Does aetiology of neonatal encephalopathy and hypoxic-ischaemic encephalopathy influence the outcome of treatment?

Neonatal encephalopathy, a clinical syndrome affecting term-born and late preterm newborn infants, increases the risk of perinatal death and long-term neurological morbidity, especially cerebral palsy. With the advent of therapeutic hypothermia, a treatment designed for hypoxic or ischaemic injury, associated mortality and morbidity rates have decreased. Unfortunately, only about one in eight neonates (95% confidence interval) who meet eligibility criteria for therapeutic cooling apparently benefit from the treatment. Studies of infants in representative populations indicate that neonatal encephalopathy is a potential result of a variety of antecedents and that asphyxial complications at birth account for only a small percentage of neonatal encephalopathy. In contrast, clinical case series suggest that a large proportion of neonatal encephalopathy is hypoxic or ischaemic, and trials of therapeutic hypothermia are specifically designed to include only infants exposed to hypoxia or ischaemia. This review addresses the differences, definitional and methodological, between infants studied and investigations undertaken, in population studies compared with cooling trials. It raises the question if there may be subgroups of infants with a clinical diagnosis of hypoxic-ischaemic encephalopathy (HIE) in whom the pathobiology of neonatal neurological depression is not fundamentally hypoxic or ischaemic and, therefore, for whom cooling may not be beneficial. In addition, it suggests approaches to future trials of cooling plus adjuvant therapy that may contribute to further improvement of care for these vulnerable neonates.