Keywordstemporal bone, hearing loss, inner ear, computed tomography, otospongiosis, otosclerosis
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 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 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.
Pretreatment and Posttreatment Imaging
Treatment of otospongiosis includes both surgical and nonsurgical options. In the setting of fenestral otospongiosis and conductive hearing loss, surgical options include stapedotomy, partial or complete stapedectomy, and/or stapes prosthesis placement. Once otospongiosis progresses to involve the cochlea and sensorineural hearing loss develops, cochlear implantation can be considered. Nonsurgical options depend on the severity of disease and range from hearing aids to medical management including third-generation bisphosphonates.
Pretreatment imaging is primarily performed with high-resolution multidetector CT or cone-beam CT. The goal of pretreatment imaging is to assist in surgical planning, including prognostication regarding success of treatment or potential complications. Important considerations include the course of the osseous canal of the facial nerve, identification of any areas of dehiscence of the facial nerve canal, the presence of a persistent stapedial artery and/or aberrant internal carotid artery, dehiscence of the jugular bulb, obliteration of the round or oval windows, and extent of disease. The prognosis for stapes-related surgeries declines with increasing extent of disease. In one case series of residual and recurrent conductive hearing loss after stapedectomy, Nadol found 23% of revision stapedectomies were attributed to obliteration of the round window. The extent of disease is also important in surgical planning for cochlear implant placement, as obliteration of the round window and/or basal turn of the cochlea may make electrode placement extremely difficult or impossible. Alternatively, extensive abnormal spongiotic bone may allow for aberrant placement of the electrode beyond the walls of the bony labyrinth.
Magnetic resonance imaging (MRI) may complement CT by identifying other potential etiologies for sensorineural hearing loss. 3D-FLAIR imaging of the temporal bone with and without intravenous contrast can be helpful for demonstrating disease activity, as areas of breakdown of the blood-labyrinthine barrier manifest as areas of contrast enhancement.
Posttreatment imaging is primarily performed with high-resolution multidetector CT or cone-beam CT. Continued hearing loss following surgical treatment should initiate further workup, including imaging to assess for the location of a stapes prosthesis ( Figs. 47.4–47.6 ) or cochlear implant. Cochlear implant positioning can also be immediately assessed intraoperatively with an oblique (Stenvers) mastoid radiograph demonstrating the cochlear lead in the expected location of the basal turn of the cochlea. Posttreatment imaging may also be performed for facial nerve symptoms, which can occur in the setting of a malpositioned cochlear nerve implant.