Cochlear and fenestral otosclerosis.

In the early stage of otoscierosis (otospongiosis), the endochondral bone is replaced by foci of spongy new bone. In its late stage, the decalcified foci recalcify into a dense ossific plaque.

Fenestral otosclerosis causes a progressive conductive hearing loss, although findings on otoscopic examination will be normal; two-thirds of such patients have tinnitus during the early course of their disease. Cochlear otosclerosis causes a progressive sensorineural hearing loss. When both types exist simultaneously, as happened in the case illustrated here (figure), a mixed pattern of hearing loss can be observed.

Fenestral otoscierosis is the more common type, and it is thought to begin in the fissula ante fenestram (anterior oval window margin).' [1] In the lytic phase, the oval window can appear to be too wide on computed tomography (CT) as a result of the osteoclastic resorption of its margins (figure).

Although cochlear otoscierosis is much less common, when it is present it is invariably associated with fenestral otosclerosis. CT will show a focal lucency of the otic capsule, giving the cochlea the appearance of a double ring on axial CT. [2] On contrast-enhanced T1-weighted magnetic resonance imaging, areas of punctate enhancement in the otic capsule might be seen.

References

(1). Harnsberger HR. The temporal bone: External, middle, and inner ear segments. In: Harnsberger HR. Handbook of Head and Neck Imaging. 2nd ed. St. Louis: Mosby, 1995:426-58.

(2.) Valvassori GE. Otosclerosis and bone dystrophies. In: Valvassori GE, Mafee MF, Carter BL, eds. Imaging of the Head and Neck. New York: Thieme Medical publishers, 1995:143-56.