Otosclerosis

Otosclerosis (also 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. 

Terminology

The term otosclerosis is somewhat of a misnomer. Much of the clinical course is characterised by lucent rather than sclerotic bony changes and hence why it is also known as otospongiosis, a term preferred by some head & neck radiologists. 

Epidemiology

Typically, patients present during their 4^th and 5^th 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 most commonly presents with hearing loss, most often conductive, but can also be sensorineural or mixed, and is frequently bilateral ^3,10,9.

Clinically, the disease is characterised by periods of remission. These may be long, with occasional flare-ups 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 pathophysiology of otosclerosis is multifactorial and incompletely understood, with genetic, viral, inflammatory, and autoimmune components ^10,119,10.

Three phases are described: early (otospongiosis), transitional, and final (otosclerosis). In the early phase, lesions consist predominantly of histiocytes, osteoblasts and osteocytes, the latter being the most active cell group. Bone around pre-existing blood vessels is absorbed, creating a better microcirculation. Eventually, osteoblasts become more involved, resulting in the formation of irregular foci of new spongy bone ¹⁰⁹. This new bone appears densely blue on haematoxylin and eosinophil staining and is known as the blue mantles of Manasse.

Subtypes

The two recognised subtypes are:

• fenestral (stapedial): ~80%
  • involves anterior oval window
  • most often at the fissula ante fenestram and the footplate of the stapes
  • hearing loss is often conductive, due to stapes thickening and fixation
• retrofenestral (cochlear): ~20%
  • cochlear involvement with demineralization of the cochlear capsule
  • hearing loss is often sensorineural, but 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

CT

Thin-slice CT through the temporal bones is the imaging modality of choice. Axial and coronal (or preferably 20 degrees 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).

Retrofenestral

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.

CT grading system (Symons and Fanning)⁹:

Various authors have used CT grading systems of otosclerosis in their studies. In 2009, a Lee et al ⁹³ article assured that the Symons and Fanning CT grading system yields excellent interobserver and intraobserver agreement:

• grade 1
  • solely fenestral, either spongiotic or sclerotic lesions, evident as a thickened stapes footplate, and/or decalcified, narrowed or enlarged round or oval windows
• grade 2
  • patchy localized cochlear disease (with or without fenestral involvement)
    • grade 2A: basal cochlear turn involvement
    • grade 2B: middle / apical turns involvement
    • grade 2C: both the basal turn and the middle / apical turns involvement
• grade 3
  • diffuse confluent cochlear involvement of the otic capsule (with or without fenestral involvement)

MRI

MRI has a limited role. In retrofenestral 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 20^th 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.

History and etymology

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