Methods for determining auditory evoked brain-stem response thresholds in human newborns

Auditory Brainstem Response Thresholds to Air- and Bone-Conducted CE-Chirps in Neonates and Adults

PURPOSE

The purpose of this study was to compare auditory brainstem response (ABR) thresholds to air- and bone-conducted CE-Chirps in neonates and adults.

METHOD

Thirty-two neonates with no physical or neurologic challenges and 20 adults with normal hearing participated. ABRs were acquired with a starting intensity of 30 dB normal hearing level (nHL). The lowest stimulus intensity level at which a wave V was identifiable and replicable was considered the ABR threshold.

RESULTS

ABR thresholds to air-conducted CE-Chirps were 9.8 dB nHL for neonates and adults. ABR thresholds to bone-conducted CE-Chirps were 3.8 and 13.8 dB nHL for neonates and adults, respectively. The difference in ABR thresholds to bone-conducted CE-Chirps was significantly different (p < .0001, ηp2 = .45). Adults had significantly larger wave V amplitudes to air- (p < .0001, ηp2 = .50) and bone-conducted (p = .013, ηp2 = .15) CE-Chirps at a stimulus intensity of 30 dB nHL. At the same intensity, adults evidenced significantly shorter wave V latencies (p < .0001, ηp2 = .49) only with air-conducted CE-chirps.

CONCLUSION

The difference in ABR thresholds and wave V latencies to air- and bone-conducted CE-Chirps between neonates and adults may be attributed to a disparity in effective signal delivery to the cochlea.

Neonatal auditory brainstem responses recorded from four electrode montages

Simultaneous auditory brainstem responses (ABRs) to click stimuli at 30 and 60 dB nHL were recorded from 16 full term neonates with four electrode arrays: vertical (Cz-Nape of neck), ipsilateral (Cz-Mi), contralateral (Cz-Mc), and horizontal (Mc-Mi). Results indicated that ABR waveforms were morphologically similar to those recorded in adults: Waves I, III, and V were clearly identifiable and of the same polarity in 15 of 16 subjects 60 dB nHL and Wave V was clearly identifiable in 14 of 16 subjects at 30 dB nHL. Although ABR waves were identified in most cases, waveforms expression was variable with different electrode recording montages. It is suggested that if a clinician must choose one montage array for recording neonatal ABRs at high intensity levels for neurodiagnostic evaluations, the ipsilateral recording array is indicated since it displays the highest expression of wave components. For screening applications or threshold searching at lower stimulus levels, however, either the ipsilateral or vertical montage would be justified. Further, caution should be employed when interpreting ABR results in neonates recorded with unconventional montages due to the variable expression of wave components.

The effects of rate and forward masking on human adult and newborn auditory evoked brainstem response thresholds

Several investigators have reported that human newborn auditory brainstem evoked response (ABR) thresholds to clicks are about 10-20 dB higher than human adult thresholds to the same stimuli (Galambos & Hecox, 1978; Shulman-Galambos & Galambos, 1979; Lasky, Rupert, & Waller, 1987). The present study replicated these results, but, in addition, addressed developmental differences in ABR thresholds due to forward masking and changes in stimulus repetition rate. Eight human newborns and seven human adults served as subjects. Newborn ABR thresholds to a 100 microseconds click which followed a 82 db(A) SPL 50 ms broadband forward masker by 10 ms were significantly more elevated relative to their unmasked ABR thresholds than were those of the adults. Relatively loud sounds seem to have a greater effect on the newborn's ability to process subsequently presented soft sounds than they do on adult processing. In contrast, changes in repetition rate of 100 microseconds clicks from 10/s to 50/s had no greater effect on newborn than adult ABR thresholds. At near threshold stimulus levels, rate effects on the ABR are not apparent in newborns nor adults.

Estimation of auditory threshold during sleep using brainstem auditory-evoked potentials

The brainstem auditory-evoked potential (BAEP) is often employed to determine auditory threshold in subjects who are unable to complete traditional behavioural testing. These responses are usually recorded during natural or drug-induced sleep. The effects of sleep on the BAEP threshold have however not been determined. BAEPs were recorded in 8 young adults in wakefulness and during natural sleep. Click stimuli were presented through a hearing aid device at either 0, 5, 10, 20 or 30 dB above the behavioural threshold. The amplitude of wave V decreased and its latency increased with decreases in stimulus intensity. The BAEP waveform was invariant in sleep. The overall mean BAEP threshold was approximately 16 dB above that determined by behavioural methods. This did not significantly vary in the sleeping subject.