberation. Both groups had normal thresholds at standard audiometric frequencies, however, the high-risk group showed significant threshold elevation at high frequencies (10–16 kHz), consistent with early stages of noise damage. Electrocochleography showed a significant difference in the ratio between the waveform peaks generated by hair cells (Summating Potential; SP) vs. cochlear neurons (Action Potential; AP), i.e. the SP/AP ratio, consistent with selective neural loss. The high-risk group also showed significantly poorer performance on word recognition in noise or with time compression and reverberation, and reported heightened reactions to sound consistent with hyperacusis. These results suggest that the SP/AP ratio may be useful in the diagnosis of “hidden hearing loss” and that, as suggested by animal models, the noise-induced loss of cochlear nerve synapses leads to deficits in hearing abilities in difficult listening situations, despite the presence of normal thresholds at standard audiometric frequencies.
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                Author and article information

                Journal
                New England Journal of Medicine
                N Engl J Med
                New England Journal of Medicine (NEJM/MMS)
                0028-4793
                1533-4406
                December 21 2017
                December 21 2017
                : 377
                : 25
                : 2465-2473
                Article
                10.1056/NEJMra1616601
                6457651
                29262274
                db834ea4-7694-4fdb-84c1-79e1580eda7f
                © 2017
                History

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