Wideband Acoustic Immittance in Adults with Unilateral Sudden Sensorineural Hearing Loss: A Within-Subject Study of Cochlear Mechanical Contributions to Ear-Canal Acoustic Energy

This study presents the first within-subject evidence that cochlear pathology alone, independent of middle-ear status, produces systematic and frequency-specific alterations in wideband acoustic immittance (WAI). Thirty adults with unilateral idiopathic sudden sensorineural hearing loss (SSNHL) underwent WAI testing of both ears. The SSNHL-affected ear was compared with the contralateral normal-hearing ear as a within-subject control, eliminating confounds related to age, sex, and ear-canal acoustics. All participants exhibited normal middle-ear function confirmed by tympanometry, absence of air–bone gaps, and normal wideband average absorbance (WBA). SSNHL-affected ears showed a bimodal absorbance pattern: significantly elevated absorbance at 1.25–2.5 kHz (Wilcoxon signed-rank, FDR-adjusted p = 0.013–0.040, r = 0.821–0.900) and significantly reduced absorbance at 4 kHz (p = 0.004, r = 0.974) and 5 kHz (p = 0.003, r = 0.989). WBA (0.375–2 kHz) was also significantly elevated (p = 0.001, r = 0.840). Absorbance at 4–5 kHz correlated negatively with high-frequency hearing-loss severity (Spearman ρ = −0.421 to −0.448, p < 0.05), indicating a dosedependent relationship between cochlear degeneration and impaired acoustic energy transmission. Resonance frequency (RF) was significantly lower in SSNHL-affected ears (660 ± 156 Hz) compared with contralateral normal-hearing ears (848 ± 185 Hz; Wilcoxon W = 41, p < 0.001, r = 0.912), consistent with increased cochlear mass loading due to intracochlear fibrosis and degeneration. These findings demonstrate that WAI captures a biomechanical signature of cochlear degeneration detectable non-invasively at the ear canal, positioning wideband absorbance and resonance frequency as potential objective indicators of cochlear mechanical integrity in SSNHL.