Relative maturation of peripheral and central regions of the neonatal brainstem was studied using brainstem auditory evoked responses in 174 healthy preterm infants (gestational age 30 -36 wk). From 30-to 42-wk postconceptional age (PCA), I-III and III-V intervals shortened with increasing age. It was difficult to detect any apparent differences in maturational rate between the two intervals. However, III-V/I-III interval ratio decreased with increasing age, indicating that from preterm to term III-V interval shortens more than I-III interval. During term period (37-to 42-wk PCA), I-III interval was similar to term controls, but III-V interval was significantly longer and III-V/I-III interval ratio was significantly greater than controls at 37-to 38-wk PCA and 39 -40 wk PCA and was the same as controls at 41-42 wk. Therefore, from 30-to 42-wk PCA maturation of central regions of the brainstem, reflected by III-V interval, is relatively faster than peripheral regions, reflected by I-III interval which seems to be already more mature than III-V interval before 30 wk. Maturation in central regions in preterm infants is relatively delayed at early term, but "catches-up" later, whereas peripheral regions already reach normal level of maturation at early term. Preterm birth slightly delays early maturation of central brainstem regions. . Previous studies have shown that the immature brainstem can be damaged by various unfavorable perinatal conditions, e.g., hypoxia-ischemia (2). Recent studies revealed that brain structure may be abnormal in preterm infants and preterm birth is associated with regional cerebral tissue reductions (3,4). It remains unclear whether the rate of maturation in the more peripheral and more central regions of the brainstem is equal or different during earlier life and what influence the preterm birth exerts on this maturation. The understanding will help improve the care and management of very young infants, particularly for those who may be at risk of developmental abnormalities.In addition to detecting auditory abnormalities, brainstem auditory evoked responses (BAERs) have been used to study functional status and maturation of the, specifically auditory, brainstem (5-7). The responses, generating from specific brainstem auditory relay nuclei, are extremely complex and are under multiple inhibiting and facilitating influences from the rest of the CNS. Maturation of the BAER is associated with myelination of axons, formation of central synaptic connections, and axonal diameter and has been found to correlate with maturation of the brainstem, as well as the auditory pathway (8 -10). During early life, the BAER undergoes tremendous maturational changes, characterized by a shortening of wave latencies and an increase in wave amplitudes of the response (11-14). Furthermore, a shortening of the intervals between BAER wave peaks (i.e., interpeak intervals) with increasing age has been well documented.In BAER wave form, wave I, III, and V, the three major and reliable waves, originate fro...

BAER is affected by stimulus rate more in younger children than in the older. Adult-like rate-dependent changes are reached at 1-2 years for wave I latency and I-III interval, and 3-4 years for wave III and V latencies and I-V and III-V intervals. Our BAER data at different click rates provide normal references for subjects of various ages.

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Development of early (I-III) and late (III-V) brainstem conduction time (BCT) in the brainstem auditory evoked responses was examined and compared in 178 appropriate-for-gestational-age (AGA) and 24 small-for-gestational-age (SGA) children from birth to 6 years of age. In AGA children, the III-V/I-III interval ratio increased with age from birth to 2 years and remained relatively steady. This indicates that the development of the early and late BCT, or probably the lower and upper brainstem is not synchronous during early childhood, with a slightly faster development of the early BCT relative to the late BCT. In SGA children, the I-V, I-III and III-V intervals showed similar developmental trends to those in AGA children. However, the III-V/I-III ratio followed a developmental course which differed remarkably from that in AGA children. The ratio decreased slightly with age up to 2 years and was consistently smaller than in normal children after 1 year, indicating that the relative development of early and late BCT deviates from normal. This finding suggests that prenatal factors responsible for intrauterine growth retardation could alter the late or long-term development of the nervous system, resulting in sub-optimal outcome.

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