Neonatal otoacoustic emissions recorded using maximum length sequence stimuli.

Abstract

Maximum length sequence (MLS) stimulation allows transient evoked otoacoustic emissions (TEOAEs) to be recorded at very high stimulation rates. Previous work has focused on recording from normally hearing adult subjects; the aim of this study was to obtain information about emissions recorded using this technique from newborns and to compare these results with those obtained from adults. The feasibility of recording from newborns on the postnatal wards also was addressed. The study comprised two parts. In the first, TEOAEs were collected at 13 stimulation rates from a selected group of babies. The second part of the study comprised only two stimulation rates, a conventional rate of 40 clicks/sec and the maximum MLS rate of 5000 clicks/sec. The neonatal MLS TEOAEs behave in a similar manner to those obtained from adult subjects. The morphology of the waveforms was similar for the conventional and MLS TEOAEs. As the stimulus rate increases, the amplitude of the emission decreases, reaching an approximate plateau by 1000 to 2000 clicks/sec. The absolute reduction in amplitude seen at the high MLS rate is related to the amplitude of the conventional TEOAE but is always approximately the same when expressed as a percentage or proportion of that amplitude. The theoretical advantages of speed and sensitivity seen for adult subjects also should hold true for the neonatal population. Although the system used to test was a prototype with none of the refinements found in commercial systems, it was possible to record adequate emissions from a ward-based population of newborns.