Pathology and mechanisms of cochlear aging

Abstract

Presbycusis, or age-related hearing loss (ARHL), occurs in most mammals with variations in the age of onset, rate of decline, and magnitude of degeneration in the central nervous system and inner ear. The affected cochlear structures include the stria vascularis and its vasculature, spiral ligament, sensory hair cells and auditory neurons. Dysfunction of the stria vascularis results in a reduced endocochlear potential. Without this potential, the cochlear amplification provided by the electro-motility of the outer hair cells is insufficient, and a high-frequency hearing-loss results. Degeneration of the sensory cells, especially the outer hair cells also leads to hearing loss due to lack of amplification. Neuronal degeneration, another hallmark of ARHL, most likely underlies difficulties with speech discrimination, especially in noisy environments. Noise exposure is a major cause of ARHL. It is well-known to cause sensory cell degeneration, especially the outer hair cells at the high frequency end of the cochlea. Even loud, but not uncomfortable, sound levels can lead to synaptopathy and ultimately neuronal degeneration. Even in the absence of a noisy environment, aged cells degenerate. This pathology most likely results from damage to mitochondria and contributes to degenerative changes in the stria vascularis, hair cells, and neurons. The genetic underpinnings of ARHL are still unknown and most likely involve various combinations of genes. At present, the only effective strategy for reducing ARHL is prevention of noise exposure. If future strategies can improve mitochondrial activity and reduce oxidative damage in old age, these should also bring relief.

Keywords: acoustic trauma; cochlear amplifier; cochlear hair cells; spiral ganglion; stria vascularis.

Figure 1

Number of total inner (closed circles) and outer hair cells (open circles) in human cochleas ranging in age from newborn to 93 years. The counts were made from surface preparations of human temporal bones by G. Bredberg (Figures 93 and 94, Bredberg, 1968). The loss of outer hair cells with age is greater than inner hair cells and starts early in life. Most cases used for this analysis were male and no information about noise exposure was given

Figure 2

Comparison of the magnitude of inner hair cell loss (solid circles represent the median percent loss of 15 animals) and type‐I auditory neuronal loss (open triangles represent the median percent loss of 14 animals) in 27–34‐month‐old rat cochleas. The figure is redrawn from Keithley and Feldman (1982)