ABSTRACT
Hearing function comprises of the transformation of sound waves into vibration, and transduction into the electrical impulse for transmission into the brain for interpretation of its location, pitch, loudness, and quality. Hearing impairment may be sensory neural, conductive, central, and mixed. Sensory neural hearing impairment is due to lesions in the cochlea and/or injury to the eighth nerve and/or brainstem; conductive hearing impairment due to lesions in the external and/or middle ear; and central due to lesions in the cortex. Pure tone audiometry evaluates hearing impairment. Audiometer delivers sounds of specific frequencies at 125, 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz and different intensities with earphones for air conduction. It also delivers sounds at 125, 250, 500, 1000, 2000, 3000, 4000 Hz with oscillator held at mastoid or forehead for bone conduction to measure hearing thresholds. The threshold for normal hearing is 0 ± 10 dB. World Health Organization defines hearing impairment as the permanent unaided hearing threshold level for the better ear of 41 dB or greater. WHO recommends average audiometric ISO value for four frequencies, that is, 500, 1000, 2000, and 4000 Hz to grade hearing impairment. “Integrated Evaluation of Disability” disability applies, WHO grading of Hearing Impairment. Persons with monaural hearing impairment can perform most of his/her life activities, whereas individuals with binaural hearing impairment cannot perform activities requiring communication. Hence, “Integrated Evaluation of Disability” assigns maximum impairment of 15% for monaural hearing impairment and 50% for binaural hearing impairment.
