Understanding Occupational Hearing Loss

Saturday, August 2, 2003 | 0

Mini-Monograph: Occupational Hearing Loss, Part III ~ How Does Noise Damage Our Hearing?

The following article is the third installment of a 4 part series for physicians and non-physicians on occupational hearing loss by Dr. Stuart Gherini, a qualified medical examiner and expert in the field of otology. Part 1 provided a brief history on occupational hearing loss and a comprehensive analysis on causes, specific types and costs associated with the condition. In Part 2 Dr. Gherini discusses identifying and evaluating occupational hearing loss and examines long-term implications. In this part, Gherini discusses exactly how noise damages hearing.

Stuart Gherini, M.D.
Otology

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) which 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. 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. 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.

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 which he related to the unprotected exposure to the firing of a prop hand gun 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).

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.

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.

Dr. Gherini is represented by MEDLink in the Sacramento, CA. area. Appointments can be made through MEDLink at: www.camedlink.com>

(c) 2001 MEDLink -- Reprinted by Permission. Additional articles may be read at www.camedlink.com . MEDLink affiliated QME physicians may be reached by e-mail at: camedlink@camedlink.com .