Intrauterine Infection and Perinatal Brain Injury (SAC Opinion Paper 3)

Scientific Advisory Committee Opinion Paper 3

This is the second edition of this Opinion Paper, which was first published in November 2002.

This paper can be downloaded as a pdf file below:

• Intrauterine infection and perinatal brain injury - SAC Opinion Paper 3

1. Introduction

The focus of research into the causes and prevention of perinatal brain injury has increasingly switched from hypoxia-ischaemia to the role of maternal infection and the fetal inflammatory response. This document summarises the key findings underpinning this change, discusses whether it is appropriate and provides advice on its possible clinical implications, as well as suggestions for future research.

2. Epidemiology

2.1 Preterm
A growing body of epidemiological data suggests that intrauterine infection can cause brain injury in babies born before 32 weeks of gestation. Magnetic resonance imaging (MRI) demonstrates that both white and grey matter are vulnerable.¹ White matter damage most often appears as diffuse excessive high signal intensity (DEHSI) on MRI, probably reflecting a diffuse astrogliosis with subsequent loss of myelin-producing oligodendrocytes; it can be observed in up to 80% preterm babies.² Multifocal necrosis resulting in cystic change in white matter (periventricular leucomalacia, PVL)³ are seen more rarely and have a strong association with cerebral palsy.

Clinical antecedents linked with this type of injury include maternal pyrexia, prolonged preterm prelabour rupture of membranes and maternal leucocytosis. In a meta-analysis, clinical chorioamnionitis was significantly associated with both PVL (RR 3.0; 95% CI, 2.2–4.0) and cerebral palsy (RR 1.9; 95% CI, 1.4–2.5).⁴ Histological evidence of chorioamnionitis, present in up to 50% of women who deliver preterm and often not associated with clinical signs, correlates significantly with PVL (RR 2.1; 95% CI, 1.5–2.9) but has a weaker association with cerebral palsy (RR 1.6; 95% CI, 0.9–2.7).⁴

Not all studies confirm the link between chorioamnionitis and cerebral palsy; for instance, the EPICure study of outcome in babies born at 25 weeks of gestation or less actually suggested that clinically suspected chorioamnionitis was associated with reduced cerebral palsy in this group.⁵ It may be that these conflicting findings are due to the fact that the key aetiological factor is not maternal infection/inflammation (chorioamnionitis) but the fetal inflammatory response. Fetal vasculitis, but not placental membrane inflammation, is associated with ultrasonographically detected echolucencies in white matter⁶ Neonatal morbidity is higher when chorioamnionitis is combined with funisitis (histological evidence of fetal inflammation)⁷ and the risk of neonatal complications is higher in babies with elevated proinflammatory cytokine levels (interleukin-6 > 11 pg/ml) in blood collected by cordocentesis (serological evidence of a fetal inflammatory response).⁸ Elevated levels of inflammatory cytokines in cord blood (IL-6) and amniotic fluid (IL-6, IL-1β and TNFα) have also been found in babies with white matter damage and cerebral palsy.^9,10 Two features, common to many of these studies, that make it difficult to unravel the true nature of the link between infection and brain injury are:

1. Associations are made with markers of inflammation (such as clinical or histological chorioamnionitis or proinflammatory cytokines) but the role of bacteria as the stimulus for inflammation is rarely addressed, perhaps because of practical difficulties with bacterial isolation and identification.
2. The outcome measure used is normally PVL or cerebral palsy. This ignores the fact that new MRI techniques demonstrate that much white matter damage is diffuse and that there other functional outcomes to white matter damage other than cerebral palsy.

2.2 Term
Although there are fewer studies addressing the relation between maternal infection and brain injury in the term baby, the association seems to be stronger than seen at earlier gestations. The summary relative risk for cerebral palsy derived from two studies of clinical chorioamnionitis was 4.7 (95% CI 1.3–16.2) and the only study examining the association with histological chorioamnionitis found a relative risk of 8.9 (95% CI 1.9–40).⁴ In addition, maternal pyrexia during labour has been related to an increased incidence of neonatal seizures and encephalopathy in the immediate postnatal period.^11,12 The combination of maternal pyrexia and fetal hypoxia-ischaemia is associated with a particularly high risk of cerebral palsy (OR 78, 95%CI 4.8–406), which points towards a possible synergy between these two factors:¹³ While this concept is supported by a number of animal studies (see below), it remains speculative until confirmed by further clinical studies. As both infection and hypoxia-ischaemia can increase levels of inflammatory cytokines, such as TNFα and the interleukins -1β and -6, the close correlation observed between cerebral palsy in term infants and increased levels of these cytokines in neonatal blood may indicate activation of a final common inflammatory pathway rather than infection specifically.¹⁴ The pathological consequences of elevated levels of proinflammatory cytokines may be modulated by increased production of other cytokines, such as IL-10 and TGFβ, that have anti-inflammatory properties. As yet, there are no studies that have investigated the balance between these two groups of cytokines in a longitudinal manner in mother and fetus.

3. Animal studies

A causal role for infection in preterm brain injury is emphasised by experiments, summarised in a review,¹⁵ showing that white matter damage can be induced in a variety of immature animals by antenatal or postnatal administration of bacteria (Escherichia coli or Gardnerella vaginalis), virus (border disease virus) or endotoxin. The potential for a transplacental effect is demonstrated by administration of a bolus dose of endotoxin to pregnant rats, which results in failure of cerebral myelination in their offspring.¹⁶ Low doses of lipopolysaccharides (LPS) that on their own have no effect on the 7-day-old neonatal rat pup (similar in maturation to the term human fetus) dramatically increase brain injury caused by a subsequent hypoxic-ischaemic insult (occurring 4–12 hours after the infectious insult), suggesting that infection may sensitise the immature brain.¹⁷ In contrast, at longer time intervals (24 hours) LPS has the opposite effect and reduces vulnerability to hypoxia-ischaemia.18 The complex nature of the relationship between infection and hypoxia is a good example of the way in which inflammation can have both a deleterious and protective role, depending on a variety of factors, including the nature of the insult, timing and host susceptibility.

4. Potential mechanisms

There are several ways in which maternal infection might lead to inflammation within the fetal brain and the loss of vulnerable neural cell populations:

• Bacterial products could cross to the fetal circulation, binding to specific cell membrane receptors, such as CD14 and toll-like receptors,^19,20 on inflammatory cells within the systemic circulation and the brain itself. This would initiate a cascade of intracellular events, including activation of transcription factors such as nuclear factor-kappa B (NF-_B) and of proinflammatory cytokines (such as TNFα, IL-1β and -6).
• Systemic or local proinflammatory cytokines, including TNFα, IL-1β and interferon-γ, have a variety of cerebral effects including a direct toxic effect on neurones and vulnerable oligodendrocyte precursor populations,^3,21 astrogliosis with release of nitric oxide and mitochondrial dysfunction22 and microglial activation.²³ The role of cytokines is complicated by data showing that some (for example, TGFβ and IL-10) have anti-inflammatory properties and can be neuroprotective²⁴ and even IL-6, a cytokine used to determine the presence of a proinflammatory response, can have neuroprotective actions.²⁵
• Animal studies describe a direct correlation between increasing brain temperature and susceptibility to a variety of neurotoxic factors. Hypothermia can be neuroprotective after hypoxia-ischaemia in neonatal animals,²⁶ while hyperthermia increases brain injury after ischaemia in adult rats.²⁷ Maternal pyrexia, resulting from both microbial infection as well as noninfective causes such as epidural anaesthesia, could therefore augment the deleterious effects of hypoxia on the fetal brain, possibly by increasing the cerebral metabolic rate and demand for oxygen.
• Systemic fetal hypotension, endothelial injury and leukocyte aggregation, may all contribute to local tissue ischaemia, especially in vulnerable areas such as deep white matter.³

Many of these mechanisms could lead directly to cell death; they may also have an indirect neurotoxic effect by sensitising the brain and lowering the threshold at which hypoxia triggers cell death/apoptosis.

5. Clinical implications

Various aspects of the management of mothers with clinical chorioamnionitis (or perhaps even a raised temperature shortly before or during labour) need to be reconsidered in light of these findings. It is important to point out that, for most of these interventions, there is currently no specific evidence on which to base a change in practice. Only those therapies or interventions that could be either cerebroprotective or toxic are considered.

5.1 Antibiotics
The association between maternal antibiotic use and perinatal brain injury, reported in one study,³ is most likely to be the result of antibiotic use acting as a marker of severe maternal infection rather than a direct action of antibiotics on vulnerable cell populations. However, the ORACLE trial finding of increased neonatal necrotising enterocolitis in pregnancies treated with co-amoxiclav compared with erythromycin and the fact that some antibiotics result in greater release of endotoxin by damaged bacteria than others, suggests that antibiotic choice may be critical.²⁸ In particular, the data raise the hypothesis that bacteriostatic (e.g. erythromycin) rather than bactericidal (e.g. co-amoxiclav) antibiotics may result in less endotoxin release and so less of a fetal inflammatory response.

5.2 Steroids
The antenatal administration of glucocorticoids to the mother appears to reduce the risk of neonatal complications such as intraventricular haemorrhage. Because of their anti-inflammatory effects, a similar protective role for steroids in prevention of white matter injury has been postulated. A large observational study suggests that this might be true for antenatal betamethasone, which was associated with a lower risk of cystic PVL than was either the absence of glucocorticoid therapy (OR 0.5; 95% CI 0.2–0.9) or exposure to dexamethasone (OR 0.3; 95% CI 0.1–0.7).²⁹ Betamethasone is therefore the steroid of choice to enhance lung maturation.

5.3 Progesterone
There is renewed interest in the use of progesterone to prolong gestation in women at risk of preterm birth, although the trials are underpowered for detection of improvements in perinatal outcome.³⁰ A study showing that medroxyprogesterone acetate (MPA), but not progesterone, reduced infection associated fetal death in a rat model of intrauterine inflammation, suggests that the anti-inflammatory properties of MPA could improve fetal outcome, including brain injury.³¹ However further work is required before a change in current management regimes are advocated.

5.4 Mode of delivery
If fetal cerebral inflammation sensitises the brain to hypoxia-ischaemia, a reasonable hypothesis would be that avoidance of labour, a period commonly associated with a degree of hypoxic fetal stress, would be neuroprotective. In the large Australian case–control study of Badawi et al.,³² elective caesarean section was associated with a highly significant reduction in term neonatal encephalopathy (OR 0.17, 95% CI 0.05–0.56) whereas emergency caesarean section was associated with an increased risk. The clinical dilemma is one of case selection, as the large majority of fetuses born to mothers with pyrexia will have a normal outcome, whatever the mode of delivery. At present, careful attention to fetal monitoring and consideration of caesarean section early in labour if there is evidence of fetal hypoxia or severe maternal chorioamnionitis may be prudent, although, evidence in the form of a randomised controlled trial is needed.

5.5 Temperature reduction
There are several reasons why manoeuvres to keep mothers normothermic during labour may protect against neonatal encephalopathy:

• maternal pyrexia intrapartum increases the risk of abnormal neonatal neurology
• animal studies suggest that hypothermia is neuroprotective
• the fetal temperature is on average 1ºC higher than maternal temperature.

Such measures could include the use of a fan, sponging, paracetamol and antibiotics. There are no data currently available to support such interventions having any benefit.

6. Research implications

Both the basic science of fetal inflammation and brain injury and the impact of clinical interventions on these processes are clearly priority areas for further research. Questions that need addressing include:

• What is the true nature of the link between infection and brain injury in the preterm baby? This could be assessed using polymerase chain reaction-based techniques for detecting and identifying bacteria in membranes and fetal blood and using new MRI techniques to provide more sophisticated information about the nature of white matter damage.
• Does the possession of genetic polymorphisms, known to modulate aspects of inflammation (for example, mannose-binding lectin and toll-receptor function) make the mother and fetus more vulnerable to infection?
• Is there a critical interval between infection and delivery that is most likely to lead to brain injury?
• Does caesarean section before the onset of labour confer any neuroprotective benefit in mothers with chorioamnionitis?
• Does cooling mothers with a pyrexia, to restore normothermia, improve neonatal outcome?
• Is there any method (for example, Doppler assessment of fetal haemodynamics, detection of inflammatory markers in amniotic fluid, maternal or fetal blood) which could define a group of fetuses at particularly high risk?

There are obvious problems with such trials/studies, including:

• the incidence of neonatal encephalopathy and cerebral palsy is low (4/1000 and 2/1000 respectively), so that large numbers would be required to demonstrate therapeutic benefit
• current markers of clinical and histological chorioamnionitis are varied and imprecise
• a wide range of outcome measures are used, including neonatal encephalopathy, cranial ultrasound findings, seizures and cerebral palsy; ascertainment of the most relevant outcomes involves long-term follow up, which may be complex and costly
• although the basic pathological processes may be similar in term and preterm fetuses, there are differences in the epidemiological data from these two groups; for example, pre-eclampsia lowers the risk of PVL in preterm infants but increases the risk of encephalopathy at term; term and preterm fetuses should therefore be investigated independently.

7. Conclusion

It is clear that inflammatory processes, increasingly demonstrated to be important in many areas of medicine, play a key role in the pathogenesis of perinatal brain injury. The challenge for obstetricians is to find effective methods to modulating fetal inflammation to reduce the prevalence of long-term neurological deficit in the newborn. We conclude that:

• maternofetal infection and fetal inflammation are linked to brain injury in both term and preterm infants: Factors such as coexisting hypoxia-ischaemia, fetal genotype, type of bacteria and timing of delivery are likely to influence outcome
• there are no randomised controlled trials that assess the impact of clinical intervention on neonatal neurological outcome following maternal chorioamnionitis; antenatal betamethasone, rather than dexamethasone, may reduce white matter damage in preterm infants
• further research is needed to establish whether mode of delivery, type of antibiotic or reduction of maternal pyrexia reduces the incidence of neonatal encephalopathy in mothers with chorioamnionitis.

8. References

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This opinion paper was produced on behalf of the Royal College of Obstetricians and Gynaecologists by: Dr DM Peebles MRCOG, London
and peer reviewed by:
Dr JM Rennie, Neonatologist, University College London Hospitals NHS Foundation; Professor D Edwards, Hammersmith Hospital; Professor PR Bennett FRCOG, Queen Charlotte’s Hospital; Professor K Costeloe, The Homerton Hospital London.
The final version is the responsibility of the Scientific Advisory Committee of the RCOG.

The review process will commence in October 2010 unless indicated earlier.