Intraoperative neuromonitoring for superior semicircular canal dehiscence and hearing outcomes.

Background: Recent findings in patients with superior semicircular canal dehiscence (SCD) have shown an elevated ratio of summating potential (SP) to action potential (AP), as measured by electrocochleography (ECochG). Changes in this ratio can be seen during surgical intervention. The objective of this study was to evaluate the utility of intraoperative ECochG and auditory brainstem response (ABR) as predictive tools for postoperative hearing outcomes after surgical plugging via middle cranial fossa approach for SCD syndrome (SCDS).

Methods: This was a review of 34 cases (33 patients) in which reproducible intraoperative ECochG recordings were obtained during surgery. Diagnosis of SCDS was based on history, physical examination, vestibular function testing, and computed tomography imaging. Simultaneous intraoperative ECochG and ABR were performed. Pure-tone audiometry was performed preoperatively and at least 1 month postoperatively, and air-bone gap (ABG) was calculated. Changes in SP/AP ratio, SP amplitude, and ABR wave I latency were compared with changes in pure-tone average and ABG before and after surgery.

Results: Median SP/AP ratio of affected ears was 0.62 (interquartile range [IQR], 0.45-0.74) and decreased immediately after surgical plugging of the affected canal to 0.42 (IQR, 0.29-0.52; p < 0.01). Contralateral SP/AP ratio before plugging was 0.33 (IQR, 0.25-0.42) and remained unchanged at the conclusion of surgery (0.30; IQR, 0.25-0.35; p = 0.32). Intraoperative changes in ABR wave I latency and SP amplitude did not predict changes in pure-tone average or ABG after surgery (p > 0.05).

Conclusion: This study confirmed the presence of an elevated SP/AP ratio in ears with SCDS. The SP/AP ratio commonly decreases during plugging. However, an intraoperative decrease in SP/AP does not appear to be sensitive to either the beneficial decrease in ABGs or the mild high-frequency sensory loss that can occur in patients undergoing surgical plugging of the superior semicircular canal. Future work will determine the value of intraoperative ECochG in predicting changes in vestibular function.