Interactive effects of aging with noise induced hearing loss.

Aging is associated with a down-regulation in metabolism, which may underlie an increased sensitivity to stress agents and a decreased repair of tissues following stress. In the auditory system this could lead to increased sensitivity to noise induced hearing loss (NIHL) with age. This study examines the effects of high intensity noise exposure (0.5-40 kHz noise at 108 dB SPL for 45 min) on young and old normal (CBA/Ca) mice and young premature presbycusis (C57BL/6) mice. Tone evoked auditory brainstem response (ABR) audiograms were obtained before and after the exposure. The animals were sacrificed and cytocochleograms were performed. The ABR threshold shifts and hair cell losses which followed noise exposure increased with increasing age in the normal (CBA/Ca) mice. Subjects which showed early presbycusis (C57BL/6) associated with vascular pathology, showed an increased sensitivity to noise induced hearing loss over normal subjects. Interestingly, in some cases the physiological loss in C57BL/6 mice was not associated with hair cell loss. These findings support the view that aging with or without hearing loss increased the sensitivity of the ear to NIHL. However, it seems that other factors may contribute to NIHL in the animals with premature presbycusis.