gms | German Medical Science

The modern view of sensorineural hearing loss was established by Schuknecht and colleagues who proposed four types of presbycusis based on cochlear histopathology: i.e. sensory (12%), strial (35%), neural (31%) and cochlear conductive (23%). According to this view, only “sensory” cases showed hair cell loss in regions appropriate to the threshold shifts. However, the cytocochleograms in the prior studies were binarized, drastically underestimating the importance of hair cell loss in presbycusis ears. In noise-damaged or aging ears, synapses between inner hair cells (IHCs) and auditory-nerve fibers (ANFs) are lost before hair cells degenerate. Cochlear synaptopathy does not elevate thresholds, but likely impairs performance on complex listening tasks. This phenomenon has been shown in animals (Sergeyenko et al., J. Neurosci, 2013) and in aged human ears (Wu et al., Journal of Neuroscience, 2018), but the prevalence of noise induced synaptopathy in human remains unclear.

We analyzed 126 human temporal bones from the Mass. Eye and Ear Temporal bone Archive. To analyze the ANF, we selected unstained tangential sections through the osseous spiral lamina limbus from each half turn, stained them with Cellmask^® and Confocal imaged them. IHC and OHC counts were derived from a high-power, DIC-based re-examination of the legacy sections stained with H & E. This method has been shown to accurately reflect the fractional hair cell loss (Wu et al., Laryngoscope, 2019).

To understand the mechanism of age-related hearing loss, the histopathological measures were combined in a statistical model aiming to predict thresholds. Most of the variance in hearing level was explained by loss of OHCs and IHCs, along with age (r2=0.631). Although strial atrophy and ANF degeneration were common in the aging ear, adding them to the model only increased the correlation minimally (r2= 0.633). This suggest that hair cell loss dominates age-related hearing loss, and that commonly accepted ideas about strial, sensory and neural presbycusis need significant modification.

We further interrogate the data for evidence of noise-induced synaptopathy in human cochleae. After adjusting for IHC survival and cochlear location, the noise-damage group showed 31% more ANF loss than controls at 50 yrs, with the intergroup difference shrinking by 8.2% for every additional 10 years of age. Thus, noise exposure causes significant primary neural degeneration. These data suggest that the noise-induced primary degeneration of cochlear nerve terminals seen in animal studies is also significant in humans.

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