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.

Key Words::

• Auditory brainstem response
• air conduction
• bone conduction
• age-related
• stimulus level correction
• calibration reference levels

Notes

Acknowledgements

This paper has been presented as a poster at the International Evoked Response Audiometry Study Group meeting held in New Orleans, June 2013, and used as the basis for an oral presentation at the British Society of Audiology Annual Conference held in Keele University, UK, in September, 2013.

Declaration of interest: The authors alone are responsible for the content and writing of this paper. There are no conflicts of interest to report.

Notes

1. WHO definition, 2012: Preterm is defined as babies born alive before 37 weeks of pregnancy are completed.

2. When using TDH39 earphones 30 dB eHL corresponds to 40 dB nHL Re: ISO 389-6 (2007). When using ER-3A insert earphones 30 dB eHL corresponds to 30 dB nHL Re: ISO 389-6 (2007). The dB eHL to dB nHL correction includes two elements: an age-related transducer correction and a correction to predict the behavioural threshold from the ABR threshold. Full details are given in the English newborn hearing screening programme guidelines for early audiological assessment (2011).