Dehiscence Size in SSCD Correlates with Air-Bone Gap Size

by Gerard Gianoli, MD •

Does the size of the dehiscence in SSCD correlate with the size of the air-bone gap?

Background: Patients with superior semicircular canal dehiscence (SSCD) present with a variety of symptoms that may include a low frequency conductive hearing loss on pure-tone audiometry. It has been postulated that this air-bone gap happens when the dehiscent superior canal acts as a third mobile window, dissipating acoustic energy. Not all patients exhibit an air-bone gap, however, and, to date, no study has correlated the size of the air-bone gap with the size of the dehiscence.

Study Design: Retrospective chart review of 23 patients (28 ears) diagnosed with SSCD

Setting: Tertiary referral center in Perth, Australia

Synopsis: Twenty-three patients (28 ears) met the study criteria of typical SSCD history; at least one physiologic test confirming SSCD (usually an abnormal vestibular evoked myogenic potential [VEMP]); and observation of SSCD on a high-resolution CT scan of the temporal bone. The dehiscent areas of the superior semicircular canal were measured to the nearest 0.5 mm and ranged from 1.0 – 6.0 mm (mean 3.5 ± 1.6mm). The air-bone gap was determined for 500, 1,000 and 2,000 Hz, both individually and as an average of the three frequencies. All six ears that did not have an air-bone gap had dehiscences measuring less than 3.0 mm. In the 22 ears that demonstrated air-bone gaps, the gaps ranged from 3.3 to 27 dB (mean 11.6 ± 5.7 dB). A linear regression analysis demonstrated a coefficient of 0.780 (p<0.001), suggesting that the larger the dehiscence, the bigger the air-bone gap.

Bottom Line: A low frequency air-bone gap was consistently documented in patients with SSCD where the dehiscence measures greater than 3 mm. The size of the air-bone gap increased with the size of the dehiscence.

Citation: Yuen HW, Boeddinghaus R, Eikelboom RH, et al. The relationship between the air-bone gap and the size of the superior semicircular canal dehiscence. Otolaryngol Head Neck Surg. 2009;141(6):689-694.