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Typically, otosclerosis causes a progressive conductive deafness due to fixation of the stapes at the oval window; the ear drums are normal.
It results from the laying down of vascular spongy bone across the joint between the margin of the stapes footplate and the oval window, thus creating an ankylosis. The foot plate is immobilised and conductive deafness ensues.
Otosclerotic foci may appear at other sites within the labyrinthine capsule. Less commonly, there is a sensorineural deafness. This is associated with masses of new bone around the cochlea and is thought to be due to the release of enzymes from the osteolytic process.
The condition is usually bilateral and symmetrical, but may be unilateral. It is slightly more common in women.