Provisional stimulus level corrections for low frequency bone-conduction ABR in babies under three months corrected age

Abstract

Objective: To estimate bone-conduction stimulus level corrections by testing the auditory brainstem response (ABR) of normally-hearing newborns. The stimuli used were low frequency tone pips calibrated to reference levels derived from ISO 389 values. Design: Tone pips were presented via supra-aural earphones and a B71 Radioear bone vibrator at 0.5 or 1 kHz. ABR thresholds from both transducers were compared at each frequency. Study sample: twenty-seven newborn hearing screening referrals (33 ears) who passed an ABR discharge criterion at 4 kHz. Results: Median air- and bone-conduction ABR threshold differences were 30 dB at 0.5 kHz and 20 dB at 1 kHz. Conclusion: The 0.5 kHz data from this study and previous studies were compared. Previous studies suggested lower figures for the bone-conduction stimulus level correction. Likely sources of this discrepancy are discussed. The average 0.5 kHz bone-conduction correction value for infants < 3 months old is about 28 dB. The correction for 1 kHz is 20 dB. We recommend that calibration reference levels used in this study be adopted and that appropriate corrections be applied to bone conduction ABR thresholds in infants < 3 months old before calculation of any air-bone gap and subsequent clinical interpretation.