Hearing Loss Health Article

Definition

Hearing loss is any degree of impairment of the ability to comprehend sound.

Description

Sound can be accurately measured. The term decibel (dB) refers to an amount of energy moving sound from its source to the ears of one or more listeners or to a microphone. A decrease of more than 10 dB in the level of sound a person can hear is significant.

Sound travels through a medium, such as air or water, as waves of compression and rarefaction. These waves are collected by the external ear and cause the tympanic membrane (ear drum) to vibrate. The chain of ossicles connected to the ear drum—the incus, malleus, and stapes—carries the vibrations to the oval window, increasing their amplitude 20 times along the way. There, the energy creates a standing wave in the watery liquid (endolymph) inside the organ of Corti. (A standing wave is one that does not move. A vibrating cup of coffee will demonstrate standing waves.) The shape of the standing wave is determined by the frequency of the sound. Many thousands of tiny nerve fibers detect the highs and lows of the standing wave. Motions in the nerve endings are converted into electrical impulses. These are transmitted to the brain via a nerve. Specialized areas of the brain interpret the signals as sound.

To summarize, sound energy passes through the air of the external ear, the bones of the middle ear, and the liquid of the inner ear. It is then translated into nerve impulses, sent to the brain through the acoustic nerve, and understood there as sound. It follows that there are five steps in the hearing process:

• air conduction through the external ear to the ear drum
• bone conduction through the middle ear to the inner ear
• water conduction inside the organ of Corti
• nerve conduction into the brain
• interpretation by the brain

Causes and symptoms

Hearing can be interrupted in several ways at each of the five steps noted above.

The external ear canal can be blocked with ear wax (cerumen), foreign objects, infection, and tumors. Overgrowth of the bone, a condition that occurs when the ear canal has been flushed with cold water repeatedly for years, can also narrow the passageway, making blockage and infection more likely. This condition occurs often among northern Californian surfers and is aptly called "surfer's ear."

The ear drum is so thin that a physician can see through it, into the middle ear. Sharp objects, pressure from an infection in the middle ear, or even a firm cuffing or slapping of the ear, can rupture the ear drum. The eardrum is also susceptible to pressure changes, such as those that occur when one is scuba diving.

Several conditions can diminish the mobility of the ossicles (small bones) in the middle ear. Otitis media (an infection in the middle ear) occurs when fluid cannot escape into the throat because of blockage of the eustachian tube. The fluid that accumulates, whether it is pus or mucus, dampens the motions of the ossicles. A disease called otosclerosis can bind the stapes in the oval window, thereby cause deafness.

All of the problems and conditions that occur in the external and middle ear are causes of conductive hearing loss. The second category, sensory hearing loss, refers to damage to the organ of Corti, or the acoustic nerve. Prolonged exposure to loud noise is the leading cause of sensory hearing loss. More than a million people have been identified as having this condition. The cause is often believed to be prolonged exposure to loud music. Occupational noise exposure is the other leading cause of noise induced hearing loss (NIHL) and is an ample reason for wearing ear protection on the job. Jobs in construction and loud offices may contribute to ear damage, as may noises of recreation, such as loud music or the engine of a motorcycle. Both types of noises may lead to loss of hairs in one's inner ear. Thousands of these tiny hairs are attached to nerve cells in the cochlea (a snail-shaped structure in the inner ear). These tiny hairs aid the conversion of sound vibrations into electrical signals, which are then transmitted to the brain. When hairs become broken or bent, the transmission of the electric signals isn't as good, and sound may become muffled. Words may become difficult to distinguish against background noises.

One-third of people over 65 have presbycusis (gradual loss of hearing that occurs as one ages) and one-half of those older than age 75 have a hearing impairment. Both NIHL and presbycusis are primarily high frequency losses. The human speech frequencies are in relatively low ranges. People with presbycusis hear noise but cannot easily make sense of it. They have particular trouble distinguishing speech from background noise. Brain infections, such as meningitis, drugs such as the amino-glycoside antibiotics (e.g., streptomycin, kanamycin, tobramycin), and Menière's disease may also cause permanent sensory hearing loss. Menière's disease combines attacks of hearing loss with attacks of vertigo. The symptoms may occur together or separately. High doses of salicylates, like aspirin and quinine, can cause a temporary high-frequency loss. Prolonged high doses can lead to permanent deafness. There is an hereditary form of sensory deafness and a congenital form most often caused by rubella (German measles).

Sudden hearing loss—at least 30dB in less than three days—is most commonly caused by cochleitis, an inflammation of the cochlea. The source of this process is thought to be viral, but as of 2001, no causative virus has been identified.

The final category of hearing loss is neural. Damage to the acoustic nerve and the parts of the brain that integrate and interpret sounds are the most likely to produce permanent hearing loss. Strokes, multiple sclerosis, and acoustic neuromas are all possible causes of neural hearing loss.

Hearing can also be diminished by extra sounds generated by the ear, most of them from the same kinds of disorders that cause hearing loss. These sounds are referred to as tinnitus and can be ringing, blowing, clicking, or anything else that no one but the affected person hears.