Otosclerosis is a disease of abnormal bone metabolism affecting the stapes, oval window and the otic capsule. It results in progressive fixation of the stapes footplate and eventually sclerosis of the cochlea in advanced cases.
The most common location of bony involvement is the bone just anterior to the oval window in a small cleft known as the fissula ante fenestram. The abnormal deposition and hardening of the bone in this region leads to fixation of the stapes footplate with concomitant conductive hearing loss. This active bone remodelling may progress to involve and damage the cochlea, through the release of proteolytic enzymes or from direct injury to the cochlea and spiral ligament. Sensorineural hearing loss may therefore result in advancing otosclerosis.
The onset of hearing loss usually starts between the ages of 15 to 45, and thereafter slowly progresses. In most cases, the hearing loss affects both ears. Women are more often affected than men. Certain ethnic groups are more prone to otosclerosis, especially Caucasians and Indians. It is less prevalent amongst the Chinese and Malays.
Clinical examination usually reveals an unremarkable external ear canal, tympanic membrane and middle ear space. Rarely there may be Schwartz sign, which is a faint reddish hue seen over the cochlear promontory visualized through the tympanic membrane. A Rinne’s Tuning Fork test would be negative, and Weber’s tuning fork test lateralises to the affected side.
A Pure Tone Audiogram would reveal an “Air-Bone Gap” wherein the Bone Conduction thresholds are better than the air conduction thresholds. The gap may be as small as 10dbHL or as large as 50-60dBHL. However, this gap narrows at the 2 KHz frequency in a pattern known as “Carhart’s notch”. High Resolution CT Temporal Bone Imaging is nowadays commonly performed to help confirm the diagnosis radiologically and to exclude other middle ear pathologies. CT scans show up lucency in the area of the fissula ante fenestram (fenestral otosclerosis) as well as any cochlear involvement of disease.
There is some research to suggest that the use of sodium fluoride is able to impede the progression of the disease. The administration of fluoride replaces the hydroxyl radical with fluoride leading to the formation of fluorapatite crystals instead of hydroxylapatite crystals which would otherwise lead to stapes fixation. However in Singapore, this medication is not readily available and not commonly prescribed as the benefits of this therapy remain controversial.
Conventional Hearing Aids can be used to improve hearing, but does so only when the device is worn. The disease is not corrected and will continue to progress leading to the need to adjust the hearing aid as hearing deteriorates. As otosclerosis usually affects those in the young adult to middle age groups, a significant number of patients are concerned with the stigma and cosmesis of wearing a hearing device.
The conventional surgical treatment of otosclerosis used to be a procedure known as Stapedectomy.
In this operation, the entire stapes including the footplate is removed and the exposed oval window covered with a vein graft. A tiny prosthesis called a stapes piston is attached to the incus long process and the other end is applied over the vein graft to transmit the acoustic vibrations.
This surgery has now been mostly replaced by a procedure known as Stapedotomy.
In this operation, the stapes suprastructure is removed leaving the stapes footplate intact. The footplate is then fenestrated using a laser or microdrill through which the stapes piston inserts into the vestibule. The benefits of a laser stapedotomy to fenestrate is that it is a non-touch technique, leading to less sensorineural trauma to the inner ear which happens during microdrilling.
There have been advances in the design of stapes pistons over the years and now there are new pistons which are able to automatically crimp onto the incus upon application of heat leading to easy and more reliable incus attachments.
When performed by an experienced ENT Otologist, the success rate of these operations is close to 90% and affords continuous good hearing 24 hours a day without any need to wear hearing devices.
When otosclerosis becomes advanced and causes severe to profound sensorineural hearing loss, conventional hearing aids and stapedotomy operations would not be able to correct the hearing loss. In these patients, thankfully cochlear implantation has been shown to be a highly effective method of restoring hearing.
