Early neonatal brain injury in histologic chorioamnionitis

Chorioamnionitis and brain injury

The limited available evidence supports a strong association of chorioamnionitis with neonatal encephalopathy and CP in the term infant. The association of chorioamnionitis with depressed Apgar scores or neonatal seizures and with CP is equivocal in the preterm infant. Different study results may be related to differences in study populations, perhaps specifically to differences in susceptibility by stages of neurologic development as well as differences in gene frequencies associated with inflammation and thrombophilia. We require further understanding of the normal roles of cytokines in brain development, pregnancy, and inflammatory homeostasis before clinical interventions directed at cytokines, their receptors, or the inflammatory process are considered.

Clinical chorioamnionitis, elevated cytokines, and brain injury in term infants

OBJECTIVES

To determine the initial inflammatory cytokine response in term infants born to mothers with clinical chorioamnionitis and to assess whether the cytokine response is associated with birth depression, abnormal neurologic examination, and hypoxic-ischemic encephalopathy (HIE).

METHODS

Infants who were exposed to chorioamnionitis and admitted to the neonatal intensive care unit (n = 61) were studied prospectively. Cytokine concentrations were measured from umbilical cord blood and at 6 and 30 hours after birth. Control values (n = 50) were determined from cord blood of healthy term infants. Enzyme-linked immunosorbent assays were performed for interleukin (IL)-1beta; IL-6; IL-8; regulated on activation, normal T-cell expressed and secreted (RANTES); macrophage inflammatory protein-1alpha; and tumor necrosis factor-alpha. Serial blinded neurologic examinations using a modified Dubowitz score were performed simultaneously at 6 and 30 hours.

RESULTS

Cord IL-6 (1071 +/- 1517 vs 65 +/- 46 pg/mL), IL-8 (2580 +/- 9834 vs 66 +/- 57 pg/mL), and RANTES (95 917 +/- 16 518 vs 54 000 +/- 14 306 pg/mL) concentrations only were higher in infants with chorioamnionitis versus control infants. IL-6 increased at 6 hours to 1451 +/- 214 pg/mL, followed by a 5-fold decline at 30 hours in contrast to progressive decreases over time in IL-8 and RANTES. There was no relationship between cytokines and birth depression. Modified Dubowitz score correlated with IL-6 at 6 hours (r = 0.5). Infants with HIE/seizures (n = 5) had significantly higher cytokine concentrations at 6 hours versus infants without either (n = 56): IL-6 (3130 vs 1219 pg/mL), IL-8 (5433 vs 780 pg/mL), and RANTES (97 396 vs 46 914 pg/mL).

CONCLUSIONS

There was a significant association between abnormalities in the neurologic examination and cytokine concentrations, with the highest cytokines concentrations observed in infants who developed HIE/seizures.

Sequelae of chorioamnionitis

Chorioamnionitis correlates to preterm delivery prior to 30 weeks of gestation. In most studies, proxies of intrauterine infection (clinical chorioamnionitis, histological chorioamnionitis, intra-amniotic increase in cytokines) are associated with acute neonatal morbidity and mortality and, at least to some degree, with neurological impairments (periventricular leukomalacia, intraventricular hemorrhage, cerebral palsy, polymicrogyria), chronic lung disease, and involution of the thymus in the preterm infant. The connection to visual impairment and cognitive deficits is uncertain or unknown. Full-term babies exposed to intrauterine infection often present with depressed Apgar scores and neonatal encephalopathy, and are at markedly increased risk of developing cerebral palsy. The infectious/inflammatory mechanisms involved are incompletely understood, and the types of microbes, as well as the genetic characteristics of the host adaptive and innate immune response, need to be better characterized.

The toll-like receptor TLR4 is necessary for lipopolysaccharide-induced oligodendrocyte injury in the CNS

The immediate or innate immune response is the first line of defense against diverse microbial pathogens and requires the expression of recently discovered toll-like receptors (TLRs). TLR4 serves as a specific receptor for lipopolysaccharide (LPS) and is localized on the surface of a subset of mammalian cells. Although innate immunity is a necessary host defense against microbial pathogens, the consequences of its activation in the CNS can be deleterious, as we show here in a developing neural model. We examined the major non-neuronal cell types in the CNS for expression of TLR4 and found that microglia expressed high levels, whereas astrocytes and oligodendrocytes expressed none. Consistent with TLR4 expression solely in microglia, we show that microglia are the only CNS glial cells that bind fluorescently tagged lipopolysaccharide. Lipopolysaccharide led to extensive oligodendrocyte death in culture only under conditions in which microglia were present. To determine whether TLR4 is necessary for lipopolysaccharide-induced oligodendrocyte death in mixed glial cultures, we studied cultures generated from mice bearing a loss-of-function mutation in the tlr4 gene. Lipopolysaccharide failed to induce oligodendrocyte death in such cultures, in contrast to the death induced in cultures from wild-type mice. Finally, stereotactic intracerebral injection of lipopolysaccharide into the developing pericallosal white matter of immature rodents resulted in loss of oligodendrocytes and hypomyelination and periventricular cysts. Our data provide a general mechanistic link between (1) lipopolysaccharide and similar microbial molecular motifs and (2) injury to oligodendrocytes and myelin as occurs in periventricular leukomalacia and multiple sclerosis.

Predictive value of skin color for illness severity in the high-risk newborn

The relationship between skin color and illness severity in the newborn remains untested. We have evaluated the predictive value of skin color readings for illness severity in a population of high-risk newborn infants. A prospective study was conducted on 107 white newborns in the intensive care unit, which were categorized as either high or low severity of illness, defined by the presence of severe neonatal morbidity. Illness severity was also determined using a Score for Neonatal Acute Physiology (SNAP). Color readings were obtained at 10 different body sites using a portable tristimulus colorimeter during the first 24 h, and color was expressed using the standard CIE L*a*b* system. Skin CIE b* values were significantly lower in the high severity group (p < 0.0001), and a significant inverse correlation with SNAP was observed (r(s) range, -0.37 to -0.71, p < 0.0001). In particular a low b* value for the abdomen was found to be a significant predictor of illness severity (92.6% sensitivity; 96.6% specificity; 96% positive predictive value; 93.7% negative predictive value; adjusted odds ratio, 14.7; 95% confidence interval, 6.4 to 33.8). Our findings indicate that skin color reflects clinical severity in the newborn and that skin colorimetry can accurately identify higher risk infants.

Neurobiology of periventricular leukomalacia in the premature infant

Brain injury in the premature infant is a problem of enormous importance. Periventricular leukomalacia (PVL) is the major neuropathologic form of this brain injury and underlies most of the neurologic morbidity encountered in survivors of premature birth. Prevention of PVL now seems ultimately achievable because of recent neurobiologic insights into pathogenesis. The pathogenesis of this lesion relates to three major interacting factors. The first two of these, an incomplete state of development of the vascular supply to the cerebral white matter, and a maturation-dependent impairment in regulation of cerebral blood flow underlie a propensity for ischemic injury to cerebral white matter. The third major pathogenetic factor is the maturation-dependent vulnerability of the oligodendroglial (OL) precursor cell that represents the major cellular target in PVL. Recent neurobiologic studies show that these cells are exquisitely vulnerable to attack by free radicals, known to be generated in abundance with ischemia-reperfusion. This vulnerability of OLs is maturation-dependent, with the OL precursor cell highly vulnerable and the mature OL resistant, and appears to relate to a developmental window characterized by a combination of deficient antioxidant defenses and active acquisition of iron during OL differentiation. The result is generation of deadly reactive oxygen species and apoptotic OL death. Important contributory factors in pathogenesis interact with this central theme of vulnerability to free radical attack. Thus, the increased likelihood of PVL in the presence of intraventricular hemorrhage could relate to increases in local iron concentrations derived from the hemorrhage. The important contributory role of maternal/fetal infection or inflammation and cytokines in the pathogenesis of PVL could be related to effects on the cerebral vasculature and cerebral hemodynamics, to generation of reactive oxygen species, or to direct toxic effects on vulnerable OL precursors. A key role for elevations in extracellular glutamate, caused by ischemia-reperfusion, is suggested by demonstrations that glutamate causes toxicity to OL precursors by both nonreceptor- and receptor-mediated mechanisms. The former involves an exacerbation of the impairment in antioxidant defenses, and the latter, an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor-mediated cell death. Most importantly, these new insights into the pathogenesis of PVL suggest potential preventive interventions. These include avoidance of cerebral ischemia by detection of infants with impaired cerebrovascular autoregulation, e.g. through the use of in vivo near-infrared spectroscopy, the use of free radical scavengers to prevent toxicity by reactive oxygen species, the administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor antagonists to prevent glutamate-mediated injury, or the use of maternal antibiotics or anticytokine agents to prevent toxicity from maternal/fetal infection or inflammation and cytokines.