Otosclerosis is a common cause of gradual hearing loss in adults. The hearing loss is usually conductive, that is, affecting the ossicles (bones of the middle ear) that conduct sound to the inner ear. Some sensorineural, or nerve loss, may also occur.
Otosclerosis is a common cause of hearing impairment and is hereditary. About 10% of adults have otosclerosis, but in only about 10% of these (or 1% of the adult population) does the otosclerosis involve the stapes (stirrup) bone where it can be detected by its interference with hearing. When otosclerosis is present in other places in the middle ear, it causes no symptoms. The disorder can be passed down from one generation to another, perhaps not manifesting itself for several generations. Since the condition is hereditary, diseases such as scarlet fever, ear infections, measles, and influenza have no relationship to its development.
Normal Ear Function:
The ear is divided into three parts: the external ear, the middle ear, and the inner ear. The external ear collects sound, the middle ear increases the sound pressure and transmits the sound to the inner ear which transforms the sound into nerve impluses. Sound waves normally pass through the ear canal and cause the ear drum to vibrate. Movements of this membrane are transmitted to the inner ear by three small bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup). In the inner ear, the vibrations stimulate hair cells. These hair cells transform the vibrations into electrical impulses that are carried by the hearing nerve to the brain where they are interpreted as sound.
This is a picture of the stapes, the third of the three bones of the
middle ear. The stapes is the smallest bone in the body. It's average
height is 3.3 mm (one-eighth inch).
Type of Hearing Impairment:
The external ear and the middle ear conduct sound in the form of vibrations to the inner ear. Within the inner ear special sensory cells, called hair cells, convert it to signals that can be processed by the brain. If there is some difficulty in the external or middle ear, a conductive hearing impairment occurs. If the trouble lies in the inner ear, a sensori-neural or nerve hearing impairment is the result. When there is difficult in both the middle and inner ear, a mixed or combined impairment exists. Mixed impairments are common in otosclerosis, but the conductive component predominates in most cases.
Hearing Impairment from Otosclerosis:
Had we been able to examine your middle and inner ear under a microscope before your hearing impairment developed, we would have seen minute areas of both softening and hardening of the bone. This process may spread to the stapes, to the inner ear, or to both of these areas.
When otosclerosis spreads to the inner ear, a sensori-neural hearing impairment may result due to interference with the inner ear function. This hearing impairment is called cochlear otosclerosis and once it develops it is permanent. On occasion, the otosclerosis may spread to the balance canals and may cause episodes of unsteadiness.
In about 10% of patients with otosclerosis, the otosclerosis spreads to the stapes bone, the final link in the middle ear chain. The stapes rests in a small groove, called the oval window, which is intimate contact with the inner ear fluids. Anything that interferes with the movement of the stapes produces a conductive hearing loss. The amount of hearing loss is due to the amount of otosclerosis involvement at this location, and can be determined only by careful hearing tests.
Treatment of Otosclerosis:
There is no local treatment to the ear itself or any medication that will improve the hearing in patients who have otosclerosis. There is some evidence that sodium fluoride decreases the rate of hearing loss in patients who are losing hearing at a rate of at least 5 dB/year.
Stapes surgery (stapedectomy or stapedotomy) is recommended for patients with otosclerosis who are appropriate candidates. The operation is performed under local or general anesthesia and can usually be carried out as an outpatient procedure. Overnight hospitalization remains an option if it becomes necessary. Convalescence is usually short. Over 95% of these operations successfully improve or restore the hearing.