Pathogenesis of neonatal asphyxia

Asphyxia results from compromised gas exchange. Pathogenesis of neonatal asphyxia involves energy crisis, lactic acidosis, excitotoxicity& free radical injury. Initial poor oxygenation leads to hypoxemia with intact organ function.Later hypoxia develops with anaerobic metabolism followed by asphyxia with involvement of major organs. The changes during asphyxia depend on organ involvement & its severity. Interference with cerebral blood flow secondary to systemic hypotension leads to failure of cerebral autoregulation thereby leading to ischemia, neuronal &oligodendoglial damage via excitoxicity. Reduced oxygen supply leads to ineffective oxidative phosphorylation, anaerobic metabolism & depletion of ATP reserves, accumulation of lactic acid & hydrogen ions & reduced cellular functions. ATP-dependent sodium-potassium  pump fail leading to disruption of ion exchange across cell membrane leading to cell injury. Types of hypoxic brain damage include Haemorrhagic lesions, Destructive lesions involving white matter & grey matter. Neuropathological patterns of injury include Selective neuronal necrosis, White matter lesions, Combined lesions & Advanced lesions including  Ulegyria, Multifocal cystic encephalopathy, Status marmaratus&Unifocalpseudocyst. Brain ischemia, inflammation & neuronal cell death are the 3 major steps in the pathogenesis of neural damage during asphyxia. Brain histology helps in timing of asphyxia which depend on aetiology & stage of neurodevelopment.Reperfusion temporarily corrects this energy failure, however it may trigger delayed neuronal death or secondary damage due to brain swelling. Ischemia & reperfusion induce both rapid & delayed changes in gene expression. Asphyxia negatively affects integrity of the genome, triggering activation of sentinel proteins that maintain genome integrity.


Introduction
Asphyxia results from compromised placental or pulmonary gas exchange. This disorder can lead to hypoxia & hypercarbia in the blood. Severe hypoxia results in anaerobic glycolysis & lactic acid production first in the peripheral tissues (muscle & heart) & then in the brain. Ischemia  be causedby (1) inadequate oxygenation of maternal blood (2) low maternal blood pressure (3) inadequate relaxation of uterus to permit placental filling (4) premature separation of placenta; (5) impedance of circulation of blood through umbilical cord (6) placental insufficiency.
Phases of injury during asphyxia: Initial poor oxygenation leads to hypoxemia where oxygen saturation in arterial blood falls without affecting organ function. Later hypoxia develops where there is reduced oxygen tension & subsequent anaerobic metabolism involving peripheral tissues.
Finally asphyxia develops where hypoxia & anaerobic metabolism involves organs like heart, brain & adrenal glands, potentially leading to metabolic acidosis. Ischemia is oxygen deficiency due to hypoperfusion. There is increased VEGF expression which leads to i) endothelial swelling; ii) endothelial exhaustion; iii) endothelial detachment; iv) loss of the endothelial barrier. At histology loss of endothelial barrier is associated with following lesion -Endothelial disfunction a. Diffuse intravascular coagulation (DIC), b. Edema of perivascular tissues, c. Loss of microvascular reactivity, d. Perivascular hemorrhages, e. Dysfunction of tneurovascular brain unit, followed by dysfunction of brain blood barrier, leading to perivascular edema& neuronal cell death.

Mechanism of Cell Injury:
Interference with cerebral blood flow secondary to systemic hypotension leads to failure of cerebral autoregulation thereby leading to ischemia, neuronal &oligodendoglial damage via excitoxicity. Reduced oxygen supply leads to ineffective oxidative phosphorylation, anaerobic metabolism & depletion of ATP reserves, accumulation of lactic acid & hydrogen ions (i.e. acidosis) & reduced cellular functions [2][3][4]. ATP-dependent sodiumpotassium pump fail leading to disruption of ion exchange across cell membrane leading to cell injury. In neurons intracellular accumulation of sodium, calcium & water occur leading to cytotoxic edema, depolarization, release of excitatory neurotransmitters from axon terminals particularly glutamate. Increased intracellular calcium activates neuronal nitric oxide synthase which induces nitric oxide mediated free radical injury. Activation of phospholipases leads to fatty acids accumulation in cytoplasm which produce oxygen free radicals by mitochondrial peroxidation [2,4]. Mitochondrial membrane depolarisation occurs followed by increment of intracellular Ca2+ leading to apoptosis [5].
Hypoxic Encephalopathy:Neonatal hypoxic-ischemic encephalopathy is an acute, nonstatic encephalopathy caused by brain hypoxia & ischemia during or closely associated with labour.

Neuropathological patterns of injury:
I. Selective neuronal necrosisoccursin areas with high energy demands like 1) Cerebral cortical areas like Precentralgyrus, postcentralgyrus&calcarine cortex, 2) Deep gray matter regions like thalamus & basal ganglia, 3) areas in hippocampus like pontine nuclei &subiculum. In severe asphyxia Global cerebral necrosis occurs involving cerebral cortex, brain stem, thalamus, spinal cord & cerebellum.

III. Combined grey & white matter lesions include
Parasagittal cerebral injury, bilateral necrosis of cerebral cortex & subjacent white matter in the watershed areas, Focal & multifocal infarcts.

IV. Advanced lesions in late stages include 1)
Ulegyria-which is distorted cerebral gyri in the parasagittal areas, 2) Multifocal cystic encephalopathywhich is sponge like brain with large septated cavities throughout cortex & white matter of both hemispheres, 3) Status marmaratus -which is marbled appearance of striatum &/or thalamus secondary to neuronal loss, astrogliosis& excessive myelination, 4) Unifocalpseudocyst, cystic periventricular leukomalacia, white matter hypoplasia, porencephaly&hydranencephaly or basket brain.
Pathologic changes of neural injury during asphyxia: Brain ischemia, inflammation & neuronal cell death are the 3 major steps in the pathogenesis of neural damage during asphyxia.
Brain ischemia: ATP depletion leads to primary cellular energy failure & initiation of cascade reactions leading to cell death. Metabolic acidosis itself may cause neuronal injury & death, particularly due to excessive release of glutamate, energy-requiring excitotoxic cascade & neuronal degeneration [2]. Metabolic acidosis also cause hypotension resulting in ischemia [7,8], impairment of myocardial function resulting in reduced cerebral blood flow [9]. Subsequent reperfusion deteriorates brain metabolism further by increasing oxidative stress damage [2] through activation of xanthine oxidase, cyclooxygenase enzymes & superoxide from mitochondria. Neuronal & endothelial nitric oxide synthetase activation leads to increase in nitric oxide production. Ultimately secondary energy failure occurs due to mitochondrial dysfunction [4,10].
Neuronal cell death: Necrosis is passive, ATPindependent process characterized by cellular swelling, mitochondrial damage, chromatin condensation, cytotoxic edema, cell lysis, cytokine release & activation of inflammatory reactions. Apoptosis is an active, ATP-dependent process of programmed cell death characterized by cell shrinkage, chromatin condensation, DNA fragmentation with no activation of inflammatory responses [4,10]. In asphyxia apoptotic cell death occurs often followed by ischemic necrotic cell death. Hypoxia mediated energy failure prevents effective completion of ATP-dependent apoptotic processes thereby leading to necrosis [17]. In reperfused areas cell death mainly consists of apoptosis [2,10]. Mild to moderate asphyxia causes apoptotic cell death whereas severe asphyxia leads to necrosis [4,10]. Excitotoxic cell injury mainly induces necrosis. Glutamate receptors which increase during hypoxia make fetal neurons more vulnerable to excitotoxicity [2,17].
Cerebral regions with high degree of glutamate receptor expression like hippocampus, cerebral cortex & deep nuclear structures & therefore more vulnerable to asphyxia [17]. Among the neural cells, Neurons are most vulnerable, followed by oligodendrocytes & astrocytes, whhereas microglia cells are less susceptible to hypoxia [2]. Grey matter is primarily involved in term infants & white matter in preterms [10]. Watershed areas in periphery of cerebral vascular supply are most susceptible to ischemia [10]. Acute, near total asphyxia causes central pattern of focal neuronal injury, affecting mainly thalamus, basal ganglia & brain stem nuclei with cortical sparing whereas prolonged, partial asphyxia lead to cerebral cortical injury in watershed areas & in parasagittal regions with relative sparing of central grey matter. Multiple cell death mediators are activated by neonatal Hypoxic injury, including various members of the Bcl-2, Bcl-2-associated X protein (BAX), Bcl-2-associated death promoter (BAD) [18,19]  Reperfusion injury: Normal transition from fetal to newborn life is associated with an immediate decrease in cerebral blood flow velocity, followed by an increase above fetal level by 24 hours of life [40]. In asphyxiated infants this alteration in cerebral blood flow is disturbed. During compromised cerebral blood flow, deprivation of cerebral oxygen & glucose supply occurs which lead to ATP depletion. This energy failure leads to loss of Na+/K+-ATPase activity ultimately leading to cytotoxic edema [41,42]. Reperfusion temporarily corrects this energy failure, however it may trigger delayed neuronal death [42,43] or secondary damage due to brain swelling. Two main sources of these metabolites are 1) oxidation of hypoxanthine to xanthine by xanthine oxidoreductase& 2) neutrophils accumulating in ischemic &reperfused tissue. In animal studies, resuscitation after ischemia leads to a period of hyperemia, followed by hypoperfusion [41,44] due to endothelial injury & swelling [45,46], granulocyte plugging of microvessels [47,48] or intravascular clotting [49].
Severe DNA damage is usually triggered by a massive degree of oxidative stress triggered by reactive oxygen species such as peroxynitrite, hydroxyl & superoxide free radicals.