The relationship between the air-bone gap and the size of superior semicircular canal dehiscence

OBJECTIVE: To examine the relationship between the air-bone gap (ABG) and the size of the superior semicircular canal dehiscence (SSCD) as measured on a computed tomography (CT) scan.

STUDY DESIGN: Case series with chart review.

SETTING: Tertiary referral center.

PATIENTS: Twenty-three patients (28 ears) diagnosed with SSCD.

MAIN OUTCOME MEASURES: The size of the dehiscence on CT scans and the ABG on pure-tone audiometry were recorded.

RESULTS: The size of the dehiscence ranged from 1.0 to 6.0 mm (mean, 3.5 +/- 1.6 mm). Six ears with a dehiscence measuring less than 3.0 mm did not have an ABG (0 dB). The remaining 18 ears showed an average ABG at 500, 1000, and 2000 Hz (AvABG(500-2000)) ranging from 3.3 to 27.0 dB (mean, 11.6 +/- 5.7 dB). The analysis of the relationship between the dehiscence size and AvABG(500-2000) revealed a correlation of R(2) = 0.828 (P < 0.001, quadratic fit) and R(2) = 0.780 (P < 0.001, linear fit). Therefore, the larger the dehiscence, the larger the ABG at lower frequencies on pure-tone audiometry.

CONCLUSIONS: In SSCD patients, an ABG is consistently shown at the low frequency when the dehiscence is larger than 3 mm. The size of the average ABG correlates with the size of the dehiscence. These findings highlight the effect of the dehiscence size on conductive hearing loss in SSCD and contribute to a better understanding of the symptomatology of patients with SSCD.