Otosclerosis

Synonym: otospongiosis

Otosclerosis (OTSC) is the single most common cause of hearing impairment. It 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 otic capsular bone is unusual in that, following early development, it does not undergo extensive remodelling. In OTSC there is a pathological increased bony turnover leading to sclerosis and failure of the sound conduction mechanism, due to ankylosis of the stapes footplate in the fenestra ovalis of the cochlea. This produces a slowly progressive 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.

OTSC is inherited in an autosomal dominant fashion with variable penetrance¹. The interplay of genes with environmental factors is thought to be critical in the phenotypic activation of genetic susceptibility². There is considerable genetic heterogeneity, with at least seven loci identified via linkage studies, as well as rare monogenic forms³.

Environmental factors implicated in the aetiology of otosclerosis include oestrogens, fluoride and viral infections such as measles. Measles virus RNA has been found in the footplate of OTSC specimens but there are other cases of stapes fixation that appear not to be associated with measles infection⁴. It is likely that a number of different pathways underlie the development of OTSC, with different genetic and environmental risk factors contributing to a similar final disease outcome².

Epidemiology

Histological OTSC is present in around 2.5% of people⁵. However, the clinical manifestations are seen in only about 0.3%. The annual incidence is thought to be declining as measles becomes a rarer infection, due to childhood immunisation programmes against the disease. In Germany, decrease in hospital treatment for OTSC has been observed since the introduction of measles vaccination⁶⁷.

Risk factors

• Race - it is more common in white and Asian populations.

• Sex - it is more commonly diagnosed in women, although not sex-linked and there are no sex differences found in pathological series.

• Age - it usually presents between teen years and middle age (typically between the ages of 15 and 35 years). However, the average age for having surgery is rising; it is thought that the childhood measles vaccination programme is altering the epidemiology of the condition.

• Positive family history - there is approximately a 1 in 4 risk if one parent is affected and a 1 in 2 risk if both parents are affected.

• Drinking non-fluoridated water - this is controversial². Some studies suggest fluoridation of water does not prevent the development of OTSC.

Presentation

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 but may cause mild dizziness in some cases.

• Very occasionally, vestibular involvement can manifest as severe, paroxysmal rotatory nystagmus akin to Ménière's disease⁸.

Signs

• Patients may have low-volume speech (enhanced bone conduction leads to the perception of their own speech as 'loud').

• Auroscopy is usually normal.

• About 10% show the Schwartze sign - reddish-blue discolouration over promontory and oval window niche, due to vascular hyperaemia of immature abnormal bone⁹.

• Tuning fork tests (Rinne's and Weber's tests) 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.

Differential diagnosis

• Other middle ear pathology - eg, 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.

• Postinfective 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 disease of bone can be differentiated from OTSC.

• 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.

Investigations

• Audiometry is the primary investigation of choice. Bone and air conduction must be tested and they typically reveal 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 OTSC from other causes of sensorineural hearing loss.

• Audiogram patterns such as Carhart's notch (a drop in bone conduction thresholds of 20-30 dB, most pronounced at 2000 Hz) and excessive sensorineural loss (beyond any likely presbycusis) may help to discriminate.

• Tympanometry may show a stiffness curve, indicating low compliance of the tympanic membrane and ossicular chain.

• CT scanning is a useful diagnostic investigation. It can reveal a variety of differential diagnoses and also improves targeting or surgical procedures¹⁰.

• 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 condition¹¹.

Management

Medical¹²

• Hearing loss can be ameliorated through the correct use of bilateral hearing aids.

• Sodium fluoride 20-120 mg daily has been used as a medical therapy, via its effect on bone metabolism, forming fluorapatite complexes. Evidence is of low quality only¹³¹⁴.

• Bisphosphonate treatment has been used as an alternative in those unable to tolerate fluoride therapy. Studies show promising results for this treatment¹⁵.

Surgical¹⁶

Surgical therapy has advanced significantly over the period of a few decades and this is now an effective and popular form of treatment. Stapedectomy (extraction of the footplate) or stapedotomy (a small hole made in the stapes footplate) aims to improve the circulation of fluid within the cochlear canal. They are short procedures that can be done as day cases. The use of minimally invasive techniques, sometimes involving lasers, has been developed. 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. It also increases quality of life of these patients by improving their social life and delaying the need for a hearing aid¹⁷. Benefits are long-lasting¹⁸. However, surgery may not completely obviate the need to use a hearing aid¹⁹.

Revision surgery (where there has been previous surgical failure or with progressive disease) is much less successful.

Complications

OTSC complications

If the condition is not treated, it leads to significant hearing loss of about 50-60 dB but total deafness is uncommon.

Stapedectomy/stapedotomy complications²⁰

• 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. Unfortunately, this rarely improves or responds to amplification.

• Facial nerve injury occurs in about 1 in 1,000.

• Perforations of the eardrum may occur but are usually easy to repair and they heal well.

• Disturbance of taste sensation, due to intraoperative manipulation of the chorda tympani, is a possible complication; however, it is usually transient.

• For a minority, tinnitus may arise de novo, or worsen.

Prognosis

OTSC treated surgically has a good prognosis in the vast majority of cases. The rate of hearing deterioration with time after stapedotomy does not exceed that due to presbyacusis²¹.