Impact of chorioamnionitis on morbidity of preterm newborns and possible therapeutic interventions

Summary

Incidence of preterm labor is progressively rising and more newborns are being saved in lower gestational ages. However, despite of advances in neonatal care are being undisputable, immaturity is still very limiting factor in normal development of a newborn because it is connected with higher incidence of perinatal and neonatal mortality as well as morbidity.

One of the most frequent causes of preterm labor is chorioamnionitis. Problem of chorioamnionitis dwells not just in activation of mechanisms leading to preterm labor but inflammation may be transmitted on the fetus too, causing serious complications that significantly worsen prognosis of premature newborn. Negative effect is not linked only to a specific microorganism itself but mainly to initiation of prooxidant and proinflammatory cascade. Because antibiotics serve only for growth inhibition or killing of bacteria and are not able to intervene with cytokines or free radicals, new therapeutic strategies aimed at cytokine and free radical inhibition are in research. Melatonine, Pentoxifylline, Erythropoietine and N-acetylcysteine seem to be most promising in this indication. These drugs may attenuate deleterious effect of inflammatory process on many organ systems and thus decrease complications connected with preterm labor caused by chorioamnionitis.

Keywords:

inflammation – Chorioamnionitis – preterm newborn – oxidative stress – neuroprotection

INTRODUCTION

Preterm labor is growing in importance for its increasing incidence worldwide. It contributes up to 11% of all live births [1]. Every year there are 15 million preterm infants born, unfortunately, according to WHO data, approximately 1.1 million of them die [2]. Statistical analyses show that prematurity is a major cause of morbidity and mortality till the age of five [1, 3, 4]. For chorioamnionitis seems to be a substantial contributor to preterm labor and neonatal infection resulting in multiorgan dysfunction, there is a strong effort to stop the cascade of pro-inflammatory processes by the means of medical interventions [1]. Majority of them are not approved for standard clinical use yet, however, preliminary results seem to be promising in some cases of medicaments.

CHORIOAMNIONITIS

Every labor is an inflammatory process in which cytokines, by stimulation of prostaglandine production, play a key role. However, in preterm labor the inflammation of gestational tissues is much more pronounced than in term one [1, 5]. Chorioamnionitis occurs in 2 to 5% of all births and its incidence increases with lower gestational ages. In 23 to 24 weeks of gestation it accounts for 65%, 30% in 29 weeks and 16% in 34 weeks of gestation [6, 7]. Clinical and laboratory signs of chorioamnionitis such as stained and odorous amniotic fluid, maternal fever, both maternal and fetal tachycardia, tenderness of uterus, leukocytosis and elevation of CRP levels are often unspecific. Nor cultivation of bacteria is fully reliable for it is frequently falsely negative. PCR seems to be more helpful, however, its availability is restricted and its cost is high. Proper diagnosis of chorioamnionitis is based on histology of placental membranes and umbilical cord. A study comparing histological diagnosis with cultivation found that only 4.6% of all histologically confirmed chorioamnionitis was culture-positive [7, 8]. It is necessary to emphasise that even culture-negative chorioamnionitis with no clinical signs present may lead not only to early neonatal infection but also to persistent activation of fetal immune response that impairs normal development of immune system of the child [6, 7].

Impact of chorioamnionitis on newborn

In preterm labor it is not just an immaturity of fetal organs but also inflammation that contributes to neonatal morbidity [1]. Fetal inflammatory response syndrome (FIRS) is a very serious complication of chorioamnionitis. It is characterized by inflammation of placental membranes and funisitis (which is a sign of progressive inflammation) with increased levels of Il-6 in umbilical cord. This inflammation starts before labor and it negatively influences immune and organ development and their postnatal function [9]. It is crucial to mark that deleterious effect of chorioamnionitis on newborn is mediated not just by pathogen itself but mainly by pro-inflammatory cytokines that stimulate free radical production [8]. Hence, inflammation and oxidative stress impair function and development of immune system, lungs, brain and it leads to recurrent infections, sepsis and NEC (necrotising enterocolitis) [8–10].

Immune system

Innate immune system starts to develop in early fetal life. However, while anti-inflammatory responses are more effective with higher gestational age, it is not the case of proinflammatory processes. This imbalance between pro-inflammatory and anti-inflammatory responses points to more deleterious effect seen in preterm infants, especially when infection is present [9]. Another indispensable fact is that intrauterine infection leads to impaired development of components of immune system even if it doesn't lead to preterm labor, thus making postnatal immune responses altered. Gravidity itself is therefore a crucial period of development of adult phenotype of immune system because stable and unchanging gene expression is being established [10, 11].

Lungs

Preterm neonates are disadvantaged for lung tissue immaturity, primary surfactant deficiency and presence of lung inflammation too [3]. Nonetheless, systemic inflammatory process may lead to secondary surfactant inactivation, known as neonatal ARDS [12]. Chorioamnionitis also modifies function of respiratory centres with imbalance between excitatory and inhibitory neurotransmission which results in prolonged inspirium and delayed expirium, bradypnea and apnea. It also modulates sensitivity of chemoreceptors on hypoxia and hypercapnia [13]. Arguments on relationship between chorioamnionitis and development of bronchopulmonary dysplasia (BPD) in newborns are controversial [7, 8]. In experimental model of sheep with E. coli infection stimulation of surfactant production has been observed, however, microvascular and alveolar simplification was present [14]. Even though meta-analyses do not provide clear correlation, the fact that intrauterine infection makes lungs more vulnerable for postnatal insults is definite [10]. Furthermore, it must be mentioned that lungs themselves are believed to function as a generator of proinflammatory responses. After pathogen stimulation they produce proinflammatory cytokines that are present in lung fluid together with leukocytes. Approximately 50% of lung fluid is transported with cytokines into amniotic fluid where they react with another immune cells, skin and after swallowing with gastrointestinal system. Hence, lungs serve as a major mediator of inflammatory process between amniotic fluid and other organs of fetus. This can be an explanation of how congenital pneumonia may lead to systemic inflammation [15, 16].

Central nervous system

Association between intrauterine infection, systemic inflammation of fetus or newborn and damage of central nervous system has been confirmed in various studies [1, 6]. Chorioamnionitis has been recognized as an independent predictive factor of brain damage and cerebral palsy also in cases when inflammation doesn't lead to preterm labor [17, 18]. Chronic psychiatric diseases, such as autism and schizophrenia, have been linked to fetal and neonatal systemic inflammation too [19, 20]. There are several possible pathomechanisms leading to brain damage. Impairment of prooligodendrocytes by proinflammatory cytokines and free radicals, restricted neurogenesis and finally, alterations of microscopic architecture of brain cell layers have been observed in experimental studies [9, 19, 20]. Hypotension during systemic inflammation also substantially contributes to brain damage [6, 13]. Loss of prooligodendrocytes results in demyelinisation that may be detected not just as periventricular leukomalatic cystic formation but also as a diffuse white matter injury. This is not visible by standard head ultrasound but by MRI scan after 37 weeks of gestation [9]. It is also worth of noting that bacterial infection increases brain vulnerability to non-infectious insults, such as hypoxia, asphyxia or other metabolic disturbances. This vulnerability lasted for 70 days in animal models [6].

POSSIBLE THERAPEUTIC INTERVENTIONS

For preterm labor becomes a large socio-economic burden with progressively increasing incidence, many therapeutic interventions have been proposed in experimental and clinical studies. Getting control over infection means not just to start with proper and well-targeted antibiotic treatment but also to cease proinflammatory and prooxidant cascade. Indeed, cytokines and free radicals are responsible for organ damage so it is crucial to stop their production. Antibiotic therapy itself doesn't seem to lower the risk of brain damage, cerebral palsy and delay in psychomotor development after systemic infection [6].

Corticosteroids

Antenatal corticosteroid administration to pregnant women between 24 to 36 weeks of gestation has undisputed benefits regarding fetal lung maturation. After administration there was 34% reduction of RDS (respiratory distress syndrome) and 31% reduction of neonatal mortality. Lower incidence of intracranial heamorrhage based on hypoxaemia has been also described in previous studies [3, 21]. One corticosteroid course is recommended because repeated doses are connected with psychomotor disturbances, reduction of brain volume and head growth [22]. Anti-inflammatory properties led to usage of corticosteroids also postnatally. Despite of this fact, its immunomodulatory effect has not been fully understood. It is believed to lower incidence of BPD and provide better haemodynamic stability [21, 23]. However, for its metabolic, cardiovascular, gastrointestinal and neurological side effects corticosteroids are not recommended during the first week of life [23].

Melatonine

Melatonine is a hormone produced by pineal gland. Except it serves as a pacemaker of circadian rhythm, it has very strong antioxidant properties too. What makes melatonine more attractive in comparison to other antioxidants is that it lacks prooxidant effect. By inhibition of neutrophil infiltration and transcription of nuclear factor κB (NF-κB) it has also anti-inflammatory properties. For a good permeability through blood brain barrier and solubility in lipids, melatonine seems to be promising in brain protection during systemic infection or severe hypoxia when cytokines and free radicals are overexpressed [24, 25]. It supports myelinisation, neuronal sprouting and reduces neuronal apoptosis, inflammation and prevents from brain cyst formation [9, 26]. With its vasoactive effect it provides organ protection (such as heart and brain) during hypoxia [21]. In a study of newborns with RDS after melatonine administration reduction of ventilatory parameters and circulating cytokines has been observed, however, a long-term outcome has not been assessed [27]. There have been no side effects perceived even after high doses administered [21].

N-acetylcysteine

N-acetylcysteine (NAC) as a mucolytic drug is already used in newborns requiring mechanical ventilation. Except this effect, antioxidant properties have been described too for it stimulates glutathion synthesis in the environment of its depletion [28]. Various studies confirmed that NAC has also anti-inflammatory and immunomodulatory properties [29, 30]. During infection there is an overproduction of free radicals that on the other hand via transcription factors enhance cytokine production, thus making inflammation process stronger [10]. NAC has a potential to counteract this process and plays an important role in improvement of lung function and brain protection. In experimental study of ARDS caused by meconium aspiration a synergistic effect of NAC and surfactant has been observed. It has been explained by its anti-inflammatory and antioxidant action that protects pneumocytes and surfactant against its inactivation [31–33]. NAC seems to be a good neuroprotective agent too for it inhibits neurotoxicity mediated by cytokines and free radicals and preserves brain morphology and oligodendrocytes. However not in all clinical trials this potency has been observed [11, 34, 35]. In experimental studies N-acetylcysteine had a positive effect on regeneration of kidneys after hypoxic injury and also decreased the incidence of congenital heart disease in mothers with pregestational diabetes mellitus [36, 37]. It also seemed to prevent development of necrotising enterocolitis [38,39]. In clinical studies in preterm infants no adverse effects have been observed [33, 40, 41]. However, N-acetylcysteine has not been recommended in adult septic patients for possible cardiovascular instability [42].

Pentoxifylline

Pentoxifylline is a phosphodiesterase inhibitor that increases intracellular levels of cAMP, responsible for cytokine synthesis. In clinical studies of preterm infants with sepsis and NEC it reduced mortality rates which was confirmed by Cochrane meta-analysis too [6, 43]. In newer meta-analysis pentoxifylline use significantly shortened hospitalisation stay of preterm infants and reduced severity of metabolic acidosis, thus making this drug beneficial while administering to premature infants [44]. On the other hand, even though it has anti-inflammatory effect, pentoxifylline nebulisation in preterm infants requiring mechanical or non-invasive ventilation with oxygen supply didn't lead to decreasing rates of BPD in 40 weeks of gestation [45]. Nowadays, some of the neonatology clinics use pentoxifylline in their guidelines for treatment of late-onset sepsis [46]. Even though it is generally well tolerated drug, bleeding complications were suggested by some studies. It can have negative impact on thrombocyte, erythrocyte levels and coagulation factors, nonetheless, in some trails this effect has not been confirmed [6, 46, 47].

Erythropoietine

The use of erythropoietine (EPO) in neonatology is nowadays limited to anaemia treatment. However, for its antioxidant, strong anti-inflammatory and antiapoptotic properties, support of neurogenesis and reduction of oligodendrocyte damage, its neuroprotective effect has been suggested by various studies [6, 9, 26]. Erythropoietine receptors have been found in astrocytes, oligodendrocytes and microglial cells. During inflammation or after asphyxia there is an upregulation of these receptors, however, with inadequate synthesis of EPO. Cells with receptors that stay unoccupied resolve in apoptotic process [24]. In children after severe asphyxia and in extremely preterm newborns with intraventricular haemorrhage (IVH) who were treated with EPO for anaemia, a better neurological development in 10 to 13 years of age was observed [48, 49]. In animal models its administration even in higher doses was safe but some clinical studies warn against its standard use for higher incidence of retinopathy of prematurity (ROP) and in adult patients after stroke and cardiopulmonal resuscitation for thrombotic complications with higher mortality rates [6, 21, 50]. These concerns were disapproved in a prospective clinical trial (phase II) where EPO was administered to preterm infants (26+0 to 31+7 weeks of gestation) for 2 days after birth. Incidence of ROP, IVH, NEC, sepsis and BPD was not higher in interventional group in comparison to placebo group. PENUT trial (phase III) is now in progress and its primary outcome will be evaluation of impact of EPO administration on mortality and cerebral palsy rates. Anti-inflammatory and neuroprotective effects of EPO will be tested (by laboratory testing and MRI imaging) as well and side effects will be noted too [51, 52].

Caffeine

Caffeine in preterm neonates has been used for prevention of apnea of prematurity over 40 years. It stimulates respiratory centres in medulla oblongata, increases sensitivity of chemoreceptors to CO₂, minute ventilation, oxygen metabolism and tone of skeletal muscles and diaphragm. It stimulates peripheral chemoreceptors too, leading to apneic pause inhibition. Caffeine also decreases the number of reintubations so it serves as a prevention of extubation failure. For blocking adenosine receptors it has also anti-inflammatory properties following by improvement of lung function. BPD reduction has been proposed if administered early after birth. Regarding cardiovascular system less PDA (patent ductus arteriosus) has been observed after caffeine therapy. Neuroprotection seems to be very promising too. It is worth of mentioning that all of these effects are not dependent on administration route. Like other drugs, caffeine may have adverse effects too such as tachycardia and tachypnea, tendency to irritability, hypertonia, gastrointestinal reflux and increased mineral loss by urine, leading to osteopenia. It is necessary to emphasise that despite its anti-inflammatory properties, high doses may turn caffeine into a drug with proinflammatory effect [24, 53].

Magnesium Sulphate

In obstetrics magnesium sulphate is used when there is a threat of preterm delivery. Even though Cochrane meta-analysis has not confirmed its tocolytic effect, neuroprotective properties of magnesium on infant have been suggested when administered antenatally. It decreases synthesis of NO-synthetase, NF-κB through NMDA-receptors and it reduces intracellular calcium intake too, preventing brain from excitotoxic damage. It leads to vasodilation of brain vessels and by antioxidant and anti-inflammatory properties reduces reperfusion injury after ischaemia. The result is preservation of white matter of the brain and decreased incidence of cerebral palsy [20, 54, 55]. Nonetheless, a recent meta-analysis has shown that if magnesium sulphate is administered to women before 31 weeks of gestation, only one out of 46 babies will profit from this treatment in terms of development of cerebral palsy [55]. Clinical study of Zeng et al. proclaimed that magnesium decreases incidence of moderate to severe cerebral palsy but on the other hand, doesn´t decrease mortality rates [56]. Possible described adverse effects are hypotension, bradycardia and AV blockade, IVH, spontaneous intestinal perforation and respiratory failure with a need of intubation [24, 57, 58].

Surfactant

The main role of surfactant is lowering alveolar surface tension and preventing alveoli from collapse, thus making breathing less strenuous. However, it has antioxidant and anti-inflammatory properties as well. It contains enzymatic and non-enzymatic antioxidants that hinder accumulation of free radicals in alveoli. Surfactant proteins A and D are responsible for direct clearance of pathogens in alveoli, inhibition of NF-κB and lymphocyte proliferation [60]. Results of experimental studies in which surfactant has been combined with other antioxidant and anti-inflammatory substances seem to be very encouraging in management of respiratory failure in preterm newborns. They provide protection for surfactant because it can be peroxidized in infectious environment very quickly and subsequently may lead to ARDS [31, 30, 59].

Vitamin E and other antioxidants

Oxidative stress that is even more detrimental in inflammatory environment, may be overwhelming for preterm newborns because of their limited antioxidant capacity. Replacement of antioxidant enzymes, such as superoxide-dismutase (SOD) seems to be promising in animal models [60] and clinical studies too. If a newborn lacks SOD and requires high fraction of inspired oxygen, lungs may be damaged easily. Endotracheal administration of SOD in combination with surfactant had a positive effect on ventilation parameters and also decreased levels of lung damage markers were found in tracheal aspirates. However, for its short-term effect it didn't have impact on incidence of BPD [61]. Other antioxidants already used in clinical practice are vitamins E and C. The best option is their combination for vitamin C supports regeneration of active form of vitamin E. Antioxidant effect of vitamin E is in its ability to incorporate into the cell membranes and protect them against peroxidation. Nowadays, it is approved as prophylactic treatment of ROP. It may serve as a potential neuroprotective agent too. In patients with Down syndrome vitamin E decreased the level of markers of lipid peroxidation in hippocampus, leading to reduction of cognitive and behavioral deficits [24]. In some trials more NEC and late-onset sepsis have been observed after vitamin E administration [62]. Studies with vitamin C were not optimistic both for its weak efficacy and for its prooxidative effect in higher doses [59].

Other drugs and therapeutic interventions

Experimental studies are constantly working on searching the best antioxidant and anti-inflammatory drugs as adjuvant substances to conventional antibiotic therapy in preterm newborns. Unfortunately, use of many of them is limited for serious side effects. One example is indomethacin that reduces IVH, PDA and brain white matter injury. Despite of its positive potency, it may counteract with platelet function and lead to bleeding complications and for reduction of intestinal blood flow to NEC or spontaneous intestinal perforation [24]. A potential effect in elimination of brain and lung damage caused by prooxidant and proinflammatory cascade may bring administration of Il-10 or antagonist for Il-1 receptor that may improve motoric functions after infection and reduce incidence of BPD [9]. Anti-inflammatory effect has been linked to docosahexaenoic acid too. It is believed that except its role in proper growth of the child it may exhibit immunomodulatory properties as well. Also, it supports intestinal colonisation with beneficial bacteria. New studies show that after its supplementation incidence of sepsis, ROP and BPD has decreased [63]. At last, a perspective intervention is in genetics when genes responsible for cytokine production are being suppressed. Despite it has a great potential, more time is needed for further research in this field [64].

CONCLUSION

Chorioamnionitis is a frequent cause of preterm labor. Its negative impact on newborn is indisputable and it is manifested in higher short-term and long-term morbidity rates. Unfortunately, mortality is often not an exception. Cytokines and free radicals have destructive effects on immature organs so it is crucial to inhibit their synthesis. A lot of potential antioxidant and anti-inflammatory drugs are in research. Some of them have been already introduced in clinical practice, but most of them need more time to ensure their efficacy and safety. To decrease mortality and morbidity rates in preterm infants every effort that leads to improvement in quality of their lives is welcomed.

Acknowledgements:

Financial support was mediated by project VEGA 1/0055/19, APVV-17-0250, APVV-15-0075, and Biomedical Center Martin, Slovak Republic, ITMS code: 26220220187, project co-financed by the European Union and the European Social Fund.

Special appreciation and gratitude  is for prof. Čalkovská and prof. Zibolen.

Došlo: 4. 5. 2020

Přijato: 14. 8. 2020

Corresponding author:

Doc. MUDr. Katarína Maťašová, PhD.

Department of Neonatology

University Hospital Martin

Jessenius Faculty of Medicine in Martin

Kollárova 2

036 59 Martin

Slovak Republic

e-mail: matasova.katarina@gmail.com