Cochlear neuropathy in human presbycusis: Confocal analysis of hidden hearing loss in post-mortem tissue.

Summary

Cochlear synaptopathy had mainly been shown in animals. Here, the same quadruple-immunostaining protocols used in mice were applied to human temporal bones harvested within 9 hours post-mortem. Quantitative analysis of five 'normal' ears (ages 54-89, no history of otologic disease) found cochlear synaptopathy and degeneration of cochlear nerve peripheral axons despite a near-normal hair cell population, suggesting this silent nerve loss is an important, previously hidden component of human age-related hearing decline (presbycusis), likely contributing to hearing-in-noise difficulty even when it doesn't move the audiogram.

Key findings

• Human temporal bones (ages 54-89) showed cochlear nerve fiber loss despite near-normal hair cell counts.
• Confirms cochlear synaptopathy occurs in human aging, not just noise-exposed mice.
• Primary cochlear nerve degeneration is not expected to move hearing thresholds.
• Proposed as a key contributor to hearing-in-noise problems in aging.