Laboratory Studies
No laboratory studies are appropriate in the evaluation of noise-induced hearing loss (NIHL).
Imaging Studies
No imaging studies are appropriate for the study of noise-induced hearing loss (NIHL).
Other Tests
See the list below:
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Audiometric testing is the only diagnostic evaluation relevant to diagnosis of noise-induced hearing loss (NIHL).
Pure-tone audiometry at the usual octave intervals should be performed. The interoctave interval of 3000 Hz should always be included as well; 3000 Hz is a sensitive area for NIHL and is a frequency that contributes significantly to speech understanding. The American Medical Association (AMA) guidelines for determining hearing handicap require the amount of hearing loss at 3000 Hz to be included in the calculation.
The speech reception threshold (SRT) should also be measured for each ear. Differences between the pure-tone average (PTA) (ie, the number of dB of hearing loss at 500, 1000, and 2000 Hz averaged) and the SRT of more than 5-10 dB should bring into question the reliability of the test. Discrimination scored below 60% suggests an etiology other than NIHL.
Screening audiometry is performed as part of hearing conservation programs (see Deterrence/Prevention).
Because NIHL is compensatable, pseudohypoacusis is more frequently a diagnostic issue when testing patients with alleged NIHL than in many other circumstances. Care should be taken to ensure that accurate, reliable, and repeatable responses are being obtained.
A high incidence of exaggerated hearing loss has been found in individuals claiming NIHL as a result of impulse noise. Objective tests of hearing threshold have been used to obtain accurate hearing thresholds in individuals suspected of exaggerating their hearing loss. Cortical evoked response audiometry (CERA) is the most valuable objective test for the following reasons:
It has good frequency specificity over the speech frequency range (ie, 500-4000 Hz).
It is noninvasive and requires only passive cooperation.
It is recorded from a higher auditory level than electrocochleography (ECochG) or brainstem electric response audiometry (BERA) and, therefore, is less subject to organic neurologic disorders.
It has a closer correlation with behavioral audiometry thresholds than BERA. CERA must be done in such individuals, especially in the presence of flat audiograms and hearing thresholds of more than 25 dB at 500 Hz.
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Sound level meters are able to evaluate 3 time-averaging characteristics: fast, slow, and peak. The slow setting should be used when measuring sound intensity for purposes of assessing occupational noises.
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OSHA standard for the maximum sound intensity (A level, slow) tolerable over certain lengths of time is listed below. These levels are referred to as the permissible exposure level (PEL).
Duration of 16 hours, 85 dBA
Duration of 8 hours, 90 dBA
Duration of 6 hours, 92 dBA
Duration of 4 hours, 95 dBA
Duration of 3 hours, 97 dBA
Duration of 2 hours, 100 dBA
Duration of 1.5 hours, 102 dBA
Duration of 1.0 hour, 105 dBA
Duration of 30 minutes, 110 dBA
Duration of 15 minutes, 115 dBA
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In the jargon of hearing conservation, "the PEL for 4 hours per day is 95 dBA." The reduction in permissible exposure time as the loudness level increases is intended to equalize the risks of NIHL. Consequently, 4 hours of exposure at 95 dB is deemed an equivalent time weighted average (TWA) exposure to 90 dBA for 8 hours.
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The exposure limits set by OSHA have been empirically derived by using both epidemiologic and laboratory data. The standards are believed to be reasonably protective, and the American Academy of Otolaryngology-Head and Neck Surgery has asserted that if exposure to occupational noise did not exceed a TWA of 85 dBA or a dose of 50%, exposure to occupational noise should be excluded as the cause of hearing loss.
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Two methods are used to assess exposure: the amount of noise in a given area can be systemically surveyed, or personal noise dosimetry can be performed.
Noise surveys are performed by placing a sound level meter in an area and measuring the loudness level in that location over an 8-hour day.
Personal dosimetry is performed by having an individual wear a microphone on the shoulder, approximately 5 inches lateral to the ear. Sophisticated computerized programs are then able to assess the amount of noise exposure that individual receives during the working day. The data obtained must be interpreted with the understanding that reflection of sound off the body adds about 2 dB to the exposure assessment, that bumping or touching the surface microphone increases the dosage measurement, and that the dosimeter is not in the control of the assessing professional.
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Anatomy of the inner ear.
