What are ears? |

Structure and Functions

The ear is composed of three parts: the outer ear, the middle ear, and the inner ear. All three parts are involved in hearing, while only the inner ear is involved in balance.

Sound can be thought of as pressure waves that travel through the air. These waves are collected by the fleshy part of the outer ear and are funneled down the ear canal to the eardrum. The eardrum, being a thin membrane, vibrates as it is hit by the sound waves. Attached to the eardrum is the first of the ossicles (the hammer or malleus), which moves when the eardrum moves. The second ossicle (the anvil or incus) is attached to the first, and the third to the second. Therefore, as the first bone moves, the others move also. The base of the third bone (the stirrup or stapes) is in contact with the oval window at the beginning of the inner ear. Movement of the oval window sets up vibrations in the fluid of the cochlea. These vibrations are detected by hair cells. Depending on their position in the cochlea, the
hair cells are sensitive to being moved by vibrations of different frequencies. When the hair of a hair cell is bent by the fluid, an impulse is generated. The impulses are transmitted to the brain via the auditory nerve. The nerve impulses are processed in the brain, and the result is the sensation of sound, in particular the sense of pitch. Thus the three parts of the ear turn sound waves into “sound” by changing air vibrations into eardrum vibrations, then ossicle movement, then fluid vibrations, and finally nerve impulses.

Disorders and Diseases

Each of the three parts of the ear can be affected by diseases that can lead to temporary or, in some cases, permanent hearing loss. Damage to the eardrum, ossicles, or any part of the ear before the cochlea results in conductive hearing loss
, as these structures conduct the sound or vibrations. Damage to the hair cells or to the auditory nerve results in sensorineural hearing loss
. Sound may be conducted normally but cannot be detected by the hair cells or transmitted as nerve impulses to the brain.

Disorders of the outer ear include cauliflower ear, blockage by earwax, otitis externa, and tumors. Cauliflower ear is a severe hematoma (bruise) to the outer ear. In some cases, the blood trapped beneath the skin does not resorb and instead turns into fibrous tissue that may become cartilaginous or even bonelike.

Earwax is secreted by the cells in the lining of the ear canal. Its function is to protect the eardrum from dust and dirt, and it normally works its way to the outer opening of the ear. The amount secreted varies from person to person. In some people, or in people who are continually exposed to dusty environments, excessive amounts of wax may be secreted and may block the ear canal sufficiently to interfere with its transmission of sound waves to the eardrum.

Otitis externa can take two forms, either localized or generalized. The localized form, a boil or abscess, is a bacterial infection that results from breaks in the lining of the ear canal and is often caused by attempts to scratch an itch in the ear or to remove wax. The generalized form can be a bacterial or fungal infection, known as otomycosis. Generalized otitis externa is also called swimmer’s ear because it often results from swimming in polluted waters or from chronic moisture in the ear canal.

Tumors of the ear can be benign (noncancerous) or malignant (cancerous) growths of either the soft tissues or the underlying bone. Bony growths, or osteomas, can cause sufficient blockage, by themselves or by leading to the buildup of earwax, to result in hearing loss.

The middle ear consists of the eardrum, three small bones called the ossicles, and the eustachian tube. The bones of the middle ear move in an air-filled cavity. Air pressure within this cavity is normally the same as the outside air pressure because air is exchanged between the middle ear and the outside world via the eustachian tube. When this tube swells and closes, as it often does with a head cold, one experiences a stuffy feeling, decreased hearing, mild pain, and sometimes ringing in the ears (tinnitus) or dizziness. The middle ear is susceptible to infection, such as otitis media
, because bacteria and viruses can sometimes enter via the eustachian tube. Young children are especially prone to middle-ear infections because a child’s eustachian tubes are shorter and more directly in line with the back of the throat than those of adults. Untreated ear infections can sometimes spread into the surrounding bone (mastoiditis) or into the brain (meningitis).

Fluid in the middle ear during an ear infection interferes with the free movement of the ossicles, causing hearing loss that, although significant while it lasts, is temporary. In other instances, there is the prolonged presence of clear fluid in the middle ear, resulting from a combination of infection or allergy and eustachian tube dysfunction, which itself can result from swelling caused by an allergy. This condition, known as glue ear or persistent middle-ear effusion, can last long enough to cause detrimental effects on speech, particularly in young children. Middle-ear infections can sometimes become chronic, as in chronic otitis media; permanent damage to the hearing can result from the ossicles being dissolved away by the pus from these chronic infections.

During a middle-ear infection, fluid can build up and increase pressure within the middle-ear cavity sufficiently to rupture (perforate) the eardrum
. Very loud noises are another form of increased pressure, in this case from the outside. If a loud noise is very sudden, such as an explosion or gunshot, then pressure cannot be equalized fast enough and the eardrum can rupture. Scuba diving without clearing one’s ears (that is, getting the eustachian tube to open and allow airflow) can also result in ruptured eardrums. Other causes of ruptured eardrums include puncture by a sharp object inserted into the ear canal to remove wax or relieve itching, a blow to the ear, or a fractured skull. Some hearing is lost when the eardrum is ruptured, but if the damage is not too severe, the eardrum heals itself and hearing returns.

The middle ear does not always fill with fluid if the eustachian tube is blocked. In some instances, the middle-ear cavity remains filled with trapped air. This trapped air is taken up by the cells lining the middle-ear cavity, decreasing the air pressure inside the middle ear and allowing the eardrum to push inward. Cells that are constantly shed from the eardrum collect in this pocket and form a ball that can become infected. This infected ball, or cholesteatoma, produces pus, which can erode the ossicles. If left untreated, the erosion can continue through the roof of the middle-ear cavity (causing brain abscesses or meningitis) or through the walls (causing abscesses behind the ear). The symptoms of a cholesteatoma go beyond the symptoms of an earache to include headache, dizziness, and weakness of the facial muscles.

Permanent conductive hearing loss can also result from calcification of the ossicles, a condition called osteosclerosis. Abnormal spongy bone can form at the base of the stirrup bone, interfering with its normal movement against the oval window. Hearing loss caused by osteosclerosis occurs gradually over ten to fifteen years, although it may be accelerated in women by pregnancy. There is a hereditary component.

The inner ear begins at the oval window, which separates the air-filled cavity at the middle ear from the fluid-filled cavities of the inner ear. The inner ear consists of the cochlea, which is involved in hearing, and the labyrinth, which maintains balance.

Disorders of the cochlea result in permanent sensorineural hearing loss. Hair cells can be damaged by the high fever accompanying some diseases such as meningitis. They may also be damaged by some drugs. The largest, and most preventable, sources of damage to the hair cells are occupational and recreational exposure to loud sounds, particularly if they are prolonged. In some occupations, the hearing loss from working without ear protection may be confined to certain frequencies of sounds, while other occupations lead to general loss at all sound frequencies. Prolonged exposure to overamplified music will likewise cause permanent hearing loss at all frequencies. This is more severe and has much earlier onset than presbycusis—the progressive loss of hearing, particularly in the high frequencies, that occurs with normal aging.

The labyrinth is the part of the ear that maintains one’s balance; therefore the major system of disorders of the labyrinth is vertigo (dizziness). Labyrinthitis is an infection, generally viral, of the labyrinth. The vertigo can be severe but is temporary.

With Ménière disease, there is an increase in the volume of fluid in the labyrinth and a corresponding increase in internal pressure, which distorts or ruptures the membrane lining. The symptoms, which include vertigo, noises in the ear, and muffled or distorted hearing especially of low tones, flare up in attacks that may last from a few hours to several days. The frequency of these attacks varies from one individual to another, with some people having episodes every few weeks and others having them every few years. This condition, which may be accompanied by migraine headaches, usually clears spontaneously but in some people may result in deafness.

Diagnostic and Treatment Techniques

The most common ear disorders, outer-ear or middle-ear infections, are diagnosed visually with an otoscope. This handheld instrument is a very bright light with a removable tip. Tips of different sizes can be attached so that the doctor can look into ear canals of various sizes. Infections or obstructions in the outer ear are readily visible. Middle-ear infections can often be discerned by the appearance of the eardrum, which may appear red and inflamed. Fluid in the middle ear can sometimes be seen through the eardrum, or its presence can be surmised if the eardrum is bulging toward the ear canal. In other cases, the eardrum will be seen to be retracted or bulging inward toward the middle-ear cavity. Holes in the eardrum can also be seen, as can scars from previous ruptures that have since healed.

Impedance testing may be used in addition to the otoscope for the diagnosis of middle-ear problems. Impedance testing is based on the fact that, when sound waves hit the eardrum, some of the energy is transmitted as vibrations of the drum, while some of the energy is reflected. If the eardrum is stretched tight by fluid pushing against it or by being retracted, it will be less mobile and will reflect more sound waves than a normal eardrum. In the simplest form, the mobility of the eardrum is tested with a small air tube and bulb attached to an otoscope. The doctor gently squeezes a puff of air into the ear canal while watching through the otoscope to see how well the eardrum moves.

A far more quantitative version of impedance testing can be done in cases of suspected hearing loss. This type of impedance testing is generally administered by an audiologist, a professional trained in administering and interpreting hearing tests. The ear canal is blocked with an earplug containing a transmitter and receiver. The transmitter releases sounds of known frequency and intensity into the ear canal while also changing the pressure in the ear canal by pumping air into it. The receiver then measures the amount of energy reflected back. The machine analyzes the efficiency of reflection at various pressures and prints out a graph. By comparison of the graph to that from an eardrum with normal mobility, conclusions can be drawn about the degree of immobility and, consequently, about the stage of the middle-ear infection. Many pediatricians or family physicians have handheld versions of this instrument, which resembles an otoscope but is capable of transmitting sound and measuring reflected sound intensity.

When an ear infection has been diagnosed, the treatment is generally with antibiotics. For outer-ear infections, drops containing antibiotic or antifungal agents are prescribed. For middle-ear infections, antibiotics are prescribed that can be taken by mouth. The patient is rechecked in about three weeks to ensure that the ear has healed.

In some cases, the ear does not heal, or the fluid in the middle ear does not go away. This can occur if a new infection starts before the ear is fully recovered or if the infecting microorganisms are resistant to the antibiotic used for treatment. In cases of chronic or repeated otitis media, a surgical procedure called a myringotomy can be performed in which a small slit is made in the eardrum to release fluid from the middle ear. Often, a small tube is inserted into the slit. These ear tubes, or tympanostomy tubes, keep the middle ear ventilated, allowing it to dry and heal. In most cases, these tubes are spontaneously pushed out by the eardrum as healing takes place, usually within three to six months. Patients must be cautious to keep water out of their ears while the tubes are in place.

A permanently damaged eardrum—from an explosion, for example—can be replaced by a graft. This procedure is called tympanoplasty, and the tissue used for the graft is generally taken from a vein from the same person. If the ossicles are damaged, they too can be replaced, in this case by metal copies of the bones. For example, when otosclerosis has damaged the stapes (stirrup) bone, hearing can often be restored by replacing it with a metal substitute.

Tumors, osteomas in the ear canal, or cholesteatomas on the eardrum may need to be removed surgically. Surgery may also be needed if infections have spread into the surrounding bone. Bone infections or abnormalities of the inner ear are diagnosed by X-rays or by computed tomography (CT) scans.

For some persons who have complete sensorineural hearing loss, some awareness of sound can be restored with a cochlear implant. This electronic device is surgically implanted and takes the place of the nonexistent hair cells in detecting sound and generating nerve impulses.

Problems of balance may sometimes be treated successfully with drugs to limit the swelling in the labyrinth. Ringing in the ears (tinnitus) is usually resolved when the underlying condition is resolved. In some cases, tinnitus is caused by drugs (large doses of aspirin, for example) and will cease when the drugs are stopped. However, it is difficult to identify the cause of most tinnitus cases, and there is no specific cure.

Doctors who specialize in the diagnosis and treatment of disorders of the ear and who do these surgeries are called otorhinolaryngologists (ear, nose, and throat doctors; also called otolaryngologists). They are medical doctors who have several years of training beyond medical school in surgery and in problems of the ear, nose, and throat.

Perspective and Prospects

The basic anatomy of the ear has been known for some time. Bartolommeo Eustachio (1520–1574), an Italian anatomist, first described the eustachian tube as well as a number of the nerves and muscles involved in the functioning of the ear. An understanding of how the ear functions to discriminate the pitch of sounds, however, was not arrived at until the twentieth century. Georg von Békésy won the Nobel Prize in Physiology or Medicine in 1961 for his work on the acoustics of the ear and how it functions to analyze sounds of varying frequencies (pitch).

Treatment of diseases of the ear has been radically changed by the advent of antibiotics. Older texts describe rupture of the eardrum by middle-ear fluid as a desired outcome of middle-ear infection, one which would ensure that the infection drained and healed, rather than becoming chronic.

Chronic ear infections used to be associated with diseases such as tuberculosis, measles, and syphilis, which themselves became far less common with the widespread use of antibiotics or vaccines. In the past, chronic ear infections were much more likely to result in mastoiditis, or infection of the air spaces of the mastoid bone, requiring surgical removal of the infected portions of the mastoid bone.

Adenoids and tonsils
were frequently removed from patients with recurrent ear infections, as these were thought to be the source of the reinfection. It is now known that these tissues are involved in the formation of immunity to infectious bacteria and viruses. Their removal is not advocated in most circumstances—except, for example, when they are large enough to block the opening of the eustachian tube.

Reconstructive surgery began in the 1950s with the development by Samuel Rosen and others of the operation to free up the calcified stapes bone in cases of otosclerosis. Today, virtually all components of the middle ear can be replaced.

While ear infections used to be much more dangerous, perhaps there is an equal danger today of taking threats to the ears too lightly. Chronic ear infections can still cause permanent hearing loss and may even become life-threatening infections if left untreated. Damage involving the inner ear remains untreatable, as do many cases of tinnitus and loss of balance. Because the largest source of inner ear damage is prolonged exposure to noise, the prevention of damage is far more effective than treatment.

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