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To understand why otosclerosis causes hearing loss, it is important to understand how the ear works. The human ear is divided into three parts: the external, middle, and inner ear.
The inner ear also contains the semicircular canals which are responsible, in part, for sensing movement and maintaining balance.
Otosclerosis is a disorder of bone that affects human beings only and involves the otic capsule (bony labyrinth) and primarily the stapes bone of the middle ear. It is the slow progressive fixation of the stapes bone in the oval window which usually causes the clinical symptoms to become noticed in the form of a conductive hearing loss. As the condition progresses a mixed conductive and sensorineural hearing loss develops. The clinical condition affects only 1% of the Caucasian population and it is more frequent in woman than in men with an approximate 2:1 ratio. It is much less common in the black population. The condition is noticed first in young adulthood (ages 20-30’s) but has been reported in individuals as young as 11 years of age. In females, often the conductive hearing loss accelerates during pregnancy perhaps due to the regulation of bone deposition by hormones. This condition may be found more commonly in certain families and is thought to be transmitted as an autosomal dominant trait with incomplete penetrance which is why it does not affect all individuals nor every generation.
The exact etiology of Otosclerosis remains a mystery, but the pathogenic features involve bone resorption, remodeling and then bone deposition. The lesions of the otic capsule transition from otospongiosis which is the highly vascular remodeling phase, to one of dense bone deposition known as the otosclerotic phase. What triggers these changes are speculative but there has been some evidence linking the disorder with early exposure to the measles virus antigen. While there are other theories suggesting that it is a metabolic bone disorder the exact cause remains elusive.
The clinical symptom that drives a patient to the doctor is the progressive conductive hearing loss that is associated with new bone depositing around the stapes footplate wedging it in the oval window so that its normal mobility is hampered. Even though sound is normally transmitted to the tympanic membrane, the ossicular chain has become stiffened by the fixation of the stapes bone. 80-90% of these otosclerotic lesions have an anterior location at the oval window in what is known as the fissula ante fenestram. However, each patient’s lesion can be different and can range from a biscuit type footplate to a sea of otosclerosis obliterating the entire footplate. Experience recognizing the position of the lesions and how to best deal with these variations is crucial in order to prevent complicating situations such as a floating footplate which makes completion of a successful procedure more difficult and hazardous. On rare occasions, the disorder is so widespread that it obliterates the entire round window which causes a non-surgically correctable conductive hearing loss.
The otologist/neurotologist (ear specialist) typically finds that the patient with clinically significant otosclerosis fits the following description:
On occasion the patient may need to undergo a CT scan to rule out other causes of these presenting symptoms but this is left to the discretion of the treating physician. However, a CT scan is not required to make the presumptive diagnosis in the majority of cases, nor is it required before performing surgery as there is a small risk of radiation exposure with a high resolution CT scans required to visualize the inner and middle ears. In some advanced cases the hypervascular phase of otosclerosis can be seen on otoscopic examination by the physician looking through the tympanic membrane and seeing a reddish blush on the promontory of the cochlea- this is the so-called “Schwartze’s sign”.
There is a differential diagnosis that must be considered every time a physician sees a medical condition and this represents the other possible causes of the patient’s presenting symptoms. Similarly, in otosclerosis there is a differential diagnosis that should be considered including the following:
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