Diseases and Disorders of the Middle Ear - Middle Ear Otosclerosis

Otosclerosis is a spongy bone-like tissue that grows at the place where the stapes (third middle ear bone) meets the oval window of the cochlea. As this growth grows, it slowly prevents the ossicles (bones of the middle ear) from moving well. Otosclerosis is the most frequent cause of middle ear hearing loss and affects about 10% of the United States population. It is most commonly seen in more women than men often during pregnancy, implying a hormonal cause. Caucasians are more likely to have this disorder than any other race. Only 1% of African Americans have otosclerosis. It is also rare among Orientals and Native Americans.

Image of otosclerosis occurring between the stapes and the oval window of the cochlea

Causes of Otosclerosis

The causes of otosclerosis are not exactly known. However, the most generally accepted theory is that it has a genetic basis, combined with hormonal production abnormalities (also from genetic factors). The gene (or genes) that cause the otosclerosis appears to be a dominant, rather than a recessive gene, but even though dominant it appears to have a characteristic called "reduced penetrance" i.e. it does not always dominate over a normal gene. Why this happens with some dominant genes is not know at this time.

Pathology of Otosclerosis

Otosclerosis can begin in early childhood or adolescence, with the majority of cases occurring after puberty.  One of the first signs is a small growth in front of the oval window, which separates the middle ear from the inner ear. As far as the sufferer is concerned, there is no apparent indication that anything is wrong as the stapes/oval window connection will continue to function and no hearing loss is apparent. The tissue may grow rapidly and become hard. The bone tissue grows over the stapes ossicle attaching it to the oval window. At first, hearing loss occurs in the low frequencies. High frequencies are affected next, followed by the loss of hearing in the middle frequencies. Otosclerosis usually begins in one ear, but the other ear often develops it as well.

There are different categories of otosclerosis. Subclinical Otosclerosis occurs when the tissue does not interfere with the ossicles. A person may have this form for many years and not know it. Another category is Clinical Otosclerosis. This type can be present in the teen years, but not detected until the young adult years. The last category of Histologic Otosclerosis occurs when the tissue is present, but it but it does not grow larger. This rare condition may or may not cause a hearing loss. It is rare for otosclerosis to occur after the age of fifty.

Symptoms of Otosclerosis

Symptoms include tinnitus and hearing loss, often at lower frequencies - deeper sounds - to begin with. You may also find that your hearing is better in noisy surroundings. People with otosclerosis tend to speak quietly.

Treatments for Otosclerosis

In the early stages of otosclerosis, or when the condition is mild, you might not need any treatment. Hearing aids are very useful initially. However, as the tissue builds up on the stapes you will gradually lose your hearing. Sodium fluoride tablets have been shown to help prevent the progression of otosclerosis, but only if the condition has also affected the inner ear.

Surgical procedures for Otosclerosis

At some point, most people usually have an operation - a stapedectomy or stapedotomy - where a tiny piston replaces the stapes so that sound can travel to the inner ear. This operation has a high success rate.

Stapedectomy

This operation aims to improve hearing by replacing the stapes - one of the ossicles - with a piston. The piston helps to restore the movement of the ossicles, so transmitting sound into the inner ear.

Most of the stapes bone is removed, leaving just the portion called the footplate, which sits in contact with the oval window. The oval window is the link between the middle and inner ear. A small hole is then drilled in the footplate and the piston is inserted so that it sits in contact with the oval window. At its other end, the piston is attached to the incus (anvil), the middle of the three ossicles.

About 85% of people find the operation a success and report a good improvement in hearing. However, there is a very small risk - about 1% - that fluid will leak from the inner ear and cause a complete loss of hearing in that ear. There is also a small risk of fluid loss in the years following surgery - for example, if you have a sudden blow to the head or sudden pressure change that causes the piston to penetrate the membrane of the oval window.

Occasionally the piston may move slightly and you will need a second operation to put it back into place. If you have this second operation there is a higher risk - about 5% - that you will lose the hearing completely in the ear that has been operated on.

Chances of Passing On Otosclerosis to a child

Because otosclerosis does have a probable genetic cause, there are rough calculations as who in the succeeding family tree will get the disorder. In genetic propensities through generations (i.e. the probability of children and grandchildren getting otosclerosis) a person may have a 50% chance of inheriting the gene, but their chance of developing otosclerosis is reduced to 20-40% because of the reduced penetrance attribute of the gene(s). What does this mean for one's children and grandchildren? A person who has one parent with otosclerosis has a 25% chance of being affected. If both parents have the disorder, the chance goes up to 50%. This same proportion and probability can be calculated for grandchildren, once the condition is know in the parents (your children). In many families otosclerosis starts after puberty, thus the hypothesis for the genetic and hormonal causes.

© 2004 - 2007 Hearing Central LLC