69 Otosclerosis

Otosclerosis is an autosomal dominant disease of incomplete penetrance affecting otic capsule bone. Mature lamellar bone is replaced by less dense woven bone of greater cellularity and vascularity that features large haversian canals, lacunae, canaliculi and marrow spaces. It presents with a conductive hearing loss, usually starting at the age of 30 to 40 years but may initially present from teenage years to old age. The tympanic membrane is normal provided the patient has no prior history of tympanomastoid disease. Informed consent in its management is pivotal. It should involve the patients reflecting on all the treatment options and risks and particularly how a poor surgical outcome may affect their quality of life. The decision to proceed with surgery must be the patients’.

69.1 Pathogenesis

Otosclerotic foci are most commonly located just anterior to the oval window in the region of the fissula ante fenestram. Symptoms occur when these foci fix the stapedial footplate and encroach upon the labyrinth. It is uncertain whether vestibulocochlear symptoms arise directly from this encroachment or from factors released by the otosclerotic plaque or from both. Foci have also been noted in the region of the fossula post fenestram, the semicircular canals, the round window, the base of the styloid process, the petrosquamous suture and in the cochlea. Labyrinthitis ossificans is a rare end-stage finding in severe otosclerosis.

The three main theories of pathogenesis are as follows:

1. It is an expression of a genetic mutation in collagen metabolism which is only phenotypically expressed in bone derived from the otic capsule.

2. It is an expression of humoral autoimmunity to type II collagen.

3. It is an expression of persistent measles virus infection of otic capsule-derived bone. Electron microscopy has demonstrated viral nucleocapsids in otosclerotic bone cells as have immunohistochemical and polymerase chain reaction studies. Anti-measles immunoglobulin G (IgG) has also been found in the perilymph of patients with otosclerosis.

69.2 Prevalence and Incidence

The clinical disease has been estimated to affect 0.3% of the population (the prevalence). Postmortem temporal bone studies have shown that there are otosclerotic foci within the otic capsule bone in 10% of the population but in only about 10% of such cases (i.e., 1% of the population) does the otosclerosis encroach upon the stapes footplate. Therefore, only 30% of such cases present clinically, presumably because some patients have only mild disease and do not seek advice or have just a mild conductive loss when self-presenting to audiology and remain undiagnosed. The incidence of clinical disease (number of new cases per year) is, of course, much lower than 0.3% as the 0.3% with clinical disease may present from teenage years to late middle age. Seventy-five percent of patients have bilateral disease.

Otosclerosis is more common in Caucasians. A female: male ratio of 2:1 has been noted, perhaps because women are more likely to seek advice because pregnancy, menstruation and the menopause may cause the disease to progress rapidly. Women are more likely to have bilateral clinical disease.

69.3 Clinical Features

The main symptoms are as follows:

• Deafness Noticed in most cases before the age of 30. Better hearing in noisy surroundings may be described—paracusis Willisii.

• Tinnitus Present in 75% of patients especially when there is a cochlear element to the deafness.

• Vertigo Mild and usually transient. Symptoms may mimic benign paroxysmal positional vertigo.

Ten percent of ears display Schwartz’s sign, a pink tinge of the tympanic membrane imparted from dilated blood vessels on the mucus membrane of the promontory. Otosclerosis causes no other otoscopy signs. Rinne’s test usually suggests a conductive hearing loss. The Weber disease may be referred to either ear depending on whether disease is unilateral or bilateral, and on the cochlear function.

69.4 Investigations

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