Although its most obvious role is in hearing, the auditory system contains organs that mediate both hearing and balance. Problems in the auditory system can range from a simple ear ache to complete deafness and may also involve difficulties with equilibrium. Each type of pathology is the result of damage in a specific part of the auditory system. Common pathologies of the auditory system are ear aches, hearing loss, ringing of the ears, and vestibular disturbances.
Many of us have experienced an auditory pathology called an ear ache, which is a dull pain in the ears. Ear aches are caused by problems in either the outer ear (the ear canal) or the middle ear (on the interior side of the ear drum). Both types of ear aches can usually be cured with antibiotics. Inflammation of the outer ear is called otitis externa and is caused by infection or physical injury of the ear canal (figure 1). Some of the more common sources of otitis externa are bacteria or fungi entering the ear through shower water or swimming pool water. Otitis externa may also result from bacteria infecting sites of physical injury, such as injuries caused by scraping the ear canal with a fingernail or a cotton swab. These infections can be quite painful, but again, treatment with antibiotics is effective.
Infection is also the culprit in middle ear pain (otitis media). The infection is often a complication accompanying upper respiratory disorder. In otitis media, bacteria or viruses infect the middle ear, causing a swelling which blocks a eustachian tube. The eustachian tubes are a pair of thin passageways that connect each middle ear to the back of the throat. They are closed most of the time, but open briefly over a thousand times a day during swallowing to drain the middle ears and to equalize the air pressure on both sides of the ear drum. When the eustachian tube is blocked by swelling during an infection, pus and other fluids cannot drain from the middle ear, and so these fluids accumulate and put painful pressure on the ear drum. A blocked eustachian tube can also cause pain during changes in external air pressure, such as changes that occur when riding in elevators or planes. Otitis media can be diagnosed with a tympanogram, a record of the movements of the ear drum. Normally, the ear drum sways in and out in response to an air puff or sound wave, but if fluid has accumulated on the internal side of the ear drum, these oscillations are dampened. Because the ear drum cannot vibrate normally, sounds seem muffled or faint. With proper treatment and antibiotics, hearing can be restored.
Figure 1. Common Pathologies of the Auditory System
While most cases of middle ear infection are straightforward, there can be complications. In some cases the pressure within the ear is high enough to cause the thin membrane of the ear drum to rupture. The trapped fluids then drain out of the ear and the pain diminishes. The body can usually repair a hole or tear in the ear drum naturally, but sometimes a skin graft (tympanoplasty) is required to close the perforation. Occasionally the body’s natural repair mechanisms can go awry, and as tears in the ear drum heal, the membrane continues to grow into the middle ear, developing into a benign tumor called a cholesteatoma. This cholesteatoma can erode the bones of the middle ear, potentially causing infections of brain tissue. Such complications underscore the importance of early and effective treatment for cholesteatoma. Physicians typically treat the infection first, and then surgically remove the middle ear tumor.
Ear infections are particularly prevalent amongst young children; in fact, ear infections are second only to the common cold in bringing children to the doctor’s office. Children with recurring, or chronic infections may be treated by the insertion of drainage tubes into the ear. For this procedure, a hole is cut in the ear drum and a tiny tube is threaded in. The tube allows drainage of the middle ear and also allows the air pressure to equalize. The tubes stay in the ear for 6 months to 3 years and fall out by themselves. Early treatment of hearing loss in children is very important to ensure proper speech and language development.
In addition to infection, hearing loss might result from head injuries, loud noise, wax buildup, or aging. Physicians group all of these forms of hearing loss into three main categories based on the site of damage; thus, hearing loss can be sensorineural, conductive or cortical.
Conductive hearing loss is a result of damage to or obstruction of the outer or middle ear. Hearing loss caused by middle ear infections and ruptured ear drums fall under this classification. In addition, otosclerosis can also cause conductive hearing loss. Otosclerosis is the overgrowth of the bones of the middle ear, specifically a bone called the stapes, which is the last link in sound conduction to the inner ear. The stapes is in contact with the oval window of the cochlea, and the movement of the stapes against the oval window eventually results in the perception of sound. If the stapes grows too much, it becomes immobilized against the oval window, and hearing loss results. Surgery is required to replace the bone with a man-made prosthetic, and, as with other types of conductive hearing loss, recovery of hearing is typically quite good.
Sensorineural hearing loss, also called “nerve deafness,” is the most common type of hearing loss, and is a gradual loss of hearing resulting from an abnormality in either the inner ear, the auditory nerve, or both. In the inner ear, deafness can result from the loss of the cochlear hair cells that transform the mechanical movement of the basilar membrane into neural signals. Meniere’s disease is another inner ear condition in which excess fluid in the inner ear causes the basilar membrane to stretch and rupture. The rupture allows fluids in the various compartments of the inner ear to mix, resulting in vertigo (a spinning sensation), tinnitus (ringing in the ears) and hearing loss that can last for several months. The basilar membrane can repair itself and hearing may return eventually, but patients with Meniere’s disease may experience recurring ruptures. Changes in diet, exercise and stress reduction are the primary treatments for preventing attacks.
Sensorineural hearing loss is usually permanent. Hearing aids, which amplify sound before sending it into the ear, may compensate for the hearing loss, and cochlear implants may be used in cases of total deafness. A cochlear implant uses a microphone located outside the body to transform sound into electrical signals that are then communicated directly to the auditory nerve. As with most prosthetic devices, hearing is not precisely normal, but significant sound perception is possible.
Some types of damage to the auditory system cause problems beyond hearing loss. Ringing in the ears is an auditory system pathology called tinnitus that often accompanies hearing loss. Researchers believe that tinnitus is caused by damage to the auditory nerve endings in the cochlea. The most common cause of this type of damage is exposure to loud noise, although normal aging and some infections can also trigger tinnitus. While there is no cure for tinnitus, high volume white noise, like static, can be used to mask the disturbing ringing.
Figure 2: Vestibular System
Finally, damage to the auditory system can affect one’s sense of balance and position. The anatomy of the inner ear is involved in maintaining equilibrium as well as in hearing. The vestibular organs in the inner ear include two swellings called the utricle and saccule, and the three semicircular canals (figure 2). Sensory cells within this group of organs respond to the direction and speed of head motion and to head position. Problems with these organs can result in vestibular symptoms including dizziness, nausea and imbalance. Benign Paroxysmal Positional Vertigo (BPPV) is a type of vestibular condition that arises from the free movement of small crystals in the inner ear that are ordinarily attached to the nerve endings. Normal degeneration or exposure to loud sounds can detach the crystals from the nerve endings. These crystals, sometimes called “ear rocks,” can block the semicircular canals when the head is moved, resulting in vestibular symptoms. Treatment is aimed at transferring the crystals to a less sensitive location by moving the head through a specific sequence of positions. Often, infections or abnormalities in the inner ear affect both hearing and equilibrium, and difficulties with both are observed.
In conclusion, abnormalities in auditory system function can cause difficulties in hearing as well as balance. Preventative measures, such as avoiding loud noises, refraining from putting anything in the ear, and promptly treating infections will help to protect auditory function for years to come.