Otosclerosis is a disorder that leads to spongy bone growth in the middle ear. It usually affects one of the ossicles (three tiny bones in the middle ear), the stapes. The stapes sits on the oval window between the middle ear and the cochlea of the inner ear. Sound waves cause the eardrum to vibrate. Together with the other ossicles, the malleus and the incus, the stapes pass sound vibrations onto the fluids of the inner ear.

Sponge-like bone tissue gradually builds up around the stapes and affects its movement. This results in conductive hearing loss. Should otosclerosis progress to structures in the cochlea, it can cause sensorineural hearing loss. It can also cause tinnitus (ear-ringing).

Otosclerosis can affect one or both ears.

Otosclerosis is believed to be a hereditary disorder. It occurs more frequently in females than in males and commonly develops in young adults.

A specialist will perform hearing tests and, in some cases, order a CT scan to diagnose the disorder. The presence of a so-called Carhart notch on an audiogram (the graph that demonstrates hearing threshold) may influence the diagnosis.

Otosclerosis forms a mechanical blockage that makes it harder for sound waves to travel through the air to reach the cochlea in the inner ear. For that reason, an air conduction test will reveal conductive hearing loss. Unique to otosclerosis patients, there may also be a dip in bone conduction at a frequency of 2000 Hz (what we refer to as the Carhart notch). This is because the stapes can no longer transmit sound vibrations to the oval window.

In most cases, surgery is the most effective treatment. During stapes surgery, called a stapedectomy, a surgeon will remove the stapes (or parts of it) and replace it with a type of prosthesis. Hearing aids may benefit people with mild cases of the disorder or who have a degree of hearing loss after surgery.