[Editor's note: This article is from 2007. Some newer treatments and current statistics are not included here. See further information on BrainWeb]
sections include: the ear and its common problems, disorders of the inner ear and auditory nerve, disorders of the central auditory system, diagnosis and treatment
Hearing loss is one of the most common neural impairments. Approximately 1 in 3,000 newborns has a profound hearing loss, and perhaps ten times that number have less than normal hearing. The prevalence of hearing loss increases with age, so that approximately 1 in 3 people over the age of 65 has enough hearing loss to interfere with daily communication. In the United States as many as 28 million individuals have a hearing impairment.
Hearing loss can result from a number of causes: genetic, congenital, infectious, traumatic, toxic, idiopathic (of unknown origin), and immune-mediated. Disorders can occur anywhere along the auditory pathway, so it is useful to examine the anatomy of hearing to better understand the myriad disorders that may affect this sense.
The Ear and Its Common Problems
Your peripheral auditory system includes the external, middle, and inner ear. The external ear consists of the auricle and the external auditory canal, which is separated from the middle ear by the tympanic membrane, or eardrum. The middle ear is an air-filled space connected to the nasal passage by the eustachian tube. It contains three bones, or ossicles—the malleus, incus, and stapes—that move as sound strikes the tympanic membrane. The inner ear includes the cochlea, the auditory nerve, and the vestibular (balance) system.
Sound is a mechanical energy, meaning it depends on something moving—in most cases, the air. That energy is transmitted to your inner ear through the ear canal and middle ear components. In the inner ear there are specialized sensory cells, called hair cells because they have structures that resemble tufts of hair, that transform the mechanical energy of sound into an electrical signal that is then transmitted through the auditory nerve to the central auditory system. The central auditory pathways ascend through the brain stem to the temporal lobes.
The inner ear is organized according to tone, or “tonotopically.” Sound energy of high pitch is analyzed and transformed into electrical signals at the base of the cochlea, nearest the middle ear, whereas sound of low pitch is analyzed at the top of the cochlea, furthest away from the middle ear.
There are only 15,000 hair cells to perform this analysis, and they pass the information to the auditory nerve, which contains only 30,000 individual nerves. Unlike other organs, such as the skin and the liver, the inner ear does not have the capacity to regenerate. Thus if your hair cells or neurons are damaged, you may lose hearing permanently.
We can divide hearing loss into two subgroups.
- Conductive hearing loss: the normal mechanisms that transmit sound through the external and middle ears become damaged.
- Sensorineural hearing loss (also called nerve deafness): the site of damage is in the inner ear, auditory nerve, or central auditory system. The most common causes of hearing loss occur in the inner ear.
A variety of disorders may produce a mechanical or conductive hearing loss. Examples include obstructions of the ear canal, perforations of the eardrum and other injuries, fluid filling the middle ear (serous otitis), chronic infection of the middle ear space, and otosclerosis, a bone disorder that causes the stapes to become fixed. These problems are relatively easy for a doctor to diagnose. In many cases, they can be remedied by cleaning out the ear, giving a person time to heal (while watching any infection carefully to be sure it does not spread to other important areas), or in some cases, performing surgery. If infections have destroyed an ossicle, a surgeon can sometimes create a new one. Conductive hearing loss is not a neurological problem, however, so it is largely beyond the scope of this book. The rest of this section will discuss conditions involving the inner ear, auditory nerve, and central auditory system.
Disorders of the Inner Ear and Auditory Nerve
A number of different diseases may produce either a sudden or progressive deafness in one or both of a person’s inner ears. A leading cause Disorders of the Inner Ear and Auditory Nerveof this form of hearing loss is certainly genetic. It is estimated that more than 50 percent of congenital or early-onset sensorineural loss has a genetic basis, and that at least one third of hearing loss that occurs later in life also has a genetic basis. Given that hearing is a complicated process, we should not be surprised that many genes are related to hearing, and thus to possible hearing loss. Researchers have identified a number of these genes, some of which have been linked to problems with the number and efficiency of hair cells.
Many drugs have toxic potential for the inner ear. The best known are generally administered intravenously and include aminoglycoside antibiotics such as gentamicin or kanamycin. However, diuretics, large quantities of aspirin, a variety of antitumor agents, and even such commonly prescribed oral antibiotics as erythromycin may occasionally produce a hearing loss. In most cases, the cause is damage to the hair cells of the inner ear.
Another possible cause of hearing loss is an acoustic neuroma: a benign, noncancerous tumor that generally develops on or near the auditory nerve. Symptoms vary with the size and location of the tumor, but include hearing loss and tinnitus—a ringing or buzzing sensation in the inner ear. Acoustic neuroma is caused by a somatic mutation in the “merlin” gene, located on chromosome 22; it may also be found bilaterally in neurofibromatosis 2. Acoustic neuromas are relatively uncommon overall, but they are among the most common tumors of the head, affecting approximately 1 out of 100 people. Surgical removal of the tumor is the only treatment that will completely eliminate the acoustic neuroma, though radiation therapy can reduce its size. Early intervention may help people retain most of their hearing.
Immune disorders may result in systemic symptoms, including hearing loss. Examples of these include Cogan’s syndrome, syphilis, lupus, polyarteritis nodosa, Wegener’s granulomatosis, and giant cell arteritis. Occasionally a disordered immune process may affect the inner ear only, producing a rapid hearing loss in one or, more commonly, both ears. Although some blood tests may indicate an immune disorder, a trial dosage of drugs that suppress the immune system temporarily, such as prednisone, may still be the best method to both identify and treat an innerear immune disorder. If an immune disorder of the inner ear is treated early enough, the sensorineural loss may be reversed or further loss prevented.
Circulatory difficulties are a rare cause of hearing loss, although bleeding into the inner ear may accompany blood diseases such as leukemia or may follow certain forms ofstroke. Transient hearing loss has also been reported in a basilar migraine.
A wide variety of other diseases of the inner ear with no specific known causes may produce hearing loss. “Sudden deafness” may occur in one or both ears without any obvious systemic symptoms or any evidence of inflammation of the middle ear. In these cases, doctors often suspect a virus. Ménière’s disease is characterized by fluctuating sensorineural hearing loss, vertigo, and tinnitus. Although its exact cause is unknown, the disease is thought to stem from a disorder of inner-ear fluids.
One of the most common causes of hearing loss in middle-aged to older people is called presbycusis, or the ordinary hearing loss of aging. This is a progressive, generally symmetrical hearing loss, usually worse in the high frequencies. It occurs with increasing age and probably has several causes: genetic, toxic, and traumatic.
Disorders of the Central Auditory System
Because a relatively small number of cells in the peripheral auditory system are critical for hearing, most cases of hearing loss for which we can name a cause start there. However, the central auditory system can have problems, too. One example is multiple sclerosis (MS). MS produces nerve lesions throughout the central nervous system. Those lesions may affect auditory thresholds (how loud a sound must be before someone can hear it) or more subtle attributes of hearing, such as sound localization and speech understanding. However, since auditory information ascends the brain stem on both sides, only lesions that affect both sides of the brain stem will produce a hearing loss.
Brain stem tumors (tumors, tumors of childhood) and strokes (ischemic, hemorrhagic) may also produce hearing loss in the central auditory system.
Diagnosis and Treatment
Many people suspect they or others are losing some of their hearing when it becomes difficult to converse, particularly in a noisy environment. Another indication is when a person must turn up the TV or radio to a level that is uncomfortable for others with normal hearing. Tinnitus, the sensation of ringing or buzzing in the inner ear, is a symptom that commonly accompanies many varieties of hearing loss. And sudden hearing loss is, of course, a cause of concern and should immediately send a person to his or her doctor.
Otolaryngologists (ear, nose, and throat doctors) and audiologists (individuals trained in hearing measurement and rehabilitation) are the medical professionals most likely to evaluate a person for hearing loss. The first step is to examine the ear to rule out such simple causes as cerumen (wax) or foreign bodies in the ear canal, perforations of the drum, or fluid in the middle ear. Then the specialist performs an audiogram, testing hearing at various frequencies, evaluating the function of the middle and inner ear, and testing the ability to hear single-syllable words (speech discrimination).
In addition to hearing tests, blood tests and imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) may be useful in diagnosing a specific cause of hearing loss. As discussed earlier, surgery can fix some problems that cause hearing loss. Changing medication may remedy others. Many infections and immune diseases can be slowed, stopped, or even reversed. In other cases, however, a person must learn to live with less hearing and, in some cases, the prospect that the problem will become progressively worse. Most patients with sensorineural hearing loss benefit to some degree from conventional hearing aids, which work by amplifying sounds. Physicians can also provide referrals to services that help the hearing-impaired.
Patients with profound sensorineural loss may regain some measure of hearing by using cochlear implants, which bypass the damaged inner ear. In essence, these tiny devices behave like the inner ear, converting mechanical energy into electrical impulses transmitted to the auditory nerve. They are not yet as sensitive to subtle differences among sounds as the army of 15,000 hair cells—they do not restore a person’s hearing. However, they can help some people hear enough to converse in speech without lipreading, and many others to do so while relying on both sound and lipreading.
Since as much as 50 percent of progressive hearing loss may have a genetic basis, we will benefit greatly in the future from better understanding the genetic causes of and predispositions to hearing loss. We will be better able to diagnose individual cases and to predict hearing loss, and even to prevent or reverse loss by correcting genetic errors.
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