Duration of rhythmic EEG patterns in neonates: new evidence for clinical and prognostic significance of brief rhythmic discharges

Objective: This study aimed at identifying the characteristics - especially the duration - of rhythmic discharges in neonatal EEG, and their associati on with clinical neonatal problems. Specifically, we aimed at testing the d iagnostic and prognostic validity of using 10 s as minimal duration for def ining electroencephalographic seizures. Design and methods: The polysomnographies of 340 neonates were reviewed, an d episodes of rhythmic discharges were identified, analyzed, and quantified . The study sample was divided into 3 groups: one in which the maximal dura tion of rhythmic discharges was shorter than 10 s (brief rhythmic discharge s, BRD), a second one in which there were rhythmic discharges longer than 1 0 s (long rhythmic discharges, LRD), and finally a group in which no rhythm ic discharge was found (No-RD). These 3 groups of subjects were compared fo r the baseline and outcome clinical data. Results: From the 340 neonates studied, 210 did not present any form of rhy thmic discharge, 67 (19.7%) had only BRD episodes, and 63 (18.5%) had at le ast one LRD episode. Prevalence of rhythmic discharges was low among health y full term newborns, and was significantly higher among preterm and high-r isk newborns, Electrophysiological characteristics of rhythmic discharges d id not differ between healthy neonates and high-risk ones. Accompanying cli nical manifestations were present in 26.3% of the LRD group, but also in 15 .9% of the BRD group. The presence of BRD was significantly associated with leukomalacia and with hypoglycemia in the cross-sectional analysis of base line data, and with an increased risk for abnormal neurodevelopmental outco me after a mean follow-up period of 47 months (adjusted relative risk = 4.9 0, P < 0.01). Conclusions: The present data demonstrate an association between BRD and cl inical history of hypoxic-ischemic encephalopathy, especially when complica ted by leukomalacia, and also with a prognosis of increased risk for abnorm al neurodevelopmental outcome. The clinical and prognostic significance of isolated BRD justifies the need to include these brief episodes in future s tudies of neonatal seizures. (C) 2000 Elsevier Science Ireland Ltd. All rig hts reserved.