A New Internal Acoustic Canal Anomaly, Fused Single Internal Acoustic Canal: A Case Report

The internal auditory canal (IAC) connects the inner ear to the posterior cranial fossa. In this case report, the authors presented an 8-month-old patient with an IAC anomaly and bilateral deafness. The patient has hypotonia and epilepsy, and there is multifocal epileptiform activity on Electroencephalography (EEG). Severe (total) sensorineural hearing loss was detected bilaterally. Temporal bone computed tomography showed that the internal acoustic canal is localized as a single entity at the midline and bifurcates into right and left branches approximately ~4 mm beyond. Following branching, the diameter of each IAC measures approximately ~2 mm. On both sides, inner ear structures are located more medially than normal, positioned close to the midline and each other. Both cochleae undergo approximately 1.5 turns, classified as cochlear hypoplasia type III according to Sennaroğlu and Bajin's classification. Magnetic resonance imaging showed that in the anterior aspect of the pons, extending from the midline to the internal acoustic canal, a paired appearance, resembling a couple, of the VIII cranial nerve is observed in close proximity to each other. The pair of the VIII nerve seems to diverge at the entrance of the internal acoustic canal, and immediately thereafter, they bifurcate into branches believed to belong to the superior and inferior vestibular nerves on both sides, but the cochlear nerve cannot be discerned. To the best of current knowledge, this is the first case in the literature as fused IAC.