• Vol. 40 No. 1, 59–64
  • 15 January 2011

15th Yahya Cohen Memorial Lecture – The Relationship between the Air-bone Gap and the Size of Superior Semicircular Canal Dehiscence



Introduction: This study aimed 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.

Materials and Methods: The study design was a case series with chart review. Twenty-three patients (28 ears) from a tertiary referral centre were diagnosed with SSCD. 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 (AvABG500-2000) ranging from 3.3 to 27.0 dB (mean, 11.6 ± 5.7 dB). The analysis of the relationship between the dehiscence size and AvABG500-2000 revealed a correlation of R2 = 0.828 (P <0.001, quadratic fit) and R2 = 0.780 (P <0.001, linear fit). Therefore, the larger the dehiscence, the larger the ABG at lower frequencies on pure-tone audiometry.

Conclusion: 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.

Superior semicircular canal dehiscence (SSCD) can result in a characteristic constellation of vestibular and auditory features, including autophony, oscillopsia, and vertigo, and characteristic eye movements induced by sound and pressure changes. Patients with SSCD also show lowered thresholds for vestibular-evoked myogenic potentials (VEMPs) and a heightened sensitivity to bone-conducted sounds in the presence of normal middle ear function and intact acoustic reflexes. These patients can have conductive hearing loss and often show an air-bone gap (ABG) at the low frequency on pure-tone audiometry testing.

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