Otosclerosis

Otosclerosis is a localized disorder of bone remodeling that affects the otic capsule of the temporal bone and commonly results in progressive conductive hearing loss. The condition is characterized by abnormal resorption and deposition of bone, most frequently involving the stapes footplate and the oval window.

Otosclerosis involves abnormal bone remodeling within the otic capsule, a region that normally remains inactive after development. The disease begins with osteoclastic bone resorption, followed by replacement of mature lamellar bone with highly vascularized connective tissue, an early phase known as otospongiosis. As the process progresses, immature, disorganized sclerotic bone accumulates, characterized by increased osteocytes and widened marrow spaces. Vascular spaces are eventually replaced by dense, irregular bone, impairing sound transmission. The fissula ante fenestram, a cleft anterior to the oval window near the cochleariform process, is the most common site of origin (80%), followed by round window (30%), pericochlear region (21%), and anterior segment of the internal auditory canal (19%). Persistence of embryonic cartilage in this region may contribute to its susceptibility. Lesions expand through vascular channels and often involve the stapes footplate or annular ligament, leading to mechanical fixation and conductive hearing loss. In some cases, extension into the cochlear endosteum disrupts inner ear function, resulting in sensorineural hearing loss, referred to as cochlear otosclerosis.

Otosclerosis is a relatively common cause of acquired hearing loss, particularly in adults. It predominantly affects individuals of European descent, with a prevalence of clinically significant disease estimated at 0.3-0.4%. Histopathologic evidence of otosclerosis is found in up to 10% of white temporal bones examined postmortem, although most cases remain asymptomatic. The condition is rare in individuals of African or Asian ancestry. Otosclerosis predominantly affects young to middle-aged adults, with a peak incidence between ages 20 and 40, and is approximately twice as common in females as in males.

A positive family history is found in up to 60% of cases, suggesting an autosomal dominant inheritance with incomplete penetrance. Viral factors, such as persistent measles virus infection, and environmental contributors have also been proposed, though not definitively established. Symptoms may worsen during pregnancy, possibly due to hormonal influences on bone metabolism.

Otosclerosis typically presents in early to mid-adulthood with a slowly progressive conductive hearing loss that is most pronounced at low frequencies. Patients often have difficulty hearing male voices or vowel-rich speech and may paradoxically hear better in noisy environments (paracusis of Willis). The loss is usually bilateral but asymmetric at onset, and about 80% of cases become bilateral over time.6 Tinnitus accompanies the hearing loss in up to 75% of patients, whereas vertigo is uncommon (10%) and, when present, suggests extension of disease into the vestibular labyrinth, particularly the semicircular canals. A sensorineural component may emerge in advanced stages, and deterioration can accelerate during pregnancy. Otoscopic examination is generally normal, though a reddish blush behind the tympanic membrane (Schwartz sign) may be visible, reflecting promontory hypervascularity.