|
Occupational Hearing Loss
ALTERNATIVE NAMES: Noise-induced hearing loss; acoustic trauma.
DEFINITION: Hearing loss induced by noise trauma.
WHAT IS GOING ON IN MY BODY? The inner ear consists of both balance and hearing components. The portion for hearing is shaped like a snail shell with approximately two and a half turns. Sound gets to the inner ear by vibrating the eardrum and the small bones for hearing that transmit the drum vibration to the inner ear. The inner ear has two fluid-filled compartments, one on the inside of the other one. When the stapes, which is shaped like a stirrup, moves in its membrane on the inner ear, it sets up a fluid wave which then causes a structure called the basement membrane to vibrate. By inducing basement membrane movement, the hair cells in the inner ear generate a signal which is then transmitted to the auditory nerve. The auditory nerve transmits the signal into the brain, where it is then interpreted as sound. The snail-shell-shaped inner ear is arranged by pitch. At the top of the snail shell low-frequency sounds are picked up. In the bottom turn of the snail shell high pitches are detected. There are approximately 25,000 individual rows of hair cells, each of which responds to a particular pitch.
Acute high-intensity or prolonged exposure to loud sounds can injure the hair cells. Acute noise injury can cause both temporary and permanent damage. Prolonged exposure to loud sounds can cause permanent injury by loss of hair cells. As hair cells are lost, the ability to pick up sounds at those frequencies diminishes, with subsequent hearing loss. The area of the inner ear which is most susceptible to sound is the high-frequency first turn of the cochlea.
WHAT ARE THE SIGNS AND SYMPTOMS? There are two basic forms of acoustic noise injury. The first one is acute noise injury – otherwise termed acute threshold shift. This form of hearing loss occurs with transient exposure to excessively loud sound. It can take the form of either exposure to consistently loud sound, such as a rock concert, around jet aircraft, or any other type of intense noise exposure. Very brief, loud explosions can also cause similar injury. What happens in an acute threshold shift is injury to the hair cells in the lower portion of the cochlea responsible for picking up high-frequency sound. When this occurs, the individual will experience a muffled perception of sound, a feeling of fullness in the ears, diminished hearing especially for high frequencies, and a high-frequency ringing. Typically within 24 to 48 hours the noise injury recovers, with restoration of the pre-injury hearing levels. However, microscopically permanent hair cell injury is present in spite of the fact that the hearing has recovered. In certain situations, particularly very loud, percussive sounds, such as explosions, artillery fire, fireworks, and gunfire muzzle blasts, can be so loud that there is an immediate and permanent loss of high-frequency hearing. There also seems to be a susceptibility of some individuals to noise exposure. It is clearly apparent that some people can have the same degree of noise exposure but do not end up with the hearing loss sequelae that others would. Should hearing not recover after acute noise trauma, this is termed a permanent threshold shift. For those individuals with chronic noise exposure, small noise injuries incurred over time accumulate with a slowly progressive high-frequency hearing loss. These individuals may not be aware after their period of noise exposure that anything is wrong with their ears or hearing. The Occupational Safety and Health Administration has established guidelines for prolonged occupational noise exposure. Individuals exposed to 85 decibels or greater per hour are required to wear some form of hearing protection. Hearing protection will be discussed below.
HOW TO PREVENT THE DISEASE: Individuals working in facilities where there is more than 85 decibels per hour of noise exposure should wear hearing protection. Others exposed to repetitive, loud noise, such as construction workers and heavy equipment operators, would also be advised to wear hearing protection. Any time an individual is working with loud equipment, such as lawn mowers, motorized string trimmers, or hand-held power equipment, hearing protection should be worn. Use of power woodworking equipment, especially in an enclosed environment, should be discouraged, but if it has to be performed hearing protection should be employed. Those who practice abusive noise behavior, such as listening to music too loudly, are also placing themselves at risk for subsequent permanent high-frequency hearing loss. Recent reports by hearing aid dispensers indicate that the average age of individuals seeking hearing aids is declining, which is an indication of the excessive noise exposure during the younger years of life. Those who are in loud noise environments should have regular hearing tests obtained. Identification of a hearing loss should prompt the affected individual to practice vigilant hearing protection. There are a number of different devices on the market which help to protect the ear from noise exposure. This would include silicone-based putties and expandable foam ear plugs, which will give approximately 20 to 25% reduction in the level of noise. The best form of hearing protection is the acoustic ear muffs in combination with ear plugs which can provide approximately 30 to 35 decibels of sound attenuation.
HOW IS IT DIAGNOSED? Standard hearing testing is the main means of diagnosis. Newer tests, such as otacoustic emissions, which actually look at the integrity of the hair cells in the inner ear, can identify hair cell function. Sometimes sluggish hair cell function will precede an absolute change in the pure tone threshold on an audiogram.
WHAT ARE THE LONG-TERM EFFECTS? The main long-term effect of occupational noise exposure is progressive high-frequency nervy hearing loss that is not reversible. These individuals often experience tinnitus (ringing of the ears) and difficulty hearing an individual's conversation in the presence of background noise.
AM I PUTTING OTHERS AT RISK? No.
WHAT ARE THE TREATMENTS? The best treatment in occupational noise exposure is prevention. Hearing aids may be employed for those individuals who are beginning to experience impaired abilities to hear.
WHAT ARE THE SIDE EFFECTS TO THE TREATMENTS? None.
WHAT HAPPENS AFTER TREATMENT? Many people with hearing aids still note that communication is impaired when there is significant background noise. Hopefully, with aggressive and consistent protection or avoidance of sound, no further damage will occur and the hearing will stabilize.
HOW DO I MONITOR THE DISEASE? For those individuals at risk who work in a noisy occupation, regular hearing testing needs to be performed.
|
