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Otosclerosis

Otosclerosis (or more appropriately known as otospongiosis) is an idiopathic slowly progressive primary disorder of the bony labyrinth. It is one of the leading causes of deafness in adults. 

Epidemiology

Typically, patients present during the 4th and 5th decades. However, because the condition tends to have symptoms that gradually worsen, it is often difficult to precisely determine onset 6. Presentation during childhood is uncommon. 

A female predilection is present with a F:M ratio of ~ 2:1. Caucasians are more frequently affected than other racial groups 6-7. In up to 50% of cases, a familial predisposition can be identified 7.

Histologic prevalence of otosclerosis has been reported between 3.4-10% in unselected caucasian temporal bones 7.

Clinical presentation

Otosclerosis commonly presents with a hearing loss which has a conductive as well as a sensorineural component and is frequently bilateral 3.

Clinically, the disease is characterised by periods of remission. These may be long, with occasional flares which can result in rapid deterioration. Usually, there is minimal or no findings at otoscopy, except in severe cases where cochlear involvement can result in hyperaemia of the cochlear promontory (Schwarze sign) 1-3. Hearing loss may be exacerbated by pregnancy 6.

Pathology

The dense, ivory-like endochondral bone is replaced by more spongy, highly vascular irregular foci of haversian bone tissue.

Sub types

The two recognised sub types are :

  • fenestral (stapedial): ~ 80 %
    • involves oval window
    • the most common lesion of this type affects the bone just anterior to the oval window, and can extend to involve the foot plate of the stapes, resulting in thickening and fixation
    • hearing loss is often conductive 
  • retro-fenestral (cochlear): ~ 20 %
    • cochlear involvement with demineralization of the cochlear capsule
    • sensorineural hearing loss (SNHL), although the mechanism by which this occurs is uncertain. 

NB : the prefix 'retro' does not mean 'posterior' but rather 'behind', as in 'deep to', the medial wall of the middle ear from the perspective of otoscopy.

Radiographic features

The term otosclerosis is somewhat of a misnomer. Much of the clinical course is characterised by lucent rather than sclerotic bony changes.

CT

Thin slice CT through the temporal bones is the imaging modality of choice. Axial and coronal (or preferably 20 degree coronal) thin-slice bone algorithm non-contrast scans are needed to adequately demonstrate the inner ear structures and subtle early changes 1-3.

The findings depend on the type of otosclerosis present.

Fenestral

Typical findings depend on the phase of the disease. During active phases, there is bone loss or demineralization just anterior to the oval window, involving a small cleft known as the fissula ante fenestram 5. During remission, the region becomes sclerotic. In severe cases, the oval window is completely filled in by a dense bony plate (with complete fixation of the stapes).

Retro-fenestral

Foci of lucency can be seen disrupting the normal sharply demarcated homogeneously dense (although not homogeneously thick) border of the cochlear otic capsule. It may be focal or may encircle the whole cochlea 1-3.

MRI

MRI has a limited role. In retro-fenestral otosclerosis, pericochlear and perilabyrinthine soft tissue intensity signal on T1 with contrast enhancement may be demonstrated. Increased T2 signal may also be present 8

Treatment and prognosis

A stapedectomy with stapes prosthesis is the treatment of choice for fenestral otosclerosis 4. In the first part of the 20th century, a procedure referred to as fenestration was performed, in which a neo-window was created in the lateral semicircular canal or vestibule to allow passage of sound waves into the inner ear, bypassing the ossicular chain. These changes should not be mistaken with labyrinthine fistulae or middle and inner ear malformations 4.

Etymology

It was first described by Valsalva (of the maneuver fame) in 1735.

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