Occupational hearing loss. The significance of asymmetrical hearing thresholds.

In 1873 consecutive patients referred for compensation assessment for presumed industrial hearing loss, 281 (15%) had an average difference in hearing threshold between the ears of 15 dB at 1/2, 1, 2 and 4 kHz. An unrelated cause was found in 115 (6%). It was due to trauma in 37 (2%), unexplained in 32 (1.7%) and attributed to noise exposure in 97 (5.2%). The possible mechanisms for noise as a cause are discussed and include shielding of one ear from noise, unequal recovery after severe noise exposure, and unequal sensitivity of the ears. After extensive investigation, including X-rays, vestibular tests and some myelograms, no treatable disorder was discovered.