Sound-Induced Cochlear Ischemia/Hypoxia as a Mechanism of Hearing Loss.

This review will briefly examine evidence supporting the hypothesis that sound causes changes in cochlear blood flow, intracochlear oxygen levels, and the morphology of cochlear blood vessels. A survey of the literature shows that traditional histopathological studies provided such evidence and that decreased cochlear blood flow can be demonstrated and measured by laser Doppler flowmetry and by direct observation of cochlear microvessels. Oxygen levels also decline and possibly to a greater degree than blood flow. There is also evidence that in certain circumstances sound can increase blood flow. Reduced blood flow, or reduced oxygenation, is critically important in an organ system with high energy needs like the cochlea. Therefore, a second hypothesis, that sound-induced reduction in CBF represents a functional ischemia, will be explored in examining the relevance of traditional ischemia/reperfusion models to cochlear damage. It is found that reactive oxygen species (free radicals and oxidizing ions) are present in sound-induced hearing loss and thus there is evidence that an ischemia/reperfusion type of injury occurs during loud sound exposures.