Purpose or Case Report: Congenital and acquired forms of temporal bone dehiscences involving the facial nerve (FN) canal, semicircular canals, and otic capsule can cause functional inner ear abnormalities and represent critical landmarks for cochlear implant surgeons. The rarest and most recently described type, cochlear-facial dehiscence (CFD), is characterized by lack of an osseous partition between the labyrinthine facial nerve canal segment and the cochlea. CFD has only been reported in adults, both by CT and histopathology. We share a case of CFD diagnosed by temporal bone CT in a pediatric patient presenting with hearing loss. This is an important condition for radiologists to be aware of when evaluating the temporal bone.

Case Report (in brief)
Our patient is a male toddler who presented at 15 months of age because he was not responding to parents’ voices. Workup revealed profound bilateral sensorineural hearing loss.
At 20 months of age, he underwent an axial high-resolution CT scan without contrast with 0.625 mm thick images of the temporal bones. Cochlear-facial dehiscence was noted bilaterally, with 0.8 mm bone defects between the labyrinthine segment of the facial nerve canal and the adjacent superolateral aspect of the cochlear mid turn (CFD). This was thought to represent a possible anatomic defect responsible for the patient’s reported sensorineural hearing loss (SNHL). The remainder of the CT examination was normal without additional areas of dehiscence.

Discussion (abbreviated):
In its normal, mature state, the facial nerve passes through a fully enveloped bony canal situated superiorly and along the lateral part of the cochlea. Inadequate facial nerve canal ossification may occur secondary to impaired epithelial-mesenchymal interactions in precursor areas at intermembranous bone ossification sites.
A presumed congenital cochlear-facial dehiscence was present in our patient bilaterally, and may have been partly or entirely responsible for his hearing loss. The defect could be from ossification failure or over-resorption of the bone. Cochlear-facial dehiscence is an important diagnosis to be primed for during the assessment of temporal bone CTs, especially in patients with findings suggesting a 3rd window phenomenon. Furthermore, it is critical to convey the presence of a CFD to the otolaryngologist prior to any consideration for cochlear implantation as it may predispose the patient to facial nerve stimulation during activation or usage of the implant.
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Conclusions: See above