Therapeutic hypothermia is recognized as an effective cytoprotectant and has been used in many medical situations, such as organ transplantation, cardiopulmonary bypass, spinal cord injury, and neonatal hypoxia ischemia. Animals that hibernate during the winter provide an example of how a reduction in body temperature and metabolism can aid survival during adverse conditions. Hypothermia can be classified based on the depth of cooling from a normal body temperature of 37–38°C. Mild hypothermia describes a body temperature in the range 32–35°C, moderate hypothermia (28–32°C), and deep hypothermia (<28°C). In general, mild to moderate degrees of hypothermia are used clinically because of a similar neuroprotective effect alongside reduced risks of medical complications, such as infection, arrhythmia, hypokalemia, coagulopathies, and bradycardia. Recent research has shown that hypothermia does not just act to slow metabolism, but rather can have an effect on a wide range of cell death and cell survival pathways leading to inhibition of apoptosis and inflammation, and stimulation of pro-survival pathways. There are many proposed mechanisms of action by which hypothermia offers neuroprotection in HIE. It has been proposed that cooling suppresses many of the pathways leading to delayed cell death, and may modify pathways in cells programmed for apoptosis, resulting in their survival. A reduction in body temperature leads to a reduction in cellular metabolic demands, and it has been reported that for every degree Celsius of temperature reduction, the cerebral metabolism is reduced by 5%. Hypothermia might also protect the neurons by attenuating the release of excitatory amino acids, ameliorating the ischemia-impaired uptake of glutamate, and by lowering production of toxic nitric oxide and free radicals.
Hypothermia is now the major treatment modality in the management of neonatal HIE in humans and involves reduction in temperature to 33°C initiated within 6 hours of delivery, which is continued for a total of 72 hours, followed by a gradual rewarming to normothermia over 12 hours. Therapeutic hypothermia aims to lower the temperature of the vulnerable deep brain structures, the basal ganglia, to 32–34°C. There are 2 methods for achieving this, whole body cooling and selective head cooling with mild systemic hypothermia. The rationale for selective head cooling is that the newborn infant's brain produces 70% of total body heat and that systemic hypothermia may be physiologically harmful to the sick neonate. However, a theoretical modeling of cooling, investigating temperature distribution within the neonatal head, found that the only situation that resulted in a significant reduction in deep brain temperature was when the core temperature was lowered to 34°.
There have been a number of randomized controlled trials that report improved outcomes for cooled infants with HIE compared to patients maintained at normothermia. The most recent meta-analysis to date evaluated a total of 13 clinical trials of therapeutic hypothermia, consisting of a total of 1,440 patients. The results showed a significant reduction in the risk of mortality or of moderate to severe neurodevelopmental disability in infants who received hypothermia compared to control which was used as the primary outcome. The secondary outcomes assessed were efficacy and safety. There was a significant reduction in the risk of cerebral palsy, severe visual deficit, cognitive delay or Mental Developmental Index (MDI) <70, and psychomotor developmental index <70 in the hypothermia groups compared to the control group. There was no difference in the risk of epilepsy or severe hearing deficit, and there was no difference in the relative risk of withdrawal of life sustaining medical treatment between the hypothermia and control group. There was an increased risk of arrhythmia and thrombocytopenia reported in the hypothermia group, but neither resulted in any clinically significant impact. No other significant differences between the groups were observed for other adverse outcomes, including organ dysfunction. The overall conclusions from this meta-analysis of 13 trials confirm that therapeutic hypothermia is associated with a reduced risk of the combined outcome of mortality or moderate to severe neurodevelopmental disability in infancy or childhood. Hypothermia was also effective in reducing individual outcomes or mortality, moderate to severe neurodevelopmental disability, severe cerebral palsy, cognitive delay, and psychomotor delay.
Therapeutic hypothermia is the only treatment that has been shown to improve neurological outcome in HIE in children, but it does not provide complete protection and there is evidence that it is not as successful in the treatment of the most severely affected neonates. Additional treatments can be administered during or after hypothermia that can extend the therapeutic window or provide an additive or synergistic effect are desperately needed, and several research groups are currently working on animal models to develop further therapies, some of which are described later in this review.
To the authors’ knowledge, there is only 1 published report of the use of therapeutic hypothermia in veterinary species. In this article, the author describes successful management of intractable seizures in a dog suffering from traumatic brain injury with a combination of high dose barbiturate therapy and controlled hypothermia. Several studies have shown that critically ill foals which are hypothermic have a worse prognosis for survival. In 1 study, the odds ratio for survival for foals with a subnormal temperature was 0.19, with a mean temperature of survivors of 38.3 ± 1.8°C versus a mean of 37.5 ± 1.7°C in the nonsurvivors. Based on this information, it would seem counterintuitive to suggest the use of therapeutic hypothermia in the equine NICU. However, it is important to distinguish therapeutic hypothermia and its benefits for brain injury, and hypothermia as a result of critical illness. One study investigated the risk of death or disability in term infants with HIE, which had increased temperature between 6 and 78 hours after birth. In this report, the controls were those that received “usual care” and were not cooled, and this group was compared to cooled infants. The authors reported that the odds of disability or death were increased by 3.6- to 4-fold for each 1°C increase in skin or esophageal temperature. Once we start to consider utilizing this treatment modality in foals, it becomes very clear how important our assessment and diagnostic workup is, as there are specific indications for the use of therapeutic hypothermia and it is certainly not to be used as a blanket treatment for any foal which is admitted to the NICU. We cannot ignore the results in both experimental animal models and human clinical trials, and we have to seriously contemplate that by keeping foals that have HI injury warm, we may be inadvertently worsening their prognosis.
Animal studies are critical to improving the care of the human fetus and newborn. The goals of animal models are to increase our understanding and knowledge of mechanisms of injury, evolution of injury, and to provide a clinically relevant template on which to develop and test new therapies. To study HIE in the term infant, the models used should, first, closely mimic the etiology of injury, express the functional outcomes seen in human infants, and accurately reflect histopathologic injury. There is, however, quite frequent failure of seemingly successful treatments in animal models of disease, to translate to human patients. Reasons for this include differing dose rates, routes of administration, and the disease model not truly reflecting what happens in the patient. As a result of this, despite all of the advances, adaptation of neuroprotective drugs to the clinical setting has been largely unsuccessful. At present, the question of how these models relate to foals remains largely unanswered, and is an area that needs further research. There are numerous therapies that have been used in the past to treat foals with HIE, and many are still used today, including dimethylsulfoxide (DMSO), thiamine, allopurinol, magnesium sulfate, mannitol, theophylline, ascorbic acid, naloxone, and alpha-tocopherol, despite a lack of supporting scientific evidence. There are several research groups across the world whose focus of attention lies with developing new therapies for the treatment of HIE in human infants to add to cooling, and the most relevant of these treatments are summarized below.
As described earlier, during hypoxic ischemia, free radicals are generated and the neonatal brain is particularly vulnerable to this damage because of its high concentration of lipids, high rate of oxygen consumption, altered balance of antioxidants, and increased availability of nonprotein bound free iron. A potential treatment strategy for protection against HI injury is to inhibit free radical production. The high affinity iron chelator desferrioxamine forms a complex with iron so that it cannot be reduced to the ferrous form, thus inhibiting free radical production. Experimental models have documented a reduction in brain injury in response to HI injury with desferrioxamine treatment. Desferrioxamine administered after induction of cerebral hypoxia ischemia, reduced injury in 7-day-old rats, and was found to significantly reduce hypoxia-ischemia-induced nonprotein bound iron formation in newborn lambs. More recently, Papazisis et al found that in newborn rats, desferrioxamine administered SC immediately postinsult had a neuroprotective effect on the hippocampus, and it decreased asphyxia-induced brain tissue concentrations of the excitotoxins glutamate and aspartate.
Erythropoietin (EPO) has also been investigated as a potential neuroprotectant for use in neonatal HI injury. The neuroprotective actions of EPO have been known for approximately 15 years, initially based on the observation that recombinant EPO (rEPO) protected neurons against hypoxia in vitro and that endogenous EPO was produced in brain astrocytes.[50, 51] The proposed mechanisms of neuroprotection include direct neurotrophic effects, decreased susceptibility to glutamate toxicity, induction of anti-apoptotic factors, decreased inflammation, increased blood flow to injured tissue, decreased nitric oxide mediated injury, and antioxidant effects. Recombinant human EPO administration to newborn rats has been shown to promote tissue protection, revascularization, and neurogenesis in neonatal HI injured brain, leading to improved neurobehavioral outcomes. The results from a pilot clinical trial by means of EPO in neonates with HIE have recently been published. A prospective case control study was performed with 45 neonates enrolled. The investigators split the neonates into 3 groups of 15, a normal healthy group, neonates with HIE treated with human rEPO (2500 IU/kg q24h for 5 days), and neonates with HIE who received no EPO treatment. They reported that by 2 weeks of age, EEG tracings had improved and nitric oxide (NO) concentrations had decreased in the EPO group. However, no changes in MRI findings were seen. By 6 months of age, the neonates treated with EPO had fewer neurologic and developmental abnormalities. There are now, however, concerns about the use of high doses of EPO to treat neonates, based on findings in adults where phase II/III trials suggest a higher death rate in patients receiving EPO compared to a placebo, as well as increased risk of serious complications, such as intracerebral hemorrhage and thrombotic events. Additional concerns exist with the use of rEPO in horses because of the development of anti-recombinant human erythropoietin (anti-rhEPO) antibodies resulting in erythroid hypoplasia and severe anemia.
The major common goal of the pharmacological treatment by means of antiepileptic drugs is to counteract abnormal brain excitability by either decreasing excitatory transmission or enhancing neuronal inhibition. An excessive release of excitatory amino acids and a reduced neuronal inhibition also occur in brain ischemia, and therefore the use of anticonvulsants may also contribute to neuroprotection.[31, 57] There is concern, however, that anticonvulsants which suppress synaptic activity in the brain have been associated with widespread apoptotic neurodegeneration throughout the brain when administered to normal immature rodents during the period of the brain growth spurt.[31, 58] Compounds that may cause neuronal apoptosis in the developing brain include NMDA antagonists (ketamine), agonists of gamma amino butyric acid A (GABA A) receptors (barbiturates and benzodiazepines), and sodium channel blockers (phenytoin). Barks et al investigated the combined effect of hypothermia with phenobarbital in 7-day-old rats with cerebral hypoxia ischemia. The authors reported that early posthypoxia ischemia, prophylactic administration of phenobarbital may augment the neuroprotective efficacy of therapeutic hypothermia, and that hypothermia may limit potential adverse effects of phenobarbital. In 2010, Meyn investigated the use of prophylactic administration of 40 mg/kg phenobarbital to infants with HIE at the commencement of whole body cooling. They reported that although a reduction in clinically detectable seizures was noted, there was no significant improvement in neurodevelopmental outcome.
One of the most recent adjunctive therapies currently being investigated is xenon. Xenon is approved for use as a general anesthetic in Europe, and there are encouraging results supporting its role as a potential neuroprotective agent. The major disadvantage of xenon in the clinical setting is the fact that it requires special respirators for administration, and is very expensive. The gas exhibits noncompetitive antagonism at the NMDA subtype of glutamate receptor. It is also thought to work via the activation of antiapoptotic effectors, Bcl-xL and Bcl-2, as well as induced expression of hypoxia inducible factor 1α and its downstream effectors, EPO and vascular endothelial growth factor (VEGF), which can interrupt the apoptotic pathway.[60, 64] Ma et al reported the use of combined xenon and therapeutic hypothermia in neonatal asphyxia. They reported that cultured neurons injured by glucose-oxygen deprivation were protected by combinations of interventions of xenon and hypothermia that, when administered alone, were not efficacious. Further studies have provided more evidence that xenon and hypothermia combine additively to provide neuroprotection in animal models of HIE[64, 65].