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Wu PZ, Liberman LD, Bennett K, de Gruttola V, O'Malley JT, Liberman MC. Primary Neural Degeneration in the Human Cochlea: Evidence for Hidden Hearing Loss in the Aging Ear. Neuroscience 2018; 407:8-20. [PMID: 30099118 DOI: 10.1016/j.neuroscience.2018.07.053] [Citation(s) in RCA: 211] [Impact Index Per Article: 35.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2018] [Revised: 07/25/2018] [Accepted: 07/30/2018] [Indexed: 01/18/2023]
Abstract
The noise-induced and age-related loss of synaptic connections between auditory-nerve fibers and cochlear hair cells is well-established from histopathology in several mammalian species; however, its prevalence in humans, as inferred from electrophysiological measures, remains controversial. Here we look for cochlear neuropathy in a temporal-bone study of "normal-aging" humans, using autopsy material from 20 subjects aged 0-89 yrs, with no history of otologic disease. Cochleas were immunostained to allow accurate quantification of surviving hair cells in the organ Corti and peripheral axons of auditory-nerve fibers. Mean loss of outer hair cells was 30-40% throughout the audiometric frequency range (0.25-8.0 kHz) in subjects over 60 yrs, with even greater losses at both apical (low-frequency) and basal (high-frequency) ends. In contrast, mean inner hair cell loss across audiometric frequencies was rarely >15%, at any age. Neural loss greatly exceeded inner hair cell loss, with 7/11 subjects over 60 yrs showing >60% loss of peripheral axons re the youngest subjects, and with the age-related slope of axonal loss outstripping the age-related loss of inner hair cells by almost 3:1. The results suggest that a large number of auditory neurons in the aging ear are disconnected from their hair cell targets. This primary neural degeneration would not affect the audiogram, but likely contributes to age-related hearing impairment, especially in noisy environments. Thus, therapies designed to regrow peripheral axons could provide clinically meaningful improvement in the aged ear.
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Affiliation(s)
- P Z Wu
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA 02114, USA; Department of Otolaryngology, Harvard Medical School, Boston, MA 02115, USA; Department of Otorhinolaryngology Head and Neck Surgery, The First Affiliated Hospital, Sun Yat-sen University, Guangzhou 510080, China
| | - L D Liberman
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA 02114, USA
| | - K Bennett
- Department of Biostatistics, Harvard TH Chan School of Public Health, Boston, MA 02115, USA
| | - V de Gruttola
- Department of Biostatistics, Harvard TH Chan School of Public Health, Boston, MA 02115, USA
| | - J T O'Malley
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA 02114, USA
| | - M C Liberman
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA 02114, USA; Department of Otolaryngology, Harvard Medical School, Boston, MA 02115, USA.
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