Impact of age and sensorineural hearing loss on sound transmission to the inner ear based on analysis of air-bone gaps

Air-bone gaps (ABGs) provide a clinical marker of middle ear transmission efficiency, yet their dependence on age and sensorineural hearing loss remains unclear. We retrospectively analyzed 24,570 audiograms (13,417 patients, 4-102 years) collected at three Swedish audiology clinics between 2018 and 2019, excluding ears with evidence of conductive pathology. ABGs were computed from pure-tone air conduction (AC) and bone conduction (BC) thresholds at 250-4000 Hz. Mean ABGs deviated significantly from 0 dB at all six test frequencies, with clinically relevant differences at 250, 1000, 3000, and 4000 Hz. Children and adolescents showed systematically larger ABGs than adults, consistent with age-dependent craniofacial and BC transmission differences. In adults, ABGs decreased with age at 500 Hz and 2 kHz and increased at 250 Hz, 3 kHz, and 4 kHz, with differences depending on whether analyses included all adults or only adults with normal hearing. These age-dependent patterns likely reflect changes in soft tissue at the BC transducer and middle ear stiffness. Threshold-related effects were strongest at 250 and 500 Hz when ABGs were referenced to AC thresholds, where BC transducer distortion and vibrotactile responses likely contributed. Across frequencies, ABG distributions were predominantly positively skewed, with values ranging between -0.21 and 0.55. These findings demonstrate that even if ABGs vary with age and hearing threshold, the influence from middle ear transmission is minor. Instead, most of the systematic variations of ABGs relate to methodological constraints, underscoring the importance of careful interpretation of ABGs in clinical diagnostics.