Prevalence of Cochlear-Facial and Other Non-Superior Semicircular Canal Third Window Dehiscence on High-Resolution Temporal Bone CT

BACKGROUND AND PURPOSE

The radiologic prevalence of superior semicircular canal dehiscence in the asymptomatic population has been widely studied, but less is known about the rates of other forms of third window dehiscence. Per the existing literature, the radiologic prevalence of cochlear-facial nerve dehiscence, for example, exceeds that seen in histologic studies, suggesting that conventional CT is unreliable for cochlear-facial dehiscence. These studies relied on nonisometric CT acquisitions, however, and underused multiplanar reformatting techniques, leading to false-positive findings. Our purpose was to determine the rate of cochlear-facial dehiscence and other non-superior semicircular canal third window dehiscences on optimized CT in asymptomatic patients.

MATERIALS AND METHODS

Sixty-four-channel temporal bone CT scans from 602 patients in emergency departments were assessed for cochlear-facial and other non-superior semicircular canal third window dehiscences by using high-resolution, multiplanar oblique reformats. Confidence intervals for dehiscence prevalence were calculated using the Newcombe 95% interval confidence method.

RESULTS

Of 602 patients, 500 were asymptomatic, while 102 had an imaging indication consistent with possible third window syndrome (symptomatic). Eight asymptomatic patients (1.6%) had cochlear-facial dehiscence, while 43 (8.4%) had jugular bulb-vestibular aqueduct dehiscence. There was no statistically significant difference between the prevalence of cochlear-facial dehiscence or jugular bulb-vestibular aqueduct dehiscence in asymptomatic patients compared with symptomatic patients. Cochlear-carotid canal, cochlear-internal auditory canal, and cochlear-petrosal sinus dehiscences were not observed.

CONCLUSIONS

Sixty-four-channel CT with multioblique reformatting is sensitive and specific for identifying cochlear-facial dehiscence, with rates similar to those in postmortem series. Jugular bulb-vestibular aqueduct dehiscence is a common incidental finding and is unlikely to produce third window physiology. Other non-superior semicircular canal third window dehiscences are rare in asymptomatic patients.

Carotid-cochlear dehiscence: a dangerous mimicker of inner ear pathologies

A 67-year-old woman was referred to the otolaryngology service after presenting to the emergency department for dizziness and loss of balance. She reported several similar episodes over the past years. Physical examination was unremarkable. A temporal bone CT scan revealed dehiscence between the bony carotid canal and the cochlea resulting in the diagnosis of carotid-cochlear dehiscence (CCD). CCD is an extremely rare condition involving the thinning of the bony canal separating the internal carotid artery from the cochlea. CCD is best diagnosed with temporal bone CT scan. Treatment options include observation as well as chemical or surgical labyrenthectomy. Despite similar clinical and diagnostic characteristics of reported CCD cases, general trends and consensus on treatment options cannot be ascertained due to the extreme rarity of this condition. Regardless of these limitations, CCD is a critical diagnosis as it mimics other inner ear conditions and poses a potential, significant surgical risk for the otolaryngologist.