Examination of peripheral maturity in the neonatal auditory system

The variability present in both otoacoustic emissions (OAEs) and auditory brainstem responses (ABRs) in neonates has made it very difficult to establish normative data for this population. It has been suggested that one significant factor contributing to this variability is maturation as a function of age. The purpose of the present investigation is to determine (1) if auditory peripheral maturity is present at birth; (2) if it is not, what type of maturational/developmental changes occur in the peripheral neonatal auditory system from preterm to fullterm; and (3) how do these changes affect OAEs and ABRs.; Longitudinal distortion-product otoacoustic emission (DPE) and ABR measures were obtained from a single ear of 18 preterm neonates in an intermediate care nursery. Two tones, f1 and f2 (f2 {dollar}>{dollar} f1), with an f2/f1 of 1.22, were used to generate DPEs at frequencies equal to 2f1 {dollar}-{dollar} f2. DPE amplitude and latency were measured for f2 values of 3, 4, and 5 kHz, and ABR waves I and III latencies, were examined. Each neonate was tested 3 times; twice upon arrival and one final time following discharge. The preterm neonates at the initial test session ranged in age from 33 to 38 weeks gestation. Data in the first two test sessions were obtained to determine within-subject test-retest variability and so were performed within 3 days of each other. The final test session was performed 4-10 weeks after the initial test sessions.; The longitudinal data revealed that, in general, for all f2 stimuli, DPE amplitude increased as a function of gestational age. Reduction in DPE latency for the 3 and 4 kHz stimuli was also observed as a function of gestational age. Furthermore, latencies of ABR waves I and III also decreased with increasing gestational age.; These findings suggest that (1) the auditory system is not fully mature at birth in the preterm neonate; (2) maturational/developmental changes continue from preterm to fullterm; (3) these changes are reflected in ABR and DPE measurements; and (4) these maturational/developmental changes may contribute to the variability present with ABRs and OAEs in the neonatal population.