Evaluation and management of superior canal dehiscence syndrome

Superior canal dehiscence (SCD) syndrome is a rare disorder involving the inner ear that was first described by Lloyd Minor in 1998. Patients with SCD syndrome can present with a range of auditory and/or vestibular signs and symptoms that are associated with a bony defect of one or both superior semicircular canals. There have been over 300 publications on SCD since its discovery, but much is still not understood about this unusual clinical condition that can profoundly influence auditory and vestibular function in pediatric and adult patients. This thesis has focused on the evaluation and management of SCD and examined the outcomes following surgical repair. We have explored unresolved issues regarding the variability in clinical presentation of SCD using a human temporal bone model as well as a large cohort of patients with SCD syndrome. We answered questions regarding the effect of SCD size and location. We examined CT-scan testing in a large clinical cohort of patients and assessed the effect of SCD size and location on auditory signs and symptoms, hearing loss and vestibular testing. We found that a larger SCD and a location closer to the ampullated end of the superior semicircular canal (SSC) appeared to correspond with auditory signs and symptoms, a larger air- bone gap (ABG) and lower cervical vestibular evoked myogenic potential (cVEMP) thresholds. This can help explain why a patient with SCD signs and symptoms and radiologically confirmed anatomic SCD does not present with similar characteristics on diagnostic testing. These findings regarding hearing loss are in accordance with the results from our human cadaveric temporal bone model. In addition, we found in our temporal bone model a large spread in hearing loss for similar SCD sizes between ears. This observation supports our clinical findings that the magnitude of the ABG does vary among patients with bony defects of the superior canal that are similar in size. We determined the management of patients with bilateral SCD, by assessing a large cohort of clinical patients, both those patients who were managed conservatively and those who underwent surgical repair of the SCD. In patients with bilateral SCD, lateralization of tuning fork testing, the size of the ABG and VEMP thresholds help to determine the more affected ear. Comparison of pre- and postoperative signs and symptoms showed an overall reduction following surgical repair of the dehiscence. Postoperative audiologic testing demonstrates a normalization of the ABG and cVEMP thresholds. However, decreased bone-conduction sensitivity and risk of surgical complications were also found. In patients with post-operative recurrence of symptoms, VEMP thresholds are found to be helpful in diagnosing its cause. This testing helps to discriminate between a recurrence and persistence of the SCD in the surgically repaired ear and unmasking of symptoms in the contralateral SCD ear. Our systematic review of the literature revealed a large heterogeneity on reporting of the surgical outcomes. We conclude that there is a need for more prospective data reporting which would ultimately help us to provide better patient counselling and surgical planning.