Noise-induced hearing loss

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Noise-induced hearing loss (NIHL) is an increasingly prevalent disorder that is the result of exposure to high intensity sounds, especially over a long period of time.

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[edit] Description

NIHL is a preventable hearing disorder that affects people of all ages and demographics. According to a position statement released by the American Academy of Audiology in 2003, "The average, otherwise healthy, person will have essentially normal hearing at least up to age 60 if his or her ears are not exposed to high noise levels" (American Academy of Audiology [AAA], 2003). Unfortunately, around 30 million adults in the United States are exposed to hazardous sound levels in the workplace (National Institute for Occupational Safety and Health [NIOSH], 2000). Among these 30 million people, one in four will acquire a permanent hearing loss as result of their occupation (AAA, 2003). Even though NIHL primarily affects the adult working population, NIHL can be identified in the adolescent and young adult population as well. “The Hearing Alliance of America reports that 15 percent of college graduates have a level of hearing loss equal to or greater than their parents” (Fausti, Wilmington, P.V. Helt, W.J. Helt, and Konrad-Martin, 2005, p. 51). The incidence of NIHL in young adults is not surprising due to the popularity of portable music devices (i.e. walkmans and iPods), concerts, and nightclubs within this population. However, through the proper use of ear protection, education, hearing conservation programs in the workplace, and audiological evaluations, NIHL is a preventable problem.

[edit] Mechanism of Causes

NIHL occurs when too much sound intensity is transmitted into and through the auditory system. An acoustic signal from an energy source, such as a radio, enters into the external auditory canal and is funneled through to the tympanic membrane. The tympanic membrane acts as an elastic diaphragm and drives the ossicular chain of the middle ear system into motion. Then the middle ear ossicles transfer mechanical energy to the cochlea by way of the stapes footplate hammering against the oval window of the cochlea. This hammering causes the fluid within the cochlea (perilymph and endolympth) to push against the stereocilia of the hair cells, which then transmit a signal to the central auditory system within the brain. When exposed to excessive sound levels or loud sounds over time, the force placed on the stereocilia of the hair cells becomes damaging, producing abnormalities of the cells.

“ Some of the abnormalities include metabolic exhaustion of the hair cells, structural changes and degeneration of structures within the hair cells, morphological changes of the cilia, ruptures of cell membranes, and complete degeneration and loss of hair cells, neural cells and supporting cells. (Gelfand, 2001, p. 202) ”

NIHL is therefore the consequence of over-stimulation of the hair cells and supporting structures. Structural damage to hair cells (primarily the outer hair cells) will result in hearing loss that can be characterized by an attenuation and distortion of incoming auditory stimuli.

[edit] Types

There are two basic types of NIHL: NIHL caused by acoustic trauma and gradual developing NIHL. NIHL caused by acoustic trauma refers to permanent cochlear damage from a one time exposure to excessive sound pressure levels. This form of NIHL is commonly from exposure to high intensity sounds such as explosions, gunfire, and firecrackers. Gradual developing NIHL refers to permanent cochlear damage from repeated exposure to loud sounds over a period of time. Unlike acoustic trauma NIHL, this form of NIHL does not occur from a single exposure to a high intensity sound pressure level. Gradual developing NIHL can be caused by multiple exposures to musical concerts, nightclubs, excessive noise in the workplace, and personal music devices. The U.S. Department of Labor’s Occupational Health and Safety Administration (OSHA) states that exposure to 85 dB(A) of noise for more than eight hours per day can result in permanent hearing loss (Occupational Health and Safety Administration [OSHA], 2002). Since decibels are based on a logarithmic scale, every 3 dB SPL increase results in a doubling of intensity, meaning hearing loss can occur at a faster rate. Therefore, gradual developing NIHL occurs from the combination of sound intensity and duration of exposure.

Both NIHL caused by acoustic trauma and gradual developing NIHL can often be characterized by a specific pattern presented in audiological findings. NIHL is generally observed to affect a person’s hearing sensitivity in the higher frequencies, especially at 4000 Hz. “Noise-induced impairments are usually associated with a notch-shaped high-frequency sensorineural loss that is worst at 4000 Hz, although the notch often occurs at 3000 or 6000 Hz, as well” (Gelfand, 2001, p. 202). The symptoms of NIHL are usually presented equally in both ears (Gelfand). Not all audiological results from patients with NIHL match the above description. Often a decline in hearing sensitivity will occur at frequencies other than at the typical 3000-6000 Hz range. Variations arise from differences in people’s ear canal resonance, the frequency of the harmful acoustic signal, and the length of exposure (Rösler, 1994). As harmful noise exposure continues, the common affected frequencies will broaden and worsen in severity (Gelfand). “NIHL usually occurs initially at high frequencies (3k, 4k, or 6k Hz), and then spreads to the low frequencies (0.5k, 1k, or 2k Hz)” (Chen, 2003, p. 55).

[edit] Prevention

The good news is that NIHL can easily be prevented through the use of some of the most simple, widely available and economical tools. This includes, but is not limited to ear protection (i.e. earplugs and earmuffs), education, and hearing conservation programs. Earplugs and earmuffs can provide the wearer with at least 5 to 10 dB SPL of attenuation (Gelfand, 2001). According to a survey by Lass, Woodford, C. Lundeen, D. Lundeen and Everly-Myers (1987), which examined high school students’ attitudes and knowledge on hearing safety, 66% of the subjects reported a positive response to wearing hearing protection devices if educated about NIHL. Unfortunately, more often than not, individuals will avoid the use of ear protection due to lack of comfort, embarrassment, and reduced sound quality.

“ However, the effectiveness of hearing protection programs is hindered by poor compliance in the use of hearing protection devices due to communication difficulties, comfort issues, individuals’ attitudes about protecting themselves from noise-induced hearing loss, and individuals’ perceptions about how others who do not use hearing protection will view them if they choose to use hearing protection. (Fausti et al., 2005, p. 51) ”

[edit] Workplace Standards

The Occupational Safety and Health Administration (OSHA) describes standards for occupational noise exposure in articles 1910.95 and 1926.52 (http://www.osha.gov). OSHA states that an employer must implement hearing conservation programs for their employees if the noise level of the workplace is equal to or above 85 dB(A) for an averaged 8 hour time period (Gelfand, 2001). “Hearing conservation programs in the workplace and in the general population seek to increase compliance and effectiveness of hearing protection protocols through audiometric screening tests and education on the dangers of noise exposure” (Fausti et al., 2005, p. 51).

[edit] Mitigation

For individuals living with NIHL, there are several management options that can improve the ability to hear and effectively communicate. Management programs for individuals with NIHL include the use of hearing aids, FM systems, and counseling. With proper amplification and counseling, the prognosis is excellent for individuals with NIHL. The prognosis has been improved with the recent advancements in digital hearing aid technology, such as directional microphones, open fit hearing aids, and more advanced algorithms. Annual audiological evaluations are recommended to monitor any changes in a patient’s hearing and to modify hearing aid prescriptions. There are no medical options at the moment for a person with NIHL. However, current research for the possible use of drug and genetic therapies look hopeful (National Institute on Deafness and Other Communication Disorders [NIDCD], 2006).

[edit] References

American Academy of Audiology. (2003). Preventing Noise-Induced Occupational Hearing Loss. Retrieved March 3, 2007, from http://www.audiology.org/searchresults.htm?query=noise induced%20hearing%20loss

Chen, & Tsai. (2003). Hearing Loss among Workers at an Oil Refinery in Taiwan. Archives of environmental health, 58(1), 55-58.

Fausti, S., Wilmington, D., Helt, P., Helt, W., & Konrad-Martin, D. (2005). Hearing Health and Care: The Need for Improvised Hearing Loss Prevention and Hearing Conservation Practices. Journal of Rehabilitation Research & Development, 42(4), 45-62.

Gelfand, S. (2001). Auditory System and Related Disorders. Essentials of Audiology: Second Edition (p. 202). New York: Thieme.

Lass N.J., Woodford C.M., Lundeen C., Lundeen D.J., & Everly-Myers D. (1987) A Survey of High School Student’s Knowledge and Awareness of Hearing, Hearing Loss, and Hearing Health. The Hearing Journal, June 15-19.

National Institute for Occupational Safety and Health. (2000). Work-Related Hearing Loss [Brochure]. Washington, DC: National Institute for Occupational Safety and Health.

National Institute on Deafness and Other Communication Disorders. (2006). Noise-Induced Hearing Loss. Retrieved March 3, 2007, from http://www.cdc.gov/niosh/topics/noise/abouthlp/workerhl.html

Occupational Safety & Health Administration. (2002). Hearing Conservation. Retrieved March 3, 2007, from http://www.osha.gov/Publications/OSHA3074/osha3074.html

Rösler, G. (1994). Progression of Hearing Loss Caused by Occupational Noise. Scandinavian Audiology 23, 13-37.


[edit] External links

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