Hearing loss
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
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