Otosclerosis

The term otosclerosis refers to a hardening of the bone surrounding the stapes, one of the three ossicles.

Inside the middle ear are the three smallest bones in the human body. These bones, called the ossicles, are hinged together and rock back and forth in response to sound vibrations from the eardrum. This rocking motion passes the vibrations through the middle ear and into the cochlea (or inner ear) where they are processed and sent along the auditory (hearing) nerve to the brain.

With otosclerosis, the bone surrounding the stapes becomes progressively fixed in the 'oval window', which divides the air in the middle ear from the fluids in the inner ear. The resulting stiffening of the whole ossicular chain inhibits the transmission of sound to the inner ear. When sound is inhibited in this way it is known as a conductive hearing loss.

Otosclerosis is one of the more common causes of deafness among people in their 20s, possibly occurring more frequently in women than men. If left untreated, the hearing loss will become progressively worse.

What causes otosclerosis?

In many cases otosclerosis runs in the family. 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.

It is possible that there are a number of different genetic forms of the condition and that there are other, non-genetic, causes. In genetic cases, otosclerosis is carried by mistakes in the 'autosomal dominant' gene. Children of those affected by genetic otosclerosis will therefore have roughly a one in six chance of developing the condition.

The underlying causes of the condition may not always be the same, but all seem to involve a fault in the normal process of rebuilding bone, which happens continually throughout the body.

Finding the gene or genes responsible for otosclerosis could help identify the chemicals which control the building process and lead to ways of preventing or treating the condition.

What is cochlear otosclerosis?

On rare occasions, the spongy bone growth associated with otosclerosis can spread from the ossicular chain into the cochlea. Here, damage to the tiny 'hair' cells can result in a sensorineural deafness. Cochlear otosclerosis is permanent and cannot be helped by surgery. However some medications may slow down the progress of the condition. Otosclerosis may also spread to the vestibular canals causing episodes of unsteadiness.