Correlation of vestibular aqueduct size with air-bone gap in enlarged vestibular aqueduct syndrome.

OBJECTIVES/HYPOTHESIS Patients with enlarged vestibular aqueduct (EVA) often demonstrate an air-bone gap (ABG) at low frequencies on audiometric testing. The mechanism for this has not been well elucidated. We investigated vestibular aqueduct (VA) size and ABG in patients with EVA, and assessed the relationship between VA diameter and ABG. STUDY DESIGN Retrospective review. METHODS We assessed 98 ears from 49 individuals diagnosed with Pendred syndrome. The VA diameters of ears in groups with (n = 60) and without (n = 38) ABG were measured using axial computed tomography images. Three different parameters of VA diameter (fundus, midpoint, and porous width) were correlated with the results of pure tone analysis. RESULTS We found that patients with ABG have larger diameters of the VA than patients without ABG (fundus width = 1.13 vs. 0.71 mm, respectively; midpoint width = 3.23 vs. 2.77 mm, respectively; porous width = 2.73 vs. 2.29 mm, respectively; P < .001, .038, and .049, respectively). A cutoff value of VA fundus diameter was set at 0.865 mm. Although <5% of the variability in the ABG was explainable by variation in the diameter of the VA fundus at low frequencies, on average, larger gaps were observed with fundus diameters of >0.865 mm. CONCLUSIONS Our findings suggest that the axial parameters of the VA yield information about ABG in patients with EVA. The data are consistent with the hypothesis that an EVA introduces a third mobile window into the inner ear, resulting in an ABG at low frequencies. LEVEL OF EVIDENCE 4. Laryngoscope, 126:1633-1638, 2016.