Perinatal Asphyxia: Timing and Mechanisms of Injury in Neonatal Encephalopathy Mark Scher, MD
Address Division of Pediatrics and Neurology, Rainbow Babies and Children’s Hospital and Case Western Reserve University, 11100 Euclid Avenue, Cleveland, OH, 44106-6090, USA. E-mail:
[email protected] Current Neurology and Neuroscience Reports 2001, 1:175–184 Current Science Inc. ISSN 1528-4042 Copyright © 2001 by Current Science Inc.
This article summarizes the recent medical literature regarding perinatal asphyxia with respect to timing and mechanisms of injury for neonates who were clinically diagnosed with an encephalopathy in the newborn period. Multiple mechanisms of injury are reviewed, including genetic vulnerability, acquired inflammatory responses, and clotting defects that can lead to ischemic-induced brain damage. Before effective treatments for fetal and neonatal brain disorders can be developed, accurate and timely diagnoses of fetal or neonatal brain injury must be achieved. Specific subsets of children can then benefit from neuroprotective strategies that can target the specific developmental aspects of brain adaptation or plasticity relative to the specific etiology and timing of injury after asphyxia.
Introduction The pediatric neurologist must consider a series of questions regarding neonatal brain disorders (Table 1). Different pathophysiologic processes may contribute to an encephalopathy: genetic, developmental, metabolic-toxic, infectious, traumatic, and neoplastic-infiltrative processes on either an acute, subacute, or chronic basis. Common clinical signs include altered states of arousal, (ie, hyperalertness with or without seizures or unresponsiveness), abnormalities of muscle tone or strength, focal neurologic deficits, or delayed developmental milestones. Such signs, however, do not necessarily suggest either the causal nature or timing of injury. Encephalopathy is a general term for a composite of clinical signs that may reflect multiple mechanisms of brain injury over any time course, or alternatively describes brain dysfunction without damage. The perinatal period is a broad time period from the beginning of
the third trimester of pregnancy through the first month of postnatal life. It is, therefore, more practical to distinguish fetal events contributing to an encephalopathy (ie, antenatal) that occurred before labor and delivery (ie, the antepartum period), compared with during parturition (ie, the intrapartum period). Antepartum brain injuries during the first half of gestation appear as encephaloclastic abnormalities on neuroimaging or neuropathologic examinations, reflecting brain maldevelopment. Injuries after 20 weeks of gestation alternatively result in destructive disease processes. Neuroimaging may help distinguish between either malformations from early maldevelopment or injury, or encephalomalacia/ atrophy from later injury. Asphyxia (ie, hypoxia-ischemia) is a commonly encountered etiology for a neonatal encephalopathy that may cause injury expressed as cerebral palsy. However, children with cerebral palsy usually have an antepartum time period during which brain injury occurred from either asphyxia or other etiologies [1]. Excellent reviews provide further details regarding the general themes of perinatal asphyxia, mechanisms of brain injury, and timing of injury [2••,3]. Because less than 20% of patients with cerebral palsy suffer brain damage exclusively after intrapartum asphyxia, the clinician must properly select the appropriate fetal or neonatal cohorts who will benefit from a neuroprotective strategy.
Mechanisms of Ischemic Brain Injury Different mechanisms of cellular ischemic injury may lead to neuronal or glial cell loss from asphyxia (Table 2). Mechanisms include both destructive pathways of necrosis and apoptosis, as well as adaptive mechanisms that protect the developing brain. Understanding maturational aspects of asphyxial stress that promote vulnerability or resistance will help create a blueprint for the development of neuroprotective strategies against brain injury.
Ionic Changes During Ischemia Early changes in pH and ionic flux compromise energy supply to ischemic neurons, leading to inhibition of the sodium-potassium adenosine triphosphate (ATP)ase,
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Table 1. Perinatal encephalopathy: issues and questions relative to asphyxia When did brain injury occur? What are the mechanisms of brain injury? Who are the fetal and neonatal populations at risk? How to distinguish brain dysfunction from damage? How to diagnose fetal vs neonatal encephalopathy? What constitutes optimal neuroprotective therapy? What is the scope of neurologic sequela?
and depolarization of the cellular membrane. Increased intracellular sodium and calcium levels, and decreased intracellular potassium result [4]. Brain tissue hypoxia modifies receptor ion channels specifically for N-methylD-aspartate (NMDA), leading to increases in active NMDA receptor sites, which are more dependent on calcium. Increases in intracellular calcium activate multiple enzymatic pathways that release oxygen free radicals, causing membrane lipid perioxidation and cell membrane dysfunction. Increased intracellular calcium leads to altered intranuclear calcium concentrations, initiating transcription of specific genes that are responsible for programmed cell death. Neuronal activities crucial for cell survival and neuronal connectivity are also based on the genetic endowment of ionic channels, which are required for these processes. Knockout mice, deficient in brain voltage-gated sodium channel mechanisms, may be more susceptible to asphyxial injury. Redundancy of sodium channel mechanisms during embryonic development may then be considered essential for postnatal survival [5].
Table 2. Mechanisms of ischemic-reperfusion of neuronal injury Ionic changes during ischemia Calcium cell damage Oxidative stress Injury due to excitotoxicity Changes in gene expression (including apoptosis) Ischemia-reperfusion injury (secondary disruption of energy metabolism) Vascular injury (free radical-induced damage) Neurotrophin factors Inflammatory mediatiors (lipid-derived factors, cytokines, neutrophils, platelet adhesion agents, etc.)
Repetitive, rather than continuous, intermittent hypoxic-ischemic stress results in less pronounced damage [8]. Prophylactic neuroprotection may be created in hypoxia-preconditioned animals by altering the neuronal response to subsequent ischemic-reperfusion injury [9].
Oxidative Stress
Brain ischemia is associated with a fall in both extracellular and intracellular pH. Whereas mild acidosis is protective, severe acidosis favors free radical production and promotes cytotoxic edema. Persistent increases in cerebral lactate concentrations can be detected in human newborns after birth asphyxia using proton magnetic resonance spectroscopy [6]. Changes in lactate/creatinine ratios are detectable within hours of an ischemic injury, but persist for weeks after the ischemic injury, which may contribute to ongoing neuronal impairment.
The brain is vulnerable to oxidative stress injury because of its high metabolic rate, which is supported almost exclusively by oxidative metabolism [10]. The brain has little antioxidant activity to counteract the excessive free radicals generated in the setting of ischemic injury. Iron is one critical component in the generation of free radicals, released from cellular transit pools and from metalloproteins by hydrolases. Iron can be sequestered if combined with oxygen peroxide and superoxide to create hydroxyl radicals. Fullerton et al. [11] showed how overexpression of superoxide dismutase may be harmful to the neonatal brain. Given the developmentally low activities of catalytic enzymes, the immature brain cannot overcompensate for overexpression of superoxide mutase, resulting in increased production of hydrogen peroxide, which exacerbates brain injury. Superoxide mutase levels may vary, depending upon which areas of the brain are affected by asphyxia. Higher levels exist in the frontal/parietal white matter, basal ganglia, and cerebellum, with lower levels in the cerebral cortex and hippocampii, suggesting that immature telencephalic white matter may be more vulnerable to asphyxia from oxidative stress [12].
Distinguishing Hypoxia from Ischemia
Neurotransmitters
Hypoxia without ischemia does not generally cause brain necrosis, although hypoxia exacerbates ischemic-induced necrosis in both the mature and immature organisms [7••]. Although hyperoxia potentially protects against focal brain injury in adult rats, newborn animals paradoxically are more vulnerable to hyperoxic-induced brain damage in specific brain regions (ie, pontosubicular nuclei).
Persistent elevations of extracellular concentrations of excitatory neurotransmitters (eg, glutamate and dopamine) potentially harm ischemia-exposed neurons. Conversely, inhibitory neurotransmitters (eg, gamma-aminobutyric acid) protect neurons from damage. This balance between excitatory and inhibitory stimulation of brain is crucial in maintaining cellular integrity.
Acidosis
Perinatal Asphyxia: Timing and Mechanisms of Injury in Neonatal Encephalopathy • Scher
Four classes of glutamate receptors, as well as a class of second messenger-linked receptors called the metabotropic receptors, function in the cell membrane. Glutamate receptor overactivity is involved in the pathophysiology of ischemic-induced injury to the brain. Developmental profiles define susceptibility of these subclasses of receptors after asphyxia [10]. One specific receptor identified by its agonist, NMDA, is preferentially affected, depending on the degree of oxygen deficit. Matute et al. [13] showed how excessive activation of specific ionotropic glutamate receptors, AMPA and kainate, are specifically expressed by oligodendrocytes in a manner that is unique from other glutamate receptors with other cell types. Specific brain regions, therefore, express different effects to asphyxia, based on cell-specific receptor sites. In addition to glutamatergic changes after asphyxia in the immature brain, monoaminergic and cholinergic pools are affected. Differential changes in neurotransmitter expression occur months after the asphyxial insult [14]. Prenatal exposure to certain substances such as nicotine alters the neurotransmitter receptor expression and contributes to asphyxial-induced brain injury [15]. Caffeine may alter the expression of adenosine, which also exacerbates asphyxial injury [16]. Hypoxia induces changes in dopamine metabolism in the immature brain, but not in mature brain; lower levels of transport and storage processes regarding dopamine metabolism exist in the mesencephalic and diencephalic cell populations within developing brains [17]. Although up-regulation of extracellular striatal dopamine occurs with mild asphyxial insults, dopamine depletion results with major insults [18].
Reperfusion Injury Approximately 6 to 12 hours after the initial ischemic injury, a secondary phase of reperfusion injury occurs, resulting in toxic effects from a variety of modulators that magnify the initial damage. Pathophysiology of ischemia-reperfusion injury has been recently reviewed, highlighting the role of microvascular dysfunction [19]. Impaired endothelia result in altered dilation of arterioles, enhanced fluid filtration, leukocyte plugging in capillaries, trafficking of leukocytes, and plasma protein extravasation in postcapillary venules. An imbalance between superoxide and nitric oxide in endothelial cells leads to the production and release of inflammatory mediators, including platelet activating and tumor necrosis factors. Enhanced biosynthesis of adhesion molecules also mediates leukocyte endothelial cell adhesion [20]. Pial arterioles become unresponsive to topical application of vasodilators, suggesting cerebral vasoparalysis in an animal model of asphyxia. Reductions in cyclic AMP then promote mitochondrial damage [21]. Nakai et al. [22] suggested that oxygenderived free radicals are responsible for the bioenergetic failure associated with reperfusion injury, rather than vasoparalysis. Neuronal cell damage during the reperfusion phase is probably multifactorial, involving the release of oxygen
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radicals, synthesis of nitric oxide, inflammatory reactions, and an imbalance between excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may also be the induction of a completely separate cellular death program, known as apoptosis [23].
Changes in Gene Expression Apoptosis is another mechanism of delayed cellular injury, similar to reperfusion ischemic injury. Necrosis is a more rapidly occurring form of cell death, attributed in part to alterations in ionic hemostasis. By contrast, apoptosis is a delayed form of cell death that occurs as a result of activation of a genetic program. Both apoptotic and necrotic pathways are distinct molecular pathways, along a continuum of cell death, sharing similar operative mechanisms [24]. For instance, excitatory amino acid release and alterations in ionic hemostasis contribute to both necrotic and apoptotic neuronal death. However, apoptosis is distinguished from necrosis by gene activation as the predominant mechanism that regulates cell survival. There is a balance in expression of pro- and antiapoptotic genes, which accounts for regional differences in vulnerability to asphyxial insults. Brains of six human infants who died after asphyxia were compared with six infants who experienced an unexpected sudden death. The former group had a greater number of apoptotic cell deaths than in the group with sudden death [25]. Certain cortical areas also may be more vulnerable to apoptosis, such as the cingulate sulcus of newborn piglets after an asphyxial insult [26]. Specific genes regulate apoptosis. Bcl-2 belongs to a family of genes that regulates programmed cell death and protects neurons from apoptotic death, as well as from free radical-mediated insults. Activation of NFk-B, for example, promotes over-expression of Bcl-2, and subsequent prevention of apoptotic cell death [27]. Down- regulation of specific caspase enzymes may also be important in inhibiting the apoptotic pathway [28,29]. Other positive and negative regulators for cell death include a cell death agonist, BAD, expressed in retina and forebrain neurons [30]. Neurotrophins are a family of polypeptide growth factors that bind receptors on cell surface, and consequently alter cell metabolism to reduce both necrotic and apoptotic cell death pathways. An immediate decrease in IGF-I gene expression after asphyxia in the neonatal rat brain, for example, allows activation of apoptotic neuronal death, which explains selective vulnerability of cell populations to these pathways during the perinatal period [31].
Immuno-inflammatory System and Asphyxial Brain Injury Epidemiologic data implicate antepartum maternal infection, with associated increases in circulatory cytokines, as an important causative factor in brain injury in children expressed as cerebral palsy [32]. This may be an important
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cause for ischemic injury to the white matter in pre-term infants, termed periventricular leukomalacia, and provides an explanation for the association of chorioamnionitis with cerebral white matter lesions and cerebral palsy in pre-term infants. Specific pro-inflammatory cytokines, such as interleukin (IL)-9 and IL-6, are higher in neonates with signs of brain injury [33]. Pathways by which the inflammatory cell response causes brain injury include alterations in the cascade of events leading to ischemic cell injury, which exacerbate excitotoxic lesions created by the asphyxial insult [34]. Elevated cytokines in the amniotic fluid or in the fetal circulation may have a humeral expression of inflammation, as well as release of inflammatory cells into the chorionic plate or umbilical cord blood vessel walls. Morphologic expressions of fetal inflammatory response will promote asphyxial injury [35,36]. Elevated levels of specific pro-inflammatory cytokines have been noted in the spinal fluid of asphyxiated infants [37], and elevated pro-inflammatory cytokines have been noted in the white matter of fetuses with extensive programmed cell death in the white matter [38,39]. Pharmacologic antagonism of the pro-inflammatory pathways may, therefore, limit the extent of irreversible brain injury [40].
Diagnosis of Fetal/Neonatal Enecphalopathy It is often difficult to determine the precise timing, as well as the predominant etiology, of the encephalopathic neonate [2••]. Epidemiologic and neuropathologic studies have identified multiple maternal/placental and fetal risk factors that may be associated with suspected prenatal brain injury. Fetal brain injury from asphyxia or other etiologies may then result in varying degrees of clinical abnormalities for pre-term or full-term neonates. Similar fetal and neonatal clinical signs may be alternatively expressed without brain injury. Serial examination findings, as well as neuroimaging and neurophysiologic evidence, can estimate the severity and persistence of structural and functional abnormalities in the encephalopathic neonate. The physician must apply an individualized diagnostic algorithm to predict longterm outcome. Identification of a subset of children who might benefit from a specific neuroresuscitative strategy remains elusive.
Antenatal Contributions to Fetal Distress and Neonatal Encephalopathy Antenatal factors predominate as the principal definable causes for neurologic or behavioral deficits, such as with 70% to 80% of children with cerebral palsy [41]. Maternal/placental and fetal conditions are associated with antepartum cerebral injuries (Table 3), such as disorders of thrombophilia [42]. Brain injury from antenatal factors may precipitate an encephalopathic clinical picture during intrapartum neonatal periods without concurrent
brain injury. It is also difficult to distinguish the symptomatic newborn with preexisting brain injury from the newborn with new or additional injury during the intrapartum period [2••].
Delivery Room Assessment: Relationship to Neonatal Encephalopathy The neurologically depressed newborn with acute brain injury detected by intrapartum fetal surveillance testing is an uncommon occurrence. Goodwin [43] emphasized the limitations of two measures of fetal distress (ie, Apgar scores and umbilical acid-base assessment) to predict neonatal encephalopathy, with or without concurrent brain damage. Similarly, fetal heart rate monitoring (FHR) has not lived up to its original expectations to predict neonatal encephalopathy or brain injury [44]. Poor specificity of FHR pattern interpretations has resulted in a search for other ancillary studies that can lower a false-positive test result. Computerized FHR analysis [45] is a recent strategy to better define features in the fetal electrocardiogram that correlate with other signs of fetal distress, such as umbilical artery pH and neurologic condition at birth. Such new technologies, however, require application of evidencebased experimental design. Current markers of intrapartum fetal stress or distress uncommonly identify infants who progress to postasphyxial encephalopathy. A number of postnatal markers offer the best opportunity to identify infants within the first hour of life who might progress to an encephalopathy (including seizures) [46]. Abnormal acid-base status and cardiopulmonary resuscitation, together with low Apgar scores, only predict 45% of medically ill infa nts will develop an encephalopathy. These tests also may identify children with preexisting brain injury who became symptomatic when stressed during a difficult intrapartum experience [47,48].
Clinical Scoring Systems to Predict Encephalopathy with or Without Brain Injury The American College of Obstetrics and Gynecology published guidelines that set forth necessary criteria to predict postasphyxial encephalopathy (ie, hypoxic-ischemic encephalopathy [HIE] ) [49]. These guidelines include the following: 1) profound umbilical artery metabolic or mixed acidemia (pH less than 7.00); 2) persistence of an Apgar score of 0 to 3 for longer then 5 minutes; 3) neonatal neurologic sequela (eg, seizures, coma, and hypotonia); and 4) multiorgan system dysfunction (eg, cardiovascular, gastrointestinal, hematologic, pulmonary, and renal). The original clinical scoring system by Sarnat and Sarnat [50] combined clinical and electrophysiologic (EEG) scores performed at 24 hours of age to grade a brain disorder or encephalopathy after asphyxia. The scoring system went from Grade I (ie, no seizures and hyperalertness), to Grade II (ie, reduced tone, seizures and decreased level
Perinatal Asphyxia: Timing and Mechanisms of Injury in Neonatal Encephalopathy • Scher
Table 3. Selected markers of possible antenatal brain injury Maternal / placental factors Polyhyrdramnios Maternal substance use Prenatal infection or trauma Maternal causes of thromophilia Placental or cord abnormalities reflecting infectious, congenital, toxic, or metabolic diseases Thyroid disease Fetal risk factors Genetic syndromes Congenital malformations Fetal infections Multiple gestation pregnancies Hydrops fetalis Subnormal head size at birth Intrauterine growth retardation
of arousal), to Grade III (ie, severely depressed EEG background patterns, flaccidity and delayed seizures). Grade I virtually predicted normal development, whereas Grade III universally predicted abnormal outcome in all neonates. Outcome predictions remain difficult for Grade II. A recent report by Sorensen [51] indicated that no infant with mild or moderate postasphyxial encephalopathy died or developed handicaps. A meta-analysis by Bohr and Greisen [52] of data from the past 30 years indicated that of 1042 term infants born after intrapartum asphyxia, only 48% suffered neurologic sequela, which was closely related to the most severe expression of a postasphyxial encephalopathy during the first week of life. Recent studies suggest that specific features of the postasphyxial encephalopathy syndrome increase the likelihood of neurological sequela, such as neonatal seizures and other organ injuries. Although such clinical or laboratory evidence defines hypoxic-ischemic encephalopathy, such factors may not accurately predict outcome, nor indicate timing when brain injury occurred. One recent study suggested that clinical seizures after asphyxia were associated with elevated nucleated erythrocytes, and indicated that a neurologic injury occurred in the antepartum period [53]. Scher et al. [54] emphasized the association of EEGconfirmed neonatal seizures with chronic placental lesions, suggesting an antepartum disease process to the placenta that affected neonatal brain function. It also cannot be assumed that organ system complications after asphyxia definitively predict brain damage. In a study of near-term fetal lambs after a known duration of asphyxia, organs were examined at a fixed time 72 hours after the insult. Renal tubular damage was seen in all degrees of asphyxia, despite only variable brain damage. Histologic changes in the heart and liver were seen only with the most severe brain damage [55]. These findings
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underscore previous observations that a redistribution of cardiac output may preserve neuronal integrity after asphyxial stress at the expense of renal or liver function [46]. Therefore, the presence of multiorgan dysfunction or injury does not specifically predict brain injury.
Other Biochemical Markers of Perinatal Asphyxia Severe asphyxia may cause accumulation of intermediary metabolites detected in blood, urine, or CSF. Recent studies emphasize novel approaches to link the presence of specific substances to increased risk for HIE, with or without brain injury. Huang et al. [56] measured urinary lactate/creatinine ratios in asphyxiated infants soon after birth, using proton nuclear magnetic resonance. 1H nuclear magnetic resonance (NMR) spectroscopy indicated an elevated ratio within 6 hours after birth, with a sensitivity of 94% and a specificity of 100% in predicting HIE. Elevated serum protein S-100 [57], platelet activating factor [58], and nucleated erythrocyte counts [59] also indicate increased risk for brain injury. Elevations in cerebrospinal fluid (CSF), erythropoietin [60], glycine [61], creatine kinase brain isoenzymes [62], and kynurenic acid [63] also may suggest brain injury. These intermediary metabolites are possible markers for the metabolic pathways involved in the mechanisms of ischemic brain injury, including alterations in neurotransmitter pools, pro-inflammatory processes, coagulation defects, and glucose utilization.
Neurophysiology Several neurophysiologic measures continue to play an important role in the determination of the severity of a neonatal encephalopathy. The interictal EEG background appears to be more predictive of outcome than seizures, as recently described for the pre-term infant [64]. A single-channel, digitized EEG device has been reintroduced, based on investigations several decades ago, to supplement multichannel recordings. EEG signals are digitally transformed and expressed in a semi-logarithmic scale at slow speed in a single channel. Continuous recordings document global EEG background patterns, sleep cycling, and seizures. Various authors suggest how this device can help select infants who might benefit from pharmacologic intervention after asphyxia, as well as predict neurologic outcome [65,66]. Evoked potentials are computer-averaged EEG signals in response to repetitive auditory, visual, or somatosensory stimuli, which assess the functional integrity of specific sensory pathways within the brain. Recent studies [67] suggesting a combination of multimodality evoked potentials provide as a strong prognostic tool for asphyxiated term newborns.
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Neuroimaging
Placental Examination
Neuroimaging can help in the assessment of severity and timing of the encephalopathic full-term and pre-term neonates. Cranial ultrasound abnormalities detect suspected brain abnormalities within the thalami and basal ganglia, as well as in the periventricular parenchymal or intraventricular regions [68]. Cystic lesions suggest an antenatal onset if detected within days after birth for pre-term neonates at risk for cerebral palsy [47]. Cranial magnetic resonance imaging (MRI) can be helpful, particularly during the early neonatal period. Newer imaging methods assess both normal and abnormal brain structures [69]. Conventional T1- and T2-weighted MR sequences are being compared with novel signaling techniques for both the pre-term and full-term infant populations; diffusion-weighted imaging, perfusion-weighted imaging, and MR spectroscopy technologies will have an important impact on the diagnosis and management of newborns with encephalopathy. Total brain volumes within cerebral grey matter, as well as both unmyelinated and myelinated white matter volumes, can be measured using three-dimensional imaging and diffusion tensor MRI in pre-term and full-term infants [70,71]. Correlations between MR imaging and histopathology have been described for pre-term infants who suffered hemorrhage or infarction [72]. Earlier white matter injury in pre-term infants result in reduced cortical grey matter volumes at term [73]. For term infants after acute asphyxia, MR diffusion-weighted imaging demonstrates advantages over conventional MRI by displaying recent asphyxial injuries to deep grey matter and perirolandic white matter regions before documentation by conventional MRI techniques. Rapid acquisition MRI sequences also document normal and abnormal fetal brain development [74,75]. Asphyxiated infants may develop abnormalities of cerebral energy metabolism that can be detected by proton or phosphorous 31 MR spectroscopy [65]. Altered ratios of high-energy compounds reflect energy dysmetabolism, which predict severity of later neurodevelopmental impairment, and correlates with low Apgar scores and regional brain injury [76]. Persistence of abnormalities of cerebral lactate weeks after the asphyxial injury suggests a progressive disruption of energy metabolism, which may exacerbate the initial injury [6]. Neonatal MRI studies may predict later MRI findings and clinical outcome for both pre-term and full-term infants [77]. Cerebral palsy in pre-term infants was predicted by altered MRI signals associated with hemorrhagic and white matter lesions. In term infants, T2-signal abnormalities, more then T1 abnormalities, predicted an unfavorable outcome, documenting poor myelination and focal/multifocal lesions on subsequent studies for children with subsequent neurologic insults [78].
Specific placental lesions help elucidate possible pathophysiologic mechanisms of brain injury, with or without neonatal encephalopathy, and reflect antenatal or intrapartum disease processes that interact with other perinatal risk factors associated with long-term neurological deficits [79]. Both gross and microscopic descriptions of placental tissue may be helpful. Placental weight/body weight ratios, umbilical cord length, and placental vascular or parenchymal abnormalities correlate with outcome [79••]. Descriptions of the size and thickness of the placenta [80], morphologic assessment of the umbilical cord by sonography [81], fetal thrombotic vasculopathy by placental examination [82], and chorioamnionitis [83••] have been associated with compromised neurologic outcome. Placental lesions reflect cellular mechanisms associated with brain injury involving the inflammatory process, or programmed cell death or necrosis after ischemia [84,39].
Therapeutic Interventions: Search for Strategies Among Many Potential Agents The therapeutic window for initiating treatment for asphyxiated full-term neonates can be as little as 6 hours [85]. Clinical features, delivery room resuscitative measures, and neurophysiologic/neuroimaging findings need to be integrated into a diagnostic algorithm to best select the optimal subset of patients who will benefit from a neuroresuscitative protocol [86••]. Therapeutic interventions have been reviewed for the full-term neonate with hypoxic-ischemic encephalopathy [86••,87], with recent discussion of proposed neuroprotective therapies for the fetus and pre-term neonate [35]. Johnston et al. [89] summarized the new experimental approaches to salvage brain tissue by aborting the delayed cascade of events related to necrosis and apoptosis after asphyxia. Whitelaw [88] critiqued all randomized trials of drug intervention for full-term newborn infants with asphyxia. No intervention was recommended for neonatal HIE on the basis of present experimental evidence. Pharmacologic and nonpharmacologic interventions after asphyxia (Table 4) are currently under investigation, based on mechanisms of injury already reviewed. Crucial to the choice of therapeutic rescue procedures will be prompt identification of neonates in the early stages of hypoxic-ischemic encephalopathy after a recent asphyxial insult. It remains problematic how to identify the majority of children who suffer brain injury on an asphyxial or nonasphyxial basis during the antepartum period [89]. Neuroprotective strategies for the fetus would hypothetically counteract coagulation defects or maternal/placental infection, which promote brain injury in utero. Novel experimental approaches might provide neuroprotection
Perinatal Asphyxia: Timing and Mechanisms of Injury in Neonatal Encephalopathy • Scher
Table 4. Proposed treatment interventions to prevent or lessen brain injury from asphyxia Pharmacologic agents under investigation Oxygen free-radical inhibitors and scavengers (allopurinol) Excitatory amino acid antagonists (magnesium sulfate) Calcium channel blockers Inhibitors of nitric oxide production Monosialogangliosides Growth factors Glucocorticosteroids Phenobarbital Viral vector delivery (gene therapy) Nonpharmacologic investigations Hyperglycemia Carbon dioxide Hypothermia Hypoxic preconditioning
through manipulation of inflammation, neurotrophin metabolism, or the coagulation process, but which may also have profound effects on neuronal development of the fetus. Selection of patients may pertain to a neuroprotective strategy that offers neuroprophylaxis versus neuronal rescue in utero or after birth [90••]. A recent critique discussed specific therapies after asphyxial injury in the newborn period [91], with methodologic problems in all studies. No positive effects on mortality or neurologic sequelae were shown. One randomized trial regarding allopurinol showed short-term benefits, but the study population was too small to assess mortality and morbidity. One small, randomized trial of hypothermia found no adverse effects, but it also had an insufficient population size to examine mortality and morbidity. No adequate trials of dexamethasone, calcium channel blockers, magnesium sulfate, or naloxone exist. Pilot studies are unclear as to the benefits of magnesium sulfate and calcium channel blockers. Thoresen [85] reviewed the possible neuroprotective properties of hypothermia. Initial studies in human adults after trauma or stroke are only speculative; animal data suggest that cooling is effective until 6 hours after asphyxia. Hypothermia may also affect multiple physiologic systems in a way that could be harmful. A large, multicenter trial of mild hypothermia after neonatal HIE is ongoing, with a planned report of developmental outcome in 2002 or 2003. Neuroresuscitative therapies require a precise understanding of mechanisms of damage and timing. Neurons can die by either necrotic or apoptotic cellular pathways. Excessive excitatory amino acids, altered intracellular calcium regulation, free radical generation, mitochondrial dysfunction, specific gene activation, changes in the availability of trophic factors, and the immuno-inflammatory system are all implicated in the therapeutic strategies to avoid brain injury in the fetus and the neonate. These various mechanisms of brain injury must be combined with
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our epidemiologic understanding of the clinical scenarios (maternal, placental, fetal, neonatal) that lead to brain injury specific to asphyxia, as well as other etiologies. Alteration of gene expression may constitute a novel approach to neuroprotection. Hypoxic preconditioning may provide neuroprotection by the regulation of specific stress-related gene expressions [9]. Preconditioning includes repetitive hypoxia [8], or use of specific proteins such as metallothioneines [92], hemeoxygenase-1, C-Fos, and activator protein-1 transcription factor complex [93]. Substances such as P53 tumor suppressor protein are important for cellular mitosis. A gene is induced to produce P53 in response to DNA damage, consequently halting cell division and allowing neuronal recovery during ischemia and reperfusion. P53 expression may be induced in some cells, causing apoptosis, or blocked in other cells to prevent programmed cell death [94]. Gene therapy using viral vectors for acute neurologic insults also has been recently discussed [95••], targeting steps in the necrotic or apoptotic pathways to block cell death.
Conclusions Mechanisms and timing of injury, diagnosis, and treatment of the fetus and neonate with asphyxia have been reviewed. Multiple mechanisms of injury may be involved in asphyxial brain damage, such as genetic vulnerability, acquired inflammatory responses, and clotting defects. The greatest challenge to treatment of fetal and neonatal brain disorders will be the development of accurate and timely diagnoses that distinguish fetal from neonatal brain disease. Specific subsets of children who might benefit from neuroprotective strategies require well-designed outcome studies that adjust for the developmental aspects of brain adaptation relative to the specific etiology and timing of injury.
References and Recommended Reading Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance 1.
Nelson KB: What proportion of cerebral palsy is related to birth asphyxia? J Pediatr 1988, 112:572–574. 2.•• Hagberg H, Mallard C: Antenatal brain injury: etiology and possibilities of prevention. Semin Neonatal 2000, 5:41–51. A useful review of the literature regarding fetal disease that predisposes to brain injury. 3. Stevenson DK, Sunshine P: Fetal and Neonatal Brain Injury. Oxford: Oxford University Press; 1997. 4. Mishra OP, Delivoria-Papadopoulos M: Cellular mechanisms of hypoxic injury in the developing brain. Brain Res Bull 1999, 48:233–238. 5. Planells-Cases R, Caprini M, Zhang J, et al.: Neuronal death and perinatal lethality in voltage-gated sodium channel alpha (II)-deficient mice. Biophys J 2000, 78:2878–2891. 6. Hanrahan JD, Cox IJ, Edwards AD, et al.: Persistent increases in cerebral lactate concentration after birth asphyxia. Pediatr Res 1998, 44:304–311.
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