An Easy and Reliable Method to Locate the Dehiscence During Middle Fossa Superior Canal Dehiscence Surgery: It is a (C)inch

Objective: The middle fossa floor lacks reliable surface landmarks. In cases of superior semicircular canal dehiscence (SSCD), multiple skull base defects may be present, further confounding the location of the labyrinth. Misidentification of the SSCD during surgery may lead to treatment failure or sensorineural hearing loss. Anecdotally, the authors have observed the distance from the lateral edge of the craniotomy to the SSCD to be consistently 1 inch. Herein, we present radiologic evidence of this practical and clinically useful relationship. Patients: All patients at two tertiary care academic referral centers with high-resolution temporal bone computed tomography (CT) evidence of SSCD were retrospectively reviewed. Intervention(s): Review of high-resolution temporal bone CT. Main Outcome Measures: The horizontal distance from the outer cortex of the squama temporalis immediately superior to the bony external auditory canal (approximating lateral edge of craniotomy) to the SSCD was measured in the coronal plane by two independent reviewers. Results: A total of 151 adult ears with SSCD were analyzed. A Shapiro-Wilk goodness-of-fit test confirmed that measurements were normally distributed. Pearson inter-rater correlation was 0.95, confirming very strong agreement. The mean distance between the outer cortex of the squama temporalis and SSCD was 25.9 mm, or 1.02 inches. Sixty-eight percent of the SSCD population would fall between 0.92 and 1.12 inches and 95% would lie between 0.83 and 1.21 inches. Conclusions: The horizontal distance from the outer cortex of the squama temporalis to the SSCD consistently approximates 1 inch. This easily remembered distance can aid surgeons in locating or confirming the SSCD during middle fossa surgery.