Otosclerosis – A common cause of middle ear deafness
What is Otosclerosis?
Otosclerosis is a disease of the hearing mechanism in the middle (and internal) ear in which the third bone of the chain of 3 bones in the middle ear gradually gets fixed due to new bone formation and decreasing the vibrations of sound entering the inner ear. (see diagram). The cause of the disease is unknown. It usually manifests after the age of 15 years and is commoner in females. In 80 % of cases, the disease affects both ears to a greater or lesser extent.
The patient or relatives notice increasing difficulty in hearing, mainly in one ear. One’s own voice may sound loud and hence the patient tends to speak softly as compared with patients with a weakness of the hearing nerve (sensori neural hearing loss) who tend to speak loudly. In some patients the first complaint is an intermittent or continuous spontaneous noise in the ear (tinnitus).
For some reason, in females, every pregnancy increases the deafness.
As the disease process advances, the otosclerosis can involve the opposite ear (in80% patients). When the inner ear is also affected by the process (cochlear Otosclerosis) a “mixed” deafness (conductive and sensori-neural) develops.
Whites are the most affected race, with the prevalence in the Black and Asian populations being much lower. Females are twice as likely as males to be affected. Usually noticeable hearing loss begins at middle-age, but can start much sooner. The hearing loss often grows worse during pregnancy.
The disease is inherited as an autosomal dominant fashion.
Special tests for hearing
Pure Tone Audiometry shows the exact levels of hearing and confirm a conductive deafness where hearing by bone conduction is better than hearing by air conduction. Acoustic Impedance Bridge studies confirm a fixation of the 3rd bone (Stapes) and also confirm normal middle ear pressure but diminished ear drum movement.
What is the treatment?
The main treatment is surgical. The ear drum is lifted up and the middle ear is entered. Under an operating microscope, the joint between the 2nd bone (Incus) and third bone (Stapes) is separated. The upper part of the Stapes bone (super structure) is removed. A very small microscopic hole of 0.6 to 0.8 mm diameter is made in footplate of the stapes bone (Stapedotomy). One end of a teflon piston is inserted through this hole, either directly or through a vein graft. The other end of the piston is hooked around the 2nd bone (Incus). The ear drum is then replaced back in the original position.
Sound vibrations now travel from the ear drum via the teflon piston.
Hearing is restored to normal on the operating table in 97 to 98% of cases.
A Hearing aid will also give satisfaction, as in any case of conductive deafness. However, it can never give the satisfaction of normal natural hearing as a successful operation does.
Otosclerosis is a slowly progressive disease. As years go by, deafness increases and the opposite ear too shows signs of involvement. In females, every pregnancy increases the deafness. Gradually, after the age of 45 or 50 years, when old age deafness sets in due to age related changes of the nerves of hearing, the hearing impairment becomes more pronounced and a definite handicap in communication.
Treatment of otosclerosis relies on two primary options: hearing aids (more recently including bone-conduction hearing aids) and a surgery called a stapedectomy. Hearing aids are usually very effective early in the course of the disease, but eventually a stapedectomy may be required fordefinitive treatment. Early attempts at hearing restoration via the simple freeing the stapes from its sclerotic attachments to the oval window were met with temporary improvement in hearing, but the conductive hearing loss would almost always recur. A stapedectomy consists of removing a portion of the sclerotic stapes footplate and replacing it with an implant that is secured to the incus. This procedure restores continuity of ossicular movement and allows transmission of sound waves from the eardrum to the inner ear.
A modern variant of this surgery called a stapedotomy, is performed by drilling a small hole in the stapes footplate with a micro-drill or a laser, and the insertion of a piston-like prothesis. The success rate of either a stapedotomy or a stapedectomy depends greatly on the skill and the familiarity with the procedure of the surgeon.
Other less successful treatment includes fluoride administration, which theoretically becomes incorporated into bone and inhibits otosclerotic progression. This treatment cannot reverse conductive hearing loss, but may slow the progression of both the conductive and sensorineural components of the disease process. Recently, some success has been reported with bisphosphonate medications, which stimulate bone-deposition without stimulating bony destruction.
Sequence of a Stapedectomy operation