Hearing impairment occurs in all age groups. In children hearing loss can be genetic or acquired as a result of infection either during fetal development or during childhood. It is estimated that up to 1 child in every 1,000 live births suffers from hearing loss. The prevalence of hearing loss in children grows to 1.5 to 2 cases per 1,000 children by the age of six. Overall, 50 percent of childhood hearing loss can be traced to genetic factors. This includes congenital conditions such as Down syndrome. Parents with familial deafness pass an increased risk of deafness to their children. Infections during pregnancy (e.g., German measles or cytomegalovirus) can cause congenital deafness. Only 6 percent of children with hearing impairment are profoundly impaired, while the majority retain some hearing ability. Children without a profound hearing loss still have difficulty with speech development and later learning.
In adults, the greatest hearing losses are due to presbycusis and noise-induced hearing loss. Presbycisus of "old hearing" affects men more than women and is estimated to affect up to 80 percent of persons over 65 years of age. Presbycusis may also be linked to noise exposure. Noise exposure is a preventable cause of hearing loss. It is also among the most commonly identified disabilities in industrialized nations. In 1996 the National Institute for Occupational Safety and Health estimated that 30 million people work with noise levels above the level of 85 decibels and 17 percent of production workers suffer some hearing loss. Hearing loss is also associated with a family history of hearing loss, a history of smoking, and presence of hypertension, diabetes, and elevated cholesterol.
The human hearing mechanism is complex. Sound waves enter the ear canal and set up movement of the eardrum, also called the tympanic membrane. The eardrum is connected to the hearing organ, or cochlea, by small bones called ossicles. The cochlea is filled with fluid and it rests in the fluid-filled inner ear. One of the ossicles is anchored to the eardrum; another to an opening in the cochlea called the oval window. The movement of the eardrum sets up movement in the bones, which in turn moves the oval window. This sets up waves in the fluid of the cochlea. Thousands of tiny hair cells that line part of the cochlea are stimulated by these waves, which are then translated to nerve impulses that travel to the brain where they are deciphered by the hearing center of the brain and perceived as sound.
Problems with hearing can be caused from problems all along the hearing pathway. Physicians generally divide hearing loss into two major types: "conductive" and "sensorineural" (sensory). Conductive losses are those involving the transmission of sound waves from the environment to the cochlea. Sensory losses involve the cochlea and its nerve cells, as well as the eighth cranial nerve, called the auditory nerve, which carries nerve impulses to the brain. Some physicians also include a third type of hearing impairment, called central, in which the brain is unable to decipher the information from the hearing complex because of stroke or brain damage.
The most common cause of conductive hearing loss is cerumen, or earwax, impaction. It is often caused by attempts at cleaning the ears with cotton swabs. Putting a swab in the ear usually pushes wax further back in the canal causing buildup over time and leading to impaction. Some patients genetically make more or harder earwax that predisposes them to impactions. Children are notorious for introducing foreign objects into their ears. These objects most often must be removed under controlled circumstances to avoid damage to the inner ear. Water can lodge behind wax buildups causing irritating noises and a sense of fullness in the ears. People who use hearing aids may also experience wax buildup, possibly because the introduction of the hearing aid affects the natural mechanism by which earwax flows from the inner ear. Irrigation and solutions are used to soften the wax so that it can be rinsed out. A physician may also use a removal scoop. Prevention includes avoidance of cotton tipped swabs and maintenance use of solutions to soften earwax.
Another cause of hearing loss is disruption or dislocation of the bones of the inner ear. This occurs when a foreign object, such as a cotton swab or pencil, is put in the ear. This can cause perforation of the eardrum and dislocation of the bones, requiring surgery. Conductive hearing loss is also caused by common cold and other upper respiratory infections, including infections of the fluid of the inner ear.
The eustachian tube drains the inner ear into the back of the throat, but when swollen because of infection of the upper respiratory tract, the fluid builds up in the inner ear. This decreases the ability of the eardrum to move and decreases hearing. The effect is usually transient and hearing is restored when the infection resolves. In young children, however, repetitive ear infections can cause speech delay because children depend on acute hearing in order to mimic language sounds.
Cholesteatoma is a buildup of skin cells around the bones of the ear preventing their movement. It usually occurs as a complication of chronic middle-ear infections and is corrected by surgery, but it must be caught early in order to avoid destruction of the ossicles by the built up material. Otosclerosis is a slowly progressive hearing loss that is more common in women than in men. It is caused by a change in the bones of the ear and is treatable with surgery if recognized early. Other causes of conductive hearing loss are rigidity of the eardrum from scarring caused by old ear infections, eardrum perforations, and barotrauma (pressure trauma) in divers and air travelers.
Generally, sensory hearing loss is less amenable to surgery and treatment, and is better dealt with through prevention. The most common cause of sensory loss is presbycusis—hearing loss associated with aging. Treatment consists of hearing aids, or "amplification."
Noise-associated hearing loss is usually preventable. Noise damage may be part of the hearing loss associated with aging. Earplugs and hearing protection in the workplace helps to prevent some hearing loss, but loud music, concerts, sporting events, and power tools and machinery used in the home also adds to the damage. Hearing protection should be used in all of these cases.
Other causes of sensory hearing loss include multiple sclerosis, tumors of the nerves, congenital deafness associated with infection, or genetic abnormalities and toxins. A tumor on the auditory nerve, known as acoustic neuroma, may be surgically removed, but depending on the position and size of the tumor, hearing may not improve. Toxic effects of drugs such as aspirin, certain antibiotics, and some cancer treatments can lead to hearing loss, and can be avoided by careful monitoring of the dosage. Sudden sensory hearing loss has many causes including viral illness, diabetes, and Meniere's syndrome.
KAREN L. HALL
(SEE ALSO: Hearing Protection; Occupational Safety and Health)
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