Develops slowly after many years of exposure. Susceptibility varies quite widely, but 10 years or more of exposure is generally required for significant hearing loss to occur. In 1990, Dobie listed criteria for the diagnosis of occupation noise-induced hearing loss (ONIHL), as follows:[11]
ONIHL is always a neurosensory loss.Can be treated with intratympanic platelets rich plasma
ONIHL is almost always bilateral.
High-frequency losses rarely exceed 75 dB, and low-frequency losses rarely exceed 40 dB.
Hearing loss does not progress after noise exposure is discontinued.
As hearing loss progresses, the rate of hearing loss decreases.
Loss is always greater at the frequencies 3000-6000 Hz than at 500-2000 Hz. Loss is usually greatest at 4000 Hz. The 4000-Hz notch is often preserved even in advanced stages.
In stable exposure conditions, losses at 3000, 4000, and 6000 Hz usually reach a maximum level in 10-15 years.
When hearing loss is limited to the high frequencies, individuals are unlikely to have difficulty in quiet conversational situations. The first difficulty the patient usually notices is trouble understanding speech when a high level of ambient background noise is present. As NIHL progresses, individuals may have difficulty understanding high-pitched voices (eg, women’s, children’s) even in quiet conversational situations. Conversation on the telephone is generally unimpaired because telephones do not use frequencies above 3000 Hz.
The aforementioned symptoms are nonspecific symptoms of high-frequency neurosensory hearing loss and do not help to distinguish among etiologies. Consequently, clinical presentation is of no use in distinguishing NIHL from early ototoxicity, genetically mediated progressive losses, or presbycusis.
Clinically, NIHL begins with a temporary threshold shift (TTS). A TTS is defined by Dobie as a temporary neurosensory hearing loss that recovers almost completely once the noxious stimulus is removed. The amount of time over which recovery occurs is unclear and controversial. Sixteen hours has been used in the past, but some people with TTS require longer periods to recover. Dobie uses a 24-hour threshold; however, some argue that days or months may be required to recover TTS, especially if the case is associated with acoustic trauma. Nonetheless, as a practical matter, Dobie’s time limit of 24 hours is commonly used.
The extent of a TTS is predictable on the basis of the causative noise’s intensity, frequency, content, and temporal pattern of exposure (ie, intermittent or continuous). Pure-tone and narrow-band stimuli result in a maximum TTS at or slightly above the center frequency of the noise producing it. However, in occupational situations, TTSs are almost always greatest between 3000-6000 Hz and are often quite narrowly focused at 4000 Hz.
High-frequency noise is much more damaging than low-frequency noise; therefore, intensity alone cannot predict risk. For this reason, a special scale has been developed for measuring environmental noise when the purpose is to assess its potential to produce hearing loss.
Continuous stimuli are more damaging than interrupted stimuli. Intermittent noise is more protective for apical lesions induced by low frequencies than for basal lesions induced by high frequencies.
A clinically important feature of TTS is that it is rarely apparent to the subject because of its relatively low magnitude and relatively high frequency. Repeated TTSs over weeks, months, and years fail to recover completely and thereby become a permanent threshold shift (PTS).
ONIHL begins with selective loss of hearing at around 4000 Hz. Thresholds are better at both higher and lower frequencies. This is recognized on an audiogram as a notch centered around 4000 Hz and, although not pathognomonic, it is the characteristic audiometric pattern of early NIHL. If exposure is continued, the notch gradually deepens and widens. Eventually, retention of good hearing in the higher frequencies is lost, and the resulting hearing loss appears only as a relatively steep high-frequency loss beginning at 3000 Hz and becoming more severe at each higher frequency over a period of many years. Persistent noise exposure progressively encroaches on the middle frequencies. In the most severe cases, even the lower frequencies may eventually become involved.
Many patients experience tinnitus associated with both TTS and PTS. Individuals who reliably have ringing in their ears after noise exposure probably have experienced an injury to the auditory system in the form of at least a TTS. Because repeated TTS slowly converts to PTS, postexposure tinnitus and TTS serve as warning signs of impending permanent NIHL.
Evidence is strong that a significant amount of individual variability exists with respect to susceptibility to NIHL. The auditory system of some individuals seems to be able to withstand longer exposure times to higher loudness levels than the auditory system of others. Thus, norms established for hearing conservation programs, although protecting the group as whole, may not protect the most sensitive individuals. The symptoms of individuals with postexposure tinnitus or hearing loss should be taken seriously. Audiograms immediately after exposure and again 24 hours later should be attained to establish the presence or absence of TTS or PTS. From time to time, such testing may need to be repeated on several occasions.
The 4-kHZ notch appears to be a consequence of several factors: (1) the fact that human hearing is more sensitive at 1-5 kHz, (2) the fact that the acoustic reflex attenuates loud noises below 2 kHz (as demonstrated by Borg), and (3) nonlinear middle ear function as a result of increased intensities.
NIHL, especially ONIHL, is generally symmetrical. Occasionally, a work environment results in asymmetrical noise exposure, as seen in tractor drivers with ONIHL in which the left ear is more frequently affected than the right ear. As tractor operators have to monitor equipment mounted on the rear side, most operators look over their right shoulder, exposing their left ear to the noise of the prime mover and exhaust while their right ear is shielded by head shadow.
However, work environments, especially indoor environments, have sufficient reverberation so as to produce essentially equal stimulation of both ears.
The most common cause of asymmetric NIHL is exposure to firearms, particularly long guns. Right-handed shooters have a more severe hearing loss in the left ear because the left ear faces the barrel while the right ear is tucked into the shoulder and is in the acoustic shadow of the head.
The physical examination is not important in the evaluation of noise-induced hearing loss (NIHL) except to rule out other causes. The physical examination should include evaluation of the tympanic membranes and external auditory canals. A neurologic examination should be performed to rule out neurologic diseases

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