ACOUSTIC INJURY

SUMMARY: Hearing can be damaged by a single very loud noise, or by repeated lesser exposure.  These two types of hearing loss typically have distinctive patterns, but chronic acoustic injury may be complicated by other factors. Tinnitus must be evaluated in context of the accompanying hearing loss.  

Acoustic injury can be acute (an incident of high decibel noise, acoustic trauma) or chronic (occupational or lifestyle). 

Injury takes the form of hearing loss with or without tinnitus.  There may also be subtle injury to the balancing mechanism1 but there is no evidence for the proposition that Noise Induced Hearing Loss (NIHL) causes Meniere's disease2. 

Hearing Loss

These two types of hearing loss typically have distinctive patterns, but chronic acoustic injury may be complicated by other factors.

Acute Acoustic Injury

Following acute acoustic trauma, a down-sloping or flat audiogram is more usual3, but the pattern of hearing-loss typical of chronic NIHL (see below) may occur.  Whereas chronic NIHL is of sensorineural type, acoustic trauma may also cause conductive deafness, with disruption of the tympanic membrane (ear-drum) or ossicles (bones). 

Chronic Acoustic Injury

Chronic NIHL has a characteristic pattern on the audiogram - a notch at 3-6kHz. 

The American College of Occupational Medicine Noise and Hearing Conservation Committee has established3 the following criteria: 

Practice Point

Characteristics of chronic, occupational Noise Induced Hearing Loss:

  1. sensorineural

  2. bilateral and symmetrical

  3. rarely profound

  4. non-progressive when noise stopped

  5. rate of loss decreases as threshold increases

  6. frequencies affected:

    a.  3-6kHz more than lower frequencies

    b.  4kHz most affected

  7. maximal losses typically after 10-15 years

  8. continuous noise more damaging than intermittent

In the medicolegal context, non-occupational NIHL must also be considered :  gunfire, loud music, open vehicles and power tools, for example. 

A frequent medicolegal problem in the more elderly is interpretation of the pattern caused by the interaction of NIHL and ageing (presbycusis, Age Induced Hearing Loss, AIHL).  Whereas the higher frequencies are affected first in simple presbycusis, the noise-damaged ear also tends show progressive hearing-loss at 2kHz with advancing age4. 

There is a wide range of individual susceptibility to chronic NIHL, and occupational noise regulations are conservative, based on the level and duration of noise that injures the most vulnerable individuals.  Individual susceptibility may be increased by taking high dosage aspirin5, or by simultaneous exposure to certain chemical pollutants 5,6. 

Tinnitus must be evaluated in context of the accompanying hearing loss.

Tinnitus is very common in the uninjured population and its occurrence increases with advancing age.  Although it may be the most disabling symptom of noise injury, Causation should not be too readily attributed to NIHL7. 

Practice Point

Tinnitus is very common in the general population and Causation by acoustic injury must be considered in context of the accompanying Noise Induced Hearing Loss

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