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Perinatal neuroprotection
ARTICLE IN PRESS Current Anaesthesia & Critical Care (2007) 18, 215–224
www.elsevier.com/locate/cacc
FOCUS ON: BASIC SCIENCE
Perinatal neuroprotection R.D. Sandersa,b,, H.J. Manningc, D. Mab, M. Mazea,b a
Department of Anaesthetics, Imperial College, London, UK Magill Department of Anaesthetics, Pain and Intensive Care Medicine, Chelsea & Westminster Hospital, Fulham Rd, London, SW10 9NH, UK c Department of Obstetrics and Gynaecology, Basingstoke and North Hampshire Foundation Trust, UK b
KEYWORDS Hypoxic encephalopathy; Ischaemic encephalopathy; NMDA receptors; Glutamate; Hypothermia
Summary The ability to protect or rescue the central nervous system from hypoxic–ischaemic injury has yet to materialise into clinical reality. However, exciting advances have recently occurred in this field and in this article the mounting evidence for perinatal neuroprotective interventions is reviewed. A multi-modal approach is advocated, based on the recent findings that hypothermia is therapeutic for sufferers of perinatal asphyxia. As anaesthetic/sedative agents need to be coadministered available data to define the best putative protective adjunct to hypothermia is also reviewed. & 2007 Elsevier Ltd. All rights reserved.
Introduction Peripheral and central nervous system (CNS) injury have a devastating impact when they occur at term after an uncomplicated gestation; both brachial plexus injury and moderate to severe hypoxic–ischaemic encephalopathy (HIE) occur at a rate of approximately 1–2 per 1000 full-term live births.1 While the peripheral nervous system retains the ability to repair itself the CNS is relatively poor at regeneration. This factor in combination with the devastating consequences of Corresponding author. Magill Department of Anaesthetics, Pain and Intensive Care Medicine, Chelsea & Westminster Hospital, Fulham Rd, London, SW10 9NH, UK. Tel.: +44 020 8746 8035/8816; fax: +44 020 8237 5109. E-mail address: [email protected] (R.D. Sanders).
CNS injuries impacts, extraordinarily, on the infant’s future development. Even though this emotive issue has provoked industrious activity by scientists and clinicians alike this field remains relatively uninformed because the goal of rescuing CNS neurons from the consequences of injury is challenging. As studies have begun to unravel the myriad of pathogenic factors that contribute to this injury it has become evident that multi-modal strategies will be required to provide long-lasting neuroprotection. Because the pathogenesis of perinatal HIE injury differs significantly from that governing adult injury, especially the relative over-abundance of apoptotic cell death,2 age-tailored neuroprotective regiments will be required. Nonetheless, significant advances have been made in this field; two large randomised controlled trials demonstrated that hypothermia provided some neuroprotection when
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ARTICLE IN PRESS 216 administered for 72 h after a neonatal asphyxial episode.3,4 These trials have laid the foundation upon which multi-modal strategies can be built and offer hope that we may one day be able to effectively combat the effects of perinatal injury. In this article we review potential therapies for sufferers of perinatal asphyxia at term.
Diagnosis of perinatal brain injury Antenatal screening identifies risk factors which may predispose the fetus to brain injury that include antenatally acquired infections, preeclampsia and thyroid disease.5 However, the diagnosis of perinatal hypoxic–ischaemic injury is still hampered by the lack of monitors that can reliably provide the required specificity and sensitivity. This has implications for both clinical management and subsequent litigation, for example birth asphyxia is believed to account for 10–20% of cases of cerebral palsy in term infants.6,7 The definition of birth asphyxia from a recent task force at the American College of Obstetrics and Gynecology (ACOG) is a clinical situation of damaging acidemia, hypoxia and metabolic acidosis with a sentinel event capable of interrupting oxygen supply to the fetus.8 The need to identify such an event has led to increased rates of cardiotocography (CTG); 85% of live births in the USA in 2002 were monitored using CTG.7 CTG patterns of reduced fetal heart rate variability and moderate to severe variable or late decelerations have been shown to correlate with episodes of fetal acidaemia.9 However, CTG suffers from large intra- and inter-operator variability and while it is a sensitive tool it lacks specificity. CTG only has a positive predictive value (PPV) of 0.2% for the prediction of cerebral palsy10 and PPV of approximately 2.6% for the prediction of asphyxia (2.6%) with standard practice.11 This false positive rate in the order of 97.4–99.8% explains in part the rise in caesarean section rates since instigation of CTG monitoring from 5% in the 1960s to approximately 25% today. Meta-analysis of 13 randomised control trials by the Cochrane group concluded that continuous CTG reduced the incidence of neonatal seizures but had no effect on the incidence of cerebral palsy or perinatal death.12 Thus, despite the upsurge in monitoring the cerebral palsy rate has remained unchanged at 1–2 per 1000 births in the last 50 years.7 It should be noted that neonatal seizures have been recently associated with neonatal cerebral infarction13 and therefore this does represent a significant advance in predicting perinatal injury.
R.D. Sanders et al. In order to reduce the high false positive rates of CTG, fetal scalp pH of p7.20 and more recently fetal scalp lactate greater than 4.2–4.8 mmol L1 have been shown to enhance detection of a compromised fetus.14,15 However, these assessments are still hampered by high false positive rates as a fetus may undergo transient episodes of asphyxia with no adverse consequences;16 in a retrospective study lactate greater than 4.8 mmol L1 has a PPV of 3.6 and pHo7.21 has a PPV of 2.8 for moderate to severe HIE.15 The definition of an acute intrapartum hypoxic event as sufficient to cause cerebral palsy has recently been defined and includes clinical criteria and umbilical arterial pH (Table 1). New techniques of assessing intrapartum fetal well-being offer hope for the future. Fetal pulse oximetry monitoring in conjunction with CTG have been investigated but this did not reduce the overall caesarean section rate and thus its further use was not endorsed by the ACOG.7 Intrapartum fetal ECG ST segment changes and umbilical artery and middle cerebral artery Doppler velocity are examples of more recent developments in monitoring.17,18 It remains to be seen though whether these will result in the evidence base required to change current practice. Nonetheless if neuroprotective interventions are to be instituted in a timely and appropriate manner early identification of an asphyxiated infant is imperative.
Pathogenesis of perinatal neuronal injury Understanding the pathogenesis of neuronal injury will allow rational development of multi-modal Table 1 Criteria to define an acute intrapartum hypoxic event as sufficient to cause cerebral palsy 1 Evidence of metabolic acidosis in fetal umbilical cord arterial blood obtained at delivery (pH o7 and base deficit X12 mmol L1) 2 Early onset of severe or moderate neonatal encephalopathy in infants born at 34 or more weeks of gestation 3 Cerebral palsy of the spastic quadriplegic or dyskinetic typea 4 Exclusion of other identifiable etiologies, such as trauma, coagulation disorders, infectious conditions, or genetic disorders a
Spastic quadriplegia and, less commonly, dyskinetic cerebral palsy are the only types of cerebral palsy associated with acute hypoxic intrapartum events. Spastic quadriplegia is not specific to intrapartum hypoxia. Hemiparetic cerebral palsy, hemiplegic cerebral palsy, spastic diplegia, and ataxia are unlikely to result from acute intrapartum hypoxia.8
ARTICLE IN PRESS Perinatal neuroprotection therapies to specifically target individual components of the injurious process. In the non-pathogenic state, the CNS has a relatively high requirement for oxygen and glucose which is mostly metabolised by oxidative phosphorylation. Indeed the neonate is known to be especially vulnerable to derangements in glucose homeostasis and avoiding hypo- and hyperglycaemia are critical as either state predisposes the CNS to injury in the young.
Primary energy failure Impairment of cerebral blood flow restricts the delivery of substrates, particularly oxygen and glucose, and thus impairs the energetics required to maintain ionic gradients in the brain. Energy depletion results in dysfunction of ATP-dependent ion channels and ion exchangers leading to cellular depolarisation and the release of excitatory neurotransmitters such as glutamate. Excess glutamatergic excitatory neurotransmission induces excitotoxic cell death as extracellular concentrations of glutamate rise by 3–10 fold. This excitotoxicity is compounded by a failure of energy-dependent glutamate uptake mechanisms which may even reverse and thereby further increase extracellular glutamate. Activation of postsynaptic glutamate receptors such as a-amino-3-hydroxy-5-methylisoxazole-4- propionic acid (AMPA/Kainate) receptors and N-methyl-D-Aspartate (NMDA) receptors mediate this injury to varying degrees in different brain regions; for example perinatal cortical injury is mediated predominantly by NMDA receptors while brainstem injury is mediated by a significant degree by AMPA receptors.19 Stimulation of the NMDA subtype (and some AMPA subtypes) of glutamatergic receptor produces a transmembrane flux of sodium and calcium cations contributing further to neuronal overactivation. Water passively follows the transmembrane flux of sodium and chloride contributing to brain swelling and oedema. Calcium initiates a series of cytoplasmatic and nuclear events that promote tissue damage profoundly. For example, overactivation of enzyme systems, e.g. proteases, lipases, endonucleases degrades cytoskeletal proteins and generates free radicals which damage membranes, mitochondria and DNA. Initially an excitotoxic form of cell death evolves which is followed by a wave of programmed cell death or apoptosis.20 This apoptotic component may occur secondarily to a loss of synaptic connectivity (due to the first wave of cell death killing an innervating cell), the intensity of the insult or type of stimulus. This process involves mitochondrial release of cytochrome C, activation of caspases
217 (aspartate-specific cysteine proteases), and other pro-apoptotic factors such as Bax. This apoptotic injury is particularly important in the very young2 and is an evolving injury, taking hours to develop. It is this form of injury that requires specific attention when designing neuroprotective regimes.
Role of inflammation in HIE Inflammatory mediators contribute significantly to the pathogenesis of HIE and the expression of both TNF-a and IL-1b are upregulated in this injury. While inflammation plays an important role in repair, excess inflammation is clearly deleterious. This becomes of clinical significance when the effect of peripartum infection is considered. Infection has been associated with CNS white matter injury, cerebral palsy and increased blood brain barrier permeability.21 Animal models have shown that lipopolysaccharide administration prior to a hypoxic–ischaemic insult exacerbates subsequent neuronal injury.22 The fetal inflammatory response appears to play a greater role than the maternal and in particular IL-6 upregulation is associated with increased risk of cerebral palsy and periventricular leukomalacia.23 IL-6 has also been reported as neuroprotective24 and thus the inflammatory milieu appears incredibly complex. Further study of how infection and the inflammatory cascade impact upon subsequent hypoxic–ischaemic injury in the neonate is required.
Secondary energy failure Following an interval of approximately 6 h a second phase of injury occurs with cerebral energy failure characterised by a fall in the phosphocreatine/ inorganic phosphate ratio. The degree of this energy failure correlates with adverse neurological outcome at 1 and 4 years.25 Similarly, this energy failure correlates with intracellular brain alkalosis, increased lactate/creatine ratio and more severe neurological outcome.26 Intracellular alkalosis may exacerbate excitotoxic injury, mitochondrial permeability, protease activation and apoptosis potentiating the ongoing pathology. The difficulty in pre-empting the first phase of injury has led to strategies to combat primarily this second phase of injury.
Hypothermic neuroprotection Hypothermia was first instituted as a therapeutic tool over 50 years ago; however, the first clinical
ARTICLE IN PRESS 218 trials to demonstrate improved outcome with hypothermia therapy in neonates were published recently.3,4 These landmark studies, like their counterparts in the adult cardiac arrest literature, have the potential to revolutionise therapy for this disease. The Cool-Cap study3 enrolled 234 infants with moderate to severe neonatal HIE and abnormal amplitude-integrated EEG (aEEG) with 116 infants receiving hypothermic therapy (34–35 1C) for 72 h from a cooling cap device; the control group of 118 infants received conventional therapy only. Hypothermia did not provide neuroprotection in the most severely injured neonates (n ¼ 46; odds ratio (OR) 1.8; 95% CI 0.49–6.4; p ¼ 0.51) but did prove beneficial in those with less evidence of injury (n ¼ 172, OR 0.42; 95% CI 0.22–0.80, p ¼ 0.009, NNT ¼ 6). In this group neuromotor disability was predominantly improved (p ¼ 0.03) rather than mortality (p ¼ 0.2) indicating that hypothermia protects neonates destined to survive and improves their functional outcome. The National Institute of Child Health and Human Development study4 randomised 208 term neonates to control or whole-body hypothermia with a target temperature of 33.5 1C for 72 h. Death or moderate or severe disability occurred in 45 of 102 infants (44%) in the hypothermia group and 64 of 103 infants (62%) in the control group (relative risk, 0.72; 95% CI, 0.54–0.95; NNT ¼ 6). There were no differences for mortality alone, disabling cerebral palsy, blindness, severe hearing impairment or the Bayley Mental Development Index or Bayley Psychomotor Development scores. However it should be noted that 41 infants in the control group experienced hyperthermia (438 1C) on at least one occasion which could have contributed to worsened outcome in this group. Further data from on-going trials are required before hypothermic therapy becomes standard of care for HIE. Hypothermia does reduce the incidence of thalamic and basal ganglia lesions secondary to HIE as assessed by magnetic resonance imaging providing further support for its neuroprotective potential.27 Thus, hypothermia represents a viable possibility in a field devoid of successful therapeutic interventions. Hypothermia’s neuroprotective effects may be secondary to its ability to target both excitotoxic and apoptotic cell death. Acting presynaptically hypothermia reduces glutamate release28 but also acts at an early stage to inhibit the apoptotic cascade to prevent neurotoxicity.29,30 Hypothermia alters the balance of the pro- and anti-apoptotic signalling cascades, preserving the anti-apoptotic
R.D. Sanders et al. protein kinase Akt and inhibiting the pro-apoptotic proteins cJun N-terminal kinase and forkhead transcription factor, and preventing cytochrome C release.29,30 In addition, hypothermia reduces the cerebral metabolic rate which may reduce the metabolic burden during the secondary energy failure phase and promotes an anti-inflammatory cascade. However, there are numerous toxic effects of hypothermia especially if not administered in a tightly controlled manner with coagulopathic and arrhythmogenic toxicity being of particular concern. Thus, clinical trials limited the lower level of hypothermia to 33.5 1C in the perinatal population.3,4 Hypothermia can increase metabolic rate by shivering, increasing oxygen consumption and sedative strategies to limit this adaptation are required. It is of interest that the protective potential of hypothermia is abrogated in the absence of adequate sedation.31 Remarkably, details about the sedative therapies employed in the hypothermia trials were not reported.3,4 It is likely that different sedatives will interact dissimilarly with hypothermic neuroprotection; for example, addition of methohexital to hypothermic neuroprotection showed no additional protective benefit over hypothermia alone32 in an animal model of focal ischaemia. Therefore, detailing which pharmacological agents can augment hypothermic neuroprotection has critical importance.
Pharmacological neuroprotection The optimal regimen to prescribe for sedation during hypothermic therapy has not been formally addressed but as hypothermia requires the involvement of anaesthetic and critical care services, this deficiency will need to be addressed. Preclinical evidence suggests that some classes of drug may be particularly effective; notably, sedatives exerting properties of either NMDA antagonism or a2 adrenoceptor activation both show considerable promise. Unfortunately, most agents tested so far have been ineffective; for example mannitol therapy has not proven successful in small clinical studies1,33 and preclinical studies.34 Similarly, dexamethasone is not recommended as it reduces cerebral perfusion pressure in line with its ability to reduce intracranial pressure.1 Furthermore, the increase in ICP observed in HIE is likely to be an epiphenomenon rather than a mechanism of injury. Calcium channel blockers are also associated with decreased cerebral flow and are not recommended for treatment of perinatal HIE.1
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Barbiturate therapy Perinatal seizures in the absence of other features of HIE are associated with significant neuronal injury and even cerebral infarction in the neonatal population.13 Effective anti-seizure treatment needs to be initiated promptly especially if it also can exhibit neuroprotective capability. Barbiturates are often employed in the treatment of neonatal seizures but it is unclear whether their anti-seizure actions translate into neuroprotection. Three small clinical trials have investigated the potential role of barbiturates to ameliorate perinatal HIE but only one showed a relative risk reduction of severe developmental disability or death.35 However, 23% patients were lost to follow up in this trial. Two other trials36,37 did not find thiopentone or pentobarbitone effective. Subsequent meta-analysis of the studies (n ¼ 77) showed no significant effect on death or severe neurodevelopmental disability.1 As avoidance of hypotension is regarded as critical in the asphyxiated infant, current evidence does not support the use of prophylactic barbiturates for perinatal neuroprotection. Barbiturates still have a role in the treatment of seizures and further study is required to investigate whether they possess neuroprotective efficacy when administered in this setting especially since these seizures have been associated with CNS infarctions. An obvious problem with drawing any conclusions from the studies available is the size of these trials when compared with those showing efficacy for hypothermic neuroprotection and future trials of pharmacological agents will require adequate power to fully assess neuroprotective efficacy. There are no studies of the neuroprotective efficacy of barbiturates in animal models of perinatal HIE and therefore, it would be prudent to test barbiturates in preclinical trials before extending the current clinical research. A significant reduction in GABAergic neuron expression is noted after human perinatal brain injury38 and therefore agonists depending on GABA receptors may therefore prove less effective for retrospective treatment of this injury.
219 pulmonary bypass40 assessed by functional and morphologic criteria. Further dose–responsive investigation of how volatile anaesthetics interact with hypothermia is required, especially as administration of significant anaesthesia in the absence of surgical stimulus may be neurotoxic41 and will also expose the neonate to significant respiratory and cardiovascular depression.
a2 Adrenoceptor agonists The a2 adrenoceptor agonists, exemplified by clonidine and dexmedetomidine, have been shown to have neuroprotective potential in animal models of HIE.42–44 Both agents reduced the size of excitotoxin-induced cortical and white-matter lesions in mouse pups injected intracerebrally with the NMDA receptor agonist ibotenate;42 these protective effects were abolished by the a2 antagonist, yohimbine, confirming that these agents protect through their activity at the a2 adrenoceptor. Recently, it has been shown that dexmedetomidine, concentration-dependently, diminished neuronal injury provoked either in vitro or in vivo in a neonatal asphyxia rat model.43 Dexmedetomidine, administered during the asphyxia, provided protection even when assessed 30 days later with functional neuromotor followup. Dexmedetomidine sedation appears particularly attractive as it possesses anti-excitotoxic, anti-apoptotic and anti-inflammatory capabilities with analgesia and minimal respiratory suppression. Whether dexmedetomidine can exert neuroprotection when provided well after the onset of injury is currently unknown. However, as dexmedetomidine can target different aspects of ongoing injury, it has potential to do be beneficial even when delivered after the initial hypoxic–ischemic insult. It should also be noted that a synergistic interaction between dexmedetomidine and the NMDA antagonist, xenon, has been noted in this model.44 The characteristics of the neuroprotective interaction between dexmedetomidine and hypothermia has not been reported.
NMDA receptor antagonists Volatile anaesthetics In preclinical studies supra-anaesthetic concentrations of volatile anaesthetics have been shown to potentiate hypothermic neuroprotection when given during an insult. Desflurane (9%) has proven effective in newborn piglet models of deep hypothermic cardiac arrest39 and low flow cardio-
The NMDA receptor antagonists were initially heralded as neuroprotectants of the future but those in routine clinical practice at present have not supported this claim despite the central role of the NMDA receptor in excitotoxic injury.45 In contrast to GABA receptor expression, certain NMDA receptor subtypes are upregulated following
ARTICLE IN PRESS 220 HIE46 suggesting that targeting this receptor in a retrospective manner may be appropriate for treatment of HIE. Magnesium is an endogenous voltage-dependent blocker of the NMDA channel and is also employed antenatally for tocolysis and the treatment of eclampsia. A retrospective case review of premature infants who were exposed antenatally to magnesium sulphate observed a reduced incidence of cerebral palsy at 3 years.47 Magnesium sulphate has been reported neuroprotective when given prepartum and intrapartum for very premature infants (at less than 30 weeks of age).48 This is not a consistent finding in the literature. There are also conflicting reports from the animal literature regarding the neuroprotective efficacy of magnesium, with lack of temperature control a frequent confound. The translatability of these findings to term infants is at best unclear.1,49 Prophylactic magnesium therapy may expose many infants unnecessarily to side-effects such as tocolysis with potentially prolonged labour, severe hypotension and respiratory depression.1 The possible role of magnesium as neuroprotectant for term infants requires further exploration. Anaesthetic NMDA antagonists in current clinical use include nitrous oxide and ketamine which are both often employed in obstetric practice. As with magnesium, their neuroprotective capabilities are variably reported,45 but nitrous oxide and ketamine have both recently been associated with neurotoxicity (apoptotic neurodegeneration) in the very young.41,50 Xenon, also an NMDA antagonist, does not produce apoptotic neurodegeneration in the young but does provide robust neuroprotection in several animal models of neuronal injury.51,52 In establishing whether xenon can provide protection during neonatal rat asphyxia in vivo and in vitro it has been shown that xenon attenuates hypoxic–ischaemic damage at concentrations of 40% and greater. Xenon offers neuroprotection at subanaesthetic concentrations.53 As perinatal asphyxia is difficult to anticipate xenon was investigated to see if it could provide neuroprotection when given after the injury. Xenon attenuated hypoxic–ischemic damage when given up to six hours after the injury and provided synergistic neuroprotection when given post-injury in combination with hypothermia (35 1C; Fig. 1). This synergistic interaction still occurs when the administration of hypothermia and xenon occurs asynchronously.54 The simplest analysis for this synergism assumes the rate of a reaction proceeds because thermal energy provides sufficient energy to overcome an ‘‘activation’’ energy. If the logarithm of the reaction rate is plotted against the reciprocal
R.D. Sanders et al. absolute temperature then the slope of the line is proportional to the activation energy. The bigger the activation energy, the smaller the reaction rate and the greater the temperature dependence. For LDH release, the value that characterises the temperature dependence is 34.8 kJ mol1. This is essentially ‘‘neuroprotection’’ due to hypothermia. It corresponds to a reaction that is moderately temperature dependent. When xenon is combined with hypothermia, this value increases to about 177 kJ mol1. The large increase in temperature dependence means that, in the presence of xenon, hypothermia is very much more effective at providing neuroprotection. To a small extent this might be expected because xenon is likely to bind better to its targets as the temperature decreases. However, this is a small effect and could only account for a small element of the enhanced temperature dependence. In some way, xenon enhances the system’s sensitivity to temperature. Whatever the mechanism, it means that a combination of xenon and hypothermia provides potent neuroprotection. Apoptosis is the predominant form of ongoing injury in this paradigm and therefore the effects of xenon and hypothermia on markers of apoptosis were evaluated. Xenon and xenon-hypothermia reduced this form of injury significantly and increased the anti-apoptotic protein Bcl-XL.53 This protection correlated with improved neuromotor function at 30 days of age indicating functional protection.53 This may account for why asynchronous application of the interventions attenuates the injury. This finding could be utilised in a clinical context with hypothermia being instituted early in life before transfer to a tertiary centre for xenon therapy which is an expensive agent currently requiring sophisticated administration apparatus. Xenon provides neuroprotection that is time-independent as it can be given before, during and after the insult, combines synergistically with hypothermia and targets excitotoxic and apoptotic injury.53 Xenon provides haemodynamic stability in adult settings; if present in the very young this will likely to be beneficial as the neonatal myocardium is extremely sensitive to the depressant effects of anaesthetic agents.52
Erythropoietin One non-sedative pharmacological neuroprotectant that is receiving attention in this field is erythropoietin. Interestingly, erythropoietin has been reported to be upregulated in umbilical cord blood
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Fig. 1 Neuroprotective effects of xenon and hypothermia in vitro. (A) Release of lactate dehydrogenase (LDH) as a marker of neuronal injury from oxygen-glucose deprived (OGD) cultured neurons (to simulate cerebral ischaemia) exposed to increasing concentrations of xenon at normothermia (37 1C) and hypothermia (33 1C); (B) LDH release from OGD-injured cultured neurons exposed to decreasing temperature with or without sub-anaesthetic xenon (12.5%); (C) isobologram illustrating the protective efficacy of combining xenon and temperature on injured cultured neurons. The neuroprotection of OGD-injured neurons is synergistically enhanced when xenon and hypothermia are provided in combination as the IC50 values for the interventions are significantly to the left of the ‘‘line of additivity’’ joining the IC50 of the individual interventions; and (D) van’t Hoff plot showing the temperature dependence of neuroprotection for hypothermia alone and hypothermia in the presence of xenon. The enthalpy change (DH) is a measure of the overall heat exchange during the process. Reproduced with permission from the Annals of Neurology.53
from babies who have suffered perinatal asphyxia.55 This may represent a defence mechanism as erythropoietin is neuroprotective when given after hypoxic–ischaemic injury.56 Erythropoietin’s effect is time dependent and toxicity has been reported especially when administered during the hypoxia.57 Nonetheless erythropoietin does exhibit neuroprotective effects likely in part by attenuating apoptosis; once the timing of the therapy is delineated more clearly erythropoietin may have a role as a retrospective neuroprotectant. It is certainly of interest that xenon administration
upregulates hypoxia-inducible factor and erythropoietin expression58 and this may also play a role in xenon’s neuroprotective capabilities.
Antioxidants and free radical scavengers Cellular damage induced by free radicals contributes significantly to reperfusion injury in HIE and this prompted trials analysing the safety of neonatal resuscitation with air rather than oxygen. Hyperoxia induced by ventilation with 100% oxygen
ARTICLE IN PRESS 222 is associated with reduced cerebral blood flow59 and the production of free radicals such as hydrogen peroxide.60 While no individual trial has shown difference in mortality with resuscitation with air rather than oxygen, recent meta-analysis has found that resuscitation with air is associated with a reduction in mortality61 (RR 0.71 (0.54, 0.94), NNT ¼ 20). At present there is insufficient data to comment on whether air resuscitation may reduce neurodevelopment delay and cerebral palsy. While the finding of a mortality difference is remarkable, it is biologically plausible and therefore requires further evaluation. Furthermore, trials of other oxygen concentrations are warranted as are trials designed at specific subgroups which may require higher oxygen concentrations such as severe asphyxia or sepsis. Other strategies to reduce free radical generation include the use of xanthine oxidase inhibitors which have shown protection of cerebral energetics when administered early during the reperfusion phase but did not attenuate brain damage or markers of apoptosis.62 Furthermore, allopurinol reduced circulating concentrations of free radicals in human infants exemplified by a reduced malondialdehyde level which is a marker of lipid peroxidation.63 As yet the use of free radical scavengers has not been fully evaluated in conjunction with other neuroprotective therapies such as hypothermia and this requires further study.
Summary Defining the optimal strategy for perinatal neuroprotection has potential to significantly improve neurocognitive outcome after asphyxial injury. The current data suggest hypothermic neuroprotection could soon become clinical reality if ongoing trials confirm this finding and help to establish this as mainstream therapy. Neonatal resuscitation with air may further improve neonatal outcome, though further clinical trials are still required. There is also great potential to augment hypothermic neuroprotection with co-administered neuroprotective sedatives. Xenon therapy requires evaluation in this context, though present costs and technological development may limit the ubiquity of this treatment. The a2 adrenoceptor agonists also show promise but retrospective efficacy with or without hypothermia needs to be established before clinical trials can be undertaken. Likewise allopurinol therapy should be evaluated in the context of hypothermia and air resuscitation. As more information assimilates on the ability of hypothermia to provide neuroprotection in this context, our knowl-
R.D. Sanders et al. edge of how to care for these neonates will expand. How we should sedate and protect these neonates requires further attention.
References 1. Whitelaw A, Thoresen M. Clinical trials of treatments after perinatal asphyxia. Curr Opin Pediatr 2002;14(6):664–8. 2. Nakajima W, Ishida A, Lange MS, Gabrielson KL, Wilson MA, Martin LJ, et al. Apoptosis has a prolonged role in the neurodegeneration after hypoxic ischemia in the newborn rat. J Neurosci 2000;20(21):7994–8004. 3. Gluckman PD, Wyatt JS, Azzopardi D, Ballard R, Edwards AD, Ferriero DM, et al. Selective head cooling with mild systemic hypothermia after neonatal encephalopathy: multicentre randomised trial. Lancet 2005;365(9460):663–70. 4. Shankaran S, Laptook AR, Ehrenkranz RA, Tyson JE, McDonald SA, Donovan EF, et al. Whole-body hypothermia for neonates with hypoxic-ischemic encephalopathy. N Engl J Med 2005;353(15):1574–84. 5. Badawi N, Kurinczuk JJ, Keogh JM, Alessandri LM, O’Sullivan F, Burton PR, et al. Antepartum risk factors for newborn encephalopathy: the Western Australian case-control study. BMJ 1998;317(7172):1549–53. 6. Nelson KB, Ellenberg JH. Antecedents of cerebral palsy. Multivariate analysis of risk. N Engl J Med 1986;315(2):81–6. 7. Graham EM, Petersen SM, Christo DK, Fox HE. Intrapartum electronic fetal heart rate monitoring and the prevention of perinatal brain injury. Obstet Gynecol 2006;108(3 Pt 1): 656–66. 8. American Academy of Pediatrics. American College of Obstetricians and Gynecologists. Neonatal encephalopathy and cerebral palsy: defining the pathogenesis and pathophysiology. Washington, DC: Elk Grove Village (IL): AAP; 2003. 9. Kubli FW, Hon EH, Khazin AF, Takemura H. Observations on heart rate and pH in the human fetus during labor. Am J Obstet Gynecol 1969;104(8):1190–206. 10. Nelson KB, Dambrosia JM, Ting TY, Grether JK. Uncertain value of electronic fetal monitoring in predicting cerebral palsy. N Engl J Med 1996;334(10):613–8. 11. Low JA, Victory R, Derrick EJ. Predictive value of electronic fetal monitoring for intrapartum fetal asphyxia with metabolic acidosis. Obstet Gynecol 1999;93(2):285–91. 12. Alfirevic Z, Devane D, Gyte GM. Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database Syst Rev 2006;3:CD006066. 13. Cowan F, Rutherford M, Groenendaal F, Eken P, Mercuri E, Bydder GM, et al. Origin and timing of brain lesions in term infants with neonatal encephalopathy. Lancet 2003; 361(9359):736–42. 14. Allen RM, Bowling FG, Oats JJ. Determining the fetal scalp lactate level that indicates the need for intervention in labour. Aust N Z J Obstet Gynaecol 2004;44(6):549–52. 15. Kruger K, Hallberg B, Blennow M, Kublickas M, Westgren M. Predictive value of fetal scalp blood lactate concentration and pH as markers of neurologic disability. Am J Obstet Gynecol 1999;181(5 Pt 1):1072–8. 16. ACOG Committee on Obstetric Practice. ACOG Committee Opinion No. 348, November 2006: umbilical cord blood gas and acid-base analysis. Obstet Gynecol 2006;108(5): 1319–22. 17. Westgate J, Harris M, Curnow JS, Greene KR. Plymouth randomized trial of cardiotocogram only versus ST waveform
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18.
19. 20. 21. 22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
plus cardiotocogram for intrapartum monitoring in 2400 cases. Am J Obstet Gynecol 1993;169(5):1151–60. Siristatidis C, Salamalekis E, Kassanos D, Loghis C, Creatsas G. Evaluation of fetal intrapartum hypoxia by middle cerebral and umbilical artery Doppler velocimetry with simultaneous cardiotocography and pulse oximetry. Arch Gynecol Obstet 2004;270(4):265–70. Johnston MV. Excitotoxicity in perinatal brain injury. Brain Pathol 2005;15(3):234–40. Olney JW. Excitotoxicity, apoptosis and neuropsychiatric disorders. Curr Opin Pharmacol 2003;3(1):101–9. Edwards AD, Tan S. Perinatal infections, prematurity and brain injury. Curr Opin Pediatr 2006;18(2):119–24. Eklind S, Mallard C, Arvidsson P, Hagberg H. Lipopolysaccharide induces both a primary and a secondary phase of sensitization in the developing rat brain. Pediatr Res 2005;58(1):112–6. Yoon BH, Romero R, Yang SH, Jun JK, Kim IO, Choi JH, Syn HC. Interleukin-6 concentrations in umbilical cord plasma are elevated in neonates with white matter lesions associated with periventricular leukomalacia. Am J Obstet Gynecol 1996;174(5):1433–40. Loddick SA, Turnbull AV, Rothwell NJ. Cerebral interleukin-6 is neuroprotective during permanent focal cerebral ischemia in the rat. J Cereb Blood Flow Metab 1998;18(2):176–9. Roth SC, Baudin J, Cady E, Johal K, Townsend JP, Wyatt JS, et al. Relation of deranged neonatal cerebral oxidative metabolism with neurodevelopmental outcome and head circumference at 4 years. Dev Med Child Neurol 1997; 39(11):718–25. Robertson NJ, Cowan FM, Cox IJ, Edwards AD. Brain alkaline intracellular pH after neonatal encephalopathy. Ann Neurol 2002;52(6):732–42. Rutherford MA, Azzopardi D, Whitelaw A, Cowan F, Renowden S, Edwards AD, et al. Mild hypothermia and the distribution of cerebral lesions in neonates with hypoxic–ischemic encephalopathy. Pediatrics 2005;116(4):1001–6. Busto R, Globus MY, Dietrich WD, Martinez E, Valdes I, Ginsberg MD. Effect of mild hypothermia on ischemiainduced release of neurotransmitters and free fatty acids in rat brain. Stroke 1989;20(7):904–10. Xu L, Yenari MA, Steinberg GK, Giffard RG. Mild hypothermia reduces apoptosis of mouse neurons in vitro early in the cascade. J Cereb Blood Flow Metab 2002;22(1):21–8. Zhao H, Shimohata T, Wang JQ, Sun G, Schaal DW, Sapolsky RM, et al. Akt contributes to neuroprotection by hypothermia against cerebral ischemia in rats. J Neurosci 2005;25(42):9794–806. Thoresen M, Satas S, Loberg EM, Whitelaw A, Acolet D, Lindgren C, et al. Twenty-four hours of mild hypothermia in unsedated newborn pigs starting after a severe global hypoxic–ischemic insult is not neuroprotective. Pediatr Res 2001;50(3):405–11. Westermaier T, Zausinger S, Baethmann A, Steiger HJ, Schmid-Elsaesser R. No additional neuroprotection provided by barbiturate-induced burst suppression under mild hypothermic conditions in rats subjected to reversible focal ischemia. J Neurosurg 2000;93(5):835–44. Levene MI, Evans DH, Forde A, Archer LN. Value of intracranial pressure monitoring of asphyxiated newborn infants. Dev Med Child Neurol 1987;29(3):311–9. Mujsce DJ, Towfighi J, Stern D, Vannucci RC. Mannitol therapy in perinatal hypoxic–ischemic brain damage in rats. Stroke 1990;21(8):1210–4. Hall RT, Hall FK, Daily DK. High-dose phenobarbital therapy in term newborn infants with severe perinatal asphyxia: a
223
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
randomized, prospective study with three-year follow-up. J Pediatr 1998;132(2):345–8. Goldberg RN, Moscoso P, Bauer CR, Bloom FL, Curless RG, Burke B, et al. Use of barbiturate therapy in severe perinatal asphyxia: a randomized controlled trial. J Pediatr 1986; 109(5):851–6. Ruth V, Virkola K, Paetau R, Raivio KO. Early high-dose phenobarbital treatment for prevention of hypoxic-ischemic brain damage in very low birth weight infants. J Pediatr 1988;112(1):81–6. Robinson S, Li Q, Dechant A, Cohen ML. Neonatal loss of gamma-aminobutyric acid pathway expression after human perinatal brain injury. J Neurosurg 2006;104(6 Suppl): 396–408. Kurth CD, Priestley M, Watzman HM, McCann J, Golden J. Desflurane confers neurologic protection for deep hypothermic circulatory arrest in newborn pigs. Anesthesiology 2001;95(4):959–64. Loepke AW, Priestley MA, Schultz SE, McCann J, Golden J, Kurth CD. Desflurane improves neurologic outcome after low-flow cardiopulmonary bypass in newborn pigs. Anesthesiology 2002;97(6):1521–7. Jevtovic-Todorovic V, Hartman RE, Izumi Y, Benshoff ND, Dikranian K, Zorumski CF, et al. Early exposure to common anesthetic agents causes widespread neurodegeneration in the developing rat brain and persistent learning deficits. J Neurosci 2003;23(3):876–82. Laudenbach V, Mantz J, Lagercrantz H, Desmonts JM, Evrard P, Gressens P. Effects of alpha(2)-adrenoceptor agonists on perinatal excitotoxic brain injury: comparison of clonidine and dexmedetomidine. Anesthesiology 2002; 96(1):134–41. Ma D, Hossain M, Rajakumaraswamy N, Arshad M, Sanders RD, Franks NP, Maze M. Dexmedetomidine produces its neuroprotective effect via the alpha 2A-adrenoceptor subtype. Eur J Pharmacol 2004;502(1–2): 87–97. Rajakumaraswamy N, Ma D, Hossain M, Sanders RD, Franks NP, Maze M. Neuroprotective interaction produced by xenon and dexmedetomidine on in vitro and in vivo neuronal injury models. Neurosci Lett 2006;409(2):128–33. Sanders RD, Ma D, Maze M. Anaesthesia induced neuroprotection. Best Pract Res Clin Anaesthesiol 2005;19(3): 461–74. Guerguerian AM, Brambrink AM, Traystman RJ, Huganir RL, Martin LJ. Altered expression and phosphorylation of Nmethyl-D-aspartate receptors in piglet striatum after hypoxia–ischemia. Brain Res Mol Brain Res 2002;104(1): 66–80. Nelson KB, Grether JK. Can magnesium sulfate reduce the risk of cerebral palsy in very low birthweight infants? Pediatrics 1995;95(2):263–9. Crowther CA, Hiller JE, Doyle LW, Haslam RR, Australasian Collaborative Trial of Magnesium Sulphate (ACTOMg SO4) Collaborative Group. Effect of magnesium sulfate given for neuroprotection before preterm birth: a randomized controlled trial. JAMA 2003;290(20):2669–76. Greenwood K, Cox P, Mehmet H, Penrice J, Amess PN, et al. Magnesium sulfate treatment after transient hypoxia-ischemia in the newborn piglet does not protect against cerebral damage. Pediatr Res 2000;48(3):346–50. Young C, Jevtovic-Todorovic V, Qin YQ, Tenkova T, Wang H, Labruyere J, et al. Potential of ketamine and midazolam, individually or in combination, to induce apoptotic neurodegeneration in the infant mouse brain. Br J Pharmacol 2005;146(2):189–97.
ARTICLE IN PRESS 224 51. Ma D, Williamson P, Januszewski A, Nogaro MC, et al. Xenon mitigates isoflurane-induced neurodegeneration in the developing rodent brain. Anesthesiology in press. 52. Sanders RD, Ma D, Maze M. Xenon: elemental anaesthesia in clinical practice. Br Med Bull 2005;71:115–35. 53. Ma D, Hossain M, Chow A, Arshad M, Battson RM, Sanders RD, et al. Xenon and hypothermia combine to provide neuroprotection from neonatal asphyxia. Ann Neurol 2005;58(2): 182–93. 54. Martin JL, Ma D, Hossain M, Xu J, Sanders RD, Franks NP, et al. Asynchronous administration of xenon and hypothermia significantly reduces brain infarction in the neonatal rat. Br J Anaesth 2007;98(2):236–40. 55. Ruth V, Autti-Ramo I, Granstrom ML, Korkman M, Raivio KO. Prediction of perinatal brain damage by cord plasma vasopressin, erythropoietin, and hypoxanthine values. J Pediatr 1988;113(5):880–5. 56. Spandou E, Papadopoulou Z, Soubasi V, Karkavelas G, Simeonidou C, Pazaiti A, et al. Erythropoietin prevents long-term sensorimotor deficits and brain injury following neonatal hypoxia-ischemia in rats. Brain Res 2005; 1045(1–2):22–30. 57. Weber A, Dzietko M, Berns M, Felderhoff-Mueser U, Heinemann U, Maier RF, et al. Neuronal damage after
R.D. Sanders et al.
58.
59.
60.
61.
62.
63.
moderate hypoxia and erythropoietin. Neurobiol Dis 2005; 20(2):594–600. Lim T, Ma D, To C, Maze M, Maxwell P. Xenon-Induced Hypoxia-inducible factor-1 alpha in preconditioning of neural and non-neural tissue. ASA abstract 2006; A1186. Niijima S, Shortland DB, Levene MI, Evans DH. Transient hyperoxia and cerebral blood flow velocity in infants born prematurely and at full term. Arch Dis Child 1988;63(10 Spec No):1126–30. Kutzsche S, Ilves P, Kirkeby OJ, Saugstad OD. Hydrogen peroxide production in leukocytes during cerebral hypoxia and reoxygenation with 100% or 21% oxygen in newborn piglets. Pediatr Res 2001;49(6):834–42. Tan A, Schulze A, O’Donnell CP, Davis PG. Air versus oxygen for resuscitation of infants at birth. Cochrane Database Syst Rev 2005(2):CD002273. Peeters-Scholte C, Braun K, Koster J, Kops N, Blomgren K, Buonocore G, et al. Effects of allopurinol and deferoxamine on reperfusion injury of the brain in newborn piglets after neonatal hypoxia–ischemia. Pediatr Res 2003;54(4):516–22. Van Bel F, Shadid M, Moison RM, Dorrepaal CA, Fontijn J, Monteiro L, et al. Effect of allopurinol on postasphyxial free radical formation, cerebral hemodynamics, and electrical brain activity. Pediatrics 1998;101(2):185–93.
www.elsevier.com/locate/cacc
FOCUS ON: BASIC SCIENCE
Perinatal neuroprotection R.D. Sandersa,b,, H.J. Manningc, D. Mab, M. Mazea,b a
Department of Anaesthetics, Imperial College, London, UK Magill Department of Anaesthetics, Pain and Intensive Care Medicine, Chelsea & Westminster Hospital, Fulham Rd, London, SW10 9NH, UK c Department of Obstetrics and Gynaecology, Basingstoke and North Hampshire Foundation Trust, UK b
KEYWORDS Hypoxic encephalopathy; Ischaemic encephalopathy; NMDA receptors; Glutamate; Hypothermia
Summary The ability to protect or rescue the central nervous system from hypoxic–ischaemic injury has yet to materialise into clinical reality. However, exciting advances have recently occurred in this field and in this article the mounting evidence for perinatal neuroprotective interventions is reviewed. A multi-modal approach is advocated, based on the recent findings that hypothermia is therapeutic for sufferers of perinatal asphyxia. As anaesthetic/sedative agents need to be coadministered available data to define the best putative protective adjunct to hypothermia is also reviewed. & 2007 Elsevier Ltd. All rights reserved.
Introduction Peripheral and central nervous system (CNS) injury have a devastating impact when they occur at term after an uncomplicated gestation; both brachial plexus injury and moderate to severe hypoxic–ischaemic encephalopathy (HIE) occur at a rate of approximately 1–2 per 1000 full-term live births.1 While the peripheral nervous system retains the ability to repair itself the CNS is relatively poor at regeneration. This factor in combination with the devastating consequences of Corresponding author. Magill Department of Anaesthetics, Pain and Intensive Care Medicine, Chelsea & Westminster Hospital, Fulham Rd, London, SW10 9NH, UK. Tel.: +44 020 8746 8035/8816; fax: +44 020 8237 5109. E-mail address: [email protected] (R.D. Sanders).
CNS injuries impacts, extraordinarily, on the infant’s future development. Even though this emotive issue has provoked industrious activity by scientists and clinicians alike this field remains relatively uninformed because the goal of rescuing CNS neurons from the consequences of injury is challenging. As studies have begun to unravel the myriad of pathogenic factors that contribute to this injury it has become evident that multi-modal strategies will be required to provide long-lasting neuroprotection. Because the pathogenesis of perinatal HIE injury differs significantly from that governing adult injury, especially the relative over-abundance of apoptotic cell death,2 age-tailored neuroprotective regiments will be required. Nonetheless, significant advances have been made in this field; two large randomised controlled trials demonstrated that hypothermia provided some neuroprotection when
0953-7112/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cacc.2007.03.012
ARTICLE IN PRESS 216 administered for 72 h after a neonatal asphyxial episode.3,4 These trials have laid the foundation upon which multi-modal strategies can be built and offer hope that we may one day be able to effectively combat the effects of perinatal injury. In this article we review potential therapies for sufferers of perinatal asphyxia at term.
Diagnosis of perinatal brain injury Antenatal screening identifies risk factors which may predispose the fetus to brain injury that include antenatally acquired infections, preeclampsia and thyroid disease.5 However, the diagnosis of perinatal hypoxic–ischaemic injury is still hampered by the lack of monitors that can reliably provide the required specificity and sensitivity. This has implications for both clinical management and subsequent litigation, for example birth asphyxia is believed to account for 10–20% of cases of cerebral palsy in term infants.6,7 The definition of birth asphyxia from a recent task force at the American College of Obstetrics and Gynecology (ACOG) is a clinical situation of damaging acidemia, hypoxia and metabolic acidosis with a sentinel event capable of interrupting oxygen supply to the fetus.8 The need to identify such an event has led to increased rates of cardiotocography (CTG); 85% of live births in the USA in 2002 were monitored using CTG.7 CTG patterns of reduced fetal heart rate variability and moderate to severe variable or late decelerations have been shown to correlate with episodes of fetal acidaemia.9 However, CTG suffers from large intra- and inter-operator variability and while it is a sensitive tool it lacks specificity. CTG only has a positive predictive value (PPV) of 0.2% for the prediction of cerebral palsy10 and PPV of approximately 2.6% for the prediction of asphyxia (2.6%) with standard practice.11 This false positive rate in the order of 97.4–99.8% explains in part the rise in caesarean section rates since instigation of CTG monitoring from 5% in the 1960s to approximately 25% today. Meta-analysis of 13 randomised control trials by the Cochrane group concluded that continuous CTG reduced the incidence of neonatal seizures but had no effect on the incidence of cerebral palsy or perinatal death.12 Thus, despite the upsurge in monitoring the cerebral palsy rate has remained unchanged at 1–2 per 1000 births in the last 50 years.7 It should be noted that neonatal seizures have been recently associated with neonatal cerebral infarction13 and therefore this does represent a significant advance in predicting perinatal injury.
R.D. Sanders et al. In order to reduce the high false positive rates of CTG, fetal scalp pH of p7.20 and more recently fetal scalp lactate greater than 4.2–4.8 mmol L1 have been shown to enhance detection of a compromised fetus.14,15 However, these assessments are still hampered by high false positive rates as a fetus may undergo transient episodes of asphyxia with no adverse consequences;16 in a retrospective study lactate greater than 4.8 mmol L1 has a PPV of 3.6 and pHo7.21 has a PPV of 2.8 for moderate to severe HIE.15 The definition of an acute intrapartum hypoxic event as sufficient to cause cerebral palsy has recently been defined and includes clinical criteria and umbilical arterial pH (Table 1). New techniques of assessing intrapartum fetal well-being offer hope for the future. Fetal pulse oximetry monitoring in conjunction with CTG have been investigated but this did not reduce the overall caesarean section rate and thus its further use was not endorsed by the ACOG.7 Intrapartum fetal ECG ST segment changes and umbilical artery and middle cerebral artery Doppler velocity are examples of more recent developments in monitoring.17,18 It remains to be seen though whether these will result in the evidence base required to change current practice. Nonetheless if neuroprotective interventions are to be instituted in a timely and appropriate manner early identification of an asphyxiated infant is imperative.
Pathogenesis of perinatal neuronal injury Understanding the pathogenesis of neuronal injury will allow rational development of multi-modal Table 1 Criteria to define an acute intrapartum hypoxic event as sufficient to cause cerebral palsy 1 Evidence of metabolic acidosis in fetal umbilical cord arterial blood obtained at delivery (pH o7 and base deficit X12 mmol L1) 2 Early onset of severe or moderate neonatal encephalopathy in infants born at 34 or more weeks of gestation 3 Cerebral palsy of the spastic quadriplegic or dyskinetic typea 4 Exclusion of other identifiable etiologies, such as trauma, coagulation disorders, infectious conditions, or genetic disorders a
Spastic quadriplegia and, less commonly, dyskinetic cerebral palsy are the only types of cerebral palsy associated with acute hypoxic intrapartum events. Spastic quadriplegia is not specific to intrapartum hypoxia. Hemiparetic cerebral palsy, hemiplegic cerebral palsy, spastic diplegia, and ataxia are unlikely to result from acute intrapartum hypoxia.8
ARTICLE IN PRESS Perinatal neuroprotection therapies to specifically target individual components of the injurious process. In the non-pathogenic state, the CNS has a relatively high requirement for oxygen and glucose which is mostly metabolised by oxidative phosphorylation. Indeed the neonate is known to be especially vulnerable to derangements in glucose homeostasis and avoiding hypo- and hyperglycaemia are critical as either state predisposes the CNS to injury in the young.
Primary energy failure Impairment of cerebral blood flow restricts the delivery of substrates, particularly oxygen and glucose, and thus impairs the energetics required to maintain ionic gradients in the brain. Energy depletion results in dysfunction of ATP-dependent ion channels and ion exchangers leading to cellular depolarisation and the release of excitatory neurotransmitters such as glutamate. Excess glutamatergic excitatory neurotransmission induces excitotoxic cell death as extracellular concentrations of glutamate rise by 3–10 fold. This excitotoxicity is compounded by a failure of energy-dependent glutamate uptake mechanisms which may even reverse and thereby further increase extracellular glutamate. Activation of postsynaptic glutamate receptors such as a-amino-3-hydroxy-5-methylisoxazole-4- propionic acid (AMPA/Kainate) receptors and N-methyl-D-Aspartate (NMDA) receptors mediate this injury to varying degrees in different brain regions; for example perinatal cortical injury is mediated predominantly by NMDA receptors while brainstem injury is mediated by a significant degree by AMPA receptors.19 Stimulation of the NMDA subtype (and some AMPA subtypes) of glutamatergic receptor produces a transmembrane flux of sodium and calcium cations contributing further to neuronal overactivation. Water passively follows the transmembrane flux of sodium and chloride contributing to brain swelling and oedema. Calcium initiates a series of cytoplasmatic and nuclear events that promote tissue damage profoundly. For example, overactivation of enzyme systems, e.g. proteases, lipases, endonucleases degrades cytoskeletal proteins and generates free radicals which damage membranes, mitochondria and DNA. Initially an excitotoxic form of cell death evolves which is followed by a wave of programmed cell death or apoptosis.20 This apoptotic component may occur secondarily to a loss of synaptic connectivity (due to the first wave of cell death killing an innervating cell), the intensity of the insult or type of stimulus. This process involves mitochondrial release of cytochrome C, activation of caspases
217 (aspartate-specific cysteine proteases), and other pro-apoptotic factors such as Bax. This apoptotic injury is particularly important in the very young2 and is an evolving injury, taking hours to develop. It is this form of injury that requires specific attention when designing neuroprotective regimes.
Role of inflammation in HIE Inflammatory mediators contribute significantly to the pathogenesis of HIE and the expression of both TNF-a and IL-1b are upregulated in this injury. While inflammation plays an important role in repair, excess inflammation is clearly deleterious. This becomes of clinical significance when the effect of peripartum infection is considered. Infection has been associated with CNS white matter injury, cerebral palsy and increased blood brain barrier permeability.21 Animal models have shown that lipopolysaccharide administration prior to a hypoxic–ischaemic insult exacerbates subsequent neuronal injury.22 The fetal inflammatory response appears to play a greater role than the maternal and in particular IL-6 upregulation is associated with increased risk of cerebral palsy and periventricular leukomalacia.23 IL-6 has also been reported as neuroprotective24 and thus the inflammatory milieu appears incredibly complex. Further study of how infection and the inflammatory cascade impact upon subsequent hypoxic–ischaemic injury in the neonate is required.
Secondary energy failure Following an interval of approximately 6 h a second phase of injury occurs with cerebral energy failure characterised by a fall in the phosphocreatine/ inorganic phosphate ratio. The degree of this energy failure correlates with adverse neurological outcome at 1 and 4 years.25 Similarly, this energy failure correlates with intracellular brain alkalosis, increased lactate/creatine ratio and more severe neurological outcome.26 Intracellular alkalosis may exacerbate excitotoxic injury, mitochondrial permeability, protease activation and apoptosis potentiating the ongoing pathology. The difficulty in pre-empting the first phase of injury has led to strategies to combat primarily this second phase of injury.
Hypothermic neuroprotection Hypothermia was first instituted as a therapeutic tool over 50 years ago; however, the first clinical
ARTICLE IN PRESS 218 trials to demonstrate improved outcome with hypothermia therapy in neonates were published recently.3,4 These landmark studies, like their counterparts in the adult cardiac arrest literature, have the potential to revolutionise therapy for this disease. The Cool-Cap study3 enrolled 234 infants with moderate to severe neonatal HIE and abnormal amplitude-integrated EEG (aEEG) with 116 infants receiving hypothermic therapy (34–35 1C) for 72 h from a cooling cap device; the control group of 118 infants received conventional therapy only. Hypothermia did not provide neuroprotection in the most severely injured neonates (n ¼ 46; odds ratio (OR) 1.8; 95% CI 0.49–6.4; p ¼ 0.51) but did prove beneficial in those with less evidence of injury (n ¼ 172, OR 0.42; 95% CI 0.22–0.80, p ¼ 0.009, NNT ¼ 6). In this group neuromotor disability was predominantly improved (p ¼ 0.03) rather than mortality (p ¼ 0.2) indicating that hypothermia protects neonates destined to survive and improves their functional outcome. The National Institute of Child Health and Human Development study4 randomised 208 term neonates to control or whole-body hypothermia with a target temperature of 33.5 1C for 72 h. Death or moderate or severe disability occurred in 45 of 102 infants (44%) in the hypothermia group and 64 of 103 infants (62%) in the control group (relative risk, 0.72; 95% CI, 0.54–0.95; NNT ¼ 6). There were no differences for mortality alone, disabling cerebral palsy, blindness, severe hearing impairment or the Bayley Mental Development Index or Bayley Psychomotor Development scores. However it should be noted that 41 infants in the control group experienced hyperthermia (438 1C) on at least one occasion which could have contributed to worsened outcome in this group. Further data from on-going trials are required before hypothermic therapy becomes standard of care for HIE. Hypothermia does reduce the incidence of thalamic and basal ganglia lesions secondary to HIE as assessed by magnetic resonance imaging providing further support for its neuroprotective potential.27 Thus, hypothermia represents a viable possibility in a field devoid of successful therapeutic interventions. Hypothermia’s neuroprotective effects may be secondary to its ability to target both excitotoxic and apoptotic cell death. Acting presynaptically hypothermia reduces glutamate release28 but also acts at an early stage to inhibit the apoptotic cascade to prevent neurotoxicity.29,30 Hypothermia alters the balance of the pro- and anti-apoptotic signalling cascades, preserving the anti-apoptotic
R.D. Sanders et al. protein kinase Akt and inhibiting the pro-apoptotic proteins cJun N-terminal kinase and forkhead transcription factor, and preventing cytochrome C release.29,30 In addition, hypothermia reduces the cerebral metabolic rate which may reduce the metabolic burden during the secondary energy failure phase and promotes an anti-inflammatory cascade. However, there are numerous toxic effects of hypothermia especially if not administered in a tightly controlled manner with coagulopathic and arrhythmogenic toxicity being of particular concern. Thus, clinical trials limited the lower level of hypothermia to 33.5 1C in the perinatal population.3,4 Hypothermia can increase metabolic rate by shivering, increasing oxygen consumption and sedative strategies to limit this adaptation are required. It is of interest that the protective potential of hypothermia is abrogated in the absence of adequate sedation.31 Remarkably, details about the sedative therapies employed in the hypothermia trials were not reported.3,4 It is likely that different sedatives will interact dissimilarly with hypothermic neuroprotection; for example, addition of methohexital to hypothermic neuroprotection showed no additional protective benefit over hypothermia alone32 in an animal model of focal ischaemia. Therefore, detailing which pharmacological agents can augment hypothermic neuroprotection has critical importance.
Pharmacological neuroprotection The optimal regimen to prescribe for sedation during hypothermic therapy has not been formally addressed but as hypothermia requires the involvement of anaesthetic and critical care services, this deficiency will need to be addressed. Preclinical evidence suggests that some classes of drug may be particularly effective; notably, sedatives exerting properties of either NMDA antagonism or a2 adrenoceptor activation both show considerable promise. Unfortunately, most agents tested so far have been ineffective; for example mannitol therapy has not proven successful in small clinical studies1,33 and preclinical studies.34 Similarly, dexamethasone is not recommended as it reduces cerebral perfusion pressure in line with its ability to reduce intracranial pressure.1 Furthermore, the increase in ICP observed in HIE is likely to be an epiphenomenon rather than a mechanism of injury. Calcium channel blockers are also associated with decreased cerebral flow and are not recommended for treatment of perinatal HIE.1
ARTICLE IN PRESS Perinatal neuroprotection
Barbiturate therapy Perinatal seizures in the absence of other features of HIE are associated with significant neuronal injury and even cerebral infarction in the neonatal population.13 Effective anti-seizure treatment needs to be initiated promptly especially if it also can exhibit neuroprotective capability. Barbiturates are often employed in the treatment of neonatal seizures but it is unclear whether their anti-seizure actions translate into neuroprotection. Three small clinical trials have investigated the potential role of barbiturates to ameliorate perinatal HIE but only one showed a relative risk reduction of severe developmental disability or death.35 However, 23% patients were lost to follow up in this trial. Two other trials36,37 did not find thiopentone or pentobarbitone effective. Subsequent meta-analysis of the studies (n ¼ 77) showed no significant effect on death or severe neurodevelopmental disability.1 As avoidance of hypotension is regarded as critical in the asphyxiated infant, current evidence does not support the use of prophylactic barbiturates for perinatal neuroprotection. Barbiturates still have a role in the treatment of seizures and further study is required to investigate whether they possess neuroprotective efficacy when administered in this setting especially since these seizures have been associated with CNS infarctions. An obvious problem with drawing any conclusions from the studies available is the size of these trials when compared with those showing efficacy for hypothermic neuroprotection and future trials of pharmacological agents will require adequate power to fully assess neuroprotective efficacy. There are no studies of the neuroprotective efficacy of barbiturates in animal models of perinatal HIE and therefore, it would be prudent to test barbiturates in preclinical trials before extending the current clinical research. A significant reduction in GABAergic neuron expression is noted after human perinatal brain injury38 and therefore agonists depending on GABA receptors may therefore prove less effective for retrospective treatment of this injury.
219 pulmonary bypass40 assessed by functional and morphologic criteria. Further dose–responsive investigation of how volatile anaesthetics interact with hypothermia is required, especially as administration of significant anaesthesia in the absence of surgical stimulus may be neurotoxic41 and will also expose the neonate to significant respiratory and cardiovascular depression.
a2 Adrenoceptor agonists The a2 adrenoceptor agonists, exemplified by clonidine and dexmedetomidine, have been shown to have neuroprotective potential in animal models of HIE.42–44 Both agents reduced the size of excitotoxin-induced cortical and white-matter lesions in mouse pups injected intracerebrally with the NMDA receptor agonist ibotenate;42 these protective effects were abolished by the a2 antagonist, yohimbine, confirming that these agents protect through their activity at the a2 adrenoceptor. Recently, it has been shown that dexmedetomidine, concentration-dependently, diminished neuronal injury provoked either in vitro or in vivo in a neonatal asphyxia rat model.43 Dexmedetomidine, administered during the asphyxia, provided protection even when assessed 30 days later with functional neuromotor followup. Dexmedetomidine sedation appears particularly attractive as it possesses anti-excitotoxic, anti-apoptotic and anti-inflammatory capabilities with analgesia and minimal respiratory suppression. Whether dexmedetomidine can exert neuroprotection when provided well after the onset of injury is currently unknown. However, as dexmedetomidine can target different aspects of ongoing injury, it has potential to do be beneficial even when delivered after the initial hypoxic–ischemic insult. It should also be noted that a synergistic interaction between dexmedetomidine and the NMDA antagonist, xenon, has been noted in this model.44 The characteristics of the neuroprotective interaction between dexmedetomidine and hypothermia has not been reported.
NMDA receptor antagonists Volatile anaesthetics In preclinical studies supra-anaesthetic concentrations of volatile anaesthetics have been shown to potentiate hypothermic neuroprotection when given during an insult. Desflurane (9%) has proven effective in newborn piglet models of deep hypothermic cardiac arrest39 and low flow cardio-
The NMDA receptor antagonists were initially heralded as neuroprotectants of the future but those in routine clinical practice at present have not supported this claim despite the central role of the NMDA receptor in excitotoxic injury.45 In contrast to GABA receptor expression, certain NMDA receptor subtypes are upregulated following
ARTICLE IN PRESS 220 HIE46 suggesting that targeting this receptor in a retrospective manner may be appropriate for treatment of HIE. Magnesium is an endogenous voltage-dependent blocker of the NMDA channel and is also employed antenatally for tocolysis and the treatment of eclampsia. A retrospective case review of premature infants who were exposed antenatally to magnesium sulphate observed a reduced incidence of cerebral palsy at 3 years.47 Magnesium sulphate has been reported neuroprotective when given prepartum and intrapartum for very premature infants (at less than 30 weeks of age).48 This is not a consistent finding in the literature. There are also conflicting reports from the animal literature regarding the neuroprotective efficacy of magnesium, with lack of temperature control a frequent confound. The translatability of these findings to term infants is at best unclear.1,49 Prophylactic magnesium therapy may expose many infants unnecessarily to side-effects such as tocolysis with potentially prolonged labour, severe hypotension and respiratory depression.1 The possible role of magnesium as neuroprotectant for term infants requires further exploration. Anaesthetic NMDA antagonists in current clinical use include nitrous oxide and ketamine which are both often employed in obstetric practice. As with magnesium, their neuroprotective capabilities are variably reported,45 but nitrous oxide and ketamine have both recently been associated with neurotoxicity (apoptotic neurodegeneration) in the very young.41,50 Xenon, also an NMDA antagonist, does not produce apoptotic neurodegeneration in the young but does provide robust neuroprotection in several animal models of neuronal injury.51,52 In establishing whether xenon can provide protection during neonatal rat asphyxia in vivo and in vitro it has been shown that xenon attenuates hypoxic–ischaemic damage at concentrations of 40% and greater. Xenon offers neuroprotection at subanaesthetic concentrations.53 As perinatal asphyxia is difficult to anticipate xenon was investigated to see if it could provide neuroprotection when given after the injury. Xenon attenuated hypoxic–ischemic damage when given up to six hours after the injury and provided synergistic neuroprotection when given post-injury in combination with hypothermia (35 1C; Fig. 1). This synergistic interaction still occurs when the administration of hypothermia and xenon occurs asynchronously.54 The simplest analysis for this synergism assumes the rate of a reaction proceeds because thermal energy provides sufficient energy to overcome an ‘‘activation’’ energy. If the logarithm of the reaction rate is plotted against the reciprocal
R.D. Sanders et al. absolute temperature then the slope of the line is proportional to the activation energy. The bigger the activation energy, the smaller the reaction rate and the greater the temperature dependence. For LDH release, the value that characterises the temperature dependence is 34.8 kJ mol1. This is essentially ‘‘neuroprotection’’ due to hypothermia. It corresponds to a reaction that is moderately temperature dependent. When xenon is combined with hypothermia, this value increases to about 177 kJ mol1. The large increase in temperature dependence means that, in the presence of xenon, hypothermia is very much more effective at providing neuroprotection. To a small extent this might be expected because xenon is likely to bind better to its targets as the temperature decreases. However, this is a small effect and could only account for a small element of the enhanced temperature dependence. In some way, xenon enhances the system’s sensitivity to temperature. Whatever the mechanism, it means that a combination of xenon and hypothermia provides potent neuroprotection. Apoptosis is the predominant form of ongoing injury in this paradigm and therefore the effects of xenon and hypothermia on markers of apoptosis were evaluated. Xenon and xenon-hypothermia reduced this form of injury significantly and increased the anti-apoptotic protein Bcl-XL.53 This protection correlated with improved neuromotor function at 30 days of age indicating functional protection.53 This may account for why asynchronous application of the interventions attenuates the injury. This finding could be utilised in a clinical context with hypothermia being instituted early in life before transfer to a tertiary centre for xenon therapy which is an expensive agent currently requiring sophisticated administration apparatus. Xenon provides neuroprotection that is time-independent as it can be given before, during and after the insult, combines synergistically with hypothermia and targets excitotoxic and apoptotic injury.53 Xenon provides haemodynamic stability in adult settings; if present in the very young this will likely to be beneficial as the neonatal myocardium is extremely sensitive to the depressant effects of anaesthetic agents.52
Erythropoietin One non-sedative pharmacological neuroprotectant that is receiving attention in this field is erythropoietin. Interestingly, erythropoietin has been reported to be upregulated in umbilical cord blood
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Fig. 1 Neuroprotective effects of xenon and hypothermia in vitro. (A) Release of lactate dehydrogenase (LDH) as a marker of neuronal injury from oxygen-glucose deprived (OGD) cultured neurons (to simulate cerebral ischaemia) exposed to increasing concentrations of xenon at normothermia (37 1C) and hypothermia (33 1C); (B) LDH release from OGD-injured cultured neurons exposed to decreasing temperature with or without sub-anaesthetic xenon (12.5%); (C) isobologram illustrating the protective efficacy of combining xenon and temperature on injured cultured neurons. The neuroprotection of OGD-injured neurons is synergistically enhanced when xenon and hypothermia are provided in combination as the IC50 values for the interventions are significantly to the left of the ‘‘line of additivity’’ joining the IC50 of the individual interventions; and (D) van’t Hoff plot showing the temperature dependence of neuroprotection for hypothermia alone and hypothermia in the presence of xenon. The enthalpy change (DH) is a measure of the overall heat exchange during the process. Reproduced with permission from the Annals of Neurology.53
from babies who have suffered perinatal asphyxia.55 This may represent a defence mechanism as erythropoietin is neuroprotective when given after hypoxic–ischaemic injury.56 Erythropoietin’s effect is time dependent and toxicity has been reported especially when administered during the hypoxia.57 Nonetheless erythropoietin does exhibit neuroprotective effects likely in part by attenuating apoptosis; once the timing of the therapy is delineated more clearly erythropoietin may have a role as a retrospective neuroprotectant. It is certainly of interest that xenon administration
upregulates hypoxia-inducible factor and erythropoietin expression58 and this may also play a role in xenon’s neuroprotective capabilities.
Antioxidants and free radical scavengers Cellular damage induced by free radicals contributes significantly to reperfusion injury in HIE and this prompted trials analysing the safety of neonatal resuscitation with air rather than oxygen. Hyperoxia induced by ventilation with 100% oxygen
ARTICLE IN PRESS 222 is associated with reduced cerebral blood flow59 and the production of free radicals such as hydrogen peroxide.60 While no individual trial has shown difference in mortality with resuscitation with air rather than oxygen, recent meta-analysis has found that resuscitation with air is associated with a reduction in mortality61 (RR 0.71 (0.54, 0.94), NNT ¼ 20). At present there is insufficient data to comment on whether air resuscitation may reduce neurodevelopment delay and cerebral palsy. While the finding of a mortality difference is remarkable, it is biologically plausible and therefore requires further evaluation. Furthermore, trials of other oxygen concentrations are warranted as are trials designed at specific subgroups which may require higher oxygen concentrations such as severe asphyxia or sepsis. Other strategies to reduce free radical generation include the use of xanthine oxidase inhibitors which have shown protection of cerebral energetics when administered early during the reperfusion phase but did not attenuate brain damage or markers of apoptosis.62 Furthermore, allopurinol reduced circulating concentrations of free radicals in human infants exemplified by a reduced malondialdehyde level which is a marker of lipid peroxidation.63 As yet the use of free radical scavengers has not been fully evaluated in conjunction with other neuroprotective therapies such as hypothermia and this requires further study.
Summary Defining the optimal strategy for perinatal neuroprotection has potential to significantly improve neurocognitive outcome after asphyxial injury. The current data suggest hypothermic neuroprotection could soon become clinical reality if ongoing trials confirm this finding and help to establish this as mainstream therapy. Neonatal resuscitation with air may further improve neonatal outcome, though further clinical trials are still required. There is also great potential to augment hypothermic neuroprotection with co-administered neuroprotective sedatives. Xenon therapy requires evaluation in this context, though present costs and technological development may limit the ubiquity of this treatment. The a2 adrenoceptor agonists also show promise but retrospective efficacy with or without hypothermia needs to be established before clinical trials can be undertaken. Likewise allopurinol therapy should be evaluated in the context of hypothermia and air resuscitation. As more information assimilates on the ability of hypothermia to provide neuroprotection in this context, our knowl-
R.D. Sanders et al. edge of how to care for these neonates will expand. How we should sedate and protect these neonates requires further attention.
References 1. Whitelaw A, Thoresen M. Clinical trials of treatments after perinatal asphyxia. Curr Opin Pediatr 2002;14(6):664–8. 2. Nakajima W, Ishida A, Lange MS, Gabrielson KL, Wilson MA, Martin LJ, et al. Apoptosis has a prolonged role in the neurodegeneration after hypoxic ischemia in the newborn rat. J Neurosci 2000;20(21):7994–8004. 3. Gluckman PD, Wyatt JS, Azzopardi D, Ballard R, Edwards AD, Ferriero DM, et al. Selective head cooling with mild systemic hypothermia after neonatal encephalopathy: multicentre randomised trial. Lancet 2005;365(9460):663–70. 4. Shankaran S, Laptook AR, Ehrenkranz RA, Tyson JE, McDonald SA, Donovan EF, et al. Whole-body hypothermia for neonates with hypoxic-ischemic encephalopathy. N Engl J Med 2005;353(15):1574–84. 5. Badawi N, Kurinczuk JJ, Keogh JM, Alessandri LM, O’Sullivan F, Burton PR, et al. Antepartum risk factors for newborn encephalopathy: the Western Australian case-control study. BMJ 1998;317(7172):1549–53. 6. Nelson KB, Ellenberg JH. Antecedents of cerebral palsy. Multivariate analysis of risk. N Engl J Med 1986;315(2):81–6. 7. Graham EM, Petersen SM, Christo DK, Fox HE. Intrapartum electronic fetal heart rate monitoring and the prevention of perinatal brain injury. Obstet Gynecol 2006;108(3 Pt 1): 656–66. 8. American Academy of Pediatrics. American College of Obstetricians and Gynecologists. Neonatal encephalopathy and cerebral palsy: defining the pathogenesis and pathophysiology. Washington, DC: Elk Grove Village (IL): AAP; 2003. 9. Kubli FW, Hon EH, Khazin AF, Takemura H. Observations on heart rate and pH in the human fetus during labor. Am J Obstet Gynecol 1969;104(8):1190–206. 10. Nelson KB, Dambrosia JM, Ting TY, Grether JK. Uncertain value of electronic fetal monitoring in predicting cerebral palsy. N Engl J Med 1996;334(10):613–8. 11. Low JA, Victory R, Derrick EJ. Predictive value of electronic fetal monitoring for intrapartum fetal asphyxia with metabolic acidosis. Obstet Gynecol 1999;93(2):285–91. 12. Alfirevic Z, Devane D, Gyte GM. Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database Syst Rev 2006;3:CD006066. 13. Cowan F, Rutherford M, Groenendaal F, Eken P, Mercuri E, Bydder GM, et al. Origin and timing of brain lesions in term infants with neonatal encephalopathy. Lancet 2003; 361(9359):736–42. 14. Allen RM, Bowling FG, Oats JJ. Determining the fetal scalp lactate level that indicates the need for intervention in labour. Aust N Z J Obstet Gynaecol 2004;44(6):549–52. 15. Kruger K, Hallberg B, Blennow M, Kublickas M, Westgren M. Predictive value of fetal scalp blood lactate concentration and pH as markers of neurologic disability. Am J Obstet Gynecol 1999;181(5 Pt 1):1072–8. 16. ACOG Committee on Obstetric Practice. ACOG Committee Opinion No. 348, November 2006: umbilical cord blood gas and acid-base analysis. Obstet Gynecol 2006;108(5): 1319–22. 17. Westgate J, Harris M, Curnow JS, Greene KR. Plymouth randomized trial of cardiotocogram only versus ST waveform
ARTICLE IN PRESS Perinatal neuroprotection
18.
19. 20. 21. 22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
plus cardiotocogram for intrapartum monitoring in 2400 cases. Am J Obstet Gynecol 1993;169(5):1151–60. Siristatidis C, Salamalekis E, Kassanos D, Loghis C, Creatsas G. Evaluation of fetal intrapartum hypoxia by middle cerebral and umbilical artery Doppler velocimetry with simultaneous cardiotocography and pulse oximetry. Arch Gynecol Obstet 2004;270(4):265–70. Johnston MV. Excitotoxicity in perinatal brain injury. Brain Pathol 2005;15(3):234–40. Olney JW. Excitotoxicity, apoptosis and neuropsychiatric disorders. Curr Opin Pharmacol 2003;3(1):101–9. Edwards AD, Tan S. Perinatal infections, prematurity and brain injury. Curr Opin Pediatr 2006;18(2):119–24. Eklind S, Mallard C, Arvidsson P, Hagberg H. Lipopolysaccharide induces both a primary and a secondary phase of sensitization in the developing rat brain. Pediatr Res 2005;58(1):112–6. Yoon BH, Romero R, Yang SH, Jun JK, Kim IO, Choi JH, Syn HC. Interleukin-6 concentrations in umbilical cord plasma are elevated in neonates with white matter lesions associated with periventricular leukomalacia. Am J Obstet Gynecol 1996;174(5):1433–40. Loddick SA, Turnbull AV, Rothwell NJ. Cerebral interleukin-6 is neuroprotective during permanent focal cerebral ischemia in the rat. J Cereb Blood Flow Metab 1998;18(2):176–9. Roth SC, Baudin J, Cady E, Johal K, Townsend JP, Wyatt JS, et al. Relation of deranged neonatal cerebral oxidative metabolism with neurodevelopmental outcome and head circumference at 4 years. Dev Med Child Neurol 1997; 39(11):718–25. Robertson NJ, Cowan FM, Cox IJ, Edwards AD. Brain alkaline intracellular pH after neonatal encephalopathy. Ann Neurol 2002;52(6):732–42. Rutherford MA, Azzopardi D, Whitelaw A, Cowan F, Renowden S, Edwards AD, et al. Mild hypothermia and the distribution of cerebral lesions in neonates with hypoxic–ischemic encephalopathy. Pediatrics 2005;116(4):1001–6. Busto R, Globus MY, Dietrich WD, Martinez E, Valdes I, Ginsberg MD. Effect of mild hypothermia on ischemiainduced release of neurotransmitters and free fatty acids in rat brain. Stroke 1989;20(7):904–10. Xu L, Yenari MA, Steinberg GK, Giffard RG. Mild hypothermia reduces apoptosis of mouse neurons in vitro early in the cascade. J Cereb Blood Flow Metab 2002;22(1):21–8. Zhao H, Shimohata T, Wang JQ, Sun G, Schaal DW, Sapolsky RM, et al. Akt contributes to neuroprotection by hypothermia against cerebral ischemia in rats. J Neurosci 2005;25(42):9794–806. Thoresen M, Satas S, Loberg EM, Whitelaw A, Acolet D, Lindgren C, et al. Twenty-four hours of mild hypothermia in unsedated newborn pigs starting after a severe global hypoxic–ischemic insult is not neuroprotective. Pediatr Res 2001;50(3):405–11. Westermaier T, Zausinger S, Baethmann A, Steiger HJ, Schmid-Elsaesser R. No additional neuroprotection provided by barbiturate-induced burst suppression under mild hypothermic conditions in rats subjected to reversible focal ischemia. J Neurosurg 2000;93(5):835–44. Levene MI, Evans DH, Forde A, Archer LN. Value of intracranial pressure monitoring of asphyxiated newborn infants. Dev Med Child Neurol 1987;29(3):311–9. Mujsce DJ, Towfighi J, Stern D, Vannucci RC. Mannitol therapy in perinatal hypoxic–ischemic brain damage in rats. Stroke 1990;21(8):1210–4. Hall RT, Hall FK, Daily DK. High-dose phenobarbital therapy in term newborn infants with severe perinatal asphyxia: a
223
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
randomized, prospective study with three-year follow-up. J Pediatr 1998;132(2):345–8. Goldberg RN, Moscoso P, Bauer CR, Bloom FL, Curless RG, Burke B, et al. Use of barbiturate therapy in severe perinatal asphyxia: a randomized controlled trial. J Pediatr 1986; 109(5):851–6. Ruth V, Virkola K, Paetau R, Raivio KO. Early high-dose phenobarbital treatment for prevention of hypoxic-ischemic brain damage in very low birth weight infants. J Pediatr 1988;112(1):81–6. Robinson S, Li Q, Dechant A, Cohen ML. Neonatal loss of gamma-aminobutyric acid pathway expression after human perinatal brain injury. J Neurosurg 2006;104(6 Suppl): 396–408. Kurth CD, Priestley M, Watzman HM, McCann J, Golden J. Desflurane confers neurologic protection for deep hypothermic circulatory arrest in newborn pigs. Anesthesiology 2001;95(4):959–64. Loepke AW, Priestley MA, Schultz SE, McCann J, Golden J, Kurth CD. Desflurane improves neurologic outcome after low-flow cardiopulmonary bypass in newborn pigs. Anesthesiology 2002;97(6):1521–7. Jevtovic-Todorovic V, Hartman RE, Izumi Y, Benshoff ND, Dikranian K, Zorumski CF, et al. Early exposure to common anesthetic agents causes widespread neurodegeneration in the developing rat brain and persistent learning deficits. J Neurosci 2003;23(3):876–82. Laudenbach V, Mantz J, Lagercrantz H, Desmonts JM, Evrard P, Gressens P. Effects of alpha(2)-adrenoceptor agonists on perinatal excitotoxic brain injury: comparison of clonidine and dexmedetomidine. Anesthesiology 2002; 96(1):134–41. Ma D, Hossain M, Rajakumaraswamy N, Arshad M, Sanders RD, Franks NP, Maze M. Dexmedetomidine produces its neuroprotective effect via the alpha 2A-adrenoceptor subtype. Eur J Pharmacol 2004;502(1–2): 87–97. Rajakumaraswamy N, Ma D, Hossain M, Sanders RD, Franks NP, Maze M. Neuroprotective interaction produced by xenon and dexmedetomidine on in vitro and in vivo neuronal injury models. Neurosci Lett 2006;409(2):128–33. Sanders RD, Ma D, Maze M. Anaesthesia induced neuroprotection. Best Pract Res Clin Anaesthesiol 2005;19(3): 461–74. Guerguerian AM, Brambrink AM, Traystman RJ, Huganir RL, Martin LJ. Altered expression and phosphorylation of Nmethyl-D-aspartate receptors in piglet striatum after hypoxia–ischemia. Brain Res Mol Brain Res 2002;104(1): 66–80. Nelson KB, Grether JK. Can magnesium sulfate reduce the risk of cerebral palsy in very low birthweight infants? Pediatrics 1995;95(2):263–9. Crowther CA, Hiller JE, Doyle LW, Haslam RR, Australasian Collaborative Trial of Magnesium Sulphate (ACTOMg SO4) Collaborative Group. Effect of magnesium sulfate given for neuroprotection before preterm birth: a randomized controlled trial. JAMA 2003;290(20):2669–76. Greenwood K, Cox P, Mehmet H, Penrice J, Amess PN, et al. Magnesium sulfate treatment after transient hypoxia-ischemia in the newborn piglet does not protect against cerebral damage. Pediatr Res 2000;48(3):346–50. Young C, Jevtovic-Todorovic V, Qin YQ, Tenkova T, Wang H, Labruyere J, et al. Potential of ketamine and midazolam, individually or in combination, to induce apoptotic neurodegeneration in the infant mouse brain. Br J Pharmacol 2005;146(2):189–97.
ARTICLE IN PRESS 224 51. Ma D, Williamson P, Januszewski A, Nogaro MC, et al. Xenon mitigates isoflurane-induced neurodegeneration in the developing rodent brain. Anesthesiology in press. 52. Sanders RD, Ma D, Maze M. Xenon: elemental anaesthesia in clinical practice. Br Med Bull 2005;71:115–35. 53. Ma D, Hossain M, Chow A, Arshad M, Battson RM, Sanders RD, et al. Xenon and hypothermia combine to provide neuroprotection from neonatal asphyxia. Ann Neurol 2005;58(2): 182–93. 54. Martin JL, Ma D, Hossain M, Xu J, Sanders RD, Franks NP, et al. Asynchronous administration of xenon and hypothermia significantly reduces brain infarction in the neonatal rat. Br J Anaesth 2007;98(2):236–40. 55. Ruth V, Autti-Ramo I, Granstrom ML, Korkman M, Raivio KO. Prediction of perinatal brain damage by cord plasma vasopressin, erythropoietin, and hypoxanthine values. J Pediatr 1988;113(5):880–5. 56. Spandou E, Papadopoulou Z, Soubasi V, Karkavelas G, Simeonidou C, Pazaiti A, et al. Erythropoietin prevents long-term sensorimotor deficits and brain injury following neonatal hypoxia-ischemia in rats. Brain Res 2005; 1045(1–2):22–30. 57. Weber A, Dzietko M, Berns M, Felderhoff-Mueser U, Heinemann U, Maier RF, et al. Neuronal damage after
R.D. Sanders et al.
58.
59.
60.
61.
62.
63.
moderate hypoxia and erythropoietin. Neurobiol Dis 2005; 20(2):594–600. Lim T, Ma D, To C, Maze M, Maxwell P. Xenon-Induced Hypoxia-inducible factor-1 alpha in preconditioning of neural and non-neural tissue. ASA abstract 2006; A1186. Niijima S, Shortland DB, Levene MI, Evans DH. Transient hyperoxia and cerebral blood flow velocity in infants born prematurely and at full term. Arch Dis Child 1988;63(10 Spec No):1126–30. Kutzsche S, Ilves P, Kirkeby OJ, Saugstad OD. Hydrogen peroxide production in leukocytes during cerebral hypoxia and reoxygenation with 100% or 21% oxygen in newborn piglets. Pediatr Res 2001;49(6):834–42. Tan A, Schulze A, O’Donnell CP, Davis PG. Air versus oxygen for resuscitation of infants at birth. Cochrane Database Syst Rev 2005(2):CD002273. Peeters-Scholte C, Braun K, Koster J, Kops N, Blomgren K, Buonocore G, et al. Effects of allopurinol and deferoxamine on reperfusion injury of the brain in newborn piglets after neonatal hypoxia–ischemia. Pediatr Res 2003;54(4):516–22. Van Bel F, Shadid M, Moison RM, Dorrepaal CA, Fontijn J, Monteiro L, et al. Effect of allopurinol on postasphyxial free radical formation, cerebral hemodynamics, and electrical brain activity. Pediatrics 1998;101(2):185–93.