Etiopathogenesis and Increased Cochlear Responsiveness in Superior Canal Dehiscence: A Novel Hypothesis

Abstract

Introduction: Superior canal dehiscence (SCD) causes cochleovestibular hyper-responsiveness due to the presence of a third window in the superior canal (SC). The etiology of SCD is controversial. An embryological perspective suggests that SCD may occur through failure of postnatal bone formation over the SC, whereas an acquired theory suggests that trauma or pressure from the overlying temporal lobe could break or gradually thin the SC. For the diagnosis of SCD syndrome, it is important to obtain physiologic confirmation of SCD in addition to demonstrate anatomical SCD. Few studies have addressed electrocochleographic findings in SCD. This study infers the etiology of SCD by analyzing temporal bone CT, and evaluates electrocochleographic findings of patients with SCD syndrome and determines their diagnostic value. Material and methods: Fourteen SCD syndrome patients were recruited. High resolution temporal bone CT scans of 12 SCD syndrome patients (13 SCD ears and 11 normal ears) and those of 34 age-matched controls (68 ears) were used for CT analysis. The control group was subdivided into an aerated group (18 ears) and a non-aerated group (50 ears), as defined by the presence of air cells above the SC. The thickness of the SC, horizontal canal (HC) and posterior canal (PC) of unaffected ear of SCD patients were compared with that of controls to evaluate SCD etiology from an embryological perspective. An acquired factor that affects the bone thinning was explored through a comparison of SC thickness in an aerated group and a non-aerated group. Pure tone audiometry, cervical vestibular evoked myogenic potentials (cVEMP) and extratympanic electrocochleography (ECoG) were performed in 13 SCD patients. Summating potential (SP) to action potential (AP) ratio was evaluated. Bone conduction (BC) thresholds and air-bone gaps (ABG) at low frequency (250, 500, 1000 Hz) were analyzed. Relationships between SP to AP ratios, as measured by ECoG, and other audiometric parameters were evaluated. Sensitivity analysis of SP/AP ratios was performed by using data of SCD syndrome patients and 19 age-matched healthy volunteers. Results: The SC of unaffected side in the SCD patients (0.41 ± 0.23 mm) was significantly thinner than that in the control group (0.64 ± 0.21 mm, p = 0.002). The HC and the PC were also thinner in the SCD patients (0.58 ± 0.11, 1.39 ± 0.31 mm) than in the control (0.70 ± 0.08 mm, 1.61 ± 0.32 mm

p < 0.0001, p = 0.005, respectively). Within the control group, the SC was significantly thicker in the aerated group (0.73 ± 0.14 mm) than in the non-aerated group (0.60 ± 0.23

p = 0.046)

however, no significant difference was observed for HC and PC thickness (aerated group, n = 18, 0.72 ± 0.07 mm, 1.67 ± 0.34

non-aerated group, n = 50, 0.70 ± 0.09, 1.59 ± 0.34 mm

p = 0.350, p = 0.428, respectively). Mean SP/AP ratio of SCD ears was significantly higher than that of unaffected ears (0.52 versus 0.25, p < 0.001) and SPs were significantly elevated in affected ears (p = 0.011), whereas APs were similar for affected and unaffected ears. SP/AP ratio showed a sensitivity of 92.3% and a specificity of 94.0% for distinguishing SCD syndrome patients given the inclusion criteria applied (symptoms, TBCT, cVEMP threshold) at a cutoff value of 0.34 (p < 0.001). SP/AP ratio was not correlated with SCD size or cVEMP threshold in affected ears. Negative absolute values of bone conduction at low frequency tended to increase with SP/AP ratio. Conclusion: The bony otic capsule is significantly thinner in SCD patients than in controls. However, even within unaffected individuals, SCs lacking overlying air cells are also thinner than those with overlying air cells. These results suggest that both embryological and acquired factors affect the occurrence of SCD. SP/AP ratio in ECoG appears to be a valuable diagnostic adjunct for functional demonstration of the third window in the otic capsule with high sensitivity and specificity, and thus, can support a clinical diagnosis of SCD when used in conjunction with clinical and radiological findings.