Information on Hearing Loss - Otosclerosis

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. The diagram provides a representation of the parts of the ear to help understand what is affected by Otosclerosis.

(1) Ear canal, leading out to the ear.
(2) Ear drum (tympanic membrane). Sound causes it to move like the head of a drum.
(3), (4), and (5) The malleus (hammer), incus (anvil) and stapes (stirrup), which are ossicles, or bones of the middle ear. The eardrum causes them to move, and in a chain reaction they move the oval window. Problems in the middle ear cause conductive hearing loss.
(6) The oval window, a drum-like membrane separating the middle ear from the inner ear. It causes fluid in the inner ear to move in the cochlea.
(7) The cochlea, which changes the movement of the fluid to nerve impulses for the auditory nerve which goes to the brain. Problems in the cochlea or along the nerve cause sensorineural (nerve) hearing loss.

The conductive hearing loss is caused by the growth of a spongy bone-like tissue that prevents the ossicles from moving well. Because of the spongy nature of this tissue, otosclerosis has also been known as "otospongiosis". Middle ear surgery can restore the conductive component of the hearing loss in many people, but it will not eliminate the sensorineural loss. Hearing aids can also be beneficial in cases when surgery is not recommended or when an individual does not wish to accept the risks of surgery. The cause of otosclerosis is not yet known, but researchers have some good clues which hopefully will provide a way to prevent the loss of hearing.

Progression of the Hearing Loss

The first sign of otosclerosis is a small growth of spongy bone tissue in the middle ear, often in front of the oval window which separates the middle ear from the inner ear. This may start as early as late childhood or adolescence. At first this tissue has many small blood vessels in it and it may grow rapidly. Later, it may become hard, or sclerotic. Conductive hearing loss occurs if this bone tissue grows around the ossicles and keeps them from moving. Often, the bone tissue in front of the oval window grows over the footplate of the stapes (stirrup) ossicle, attaching and fixing it to the oval window area. If the otosclerotic tissue does not interfere with the ossicles, it is called "subclinical", and a person may have this for years and not know it. The term "histologic otosclerosis" means that the otosclerotic tissue is present, whether or not it causes hearing loss. "Clinical" otosclerosis may be evident in the teen years, but usually is noticed in young adult years, and it is rare for it to start after age 50.

Hearing loss generally occurs in the low frequencies first, and is only conductive. The high frequencies are usually affected next, followed by hearing loss in the middle frequencies. The hearing loss may progress rapidly in some people, but in others it may stay the same for a number of years, and then progressively get worse. It may also start in one ear first, but often the other ear develops otosclerosis as well. A woman may find that it gets worse with pregnancy which may be due to hormones. The ability to understand speech will be affected as the hearing loss progresses, but it may not be as bad as the degree of loss would suggest.

The maximum amount of hearing loss that can be caused by the middle ear conductive problems alone is 50-60 dB, that is, in the moderate range. Most people also have ringing in their ears (tinnitus), and some people have vestibular problems such as dizziness or poor balance. Both of these problems may disappear with age.

As the conductive hearing loss develops, a sensorineural hearing loss may also begin, especially in the high frequencies. The term "cochlear otosclerosis" has been used to refer to this sensorineural hearing loss. It may be due to abnormal blood flow or enzymes produced by the otosclerostic process in the middle ear rather than to the growth of otosclerotic tissue in the inner ear. It is very rare for otosclerosis to cause sensorineural hearing loss without conductive loss. The addition of a nerve loss to the conductive loss can cause the overall mixed hearing loss to progress down to the severe and profound ranges as the person reaches their 60s and 70s.

How Many People Have Otosclerosis?

The frequency of otosclerosis depends on age, race, and sex, and whether one is referring to "histologic" or "clinical" otosclerosis. The number of people with clinical otosclerosis will increase with age; caucasians are more likely to have otosclerosis than people of other races, and women are more likely to develop hearing loss than men.

Between the ages of 30 and 50, some studies have shown that about 10% to as high as 18% of all white women and 7-9% of all white men have histologic signs of otosclerosis. Less than 10% of these people will actually seek help for hearing loss (that is, about 1-2% of all white women and less than 1% of white men). It is often stated that twice as many women as men develop hearing loss from otosclerosis, but other studies show less difference between men and women. In Blacks, only about 1% of people show histologic otosclerosis, so the rate of clinical problems is much lower than in whites. Oriental people have about half as many cases of otosclerosis as whites, and it is also very rare in Native Americans.

What is the Cause of Otosclerosis?

Otosclerosis runs in families in many cases, but the genetic cause is not clear. It is very possible that there are a number of different genetic types of otosclerosis, as well as other causes that are not genetic. The most common theory has been that there is a dominant gene that can cause otosclerosis, but that it is not completely penetrant.

It may be helpful here to briefly review dominant inheritance. All of our genes come in pairs, with one from each pair inherited from the father and one from the mother. One gene from each pair is passed on to a child. A dominant gene is one that can cause otosclerosis even if the other gene in the pair is for hearing; it can "dominate" over the other hearing gene. One would expect, then, that the child of a person with otosclerosis would have a 50% chance of developing it. However, this is not the case. The gene appears to have "reduced penetrance", meaning that it does not always dominate over the normal gene. A person may have a 50% chance of inheriting the gene, but their actual chance of developing otosclerosis if they do get the gene may be much less, on the order of 20 to 40%.

People in the same family who have otosclerosis usually have similar symptoms; that is, it will start at about the same age and progress at about the same rate, but these features can be quite different when different families are compared. This suggests the families may have different genes. Some of these may be dominant, with different degrees of penetrance, but others may be recessive. (In recessive inheritance, both genes in the pair must code for otosclerosis for the person to develop it, and the recurrence risks are less than for dominant inheritance.) This makes it very difficult to predict the exact risk a person may have of developing otosclerosis. On average, it has been said that a person who has one parent with otosclerosis has a 25% chance of being affected, and if both parents had otosclerosis the risk goes up to 50%.

It is also difficult to find the underlying cause of otosclerosis, since it may not be the same in everyone. Bone is continually being broken down and rebuilt in a process called remodeling. It appears that the rebuilding process is faulty in otosclerosis, so that the new bone is not assembled correctly and the spongy, overgrown tissue results. This could be due to genetic defects in the enzymes; the chemicals that direct the remodeling process. If this is true and researchers can learn which enzymes are involved, it may be possible to prevent the abnormal growth of tissue.

Finding the gene or genes that cause otosclerosis may also help in identifying the process that is responsible for the disease and developing a way to prevent it. Eliminating the gene itself would be more difficult, and efforts in that direction would probably be targeted at the gene in the cells of the ear rather than in preventing the transmission of the gene from generation to generation. Geneticists are quite reluctant to alter the genetic material as it is passed on, since the entire function of the gene in other parts of the body and in other environmental conditions may not be known. The extinction of a gene in future generations may hold unknown risks.

Selected Bibliography

Gordon, M.A. (1989) The genetics of Otosclerosis: a review. American Journal of Otolarngology 10(6):426-438.

Gristwood, R.E. (1988) Otosclerosis (Otospongiosis): General Considerations. In Otologic Medicine & Surgery, P.W. Alberti, R.J. Rubin, Eds. New York: Churchill Livingstone, pp. 911-941.

Houck J.R. and Harker, L.A. (1986) Otosclerosis. In Otolaryngology - Head and Neck Surgery, Volume 4, Ear and Skull Base. C.W. Cummings, J.M. Fredickson, L.A. Harker, C.J. Krause, D.E. Schuller, Eds. St. Louis: C.V. Mosby.

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Date Originally Created: Spring of 1992

The information presented here first appeared in publications of the Boys Town National Research Register for Hereditary Hearing Loss, the National Institute on Deafness and Other Communication Disorders (NIDCD), Hereditary Hearing Impairment Resource Registry (HHIRR), or the Boys Town Research Registry for Hereditary Hearing Loss.

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