Cerebral magnetic resonance biomarkers in neonatal encephalopathy: a meta-analysis

Potential biomarkers for hypoxic-ischemic encephalopathy

Cerebral hypothermia reduces brain injury and improves behavioral recovery after hypoxia-ischemia (HI) at birth. However, using current enrolment criteria many infants are not helped, and conversely, a significant proportion of control infants survive without disability. In order to further improve treatment we need better biomarkers of injury. A 'true' biomarker for the phase of evolving, 'treatable' injury would allow us to identify not only whether infants are at risk of damage, but also whether they are still able to benefit from intervention. Even a less specific measure that allowed either more precise early identification of infants at risk of adverse neurodevelopmental outcome would reduce the variance of outcome of trials, improving trial power while reducing the number of infants unnecessarily treated. Finally, valid short-term surrogates for long term outcome after treatment would allow more rapid completion of preliminary evaluation and thus allow new strategies to be tested more rapidly. Experimental studies have demonstrated that there is a relatively limited 'window of opportunity' for effective treatment (up to about 6-8h after HI, the 'latent phase'), before secondary cell death begins. We critically evaluate the utility of proposed biochemical, electronic monitoring, and imaging biomarkers against this framework. This review highlights the two central limitations of most presently available biomarkers: that they are most precise for infants with severe injury who are already easily identified, and that their correlation is strongest at times well after the latent phase, when injury is no longer 'treatable'. This is an important area for further research.

Patterns of neonatal hypoxic-ischaemic brain injury

Enormous progress has been made in assessing the neonatal brain, using magnetic resonance imaging (MRI). In this review, we will describe the use of MRI and proton magnetic resonance spectroscopy in detecting different patterns of brain injury in (full-term) human neonates following hypoxic–ischaemic brain injury and indicate the relevance of these findings in predicting neurodevelopmental outcome.

Tract-based spatial statistics of magnetic resonance images to assess disease and treatment effects in perinatal asphyxial encephalopathy

Biomarkers are required for efficient trials of neuroprotective interventions after perinatal asphyxia. This study aimed to determine whether diffusion tensor imaging (DTI) analyzed by tract-based spatial statistics (TBSS) may be a suitable biomarker of disease and treatment effects after perinatal asphyxia in small groups of patients. We performed TBSS from DTI obtained at 3 T from eight healthy control infants, 10 untreated and 10 hypothermia-treated infants with neonatal encephalopathy. Median (range) postnatal age at scan was 1 d (1-21) in the healthy infants, 6 d (4-20) in the cooled, and 7 d (4-18) in noncooled infants. Compared with the control group, fractional anisotropy (FA) was significantly reduced not only in several white matter tracts in the noncooled infants but also in the internal capsule in the cooled group. Noncooled infants had significantly lower FA than the cooled treated infants, indicating more extensive damage, in the anterior and posterior limbs of the internal capsule, the corpus callosum, and optic radiations. We conclude that perinatal hypoxic ischemic encephalopathy is associated with widespread white matter abnormalities that are reduced by moderate hypothermia. DTI analyzed by TBSS detects this treatment effect and is therefore a qualified biomarker for the early evaluation of neuroprotective interventions.

MRI and withdrawal of life support from newborn infants with hypoxic-ischemic encephalopathy

The majority of deaths in infants with hypoxic-ischemic encephalopathy (HIE) follow decisions to withdraw life-sustaining treatment. Clinicians use prognostic tests including MRI to help determine prognosis and decide whether to consider treatment withdrawal. A recently published meta-analysis provided valuable information on the prognostic utility of magnetic resonance (MR) biomarkers in HIE and suggested, in particular, that proton MR spectroscopy is the most accurate predictor of neurodevelopmental outcome. How should this evidence influence treatment-limitation decisions? In this article I outline serious limitations in existing prognostic studies of HIE, including small sample size, selection bias, vague and overly inclusive outcome assessment, and potential self-fulfilling prophecies. Such limitations make it difficult to answer the most important prognostic question. Reanalysis of published data reveals that severe abnormalities on conventional MRI in the first week have a sensitivity of 71% (95% confidence interval: 59%-91%) and specificity of 84% (95% confidence interval: 68%-93%) for very adverse outcome in infants with moderate encephalopathy. On current evidence, MR biomarkers alone are not sufficiently accurate to direct treatment-limitation decisions. Although there may be a role for using MRI or MR spectroscopy in combination with other prognostic markers to identify infants with very adverse outcome, it is not possible from meta-analysis to define this group clearly. There is an urgent need for improved prognostic research into HIE.

Therapeutic hypothermia for neonatal hypoxic ischaemic encephalopathy

There is now a strong evidence base supporting therapeutic hypothermia for infants with moderate or severe neonatal hypoxic ischaemic encephalopathy. Experimental and clinical data indicate that induced hypothermia reduces cerebral hypoxic ischaemic injury and randomized clinical trials in newborns with hypoxic ischaemic encephalopathy confirm improved neurological outcomes and survival at 18 months of age with therapeutic hypothermia. Studies are on-going to confirm whether these benefits are maintained in later childhood. Efforts are now focused on optimal implementation of therapeutic hypothermia in clinical practice: training in the assessment of severity of encephalopathy; initiation and maintenance of hypothermia before admission to a cooling facility; care of the infant during cooling; and appropriate investigation and follow-up are crucial for optimizing neurological outcomes. The establishment of registries of infants with hypoxic ischaemic encephalopathy and audit are important for guiding clinical practice.

Experimental treatments for hypoxic ischaemic encephalopathy

Hypoxic ischaemic encephalopathy continues to be a significant cause of death and disability worldwide. In the last 1-2 years, therapeutic hypothermia has entered clinical practice in industrialized countries and neuroprotection of the newborn has become a reality. The benefits and safety of cooling under intensive care settings have been shown consistently in trials; therapeutic hypothermia reduces death and neurological impairment at 18 months with a number needed to treat of approximately nine. Unfortunately, around half the infants who receive therapeutic hypothermia still have abnormal outcomes. Recent experimental data suggest that the addition of another agent to cooling may enhance overall protection either additively or synergistically. This review discusses agents such as inhaled xenon, N-acetylcysteine, melatonin, erythropoietin and anticonvulsants. The role of biomarkers to speed up clinical translation is discussed, in particular, the use of the cerebral magnetic resonance spectroscopy lactate/N-acetyl aspartate peak area ratios to provide early prognostic information. Finally, potential future therapies such as regeneration/repair and postconditioning are discussed.

MRI of perinatal brain injury

MRI is invaluable in assessing the neonatal brain following suspected perinatal injury. Good quality imaging requires adaptations to both the hardware and the sequences used for adults or older children. The perinatal and postnatal details often predict the pattern of lesions sustained and should be available to aid interpretation of the imaging findings. Perinatal lesions, the pattern of which can predict neurodevelopmental outcome, are at their most obvious on conventional imaging between 1 and 2 weeks from birth. Very early imaging during the first week may be useful to make management decisions in ventilated neonates but brain abnormalities may still be subtle using conventional sequences. Diffusion-weighted imaging (DWI) is very useful for the early identification of ischaemic tissue in the neonatal brain but may underestimate the final extent of injury, particularly basal ganglia and thalamic lesions. MR imaging is an excellent predictor of outcome following perinatal brain injury and can therefore be used as a biomarker in interventional trials designed to reduce injury and improve neurodevelopmental outcome.

Patterns of neonatal hypoxic-ischaemic brain injury

Enormous progress has been made in assessing the neonatal brain, using magnetic resonance imaging (MRI). In this review, we will describe the use of MRI and proton magnetic resonance spectroscopy in detecting different patterns of brain injury in (full-term) human neonates following hypoxic-ischaemic brain injury and indicate the relevance of these findings in predicting neurodevelopmental outcome.