Noise-Induced Hearing Loss Clinical Presentation

• Noise-induced hearing loss (NIHL) 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.
  • 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.

Noise-induced hearing loss (NIHL) is caused by high levels of ambient noise. OSHA has determined that exposure to loudness levels lower than 85 dBA continuously for an 8-hour workday is unlikely to cause harm. However, sensitive individuals may experience hearing loss even at this or slightly lower levels.

• Experimental data suggest that when exposure is continuous, injury is a consequence of the total amount of energy to which cochlear tissues are exposed. Thus, if sound energy is doubled, the risk of injury can be kept constant if the exposure time is cut roughly in half. Because each 3 dB of loudness increase represents a doubling of sound energy, the amount of damage expected from 8 hours of exposure to 100 dB should be about the same as the amount of damage sustained from 4 hours of exposure to 105 dB. However, this relationship applies only when exposure is constant. Even relatively brief interruptions significantly decrease the amount of damage that is to be expected.
• Moreover, this trade-off between intensity and duration becomes meaningless once the elastic limit of inner ear tissue is exceeded. At this point, rules governing impulse noise come into play. The point at which this occurs in humans is unclear. Available data suggest that brief exposure to relatively high-intensity impulse noise produces less damage than expected from extrapolating the intensity-duration curves established for steady-state noise. For example, young healthy air force personnel exposed for 0.4 seconds to a noise of 153 dB suffered only very slight TTS, much less damage than would have been expected from data obtained from continuous-exposure studies.
• Causative acoustic stimuli can be divided into continuous and intermittent stimuli, which are usually associated with classic NIHL. Intermittent noise is defined as loudness levels that fluctuate more than 20 dBA.
• Acoustic trauma is an extremely loud noise usually resulting in immediate, permanent hearing loss. Such transient noise stimuli are generally less than 0.2 seconds in duration. The 2 types of transient noises are impulse noise, which is usually the result of an explosion, and impact noise, which results from a collision (usually metal on metal). Impact noises are often associated with echoes and reverberations, which produce acoustic peaks and troughs.
• Assessing the degree of noise exposure an individual experiences can be extremely difficult. In most working environments, noise is not continuously sustained and is therefore intermittent. Moreover, many individuals are mobile and move through noise environments of different intensities for various periods during the workday. The American National Standards Institute (ANSI) and the International Organization for Standards (ISO) have set detailed standards for measuring environmental noise.
• Overall, the degree of NIHL is influenced by the following:

  • Intensity of the noise (dBA)
  • The temporal pattern of the noise (continuous, intermittent, transient)
  • The spectral pattern of the noise (frequency content)
  • The duration of exposure to the noise (time weighted average [TWA])
  • Individual susceptibility to the noise
• Continuous exposure to 100 dBA can be expected to produce, on average, the following levels of hearing loss:

  • Five years, 5 dB
  • Twenty years, 14 dB
  • Forty years, 19 dB
• Various nonoccupational noises can produce hearing loss. However, these exposures generally produce minor losses because the exposure times are short. These include the following:

  • Leaf blowers
  • Lawn mowers
  • Chain saws
  • Rock concerts
  • Jet noise
  • Private aircraft
  • Snowmobiles
  • Jet skis
  • Motorcycles
• A study of impulse noise in soldiers exposed to weapon-related noise levels (1.6-16 kHz) found that, after their military service, the soldiers' hearing had significantly deteriorated (an average of 6 dB exclusively at 10 and 12 kHz). Transiently evoked otoacoustic emission (TEOAE) reduction was registered predominantly at 2, 3, and 4 kHz, with greatest decrease at 2 kHz (P < 0.02). Reduced TEOAE levels in soldiers exposed to noise may be the first sign of potential hearing loss.
• Although portable radios and cassette, CD, or MP3 players are capable of producing loudness levels greater than 85 dB, they are not commonly adjusted to such high levels, even by adolescents; when they are, exposure times are generally short compared with an 8-hour workday. Dobie has noted an exception to this observation. When portable cassette players are used in the workplace, exposure from the cassette player may be added to the workplace noise and increase the potential for injury.
• Most nonoccupational NIHL is the result of firearm noise. Firearms can produce noise levels of up to 170 dB. Men who had a quiet work environment and engaged in shooting sports had, on average, hearing loss equivalent to that of individuals who had worked for 20 years in a factory with an 89-dBA noise level.