Causes and Symptoms The hearing mechanism, one of the most intricate and delicate structures of the human body, consists of three sections: the outer ear, the middle ear, and the inner ear. The outer ear converts sound waves into the mechanical motion of the eardrum (tympanic membrane), and the middle ear transmits this mechanical motion to the inner ear, where it is transformed into nerve impulses sent to the brain.
The outer ear consists of the visible portion, the ear canal, and the eardrum. The middle ear is a small chamber containing three tiny bones—the auditory ossicles, termed malleus (hammer), incus (anvil), and stapes (stirrup)—which transmit the vibrations of the eardrum (attached to the hammer) into the inner ear. The chamber is connected to the back of the throat by the Eustachian tube, which allows equalization with the external air pressure. The inner ear, or cochlea, is a fluid-filled cavity containing the complex structure necessary to convert the mechanical vibrations of the cochlear fluid into nerve pulses. The cochlea, shaped something like a snail’s shell, is divided lengthwise by a slightly flexible partition into upper and lower chambers. The upper chamber begins at the oval window, to which the stirrup is attached. When the oval window is pushed or pulled by the stirrup, vibrations of the eardrum are transformed into cochlear fluid vibrations.
The lower surface of the cochlear partition, the basilar membrane, is set into vibration by the pressure difference between the fluids of the upper and lower ducts. Lying on the basilar membrane is the organ of Corti, containing tens of thousands of hair
cells attached to the nerve transmission lines leading to the brain. When the basilar membrane vibrates, the cilia of these cells are bent, stimulating them to produce electrochemical impulses. These impulses travel along the auditory nerve to the brain, where they are interpreted as sound.
Although well protected against normal environmental exposure, the ear, because of its delicate nature, is subject to various infections and disorders. These disorders, which usually lead to some
hearing loss, can occur in any of the three parts of the ear.
The ear canal can be blocked by a buildup of waxy secretions or by infection. Although
earwax serves the useful purpose of trapping foreign particles that might otherwise be deposited on the eardrum, if the canal becomes clogged with an excess of wax, less sound will reach the eardrum, and hearing will be impaired.
Swimmer’s ear, or
otitis externa, is an inflammation caused by contaminated water that has not been completely drained from the ear canal. A moist condition in a region with little light favors fungal growth. Symptoms of swimmer’s ear include an itchy and tender ear canal and a small amount of foul-smelling drainage. If the canal is allowed to become clogged by the concomitant swelling, hearing will be noticeably impaired.
A perforated eardrum may result from a sharp blow to the side of the head, an infection, the insertion of objects into the ear, or a sudden change in air pressure (such as a nearby explosion). Small perforations are usually self-healing, but larger tears require medical treatment.
Inflammation of the middle ear, acute
otitis media, is one of the most common ear infections, especially among children. Infection usually spreads from the throat to the middle ear through the Eustachian tube. Children are particularly susceptible to this problem because their short Eustachian tubes afford bacteria in the throat easy access to the middle ear. When the middle ear becomes infected, pus begins to accumulate, forcing the eardrum outward. This pressure stretches the auditory ossicles to their limit and tenses the ligaments so that vibration conduction is severely impaired. Untreated, this condition may eventually rupture the eardrum or permanently damage the ossicular chain. Furthermore, the pus from the infection may invade nearby structures, including the facial nerve, the mastoid bones, the inner ear, or even the brain. The most common symptom of otitis is a sudden severe pain and an impairment of hearing resulting from the reduced mobility of the eardrum and the ossicles.
Secretory otitis media is caused by occlusion of the Eustachian tube as a result of conditions such as a head cold, diseased tonsils and adenoids, sinusitis, improper blowing of the nose, or riding in unpressurized airplanes. People with allergic nasal blockage are particularly prone to this condition. The blocked Eustachian tube causes the middle-ear cavity to fill with a pale yellow, noninfected discharge which exerts pressure on the eardrum, causing pain and impairment of hearing. Eventually, the middle-ear cavity is completely filled with fluid instead of air, impeding the movement of the ossicles and causing hearing impairment.
A mild, temporary hearing impairment resulting from airplane flights is termed aero-otitis media. This disorder results when a head cold or allergic reaction does not permit the Eustachian tube to equalize the air pressure in the middle ear with atmospheric pressure when a rapid change in altitude occurs. As the pressure outside the eardrum becomes greater than the pressure within, the membrane is forced inward, while the opening of the tube into the upper part of the throat is closed by the increased pressure. Symptoms are a severe sense of pressure in the ear, pain, and hearing impairment. Although the pressure difference may cause the eardrum to rupture, more often the pain continues until the middle ear fills with fluid or the tube opens to equalize pressure.
Chronic otitis media may result from inadequate drainage of pus during the acute form of this disease or from a permanent eardrum
perforation that allows dust, water, and bacteria easy access to the middle-ear cavity. The main symptoms of this disease are fluids discharging from the outer ear and hearing loss. Perforations of the eardrum result in hearing loss because of the reduced vibrating surface and a buildup of fibrous tissue that further induces conductive losses. In some cases, an infection may heal but still cause hearing loss by immobilizing the ossicles. There are two distinct types of chronic otitis, one relatively harmless and the other quite dangerous. An odorless, stringy discharge from the mucous membrane lining the middle ear characterizes the harmless type. The dangerous type is characterized by a foul-smelling discharge coming from a bone-invading process beneath the mucous lining. If neglected, this process can lead to serious complications, such as meningitis, paralysis of the facial
nerve, or complete sensorineural deafness.
The ossicles may be disrupted by infection or by a jarring blow to the head. Most often, a separation of the linkage occurs at the weakest point, where the anvil joins the stirrup. A partial separation results in a mild hearing loss, while complete separation causes severe hearing impairment.
Disablement of the mechanical linkage of the middle ear may also occur if the stirrup becomes calcified, a condition known as otosclerosis. The normal bone is resorbed and replaced by very irregular, often richly vascularized bone. The increased stiffness of the stirrup produces conductive hearing loss. In extreme cases, the stirrup becomes completely immobile and must be surgically removed. Although the exact cause of this disease is unknown, it seems to be hereditary. About half of the cases occur in families in which one or more relatives have the same condition, and it occurs more frequently in females than in males. There is also some evidence that the condition may be triggered by a lack of fluoride in drinking water and that increasing the intake of fluoride may retard the calcification process.
Tinnitus
is characterized by ringing, hissing, or clicking noises in the ear that seem to come and go spontaneously without any sound stimulus. While tinnitus is not a disease of the ear, it is a common symptom of various ear problems. Possible causes of tinnitus are earwax lodged against the eardrum, a perforated or inflamed eardrum, otosclerosis, high aspirin dosage, or excessive use of the telephone. Tinnitus is most serious when caused by an inner-ear problem or by exposure to very intense sounds, and it often accompanies hearing loss at high frequencies.
Ménière’s disease is caused by production of excess cochlear fluid, which increases the pressure in the cochlea. This condition may be precipitated by allergy, infection, kidney disease, or a number of other causes, including severe stress. The increased pressure is exerted on the walls of the semicircular canals as well as on the cochlear partition. The excess pressure in the semicircular canals (the organs of balance) is interpreted by the brain as a rapid spinning motion, and the victim experiences abrupt attacks of
vertigo and nausea. The excess pressure in the cochlear partition has the same effect as a very loud sound and rapidly destroys hair cells. A single attack causes a noticeable hearing loss and could result in total deafness without prompt treatment.
Of all ear diseases, damage to the hair cells in the cochlea causes the most serious impairment. Cilia may be destroyed by high fevers or from a sudden or prolonged exposure to intensely loud sounds. Problems include destroyed or missing hair cells, hair cells that fire spontaneously, and damaged hair cells that require unusually strong stimuli to excite them. At the present time, there is no means of repairing damaged cilia or of replacing those that have been lost.
Viral nerve deafness is a result of a viral infection in one or both ears. The mumps virus is one of the most common causes of severe nerve damage, with the measles and influenza viruses as secondary causes.
Ototoxic (ear-poisoning) drugs can cause temporary or permanent hearing impairment by damaging auditory nerve tissues, although susceptibility is highly individualistic. A temporary decrease of hearing (in addition to tinnitus) accompanies the ingestion of large quantities of aspirin or quinine. Certain antibiotics, such as those of the mycin family, may also cause permanent damage to the auditory nerves.
Repeated exposure to loud noise (in excess of 90 decibels) will cause a gradual deterioration of hearing by destroying cilia. The extent of damage, however, depends on the loudness and the duration of the sound. Rock bands often exceed 110 decibels; farm machinery averages 100 decibels.
Presbycusis (hearing loss with age) is the inability to hear high-frequency sounds because of the increasing deterioration of the hair cells. By age thirty, a perceptible high-frequency hearing loss is present. This deterioration progresses into old age, often resulting in severe impairment. The problem is accelerated by frequent unprotected exposure to noisy environments. The extent of damage depends on the frequency, intensity, and duration of exposure, as well as on the individual’s predisposition to hearing loss.
Treatment and Therapy The simplest ear problems to treat are a buildup of earwax, swimmer’s ear, and a perforated eardrum. A large accumulation of wax in the ear canal is best removed by having a medical professional flush the ear with a warm solution under pressure. One should never attempt to remove wax plugs with a sharp instrument. A small accumulation of earwax may be softened with a few drops of baby oil left in the ear overnight and then washed out with warm water and a soft rubber ear syringe. Swimmer’s ear can usually be prevented by thoroughly draining the ears after swimming. The disease can be treated with an application of antibiotic ear drops after the ear canal has been thoroughly cleaned. A small perforation of the eardrum will usually heal itself. Larger tears, however, require an operation, tympanoplasty, that grafts a piece of skin over the perforation.
Fortunately, the bacteria that usually cause acute otitis respond quickly to antibiotics. Although antibiotics may relieve the symptoms, complications can arise unless the pus is thoroughly drained. The two-part treatment—draining the fluid from the middle ear and antibiotic therapy—resolves the acute otitis infection within a week. Secretory otitis is cured by finding and removing the cause of the occluded Eustachian tube. The serous fluid is then removed by means of an aspirating needle or by an incision in the eardrum so as to inflate the tube by forcing air through it. In some cases, a tiny polyethylene tube is inserted through the eardrum to aid in reestablishing normal ventilation. If the Eustachian tube remains inadequate, a small plastic grommet may be inserted. The improvement in hearing is often immediate and dramatic. The pain and hearing loss of aero-otitis is usually temporary and disappears of its own accord. If, during or immediately after flight, yawning or swallowing does not allow the Eustachian tube to open and equalize the pressure, medicine or surgical puncture of the eardrum may be required. The harmless form of chronic otitis is treated with applied medications to kill the bacteria and to dry the chronic drainage. The eardrum perforation may then be closed to restore the functioning of the ear and to recover hearing. The more dangerous chronic form of this disease does not respond well to antibacterial agents, but careful x-ray examination allows diagnosis and surgical removal of the bone-eroding cyst.
Ossicular interruption can be surgically treated to restore the conductive link by repositioning the separated bones. This relatively simple operation has a very high success rate. Otosclerosis is treated by operating on the stirrup in one of several ways. The stirrup can be mechanically freed by fracturing the calcified foot plate or by fracturing the foot plate and one of the arms. Although this operation is usually successful, recalcification often occurs. Alternatively, the stirrup can be completely removed and replaced with a prosthesis of wire or silicon, yielding excellent and permanent results.
Since tinnitus has many possible, and often not readily identifiable, causes, few cases are treated successfully. The tinnitus masker has been invented to help sufferers live with this annoyance. The masker, a noise generator similar in appearance to a hearing aid, produces a constant, gentle humming sound that masks the tinnitus.
Ménière’s disease, usually treated with drugs and a restricted diet, may also require surgical correction to relieve the excess pressure in severe cases. If this procedure is unsuccessful, the nerves of the inner ear may be cut. In drastic cases, the entire inner ear may be removed.
Presently there is no cure for damaged hair cells; the only treatment is to use a hearing aid. It is more advantageous to take preventive measures, such as reducing noise at the source, replacing noisy equipment with quieter models, or using ear-protection devices. Recreational exposure to loud music should be severely curtailed, if not completely eliminated.
Perspective and Prospects For many centuries, treatment of the ear was associated with that of the eye. In the nineteenth century, the development of the laryngoscope (to examine the larynx) and the otoscope (to examine the ears) enabled doctors to examine and treat disorders such as croup, sore throat, and draining ears, which eventually led to the control of these diseases. As an offshoot of the medical advances made possible by these technological devices, the connection between the ear and throat became known, and otologists became associated with laryngologists.
The study of ear diseases did not develop scientifically until the early nineteenth century, when Jean-Marc-Gaspard Itard and Prosper Ménière made systematic investigations of ear physiology and disease. In 1853, William R. Wilde of Dublin published the first scientific treatise on ear diseases and treatments, setting the field on a firm scientific foundation. Meanwhile, the scientific investigation of the diseased larynx was aided by the laryngoscope, invented in 1855 by Manuel Garcia, a Spanish singing teacher who used his invention as a teaching aid. During the late nineteenth century, this instrument was adopted for detailed studies of larynx pathology by Ludwig Türck and Jan Czermak, who also adapted this instrument to investigate the nasal cavity, which established the link between laryngology and rhinology. Friedrich Voltolini, one of Czermak’s assistants, further modified the instrument so that it could be used in conjunction with the otoscope. In 1921, Carl Nylen pioneered the use of a high-powered binocular microscope to perform ear surgery. The operating microscope opened the way for delicate
operations on the tiny bones of the middle ear. With the founding of the American Board of Otology in 1924, otology (later otolaryngology) became the second medical specialty to be formally established in North America.
Prior to World War II, the leading cause of deafness was the various forms of ear infection. Advances in technology and medicine have now brought ear infections under control. Today the leading type of hearing loss in industrialized countries is conductive loss, which occurs in those who are genetically predisposed to such loss and who have had lifetime exposure to noise and excessively loud sounds. In the future, protective devices and reasonable precautions against extensive exposure to loud sounds should reduce the incidence of hearing loss to even lower levels.
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