Establishment of normal values for flash visual evoked potentials (VEPs) in preterm infants: a longitudinal study with special reference to two components of the N1 wave.

To establish normal values for flash visual evoked potentials (VEPs) in the preterm period, we analyzed 356 records from 63 neurologically normal infants (26-35 weeks gestation) followed longitudinally. Using both sleep and drowsy state recording within the same session and a mean of 5.7 weekly recording sessions per infant, we hypothesized that the prominent negative wave (N1) consisted of 2 negative components, N1a (early) and N1b (late). This hypothesis could explain the previously reported variability in VEP indices. With a precise definition of the N1a peak, we were able to establish reference ranges for N1a peak latencies at various postmenstrual ages (PMA) which, unlike those previously reported, are narrow enough to be clinically useful. From a cross-sectional analysis we found that the N1a peak latency decreased with maturation at about 4.6 msec/week between 30 and 40 weeks PMA (P < 0.001). We also analyzed the N1 wave form and demonstrated its developmental maturation during this period. There were significant decreases in the amplitudes of both the N1a and N1b peaks with maturation (P < 0.001), but the decrease of the N1a amplitude was steeper than that of N1b. The N1 wave form changed from a wave in which the early peak (N1a) was higher than the late peak (N1b) into the reverse with N1b higher than N1a. This wave change may be related to developmental processes in the visual system. Longitudinal follow-up revealed that extra-uterine life may accelerate the maturation of the N1 wave form but has no effect on the decrease of the absolute values of peak latencies.