Otosclerosis

Synonyms: OTSC, otospongiosis

This is a genetically mediated metabolic dysplasia, affecting the bony tissue of the otic capsule (the skeletal elements enclosing the inner ear mechanism), and the auditory ossicles – the malleus (hammer), incus (anvil) and stapes (stirrup). Abnormal bone vascularisation and overgrowth occurs, leading to sclerosis and failure of the sound conduction mechanism, due to ankylosis of the stapes foot-plate in the fenestra ovalis of the cochlea. This produces a conductive hearing impairment. In some cases, the disease process may also extend from bony tissue to the cochlea itself, causing a mixed conductive and sensorineural hearing loss.

It is inherited in an autosomal dominant fashion with variable penetrance. There is genetic heterogeneity, with at least 5 loci on chromosomes 15, 16, 6 & 7.^1,2 Measles infection is thought to play a role in phenotypic activation of the genetic susceptibility, and possibly as a pathological agent in the form of subacute chronic infection of the middle ear.³ Some cases of stapes fixation appear not to be associated with measles infection, without the classic histological appearance and molecular diagnostic evidence of viral tissue presence. Certain authors advocate that these cases are due to alternative causes and should be classified as pseudo-otosclerosis.³

The condition is thought to affect up to 10% of the white population in the US, based on post-mortem series, but only appears to cause clinical problems due to stapes fixation in about 1 in 10 of those affected.^4,5 A UK-based study estimated population prevalence of clinically-relevant otosclerosis, diagnosed presumptively, at 2.1%.⁶ The annual incidence is thought to be declining as measles becomes a rarer infection, due to recent childhood immunisation programs against the disease.

Risk factors

• Race - commoner in white and asian populations
• Sex - more commonly diagnosed in women, though not sex-linked and no sex differences found in pathological series⁵
• Age - presents between teen years and middle age (typically 15-35)
• Positive family history
• Drinking non-fluoridated water - controversial, some studies suggest fluoridation of water does not prevent the development of otosclerosis⁷

Symptoms

• The hearing loss may progress at a variable rate and tinnitus tends to worsen as the hearing loss worsens.
• The hearing loss is usually low tone in nature – deep, male voices may be particularly difficult to discriminate.
• Vestibular symptoms are usually absent or mild but may cause mild dizziness in some cases.
• Very occasionally, vestibular involvement can manifest as severe, paroxysmal rotatory nystagmus akin to Ménière's disease.

Signs

• Patients may have low-volume speech.
• Auroscopy is usually normal.
• About 10% show Schwartze's sign – reddish-blue discolouration over promontory and oval window niche, due to vascular hyperaemia of immature abnormal bone.
• Tuning fork tests (Rinne and Weber) reveal conductive pattern deafness in the majority of cases.
• Where the cochlea is involved there may be a mixed conductive/sensory pattern of hearing loss.

• Other middle ear pathology, e.g. chronic suppurative otitis media, chronic serous otitis media (glue ear).
• Damage to ossicles, particularly the long process of incus in chronic infection.
• Congenital stapes fixation which is non-progressive and usually noted in the first decade of life.
• Ménière's disease in cases with severe vestibular symptoms.
• Post-infective tympanosclerosis usually follows chronic infection.
• Paget's disease of bone can cause a clinically indistinguishable syndrome, but will usually have other features of the disease. Ask about change in hat size and bony pain. Histologically Paget's can be differentiated from otosclerosis.
• Osteogenesis imperfecta can have similar effects on the ossicles (and histologically is indistinguishable) but its typical features, such as the presence of blue sclerae or other bony abnormalities, may help to detect the underlying condition.

• Audiometry is the primary investigation of choice. Bone and air conduction must be tested and typically reveals a purely conductive, predominantly low-tone loss.
• If the cochlea is also involved there will be mixed sensorineural/conductive loss. It is difficult to distinguish the sensorineural loss of otosclerosis from other causes of sensorineural hearing loss.
• Audiogram patterns such as Carhart's notch (a specific pattern shown only in otosclerosis) and excessive sensorineural loss (beyond any likely presbycusis) may help to discriminate.
• Tympanometry (Type A 'stiffness' response found) can help to eliminate other possibilities and confirm otosclerosis as the primary cause of the hearing loss.
• Fine-slice CT-scanning may be used to demonstrate new abnormal bone deposition in the temporal bone via the so-called 'halo' sign, which appears to be a useful diagnostic adjunct in this condition.⁸

• Hearing loss can be ameliorated through the correct use of bilateral hearing aids.
• Sodium fluoride 20–120 mg daily has been used as a successful medical therapy, via its effect on bone metabolism, forming fluorapatite complexes. It may be most effective for those with sensorineural loss, but its overall efficacy is uncertain and relatively untested.⁹
• Surgical therapy has advanced significantly over the last few decades and this is now the most effective and most popular form of treatment. Stapedectomy or stapedotomy can be carried out in less than an hour and may be done as a day case. The eardrum is reflected anteriorly and the stapes foot-plate is freed from the fenestra ovalis. After resection of the stapes (or its partial resection in stapedotomy), a graft made from a hand vein, or perichondrium from the ear cartilage, is used to seal and connect the fenestra ovalis to the process of the incus. Up to 95% of patients experience elimination of their conductive hearing loss (defined as < 10 dB air-bone gap on audiometry) with little to choose between the two procedures.⁵ It also increases quality of life of these patients by improving their social life and delaying the need for a hearing aid.¹⁰

• Of otosclerosis:
  • If the condition is not treated, it leads to significant hearing loss of about 50–60 dB but total deafness is uncommon.
• Of stapedectomy/stapedotomy:
  • Stapedectomy/stapedotomy can cause total unilateral sensorineural hearing loss in about 1–2% of cases. The exact cause for this is uncertain and not connected with any particular surgical errors. This postoperative catastrophic hearing loss rarely improves or responds to amplification.
  • Facial nerve injury occurs in < 1% of cases (probably about 1 in 1,000).
  • Perforations of the eardrum may occur, but are usually easy to repair and heal well.
  • Disturbance of taste sensation, due to intraoperative manipulation of the chorda tympani, is a possible complication, but usually transient.

Excellent prognosis with surgical treatment in vast majority of cases. The rate of hearing deterioration with time after stapedotomy did not exceed that due to presbyacusis.¹¹

Measles vaccination as part of MMR program should reduce the disease's prevalence over the next few decades, if it is as intimately involved with its pathogenesis as is currently thought and this has been demonstrated in some countries, for example, Germany.¹²