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Case Presentation
A 52-year-old male presented with a two-year history of progressive dizziness and sound-induced vertigo (Tullio phenomenon). The patient reported that loud noises, particularly high-pitched sounds, triggered a sensation of visual oscillation (oscillopsia) and imbalance. No hearing loss was reported, though the patient admitted to occasional autophony. Physical examination revealed a positive Hennebert sign (nystagmus induced by tragal pressure) and a noticeable sound-evoked eye movement. Audiometry was within normal limits for the patient's age, with no conductive hearing loss.
Imaging Findings
High-resolution computed tomography (CT) of the temporal bones, performed with 0.5 mm slice thickness and multiplanar reformation, revealed a bony defect along the posterior semicircular canal. The defect measured approximately 3.2 mm in length, located on the medial aspect of the canal, adjacent to the posterior fossa dura. The superior semicircular canal and tegmen tympani were intact. No evidence of middle ear effusion or cholesteatoma was noted. The imaging findings were consistent with a diagnosis of posterior semicircular canal dehiscence, a rare variant of the more common superior semicircular canal dehiscence syndrome.
Vestibular Testing
Vestibular-evoked myogenic potentials (VEMPs) were obtained. Ocular VEMPs (oVEMP) showed lowered thresholds and enhanced amplitudes on the affected side compared to the contralateral ear. Cervical VEMPs (cVEMP) were also abnormal, demonstrating a reduced threshold for the sound-evoked response. These findings supported a third-window phenomenon in the posterior semicircular canal.
Management and Outcome
Given the patient's significant symptoms and functional impairment, surgical intervention was pursued. A transmastoid approach was performed to expose the posterior semicircular canal. The dehiscence was identified and repaired using a combination of bone wax, fascia, and a small bone pate graft. The patient tolerated the procedure well. At the six-month follow-up, the patient reported complete resolution of sound-induced vertigo and oscillopsia. The autophony resolved. Repeat audiometry showed no change in hearing thresholds. The patient returned to normal daily activities without restriction.
Discussion
Posterior canal dehiscence is an infrequently diagnosed cause of vestibular symptoms. It shares many clinical features with superior semicircular canal dehiscence, including sound- and pressure-induced vertigo, but the location of the dehiscent bone is distinct. Diagnosis relies on a high index of suspicion, detailed vestibular testing (particularly VEMP), and dedicated temporal bone CT imaging. Surgical repair can be highly effective for patients with disabling symptoms, though careful patient selection is essential. This case highlights the importance of considering posterior canal variants in patients with typical third-window symptoms but normal superior canal imaging.