Hypoxic Ischemic Encephalopathy (HIE) is a well-recognised cause of brain injury in newborns, with an incidence of 1.5 per 1000 live births 1. In Ireland and the UK, HIE is the third commonest cause of neonatal mortality, accounting for 9% of all deaths, and 21% of term deaths 2. HIE is defined as a condition that occurs when the brain is deprived of an adequate oxygen supply, due to either reduced cerebral oxygen concentration (hypoxia) or blood supply (ischemia). It is clinically graded as mild, moderate, or severe, based on the neurological features of the infant.
The pathophysiology of brain injury in HIE is complex, but it can be summarised as a two phase process, a primary insult caused by hypoxia or/and ischemia, and a secondary insult that occurs following circulatory restoration, which is termed reperfusion injury 3. The primary insult is marked by a primary energy failure, caused by a decline in high energy phosphate levels and an increase in cerebral lactate. After a latent period, a secondary energy failure occurs 6 to 48 hours following the initial insult, leading to further brain damage due to mitochondrial dysfunction. During this phase, the brain pH shifts to alkalosis despite a high lactate level 4. This evolving process can affect all organs and lead to multiorgan dysfunction. Signs of injury to the cardiovascular system may include reduced myocardial contractility, severe hypotension, passive cardiac dilatation, and tricuspid regurgitation 5, which during our study will be evaluated using non-invasive cardiac output monitoring and echocardiogram.
Regarding the HIE treatment, publications of several international trials, and their subsequent meta-analysis have shown that early induced hypothermia is beneficial in HIE, improving survival and reducing neurological disability 6-7. Induced hypothermia has now become a standard of care in moderate/severe HIE. However, to be effective it must be commenced within 6 hours of delivery, before the secondary energy failure occurs. In this narrow window of time the population who would benefit from treatment must be identified, resuscitated, stabilised and started on therapeutic hypothermia.
Unfortunately, the infant’s condition at birth doesn’t correlate well with the degree of the injury, nor with the neurodevelopmental outcome. Relying on the perinatal events and the clinical picture in the first 6 hours of life to make an accurate decision regarding therapeutic hypothermia can prove to be unreliable in some cases. With this current study we are hoping to identify more rapidly and accurately those infants at highest risk of HIE injury and reliably predict the neurodevelopmental outcome. In order to achieve this, we will be collecting and analysing a series of blood biomarkers. Additional information will be gathered for the prediction of neurodevelopmental outcome in our infants with HIE including continuous electroencephalography (EEG), transcutaneous carbon dioxide (CO2) monitoring, near infrared spectroscopy (NIRS) and magnetic resonance (MR).
Cardiovascular function will be assessed in all infants using non-invasive cardiac output (NICOM) and echocardiography.
Another focus of our study will be on seizure activity post hypoxic-ischaemic injury. HIE is one of the main causes of seizures in the neonatal period, which have been shown to add to the initial brain injury and relate to a worse neurodevelopmental outcome 8. To detect seizure activity and measure seizure burden in our cohort we are using the gold standard, continuous video-EEG monitoring. To better understand the pathophysiological process of seizure in HIE, we will look for correlation between CO2 levels (using transcutaneous CO2 monitoring), cerebral oxygenation (using NIRS) and brain energy metabolism (using magnetic resonance spectroscopy).
We are hoping this study will also aid in the development of future therapeutic strategies that will improve the outcome of infants with hypoxic-ischaemic injury.