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TimoHirvonen

Otosclerosis

Otosclerosis is a primary focal disease affecting the labyrinth capsule of the inner ear and the auditory ossicles. It has no association with known skeletal diseases with the exception of osteogenesis imperfecta.
In otosclerosis, the patient has a conductive hearing impairment and an intact, pale eardrum. There is no other obvious aetiological cause (e.g. trauma) in the patient history.

The prevalence of clinical otosclerosis is about 0.3% and, in addition, 10% of the population may have foci of otosclerosis in the labyrinth capsule without clinical disease (estimation based on autopsy findings).
The age of onset is usually between 30 and 40 years. Earlier onset is rare.
The disease is more common in women than in men (2:1).
Genetic factors have an influence on the risk of developing the disease. Hereditary background is apparent in about half of the cases. In the sister or daughter of a patient with otosclerosis the average risk is 20%, while in the brother or son the risk is 10%.

The most important symptom is a gradually progressing hearing impairment that rarely affects both ears at once. The better ear usually becomes affected years after the first one.
Tinnitus occurs in the majority of patients. The tinnitus is typically low-pitched. Approximately 10% of patients may also suffer from balance disturbances or vertigo.

An intact, pale eardrum with normal mobility is observed at otoscopy. Sequelae of otitis media from childhood may be visible in the eardrum and make the diagnosis more difficult.
Findings typical of a conductive hearing impairment are detected in functional examination.
- Identification of spoken and whispered words is impaired. Words containing low-pitched syllables (vowels like a, o, u) are particularly difficult to hear.
- The Weber' tuning fork test lateralizes to the more severely affected ear, and the Rinne's test is abnormal (negative).
- The typical audiogram shows a flat air conduction that may be impaired to the conductive maximum of 60 dB. In pure stapedial fixation with a normal internal ear the bone conduction may be nearly normal.
- The bone conduction typically shows Carhart's notch, with a hearing impairment at 2 000 Hz.
- A combined hearing impairment is common, affecting both air and bone conduction. At low frequencies there is, however, a marked difference between air and bone conduction.
- In speech audiometry the speech threshold is impaired as much as the air conduction threshold. Speech discrimination is good in pure stapedial fixation, but it may be affected in combined hearing impairment.
- Acoustic impedance testing reveals a normal middle ear pressure and absence of the stapedial reflex.
- A CT scan (sometimes MRI) of the ear is indicated for differential diagnosis at least in unclear cases.

There is no pharmacological or other treatment that prevents the formation of otosclerotic foci.
In progressive hearing impairment of inner ear type, large doses of fluoride or bisphosphonates have been tried with the aim of slowing down or halting the inner ear damage. There is no evidence on the long-term effectiveness of these pharmacological interventions.

Operative treatment, in which the fixed stapedial footplate is opened and the stapes is replaced with a prosthesis (stapedotomy), usually provides excellent results. The conductive hearing impairment in the speech frequencies is corrected (near to normal, i.e. below 10 dB) in more than 90% of cases, and the low-pitched tinnitus is attenuated in approximately 80% of patients.
Hearing aids are also well suited for patients with otosclerosis and they usually restore hearing to a socially adequate level.
For example the following criteria may be used as indications for surgery:
- Air conduction threshold is 30 dB PTA (pure tone average; mean at 0.5, 1 and 2 kHz) or worse, there is at least 15 dB conduction deficit, and the Rinne test is negative.
- The probable hearing threshold after operation is expected to be 30 dB or better, or not more than 15 dB worse than in the better ear.
- The patient prefers operative treatment to rehabilitation with hearing aids.
If the combined result of stapedotomy and hearing aid is not sufficient in advanced otosclerosis, a cochlear implant may be considered.

Kabbara B, Gauche C, Calmels MN et al. Decisive criteria between stapedotomy and cochlear implantation in patients with far advanced otosclerosis. Otol Neurotol 2015;36(3):e73-8. [PubMed]