Otosclerosis is a condition where there is remodelling of the small bones in the middle ear, leading to conductive hearing loss. Oto- refers to the ears, and -sclerosis means hardening. It usually presents before the age of 40 years.
The development of otosclerosis is thought to result from a combination of environmental and genetic factors, although the exact mechanism is not understood. It can be inherited in an autosomal dominant pattern. However, no specific genetic mutations have been identified. It is more common in women.
Pathophysiology
The auditory ossicles are the tiny bones in the middle ear that transmit sound vibrations from the tympanic membrane to the cochlea. They are the malleus, incus and stapes. The stapes is connected to the oval window (fenestra ovalis) of the cochlea, where it transmits vibrations into the cochlea, which converts them into sensory signals.
In patients with otosclerosis, these tiny bones in the middle are affected by abnormal bone remodelling and formation. This mainly affects the base of the stapes, where it attaches to the oval window, causing stiffening and fixation and preventing it from transmitting sound effectively. It causes conductive hearing loss.
Presentation
The typical presentation is a patient under 40 years presenting with unilateral or bilateral:
- Hearing loss
- Tinnitus
It tends to affect the hearing of lower-pitched sounds more than higher-pitched sounds. Female speech may be easier to hear than male speech (due to the generally higher pitch). This is the reverse of the pattern seen in presbycusis.
Due to conductive hearing loss with intact sensory hearing, the patient can experience their voice as being loud compared to the environment (due to bone conduction of their voice). This can lead to them talking quietly.
Examination
Otoscopy is normal.
Weber’s test is normal if the otosclerosis is bilateral, meaning that when the tuning fork is applied to the centre of the forehead, they will hear the sound equally in both ears. If the otosclerosis is unilateral or affects one ear more than the other, the sound will be louder in the more affected ear.
Rinne’s test will show conductive hearing loss. The sound will be easily heard when the tuning fork is applied to the mastoid process (bone conduction). When the patient stops being able to hear the sound during bone conduction, and the tuning fork is removed from the mastoid process and held close to the ear canal, they will still not hear the sound (air conduction is worse than bone conduction).
Investigations
Audiometry is the initial investigation of choice. Otosclerosis will show a conductive hearing loss pattern. Bone conduction readings will be normal (between 0 and 20 dB). However, air conduction readings will be greater than 20 dB, plotted below the 20 dB line on the chart. Hearing loss tends to be greater at lower frequencies.
Tympanometry will show generally reduced admittance (absorption) of sound. The tympanic membrane is stiff and non-compliant and does not absorb sound, reflecting most of it back.
High-resolution CT scans can detect boney changes associated with otosclerosis, although they are not always required.
Management
The options for management are:
- Conservative, with the use of hearing aids
- Surgical (stapedectomy or stapedotomy)
Surgical management is generally successful and can potentially restore hearing to normal. It involves lifting the tympanic membrane and some of the surrounding skin out of the way to access the middle ear through the ear canal.
Stapedectomy involves removing the entire stapes bone and replacing it with a prosthesis. The prosthesis attaches to the oval window and hooks around the incus, transmitting the sound from the incus to the cochlea in the same way the stapes normally would.
Stapedotomy involves removing part of the stapes bone and leaving the base of the stapes (the footplate) attached to the oval window. A small hole is made in the base of the stapes for the prosthesis to enter. A prosthesis is added to transmit sound from the incus to the cochlea.
Last updated July 2021