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The following is a typical case history of an history individual with permanent NIHL:

A 48-year-old man had chief complaints of constant, high-pitched tinnitus and progressive hearing loss in both ears over the previous 2 years. He reported some difficulty hearing in quiet surroundings but noticed marked difficulty understanding speech in noisy environments. He did not report any previous serious illnesses, accidents, atypical drug use, or problems with his ears. For the past 8 years, he had worked in a noisy textile mill, where he said that he “occasionally” wore hearing protective devices. The patient had not been exposed to other hazardous noises off the job, such as gunfire or motorbikes.

The diagnosis of NIHL comes under the domain of the audiologist, whose primary responsibility is the identification and measurement of hearing loss and the rehabilitation of those with hearing impairment. By measuring auditory thresholds in decibels (relative to a normal hearing level or 0 dB HL) for pure tones as a function of frequency, an audiogram (a frequency-intensity graph) is generated. Hearing level (HL) is a term used to designate an individual’s hearing threshold at a given test frequency, referenced to an audiometric zero level. The audiogram will help answer the following questions: (1) Is there evidence of hearing loss? (2) If so, what is the severity of the loss? (3) What is the nature of the loss (conductive, sensorineural, or mixed)? and (4) Can the use of a hearing aid(s) benefit the hearing-impaired individual? A typical normal audiogram and an audiogram from an individual with an NIHL are shown in Fig. 16–5.

Hearing loss induced by most industrial noise characteristically produces a bilateral symmetrical loss that is progressive in nature so long as the individual is continuously exposed to hazardous noise levels (Fig. 16–5). In the initial stages of development, the loss usually occurs at frequencies lying between 3,000 and 6,000 Hz. The maximum loss is usually centered at 4,000 Hz. The audiometric configuration, therefore, is characterized by a downward slope with greater loss in the high-frequency region (3,000–6,000 Hz) than in the low- and mid-frequency regions (250–2,000 Hz). As the NIHL accumulates following further exposure, the 4,000-Hz loss increases in magnitude and the adjacent (higher and lower) frequencies also become increasingly affected. The progressive nature of NIHL may eventually result in a moderate to severe impairment across most of the usable hearing frequency range (250–8000 Hz) unless preventive measures are taken to reduce the degree of hazard imposed by the noise.

Although the diagnosis of a permanent NIHL may be indicated by the audiometric configuration of the hearing loss (the 4,000-Hz notch), it would be premature to make a definitive diagnosis unless additional factors are considered, such as: (1) What is the duration, type, and time-weighted average of the individual’s noise exposure? (2) What is the individual’s hearing both before and after exposure? (3) What is the age and general health of the individual? (4) Are there any other disorders that may result in

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