A Method of Locating the Dehiscence during Middle Fossa Approach for Superior Semicircular Canal Dehiscence Surgery

Introduction: Superior semicircular canal dehiscence (SSCD) describes a rare syndrome of auditory and vestibular symptoms due to a bony defect between the middle cranial fossa and the superior semicircular canal. One of the challenges during middle fossa approach for SSCD repair has been the lack of a consistent landmark and the variable relationship between the arcuate eminence and superior semicircular canal. Here we propose a novel reference point centered 7 mm above the midpoint of the external auditory canal at the level of the zygoma to be used as the craniectomy site during SSCD surgery (Fig. 1).

Methods: A retrospective review was conducted to identify patients with radiologically confirmed SSCD who underwent surgical repair via a middle fossa approach. ITK-SNAP (v3.4.0) software was used to identify the area of dehiscence and measure its distance to the proposed reference point. Statistical analysis was conducted using student’s t-test and Shapiro-Wilk goodness-of-fit test to evaluate for differences by sex and test for normality, respectively.

Results: A total of 72 cases of SSCD were identified in 60 patients. Average age was 50.8 years. Females comprised 66.7% and males comprised 33.3% of the study. Average distance for all patients was 28.84 ± 2.22 mm (range: 22.96–33.43, 95% CI: 28.32–29.36). Average distance for females was 29.08 mm (range: 24.56–33.43) versus 28.26 mm (range: 22.96–32.36) for males. There was no statistical difference in distance by sex (p = 0.174). The distance measurements followed a normal distribution (Shapiro-Wilk statistic = 0.988; p = 0.722) with 95% of the patients between 24.49 to 33.10 mm.

Conclusion: The present study analyzed a potential reference point to be used during a middle fossa approach for SSCD surgery. The distance from this reference point to the SSCD was found to be consistent among patients and may serve as a readily identifiable landmark to aid in localizing the dehiscence during SSCD surgery.

Fig. 1 The intersection of a parallel line drawn at the inferior border of the zygomatic root and a perpendicular line drawn at the midpoint of the external auditory canal marks the location of the craniectomy.