"Linear" and "derived" otoacoustic emissions in newborns: a comparative study.

OBJECTIVE: To investigate the effects of a specific aspect of the acquisition procedure, the averaging technique, on the evaluation of click-evoked otoacoustic emissions (CEOAEs) in newborns. DESIGN: CEOAEs were recorded by an Otodynamic ILO88 system from 89 full-term newborns at the third day after delivery. For each ear and in the same test session, CEOAEs were evoked by 75 to 85 dB pSPL acoustic clicks and averaged according to two different modes: the "linear" (classic average) and the "derived" mode, which allows the cancellation of linear behaving components (such as acoustic artifacts). All examined ears had a normal auditory function as assessed by conventional ABR between the ages of 2 and 4 mo. CEOAEs obtained by both averaging techniques were compared on the basis of several quantitative parameters: the waveform similarity; the levels of signal and noise and the inter-test reproducibility of the broadband response and of four different frequency bands centered at 1.6, 2.4, 3.2, and 4 kHz; the amplitude as a function of time; the test time. To eliminate the contribution of the stimulus artifact, linear CEOAEs were windowed 6 to 20 msec, whereas derived emissions were windowed using the default ILO88 window (2.5 to 20 msec). Additionally, CEOAEs were classified as "pass" or "fail" accordingly to screening criteria used in the daily clinical practice. RESULTS: Linear and derived emissions had very similar wave shapes and no time shifts during the first 12.5 msec. On the contrary, clear differences in the waveforms and time shifts were observed at longer latencies. The use of both averaging techniques resulted in identical CEOAE levels for both the broadband response and for the first two tested frequencies. For the last two frequencies, emission levels were lower when averaged with the linear technique owing to the use of the time window 6 to 20 msec, which reduces the amplitudes of high-frequency components. The residual noise in derived traces is 6 dB higher than that from linear traces. Also, derived CEOAEs had a lower inter-test reproducibility in both the broadband compound emission and in the four frequency bands examined here. The greatest difference in reproducibility was observed at the lowest band (1.2 to 2 kHz). Scoring of emissions was influenced by the averaging technique: 14% CEOAEs obtained with linear averaging and scored as passes were classified as fails when averaged with the derived mode. Moreover, if a CEOAE was scored as pass when using the derived technique, it also was scored as pass when using linear averaging. The increased number of false positives most likely was due to the higher noise floor/lower signal to noise ratio (SNR) of CEOAEs obtained with the derived technique. CONCLUSIONS: In the tested newborns and at the levels of stimulation used in this study, the emissions obtained with the derived technique were noisier than those obtained with the linear technique, this being intrinsically due to the type of averaging. Therefore, screening criteria based on the evaluation of the SNR (or similar parameters) could be influenced by the type of averaging used during the acquisition.