Hearing impairment is the most common disability in the United States and the third most prevalent in those over age 65. Mechanical or nervous impediment to the transmission of sound waves can produce hearing loss. The major forms are classified as conductive, sensorineural, or mixed.
In conductive hearing loss, sound is interrupted as it travels from the external canal to the inner ear (the junction of the stapes and the oval window). In sensorineural hearing loss, sound wave transmission is interrupted between the inner ear and the brain. The most common type of sensorineural hearing loss, presbycusis, is prevalent in adults over age 50 and can't be reversed or corrected. Mixed hearing loss combines dysfunction of conduction and sensorineural transmission.
Congenital hearing loss can be conductive or sensorineural. Premature or low-birth-weight infants with congenital hearing loss are most likely to have structural or functional hearing impairments; infants with serum bilirubin levels greater than 20 mg/dl also risk hearing impairment from the toxic effects of these high levels on the brain.
Sudden hearing loss, which can occur in a patient with no previous hearing loss, can be conductive, sensorineural, or mixed and usually affects only one ear. Depending on the cause, prompt treatment (within 48 hours) may restore hearing.
Noise-induced hearing loss may be transient or permanent. Such hearing loss is common in workers subjected to constant industrial noise and in military personnel, hunters, and rock musicians.
Hearing loss may be partial or total and is calculated using the American Medical Association formula: Hearing is 1.5% impaired for every decibel (dB) that the pure tone average exceeds 25 dB.
The causes of hearing loss may be congenital (present at birth) - genetic, use of ototoxic drugs during pregnancy, prenatal rubella in expectant mothers, infections during pregnancy, perinatal anoxia (fetal oxygen lack), or Rh blood disease. Or, the cause may be acquired hearing loss noise exposure, presbycusis, infections that affect the middle ear and inner ear such as mumps, measles and influenza, middle ear infections, ototoxicity, head injuries, benign tumors of the hearing nerve (acoustic neuroma), and cancers.
Signs and Symptoms
Although congenital hearing loss may produce no obvious signs of hearing impairment at birth, the infant generally demonstrates deficient response to auditory stimuli within 2 to 3 days. In an older child, the patient history may describe a hearing loss that impairs speech development. Rinne and Weber's tests may indicate if the hearing loss is conductive or sensorineural.
In conductive hearing loss, the history may uncover a recent upper respiratory tract infection. Weber's test is positive, and the Rinne test also may be positive (a positive Rinne test also may indicate sensorineural hearing loss).
A patient with sudden deafness may report recent exposure to loud noise or brief exposure to an extremely loud noise. The patient may complain of persistent tinnitus and transient vertigo. Audiometric tests indicate that the patient has a loss of perception of certain frequencies (around 4,000 Hz) or, if he's experienced lengthy exposure, loss of perception of all frequencies. Weber's and Rinne tests may indicate conductive or sensorineural hearing loss.
In sensorineural hearing loss due to presbycusis, the patient history is probably the most valuable assessment tool becouse the patient may not have noticed the hearing loss or may deny it. The history also may expose the use of ototoxic substances. Hearing tests reveal a loss that is usually in the high-frequency tones. The patient may report a history of tinnitus. A positive Rinne test may indicate sensorineural hearing loss.
Auditory brain response is used to measure activity in the auditory nerve and brain stem. If the test results are positive or inconclusive, additional tests may be ordered.
A computed tomography scan helps to evaluate vestibular and auditory pathways, and pure tone audiometry is used to assess the presence and degree of hearing loss.
Magnetic resonance imaging is used to evaluate brain condition and helps detect acoustic tumors or lesions. Electronystagmography is used to evaluate vestibular function.
Otoscopic or microscopic examination can be used to diagnose middle ear disorders or remove debris of infection.
Treatment for patients with hearing loss varies with the type and cause of impairment and may include medication to treat infections and dissolve cerumen, surgery (stapedectomy, tympanoplasty, cochlear implant, and myringotomy), hearing aids or other effective means of aiding communication, and antibiotics and decongestants for hearing loss due to otitis media. Analgesics may be given for pain. Antipyretics may be given for fever. Sedatives may be given to small children for comfort.
Treatment for sudden deafness requires prompt identification of the underlying cause.
For noise-induced hearing loss, overnight rest usually restores normal hearing in the patient exposed to noise levels greater than 90 dB for several hours but who hasn't been exposed to such noise repeatedly. As hearing deteriorates, treatment should include speech and hearing rehabilitation because hearing aids rarely help.
Presbycusis may necessitate a hearing aid.
Dietary measures can help to prevent further hearing loss. Studies suggest that people with high cholesterol levels have greater hearing loss as they age than people with low cholesterol levels.
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