Thys and Van Camp report that otosclerosis is a major cause of acquired hearing loss in adults. The otic capsule in humans is unique and generally undergoes little "remodeling" after development. That is, in normal otic capsules, bone turnover is highly suppressed. Thys and Van Camp state otosclerosis is a process of pathologic increased bone turnover, most often leading to stapes fixation (described in 1740 by Valsalva) and subsequent conductive hearing loss. Although genetic factors play a significant role in the manifestation of otosclerosis, 40 to 50 percent of all clinical cases have been reported to be sporadic.
The authors note a study of 236 temporal bones (of European origin) demonstrated a prevalence rate for histologic otosclerosis of 2.5 percent, with no gender specific preference, although prevalence varies across ethnicities. Clinical otosclerosis (otosclerosis that causes hearing loss) is thought to occur in 0.3 to 0.4 percent of the same population and occurs more frequently in women than men (1.4:1, up to 2:1). Otosclerosis most often starts between the ages of 20 and 40 years and 90 percent of all cases are diagnosed prior to age 50, although recent data indicates the average age may be increasing.
Clinical otosclerosis is a progressive conductive hearing loss and is bilateral in 85 percent of all cases. In some 10 percent of all cases a sensorineural component is found. The authors state that otosclerosis can be divided into three categories: (1) classic (conductive hearing loss due to stapes fixation), (2) mixed (stapes fixation and cochlear involvement), and (3) pure sensorineural loss without stapes fixation. Thys and Van Camp note that the "Carhart Notch" was first described in 1950 by Raymond Carhart and is due to stapes fixation. They note that when a Carhart Notch is present, true bone conduction levels are not necessarily correlated with inner ear function.
The most frequent symptom associated with otosclerosis is tinnitus. The impact of environmental factors (estrogen, oral contraceptives, sodium fluoride, measles virus, etc) have been implicated previously and they remain controversial and unresolved.
For More Information, References, and Recommendations
Thys M, Van Camp G. (2009) Genetics of Otosclerosis. Otology & Neurotology. (30)8:1021-1032.