Audiometric Notching at 4 kHz: Good Screening Test for Assessment of Early Onset of Occupational Hearing Loss

Abstract

Context: In noise induced hearing loss (NIHL), cochlear hair cells are damaged by exposure to loud sound. The basilar membrane is tonotopically organized with the lowest frequencies preferentially transduced at the apex and the highest frequencies at the base. On the basilar membrane, the outer hair cells area corresponding to the 4 kHz and the adjacent areas of 3 and 6 kHz are most susceptible to damage. Aims:To screen the factory workers for early onset of NIHL. Subjects and Methods: Study was done over a period of 2 years that is, from September 01, 2011 to August 31, 2013 on 341 steel factory workers. Workers having a minimum of 5 years of noise exposure were included whereas workers having age >45 years or with conductive hearing loss that is, air-bone gap >10 dB were excluded from the study. Results: It was found that factory workers showed normal hearing, when calculated as average of 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz but they have started developing the early changes of NIHL in the form of notching at 4000 Hz frequency described as threshold shifts. It was seen that 198 right ears out of the total 341 studied showed threshold shifts >25 dB in 4 kHz frequency, even then 70 (35.35%) right ears showed hearing in normal range. In case of left ears 198 out of the total 341 studied showed threshold shifts >25 dB in 4 kHz frequency, even then 97 (48.99%) left ears showed hearing in normal range.