Detection and Clinical Diagnosis of Noise-Induced Hearing Loss by Otoacoustic Emissions

The purpose of this study was to explore the application of the click-evoked and distortion products otoacoustic emissions (CEOAEs and DPOAEs, respectively) in the diagnosis and detection of noise-induced hearing loss (NIHL). The study group consisted of 283 noise-exposed subjects and 176 subjects with a history of noise exposure but with a normal audiogram. Findings were also compared with those in 310 young military recruits with no reported history of noise exposure and normal bilateral audiogram. In general, the features of noise-induced emissions loss (NIEL) closely resembled the behavioural NIHL parameters: both were bilateral and both affected primarily the high frequencies, with a "notch" at around 3 kHz in the DPOAEs. On average, CEOAEs were recorded up to 2 kHz, indicating that up to this frequency range (speech area), cochlear functioning is intact and the hearing threshold s better than 25 dBHL. A clear association between the OAEs and the severity of the NIHL was noted. As the severity of NIHL increased, the emissions range became narrower and the amplitude smaller. OAEs were found to be more sensitive to noise damage than behavioural audiometry. NIEL was found in subjects with normal audiograms but with a history of noise exposure. Owing to their objectivity and sensitivity, OAEs may sometimes provide indispensable information in medico-legal cases, in which the configuration of the audiometric threshold is needed to obtain an accurate diagnosis of NIHL and compensation is proportional to the severity of NIHL. Furthermore, OAE testing between ears with and without NIHL revealed a high sensitivity (79 - 95%) and specificity (84 - 87%). This study shows that OAEs provide objectivity and greater accuracy, complementing the behavioural audiogram in the diagnosis and monitoring of the cochlear status following noise exposure.