Head and Neck Imaging

Otosclerosis

osteodystrophic disease of the labyrinthine capsule. The osteosclerotic foci may be single or multiple. Histologically the osteosclerotic foci arise in the endochondral layer of the labyrinthine capsule. These foci correspond to spongy irregular bone with numerous blood vessels, osteoblasts and osteoclasts, hence the more appropriate term otospongiosis. These spongy foci tend to recalcify, becoming more dense and relatively avascular and acellular.

There appears to be a hereditary factor in the aetiology. It usually begins in young adults, typically without a previous history of ear disease, as a gradually progressive conductive hearing loss or a mixed conductive and sensorineural hearing loss. It is commonly a symmetrical disease, though patients do not always have symptoms in both ears. Two major categories are distinguished, fenestral and retrofenestral.

Fenestral otosclerosis

This is the most common form of otosclerosis, involving a focus in the labyrinthine capsule just anterior to the oval window. It tends to extend posteriorly, fixing the stapes footplate and sometimes invading and thickening the footplate. The stapes footplate fixation causes conductive hearing loss. CT shows this otosclerotic focus as a lucent area just anterior to the stapes footplate (Fig.1); sometimes footplate thickening can be recognized. When the otosclerotic focus grows, it may obliterate the oval window and bulge into the middle ear cavity. Similar abnormalities are commonly found in the opposite ear, even when asymptomatic. The treatment of fenestral otosclerosis is stapedectomy, followed by insertion of a prosthesis. Complications related to this procedure can be evaluated using CT and MRI (see stapedectomy).

Retrofenestral otosclerosis

In this form, also called cochlear otosclerosis, foci involve the labyrinthine capsule around the cochlea, but they may also be seen around the vestibule and semicircular canals and around the internal auditory canal. Retrofenestral otosclerosis causes sensorineural hearing loss, but the mechanism is poorly understood. CT shows foci of demineralization in the labyrinthine capsule; sometimes the cochlea is completely surrounded by a ring of hypodense bone (Fig.2). A similar image may be seen in osteogenesis imperfecta. Patients with retrofenestral otosclerosis usually have also fenestral disease. As these patients present with sensorineural hearing loss, they may be first referred for MRI to exclude an acoustic schwannoma; MRI shows variable enhancement in the otosclerotic foci, presumably due to pooling of gadolinium in the otospongiotic bone (Fig.3).

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Fig.2

Coronal CT image of right temporal bone. Ring of hypodense bone (arrowheads) surrounding the cochlea; similar findings on left side (not shown). Retrofenestral otosclerosis.
Otosclerosis, Fig.1 (a)
Otosclerosis, Fig.1 (b)
Otosclerosis, Fig.2
Otosclerosis, Fig.3 (a)
Otosclerosis, Fig.3 (b)
Otosclerosis, Fig.3 (c)