Otospongiosis - Clinical Tree

Introduction

Otospongiosis, also known as otosclerosis, is a progressive primary bone disorder of abnormal bone resorption and deposition. The majority of cases result in a progressive conductive hearing loss with severe cases evolving to a combination of conductive and sensorineural hearing loss. The clinical prevalence of otospongiosis is estimated to be 0.3%, although histologic studies have reported a prevalence between 8% and 11%, indicating that many cases are clinically silent. Otospongiosis commonly affects middle-aged adults between the third and fifth decades of life; it has a female predominance and approximately 85% of cases are bilateral. Although the etiology of otospongiosis remains unknown, current theories under investigation include genetic causes, such as human leukocyte antigen (HLA) associations and mutations of collagen genes, viral etiologies (measles), autoimmune processes (response to collagen), and hormone-mediated pathways.

Regardless of the potential etiology, otospongiosis manifests as resorption of the otic capsule by osteoclasts and deposition of bone by osteoblasts. Although this process of remodeling is normal in other bones in the human body, it is unusual and abnormal in the otic capsule, the densest bone in the human body. The active, vascular phase is also known as the spongiotic phase and is characterized by active osteoclasts and an advancing front of otospongiosis in the otic capsule. Histologically, there is dilatation of the endoplasmic reticulum, resulting in the lucent appearance seen on computed tomography (CT). The inactive or sclerotic phase is characterized by the absence of osteoclasts and predominance of osteoblasts, with the previously dilated vascular spaces having been obliterated by bone.

One classification system of otospongiosis differentiates between clinical otospongiosis and histologic otospongiosis based respectively on the presence or absence of stapes footplate fixation. Involvement of the stapes footplate in otospongiosis results in fixation of the stapes and thus a conductive hearing loss. The most common location of otospongiosis is anterior to the oval window near the fissula ante fenestram (or fenestral otospongiosis; Figs. 47.1[1] and 47.2 ). If the disease progresses to include the bony labyrinth beyond the region of the fissula ante fenestram into the pericochlear otic capsule, the term retrofenestral otospongiosis is often used ( Figs. 47.1[3] and 47.3 ). However, definitions and their use in the literature are variable and it is controversial whether round window involvement is considered fenestral or retrofenestral. For this reason, some prefer the terms fenestral otospongiosis and cochlear otospongiosis . Regardless, retrofenestral (cochlear) otospongiosis may result in a mixed conductive and sensorineural hearing loss or vestibular symptoms depending on the structures involved. Retrofenestral otospongiosis is rarely observed in isolation; rather, it is much more commonly seen in combination with fenestral otospongiosis.