Hearing loss

Hearing loss: Excerpt from Handbook of Diseases

Loss of hearing results from a mechanical or nervous impediment to the transmission of sound waves. Hearing loss is classified into three major forms:

❑ Conductive loss is the interrupted passage of sound from the external ear to the junction of the stapes and oval window.

❑ Sensorineural loss is impaired cochlear or acoustic (eighth cranial) nerve dysfunction, causing failure of transmission of sound impulses within the inner ear or brain.

❑ Mixed loss is a combination of conductive and sensorineural hearing loss.

Hearing loss may be partial or complete and is calculated from the American Medical Association formula: Hearing is 1.5% impaired for every decibel (dB) that the pure tone average exceeds 25 dB.

Causes

Hearing loss may be congenital, or it may be caused by drugs, illness, loud noise, or aging.

Congenital hearing loss

Hearing loss may be transmitted as a dominant, autosomal dominant, autosomal recessive, or sex-linked recessive trait. In neonates, it may also result from trauma, toxicity, or infection during pregnancy or delivery.

Predisposing factors include a family history of hearing loss or known hereditary disorders (such as otosclerosis), maternal exposure to rubella or syphilis during pregnancy, use of ototoxic drugs during pregnancy, prolonged fetal anoxia during delivery, and congenital abnormalities of the ears, nose, or throat.

Premature or low-birth-weight infants are most likely to have structural or functional hearing impairments; those with serum bilirubin levels greater than 20 mg/dl also risk hearing impairment from the toxic effect of high serum bilirubin levels on the brain.

In addition, trauma during delivery may cause intracranial hemorrhage and damage the cochlea or acoustic nerve.

Sudden hearing loss

Sudden hearing loss may occur in a person with no prior hearing impairment. This condition is considered a medical emergency because prompt treatment may restore full hearing. Its causes and predisposing factors may include:

❑ acute infections, especially mumps (the most common cause of unilateral sensorineural hearing loss in children) and other bacterial and viral infections, such as rubella, rubeola, influenza, herpes zoster, and infectious mononucleosis, and mycoplasmal infections

❑ metabolic disorders, such as diabetes mellitus, hypothyroidism, and hyperlipoproteinemia

❑ vascular disorders, such as hypertension and arteriosclerosis

❑ head trauma or brain tumors

❑ ototoxic drugs, such as tobramycin, streptomycin, quinine, gentamicin, furosemide, and ethacrynic acid

❑ neurologic disorders, such as multiple sclerosis and neurosyphilis

❑ blood dyscrasias, such as leukemia and hypercoagulation.

Noise-induced hearing loss

Noise-induced hearing loss is caused by a loud noise and may be transient or permanent, and may follow prolonged exposure to loud noise (85 to 90 dB) or brief exposure to extremely loud noise (greater than 90 dB). Such hearing loss is common in workers subjected to constant industrial noise and in military personnel, hunters, and rock musicians.

Presbycusis

An otologic effect of aging, presbycusis results from a loss of hair cells in the organ of Corti. This disorder causes sensorineural hearing loss, usually of high-frequency tones.

Signs and symptoms

The four types of hearing loss have different signs and symptoms:

❑ Although congenital hearing loss may produce no obvious signs of hearing impairment at birth, a deficient response to auditory stimuli generally becomes apparent within 2 to 3 days of birth. As the child grows older, hearing loss impairs speech development.

❑ Sudden hearing loss may be conductive, sensorineural, or mixed, depending on its cause. Associated clinical features depend on the underlying cause.

❑ Noise-induced hearing loss causes sensorineural damage, the extent of which depends on the duration and intensity of the noise. Initially, the patient loses perception of certain frequencies (around 4,000 Hz) but with continued exposure eventually loses perception of all frequencies.

❑ Presbycusis usually produces tinnitus and the inability to understand the spoken word.

Diagnosis

Patient, family, and occupational histories and a complete audiologic examination usually provide ample evidence of hearing loss and suggest possible causes or predisposing factors. The Weber and Rinne tests and other specialized audiologic tests differentiate between conductive and sensorineural hearing loss.

Treatment

Measures for treating each type of hearing loss vary.

Congenital hearing loss

After identifying the underlying cause, therapy for congenital hearing loss refractory to surgery consists of developing the patient’s ability to communicate through sign language, speech reading, or other effective means.

Measures to prevent congenital hearing loss include aggressively immunizing children against rubella to reduce the risk of maternal exposure during pregnancy; educating pregnant women about the dangers of exposure to drugs, chemicals, or infection; and careful monitoring of the fetus during labor and delivery to prevent fetal anoxia.

Sudden hearing loss

Treatment of sudden hearing loss requires prompt identification of the underlying cause. Prevention requires educating patients and health care professionals about the many causes of sudden hearing loss and the ways to recognize and treat them.

Noise-induced hearing loss

Overnight rest usually restores normal hearing in those who have been exposed to noise levels greater than 90 dB for several hours but not in those who have been exposed to such noise repeatedly. As hearing deteriorates, treatment must include speech and hearing rehabilitation because hearing aids are seldom helpful.

Prevention of noise-induced hearing loss requires public recognition of the dangers of noise exposure and insistence on the use, as mandated by law, of protective devices, such as earplugs, during occupational exposure to noise.

Presbycusis

Patients with presbycusis usually require a hearing aid.

Special considerations

❑ When speaking to a patient with hearing loss who can read lips, stand directly in front of him with the light on your face and speak slowly and distinctly. Approach the patient within his visual range, and elicit his attention by raising your arm or waving; touching him may be unnecessarily startling.

❑ Don’t smile, chew gum, or cover your mouth when talking. It makes lip reading more difficult.

❑ Make other staff members and hospital personnel aware of the patient’s handicap and his established method of communication.

❑ Carefully explain all diagnostic tests and hospital procedures in a way the patient understands.

Clinical tip  Write out important statements or information for the patient. He may be embarrassed to tell you he didn’t hear everything you said.

❑ Make sure the patient with a hearing loss is in an area in which he can observe unit activities and people approaching because such a patient depends totally on visual clues.

❑ When addressing an older patient, speak slowly and distinctly in a low tone; avoid shouting.

❑ Provide emotional support and encouragement to the patient who is learning to use a hearing aid. Teach him how the aid works and how to maintain it.

❑ Refer children with suspected hearing loss to an audiologist or otolaryngologist for further evaluation.

❑ To help prevent hearing loss, watch for signs of hearing impairment in patients receiving ototoxic drugs. Emphasize the danger of excessive exposure to noise; stress the danger of exposure to drugs, chemicals, and infection (especially rubella) to pregnant women; and encourage the use of protective devices in a noisy environment.

Book Source Details

• Book Title: Handbook of Diseases
• Author(s): Springhouse
• Year of Publication: 2003
• Copyright Details: Handbook of Diseases, Copyright © 2003 Lippincott Williams & Wilkins.

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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.

More About This Book: Title: Handbook of Diseases Authors: Springhouse Publisher: Lippincott Williams & Wilkins Copyright: 2003 ISBN: 1-58255-266-5

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