Basic Mechanisms Underlying Noise-Induced Hearing Loss

The basic concept of noise-induced hearing loss (NIHL) is relatively simple: the development of a hearing dysfunction following exposure to a loud sound. However, there are many complexities in the types of hearing dysfunctions that can follow noise and the mechanisms underlying these changes. There have been several classical ways of differentiating types of NIHL. One such division is based on changes in the thresholds for detection of a sound, a reflexive behavioral response to a sound, an auditory brain stem response (ABR) evoked by sound, or some other sound-evoked physiological response in the central auditory pathways. If the amount of signal needed to generate detection or response becomes greater following a noise, this is considered a threshold shift. The threshold shift following noise can be temporary (a temporary threshold shift or TTS) or permanent (a permanent threshold shift or PTS). A PTS generally involves loss of sensory cells and can be further divided into cell death following apoptosis versus cell death by necrosis. TTS and the two types of PTS are all considered to have different causes and underlying mechanisms.