Effects of bone oscillator coupling method, placement location, and occlusion on bone-conduction auditory steady-state responses in infants.

OBJECTIVE The aim of these experiments was to investigate procedures used when estimating bone-conduction thresholds in infants. The objectives were: (i) to investigate the variability in force applied using two common bone-oscillator coupling methods and to determine whether coupling method affects threshold estimation, (ii) to examine effects of bone-oscillator placement on bone-conduction ASSR thresholds, and (iii) to determine whether the occlusion effect is present in infants by comparing bone-conduction ASSR thresholds for unoccluded and occluded ears. DESIGN Experiment 1A: The variability in the amount of force applied to the bone oscillator by trained assistants (n = 4) for elastic-band and hand-held coupling methods was measured. Experiment 1B: Bone-conduction behavioral thresholds in 10 adults were compared for two coupling methods. Experiment 1C: ASSR thresholds and amplitudes to multiple bone-conduction stimuli were compared in 10 infants (mean age: 17 wk) using two coupling methods. Experiment 2: Bone-conduction ASSR thresholds and amplitudes were compared for temporal, mastoid and forehead oscillator placements in 15 preterm infants (mean age: 35 wk postconceptual age (PCA)). Experiment 3: Bone-conduction ASSR thresholds, amplitudes and phase delays were compared in 13 infants (mean age: 15 wk) for an unoccluded and occluded test ear. All infants that participated had passed a hearing screening test. RESULTS Experiment 1A: Coupling method did not significantly affect the variability in force applied to the oscillator. Experiment 1B: There were no differences in adult bone-conduction behavioural thresholds between coupling methods. Experiment 1C: There was no significant difference between oscillator coupling method or significant frequency x coupling method interaction for ASSR thresholds or amplitudes in the young infants tested. However, there was a nonsignificant 9-dB better threshold at 4000 Hz for the elastic-band method. Experiment 2: Mean bone-conduction ASSR thresholds for the preterm infants were not significantly different for the temporal and mastoid placements. Mean ASSR thresholds for the forehead placement were significantly higher compared to the other two placements (12-18 dB higher on average). Mean ASSR amplitudes were significantly larger for the temporal and mastoid placements compared to the forehead placement. Experiment 3: There was no difference in mean ASSR thresholds, amplitudes or phase delays for the unoccluded versus occluded conditions. CONCLUSIONS Trained assistants can apply an appropriate amount of force to the bone oscillator using either the elastic-band or hand-held method. Coupling method has no significant effect on estimation of bone-conduction thresholds; therefore, either may be used clinically provided assistants are appropriately trained. For preterm infants, there are no differences in ASSRs when the oscillator is positioned at the temporal or mastoid placement. However, thresholds are higher and amplitudes are smaller for the forehead placement, consequently, a forehead placement should be avoided for clinical testing. There does not appear to be a significant occlusion effect in young infants; therefore, it may be possible to do bone-conduction testing with ears unoccluded or occluded without applying a correction factor, although further research is needed to confirm this finding.