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Otosclerosis is a progressive degenerative condition of the temporal bone which can result in hearing loss.
Chronic conductive hearing loss (CHL) is the finding in almost all cases of otosclerosis (in fact should a person present with sensorineural hearing loss they would likely never be diagnosed with otosclerosis). This usually will begin in one ear but will eventually affect both ears with a variable course. On audiometry, the hearing loss is characteristically low-frequency, with higher frequencies being affected later. Sensorineural hearing loss (SNHL) has also been noted in patients with otosclerosis; this is usually a high-frequency loss, and usually manifests late in the disease.
Approximately 0.5% of the population will eventually be diagnosed with otosclerosis. Interestingly, post mortem studies show that as many as 10% of people may have otosclerotic lesions of their temporal bone, but apparently never had symptoms warranting a diagnosis. 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 disease can be considered to be heritable, but its penetrance and the degree of expression is so highly variable that it may be difficult to detect an inheritance pattern. Most of the implicated genes are transmitted in an autosomal dominant fashion.
The pathophysiology of otosclerosis is complex. The key lesions of otosclerosis are multifocal areas of sclerosis within the endochondral temporal bone. These lesions share some characteristics with Paget’s Disease, but they are not thought to be otherwise related. Histopathologic studies have all been done on cadaveric temporal bones, so only inferences can be made about progression of the disease histologically. This being said, it seems that the lesions go through an active “spongiotic” / hypervascular phase before developing into “sclerotic” phase lesions. There have been many genes and proteins identified that, when mutated, may lead to these lesions. Also there is mounting evidence that measles virus is present within the otosclerotic foci, implicating an infectious etiology (this has also been noted in Paget’s Disease).
CHL in otosclerosis is caused by two main sites of involvement of the sclerotic (or scar-like) lesions. The best understood mechanism is fixation of the stapes footplate to the oval window of the cochlea. This greatly impairs movement of the stapes and therefore transmission of sound into the inner ear (“ossicular coupling”). Additionally the cochlea’s round window can also become sclerotic, and in a similar way impair movement of sound pressure waves through the inner ear (“acoustic coupling”).
SNHL in otosclerosis is controversial. Over the past century, leading otologists and neurotologic researchers have argued whether the finding of SNHL late in the course of otosclerosis is due to otosclerosis or simply to typical presbycusis. There are certainly a few well documented instances of sclerotic lesions directly obliterating sensory structures within the cochlea and spiral ligament, which have been photographed and reported post-mortem. Other supporting data includes a consistent loss of cochlear hair cells in patients with otosclerosis; these cells being the chief sensory organs of sound reception. A suggested mechanism for this is the release of hydrolytic enzymes into the inner ear structures by the spongiotic lesions.
Treatment of otosclerosis relies on two primary options: 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 for definitive treatment. Modern stapedectomy consists of removing a portion of the sclerotic stapes footplate and replacing it with an implant that is secured to the incus. When possible, the stapes may be able to be freed from its sclerotic attachments to the oval window, eliminating the need for an implant. Both of these methods restore continuity of ossicular movement and allow transmission of sound waves to the inner ear.
Other less successful treatment includes fluoride administration, which theoretically becomes incorporated into bone and inhibits otosclerotic progression. Recently, some success has been reported with bisphosphonate medications, which stimulate bone-deposition without stimulating bony destruction.