Biology Reference
In-Depth Information
capsule. Approximately 8% of temporal bones from the Caucasian popu-
lation show evidence of histologic otosclerosis, although only 1% of the
Caucasian population manifests hearing loss associated with clinical oto-
sclerosis (Altmann et al. 1967). The same study reported a lower prevalence
of histologic otosclerosis in black, Asian, and American Indian populations
(Altmann et al. 1967).
The hearing loss is typically conductive, but may progress to a profound
mixed loss in later stages of the disease. The conductive component is
caused by fixation of the stapes footplate in the oval window by otoscle-
rotic tissue. The etiology of the sensorineural loss, termed “cochlear oto-
sclerosis,” is not well understood, but has been postulated to be caused by
direct mechanical effects, or by metabolic or vascular factors associated
with the otosclerotic process within the cochlea. Fortunately, the conduc-
tive hearing loss may be reduced or eliminated by modern surgical tech-
niques that re-establish efficient sound transduction from the ossicular
chain to the vestibule (Shea 1998). Cochlear otosclerosis is not affected by
these procedures, but its progression can be retarded by the oral adminis-
tration of sodium fluoride (Causse et al. 1993).
Although 40 to 50% of cases appear to be sporadic, the hereditary nature
of otosclerosis in other cases is well established and was recognized by
Toynbee as early as 1861 (Toynbee 1861). A genetic etiology was also
strongly suggested by the high concordance rate observed for monozygotic
twins with otosclerosis (Fowler 1966). Most studies have concluded that
inheritance of otosclerosis is autosomal dominant with reduced penetrance
(Causse and Causse 1984; Gapany-Gapanaviscius 1975; Larsson 1960; Mor-
rison 1967). However, digenic inheritance of autosomal recessive genes
(Bauer and Stein 1925), as well as autosomal recessive and X-linked dom-
inant genes (Hernandez-Orozco and Courtney 1964) have been proposed.
These data, as well as other epidemiologic, clinical, and molecular studies
indicate
in toto
that otosclerosis is not a simple monogenic Mendelian trait,
but has a multifactorial, if not multigenic, etiology and pathogenesis.
Several different lines of evidence have implicated nongenetic factors.
There is a slight preponderance of females among reported cases of oto-
sclerosis and numerous reports of hearing loss exacerbation during preg-
nancy, suggesting an influence of sex hormones on progression, but not
necessarily prevalence, of the otosclerotic process. Other studies have
addressed the possibility of a viral etiology for otosclerosis. Mumps, rubella,
and measles virus antigens have all been detected in otosclerotic foci, and
recent studies utilizing RT-PCR have demonstrated measles virus RNA
in otosclerotic temporal bones (McKenna et al. 1996; Niedermeyer and
Arnold 1995). Viral material was not detected in control temporal bone
specimens in these analyses, supporting the hypothesis of a specific associ-
ation of viral infection with otosclerosis, although the evidence does not
establish a causal link. Finally, others have implicated immune mechanisms
in otosclerosis, including autoimmunity to type II collagen (Yoo 1984). Oto-
