Understanding Occupational Hearing Loss - Part 2

Sunday, July 20, 2003 | 0

Understanding Occupational Hearing Loss - Part 2

Mini-Monograph: Occupational Hearing Loss, Part II: Identifying and Evaluating Occupational Hearing Loss

Stuart Gherini, M.D.
Otology

Conductive Hearing Loss

To evaluate if a conductive hearing loss exists (when sound has difficulty being conducted through the ear canal and middle ear) "bone conduction" is tested. A small oscillator is placed over the mastoid bone behind the ear. Sound vibrations are transferred from the oscillator, through the skull, and directly into the inner ear bypassing the outer and middle ear. This testing results in "bone conduction thresholds," a measure of the inner ear function (represented on the audiogram by < for the right ear and > for the left ear). When the outer and middle ear are normal, air conduction hearing is equal to bone conduction hearing and the symbols for air conduction and bone conduction will overlap.

When there is a conductive hearing loss, bone conduction will be better (closer to the 0 dB line) than air conduction, the symbols for air conduction and bone conduction will not overlap, and an "air-bone gap" exists. The greater the conductive problem: the bigger the air-bone gap. To an otologist, various patterns on the audiogram indicate various causes of hearing loss.

Repetitive Noise Exposure

Noise-induced hearing loss (NIHL) is the permanent end result of repeated exposure to sounds that are both too intense and too long in duration (1). There are "temporary threshold shifts" in which our hearing is temporarily diminished or stunned following a brief intense noise exposure, after which it returns to normal. There are varying susceptibilities to noise damage among various individuals. For all of us, though, eventually too much noise over too long a period of time leads to permanent hearing loss.

Middle ear damage from noise exposure is rare and is usually only seen in explosions or blast injuries which can cause perforation or rupture of the ear drum. Most noise damage to the hearing occurs within the inner ear. Unlike presbycusis, (age), that affects several structures of the inner ear including the hair cells, the auditory nerve, and the blood supply; in NIHL the hair cells are the only casualty (2). Once destroyed, hair cells cannot regenerate. Broad-spectrum sound, such as is seen in heavy industry, typically has its greatest deleterious effect on those frequencies between 3000 and 6000 Hz. In the early stages of NIHL, the loss is first seen at 4000 Hz.

As the noise exposure continues over the years, the neighboring frequencies of 3000 and 6000 Hz and, later, 2000 and 8000 Hz may become involved as well.

Note how in the early years of noise exposure the hearing loss is confined to 4000Hz. Given continuous unprotected exposure to hazardous levels of noise as years progress, the dip at 4000 Hz deepens and widens. The lower frequencies are much more resistant to damage. A characteristic of industrial NIHL is that its onset is usually seen on audiometric examinations within the first five years of employment (provided that annual hearing tests are performed). Once the noise exposure stops, the damage stops; prior noise exposure does not sneak up on the ears many years later to suddenly cause a hearing loss.

Normal speech is about 60-65 dB. OSHA regulations forbid unprotected exposures exceeding 90 dB time weighted average. This means a worker should not be allowed to be around 90dB of noise (unprotected) for more than eight hours a day. For every five decibels of increased noise the permissible exposure is cut in half. OSHA regulations permit eight hours of exposure to 90dB of noise but only four hours of exposure to 95dB of noise or two hours to 100dB of noise.

Blast Injuries / Explosions

A single exposure to a very intense noise can damage the hearing without rupturing the eardrum. We refer to a single intense noise exposure as "acoustic trauma." This can occur following an explosion or unprotected exposure to the intense crack of a pistol. President Reagan suffered a hearing loss that he related to unprotected exposure from the firing of a prop handgun near his ear during the filming of a Western, (personal communication from Howard P. House, MD).

Unlike NIHL, due to repetitive noise exposure where the loss is usually first seen at 4000 Hz, acoustic trauma may result in a variety of different patterns on audiometric examination. For example, in the early days of cordless telephones, the telephone bell actually rang through the earpiece. Prior to using the handpiece, the user would have to switch the bell to "off." When placing a call, the user might have placed the handpiece against the ear without switching the bell off at the same time that a call was coming in. This gave the telephone user a blast of noise directly into the ear. A series of articles noted that the maximal hearing loss from early cordless telephones was centered at 1000 Hz (as opposed to the maximal hearing loss being centered at 4000 Hz with NIHL) (4).

Long-term Implications Over time, NIHL follows a decelerating course; the greatest change in hearing per year occurs during the early years of exposure (3). This gives rise to the saying among otologists that, "You can't kill the same hair cell twice." This is in direct contrast to hearing loss due to age, (presbycusis), which follows an accelerating pattern, which means the older we get, the faster we lose our hearing. Typically, in NIHL, the hearing pattern seen on audiometric examination is symmetrical. Roughly the same amount of hearing will be lost in each ear at each given frequency.

In acoustic trauma, a variety of different audiogram shapes can be seen (5). Of these, a down-sloping and flat audiogram are the most common (6). Hearing loss from acoustic trauma may improve over four to six months.

According to the American College of Occupational Medicine, (10) NIHL has the following characteristics:

* It is always sensorineural, affecting the hair cells in the inner ear.
* It is almost always bilateral (both ears). Audiometric patterns are usually similar bilaterally.
* It always never produces a profound hearing loss (>90dB). Usually, low frequency limits are about 40 dB and high frequency limits are about 75 dB.
* Once the exposure to noise is discontinued, there is no significant further progression of hearing loss as a result of noise exposure.
* Previous NIHL does not make the ear more sensitive to future noise exposure. As the hearing threshold increases, the rate of loss decreases (decelerating pattern).
* The earliest damage to the inner ear reflects the loss at 3000, 4000, and 6000 Hz. There is always far more loss at 3000, 4000 and 6000 Hz than at 500, 1000, and 2000 Hz. The greatest loss usually occurs at 4000 Hz. The higher and lower frequencies take longer to be affected than the 3000 to 6000 range.
* Given stable exposure conditions, losses at 3000, 4000, and 6000 Hz will usually reach a maximal level in 10-15 years.
* Continuous noise exposure over the years is more damaging than interrupted exposure to noise, which permits the ear a rest period.

References

(1) Dobie, R.A. Medical-legal evaluation of hearing loss. New York: Van Nostrand Reinhold, 1993.
(2) Lee, K.J. Essential otolaryngology. New York: Medical Examination Publishing Co., Inc., 1977.
(3) Dobie, R.A. Medical-legal evaluation of hearing loss. New York: Van Nostrand Reinhold, 1993.
(4) Singleton, GT, Whitaker, DL, Keim, RJ, Kemke, FJ, "Cordless telephones: a threat to hearing." Ann Rhinol. Laryngol. 1984; 93 (6 pt 1): 565-568.
(5) ibid
(6) Teter, DJ, Newell, RC. Aspinall, KB. "Audiometric configurations associated with blast trauma." Laryngoscope 1970; 80 (7): 1122-1132.