Thickness of the bony otic capsule: etiopathogenetic perspectives on superior canal dehiscence syndrome.

The etiology of superior canal dehiscence (SCD) is controversial. An embryological perspective suggests that SCD may occur through the failure of postnatal bone formation over the superior semicircular canal (SC), whereas an acquired theory suggests that trauma or pressure from the overlying temporal lobe could break or gradually thin the SC. We infer the etiology of SCD by comparing the thickness of the bony otic capsule of the unaffected side of SCD patients with that of non-SCD participants. Twelve SCD patients (13 SCD ears and 11 normal ears) and 34 age-matched controls (68 ears) were included. The control group was subdivided into an aerated group (49 ears) and a nonaerated group (19 ears), as defined by the presence of air cells above the SC. A high-resolution temporal bone CT was performed in all participants. The thicknesses of the SC, horizontal canal (HC) and posterior canal (PC) of the unaffected ears of SCD patients were compared with those of the controls. The SC of the unaffected side in the SCD group (n = 11, 0.41 ± 0.23 mm) was significantly thinner than the one in the control group (n = 68, 0.64 ± 0.21 mm, p = 0.002). The HC and PC were also thinner in the SCD group (n = 24, 0.58 ± 0.11 and 1.39 ± 0.31 mm, respectively) than in the controls (0.70 ± 0.08 and 1.61 ± 0.32 mm; p < 0.0001 and p = 0.005, respectively). The SC, HC and PC thicknesses were also compared between the aerated and nonaerated ears within the control group. The SC was significantly thicker in the aerated group (0.73 ± 0.14 mm) than in the nonaerated group (0.60 ± 0.23 mm; p = 0.046); however, no significant difference was observed for the HC and PC thickness (aerated group, n = 49, 0.72 ± 0.07 and 1.67 ± 0.34 mm; nonaerated group, n = 19, 0.70 ± 0.09 and 1.59 ± 0.34 mm; p = 0.350 and p = 0.428, respectively). The bony otic capsule was significantly thinner in the SCD patients than in the controls. However, even within unaffected individuals, SCs lacking overlying air cells were also thinner than those with overlying air cells. These results suggest that both embryological and acquired factors affect the occurrence of SCD.