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The University of Texas Health Science Center San Antonio, TX 78229-3900
Otosclerosis is an osseous dysplasia limited to the temporal bone that is of unknown etiology [1, 2]. Typically, otosclerosis presents with slowly progressive hearing loss, which is usually bilateral [3]. Seventy percent of patients inherit the disease (autosomal dominant with incomplete penetrance); the remaining cases are sporadic [1]. Otosclerosis is more frequent in women and is seen most often between the ages of 15 and 45 years [3]. The disease is rare in children younger than 10 years old, and only a few reports have described congenital otosclerosis [4]. We report a 7-year-old girl with bilateral conductive hearing loss and CT findings compatible with congenital otosclerosis. CT also revealed bilateral aberrant internal carotid arteries, an unrelated but extremely important entity to diagnose.
A 7-year-old girl presented with a 4-year history of hearing loss. There was no history of recurrent ear infections and no family history of hearing problems. Her birth history was unremarkable and there was no identifiable associated syndrome. Bilateral otoscopic examination revealed normal external auditory canals and tympanic membranes. An audiogram showed bilateral conductive hearing loss that was more severe on the right side. High-resolution temporal bone CT was performed that revealed bony plaques covering the regions of the oval and round windows bilaterally and linear radiolucency encircling bilateral cochleae, findings consistent with otosclerosis (Figs. 7A and 7B). In addition, there was an abnormally lateral course of the petrous portion of the bilateral internal carotid arteries that protruded into the tympanic cavity, representing bilateral aberrant internal carotid arteries (Fig. 7A,7B,7C). The patient underwent surgical exploration of the right middle ear, where a thickened stapes footplate with fixation to the oval window was found. A stapedectomy with placement of a stapes prosthesis was performed. A similar procedure is contemplated on the left ear at a later date.
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The inner ear is composed of the membranous labyrinth and the surrounding osseous labyrinth (otic capsule), which is part of the dense petrous portion of the temporal bone [1]. The otic capsule has three layers: endosteal, endochondral, and periosteal. Otosclerosis occurs when the dense endochondral bone is replaced by spongy, highly vascular, and irregular foci of haversian bone tissue, hence the term "otospongiosis." In most cases, the otosclerotic focus is located anterior to the oval window, in a region called the fissula ante fenestram. This type of disease is called fenestral otosclerosis, which is the most frequent type and which involves the lateral wall of the labyrinth, including the promontory and the oval and round window niches. When the process involves mostly the cochlea, it is called retrofenestral otosclerosis, which rarely occurs without fenestral involvement, as opposed to fenestral process, which commonly occurs as an isolated phenomenon.
Clinically, most cases of otosclerosis produce conductive hearing loss. High-resolution CT with bone algorithm is the imaging modality of choice for patients with conductive hearing loss and suspected otosclerosis. In the fenestral type, abnormalities of the oval window can be seen in 80-90% of cases. Findings include narrowing or complete obliteration of the window by a thick bony plate, which causes fixation of the footplate of the stapes in the oval window. In the retrofenestral type, there is a confluent spongiotic cochlear focus that appears as a pericochlear radiolucency or a double ring of hypodensity around the cochlea, which is well seen on CT [1, 2]. The differential diagnosis includes osteogenesis imperfecta and Paget's disease [4]. However, in our patient these two entities were excluded on the basis of normal radiographic findings of the long bones and the absence of clinical findings associated with those diseases.
This patient was also found to have bilateral aberrant internal carotid arteries. To the best of our knowledge, there has been only one case report about this entity occurring bilaterally [5].
The aberrant course of the internal carotid artery can be explained by an embryologic malformation of the first and second brachial arches, which results in the persistence of embryologic vessels and anastomoses in the middle ear. Because of the potentially serious consequences of an incorrect diagnosis, it is important that an aberrant internal carotid artery be considered and differentiated from the more commonly encountered lesions in the middle ear such as glomus tumor, cholesterol granuloma, cholesteatoma, and dehiscent jugular bulb. All these lesions may appear similar on clinical examination. Even if not apparent clinically, this rare normal variant should be reported to the clinician before middle ear surgery to prevent a potential catastrophe.
This article describes a case of congenital otosclerosis in a child with bilateral aberrant internal carotid arteries. Both these conditions can reliably be diagnosed with highresolution temporal bone CT.
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