Hearing threshold as measured by auditory brain stem response in human neonates.

OBJECTIVES

This article evaluates the concept of auditory threshold and discusses the limitations of assessing threshold in human neonates. The advantages and limitations of assessing neonatal threshold by means of auditory brain stem response (ABR) are discussed, and data from several studies of newborn ABR threshold are compared. The authors report data from their own study designed to compare adult and neonatal ABR threshold using tonal stimuli.

EXPERIMENTAL DESIGN

Several studies are compared. Data from the authors are ABR thresholds for tone bursts of 0.5, 1.5, 4, and 8 kHz, determined from 2-channel recordings in full-term neonates and adults. Stimuli were calibrated in SPL by means of a probe microphone inserted into the ear canal along with the insert transducer of each subject.

RESULTS

All studies find a degree of threshold elevation in neonates relative to adult threshold. Neonatal ABR thresholds from our laboratory for stimuli from 500 through 8000 Hz are elevated relative to adult thresholds by 5 to 25 dB. Threshold elevation in our data and in other studies has found that neonatal ABR thresholds to high-frequency stimuli show the largest elevation relative to adults and low-frequency stimuli the most mature.

CONCLUSIONS

Thresholds of neonates, as measured by the ABR, are immature especially for high-frequency stimuli. Proper stimulus calibrations, which removes the influence of ear canal resonance, are important for comparisons of data across age groups. Developmental differences in the conductive mechanism and neural immaturity are the most harmonious explanations for elevation of neonatal ABR thresholds.