PLACENTAL CONTRIBUTION TO NEONATAL ENCEPHALOPATHY

Abstract

Placental assessment, although currently underused, can inform our understanding of the etiology and timing of Neonatal Encephalopathy (NE). We review our current understanding of the links between placental dysfunction and NE and how this information may inform clinical decisions, now and in the future, emphasizing the four major placental lesions associated with NE. In addition, we discuss maternal and fetal factors that are hypothesized to contribute to specific placental pathologies, especially innate or acquired thrombophilias. We outline the importance of assessing placenta across trimesters and after delivery. As this field continues to evolve, currently available placental histopathological examination methods may need to be combined with advanced prenatal molecular and imaging assessments of placenta and be applied in well-designed studies in large representative populations to better define the links between placental dysfunction and NE.

Overview of Placental Pathology associated with NE

The placentas of neonates with NE demonstrate a very high rate of abnormalities (Table 1). The precise nature and frequency of these placental findings, and efforts to understand the contribution of specific placental patterns to fetal brain injury, have been impeded by a number of factors, prominent among which is the variable use of terms and pathological identification of placental lesions ⁴. The 2016 Amsterdam consensus document has reframed these efforts by clearly isolating and defining the four major lesion categories associated with NE and their diagnostic characteristics (Figure 1): i.e., maternal vascular malperfusion (MVM), fetal vascular malperfusion (FVM), acute chorioamnionitis (ACA), and chronic villitis (villitis of unknown etiology, VUE), eliminating much of the prior ambiguity and subjectivity regarding the nature and severity of placental lesions that may be linked to NE ⁵.

Table 1.

Placental Lesions	Term infant associated neuropathology
Maternal vascular malperfusion	Fetal demise with neuronal necrosis7 Term infants with NE15
Fetal vascular malperfusion (extensive avascular villi +/- umbilical cord lesions)	Fetal demise with neuronal necrosis8–10 Neonatal encephalopathy, 2, 11, 12, 15 Cerebral palsy13,14
Acute chorioamnionitis (frequently with high grade fetal inflammatory response)	Cerebral palsy18,19 Neonatal encephalopathy1,22,23
Chronic villitis (high grade with obliterative fetal vascular changes)	Fetal demise with neuronal necrosis8,9 Neonatal encephalopathy28,30 Cerebral palsy13, 16 Basal ganglia and thalamic injury24
Multiple/mixed lesions:	Fetal demise with neuronal necrosis8 Cerebral palsy13 Growth restriction and NE26 Neonatal encephalopathy with abnormal MRI23

Figure 1.

Four major patterns of placental injury. (a) Maternal vascular malperfusion is characterized by villous crowding (center) with increased syncytial knots and villous agglutination combined with surrounding relative paucity of branching villi (4X, H&E stain). (b) Fetal vascular malperfusion (fetal thrombotic vasculopathy) shows uniformly hyalinized avascular terminal villi branching off a stem villus with stem vessel luminal obliteration (4X, H&E stain). (c) Acute chorioamnionitis with a severe grade 2 fetal inflammatory response in chorionic vessels as indicated by confluent neutrophilic infiltration of the fetal vessel walls on the side facing the amniotic fluid cavity (4X, H&E stain). (d) Chronic villitis, high grade, with stem vessel obliteration (obliterative fetal vasculopathy) shows a lymphohistiocytic infiltrate of maternally derived immune cells in the fetal connective tissue stroma of stem and terminal villi resulting in secondary occlusion of stem villous arteries (10X. H&E stain).

There are other important limitations of current information relating placental characteristics to outcome in the child ⁶. Many of the available studies had small samples, used varying definitions of NE, were based on secondary review of placental reports, lacked specificity in the exact nature of placental lesions, and/or were based on cases submitted for medicolegal review, which were likely to be atypical cases of NE. Few studies have had detailed information on both infant characteristics and placental findings, as available studies have come chiefly from academic neonatologists in referral centers, providing detailed information on the infant in the neonatal period, but not systematic study of placentas, or from case series from perinatal pathologists with observations that are rich in placental descriptions but not ample in clinical data. Few studies have come from population-based samples, which are far more likely to be representative of the whole population of neonates with NE. Other outstanding issues include poor interobserver diagnostic reliability with regard to placental histology, uncertainty concerning which histologic features within a given diagnosis are most closely related to outcome , and the appropriate scaling of severity of the placental findings. Finally, because neurologic outcome is of key importance, but cannot reliably be determined in the neonatal period, placentas are often discarded before infants are recognized to be at risk. To relate placental characteristics to long-term outcome requires longitudinal studies, preferably in population-based samples, or other demanding study designs.

Placental pathology associated with NE: Vasculopathy

Maternal vascular malperfusion is a constellation of gross and microscopic findings indicative of chronically compromised maternal blood flow through abnormal spiral arteries leading to ischemic damage and reduced placental growth (Figure 1a). Fetal vascular malperfusion is a similar constellation of findings indicative of chronically compromised fetal blood flow in the umbilical cord leading to stasis, thrombosis, and villous stromal-vascular damage within the placenta (Figure 1b). Umbilical cord lesions leading to fetal vascular malperfusion include fetal entanglements and knots, hypercoiling, excessive length, tethering by a amnionic web, and peripheral insertion site. At the most extreme end of the spectrum, maternal vascular malperfusion is associated with fetal death ⁷, as is fetal vascular malperfusion ^8–10. Fetal vascular malperfusion has been associated with NE ^{2, 11, 12} and later cerebral palsy ^13–15 in observational studies. In two recent case-control studies that used the Amsterdam criteria, both maternal vascular malperfusion and fetal vascular malperfusion were significantly increased in neonates with NE. Vik et al. compared placentas from 73 neonates with NE of all severities with gestational age-matched controls ¹, and the only microscopic finding associated with NE was scattered small foci of avascular villi, an indicator of low grade global fetal vascular malperfusion, caused by chronic partial intermittent umbilical cord obstruction. Trends for increases in segmental and high-grade fetal vascular malperfusion were also noted. In another study, comparing 46 neonates with NE as defined by National Institute of Child Health and Human Development (NICHD) Neonatal Network criteria and contemporaneous controls matched for gestational age and admitted to the neonatal intensive care unit (NICU) for other indications ¹⁵, both fetal vascular malperfusion and maternal vascular malperfusion were increased in neonates with NE.

Placental pathology associated with NE: Inflammation

Acute chorioamnionitis with or without fetal vasculitis is the histologic acute inflammatory response to pathogenic organisms in the amniotic fluid, the placenta and/or the fetus (Figure 1c). While manifesting in the placenta, these responses arise from the maternal and fetal immune systems and, to a large degree, placental findings constitute biomarkers for excessive levels of circulating cytokines that can cause fetal injury. VUE is a non-infectious antigen-specific maternal T-cell mediated immune response that is thought to be a response to class 2 Major Histocompatibility Complex antigens, as well as other yet to be identified antigens, within the fetal stromal vascular compartment of the placenta ¹⁶ (Figure 1d). In addition to direct damage to the villous stromal vascular compartment, VUE can cause obliteration of fetal stem vessels in the placenta and be associated with activated lymphocytes and cytokines in the fetal circulation. Whether acute or chronic inflammation itself creates brain injury or whether it increases the vulnerability of the fetus to hypoxia-ischemia has not yet been elucidated.

Over the years, many clinical studies have described the association of placental inflammation, elevated cytokines and NE ^17–19, but studies in large representative populations are limited. ^{15, 16} Translational models indicate a different response to inflammation, depending on the time of exposure during pregnancy ^{20, 21}. Many clinical studies demonstrated multiple inflammatory placental findings in term neonates diagnosed with NE, including acute chorioamnionitis ^{22, 23} and VUE ^{1, 13, 14, 22, 24}. In a population-based Canadian registry ¹, acute chorioamnionitis was diagnosed in 34% of term-born neonates with NE developing cerebral palsy (CP). In a methodology rigorous study, McDonald et al. ² found that of 93 neonates with NE, 31.2% had acute chorioamnionitis/funisitis versus 4.4% of controls. Several studies have linked high-grade VUE with increased risk of NE when compared to those without VUE ^{2, 24, 25}.

Association with NE severity, neuroimaging and outcome

Several important principles have emerged from previous studies of placental pathology and a variety of adverse brain outcomes observed in the neonatal period or later. First, patterns of injury affecting fetoplacental vessels are in general stronger risk factors than those affecting maternal vessels ¹⁴. Second, multiple placental lesions can act synergistically to increase the risk of brain injury ²⁶. Third, the combination of an acute pattern of injury superimposed on chronic underlying placental pathology is highly prevalent in cases of brain injury ¹³.

Chang et al. reported that NE in the absence of sentinel events was associated with a higher prevalence of placental vascular and/or inflammatory pathology than NE following a sentinel event ²⁷. Hayes et al. found that acute chorioamnionitis was a significant predictor of mild NE only ²⁵. However, in a large retrospective cohort study, Mir et al. ²² studied 120 inborn neonates with NE and perinatal acidosis admitted to the NICU, and demonstrated that 65% had a major placental pathology. These major placental pathologies included (1) intervillous fibrin deposition/retroplacental hemorrhage/infarction involving 20% of placental volume; (2) fetal vascular malperfusion with greater than 1 focus of avascular villi/fetal thrombotic vasculopathy; (3) acute chorioamnionitis including fetal vasculitis; and (4) high-grade patchy/diffuse VUE. Univariate logistic regression analysis showed an association with a diagnosis of any major placental pathology (odds ratio, 3.5; 95% confidence interval, 1.1–11.4) and abnormal outcomes following cooling. In a multivariable regression analysis, diffuse VUE had a significant odds ratio of about 9 associated with abnormal neurodevelopmental 2-year outcomes following hypothermia. ²²

Wu et al. found a significant association between abnormal placental lesions, as defined by the Amsterdam classification, and subcortical injury ²⁸. Hartemann et al. investigated the association of placental lesions with specific patterns of brain injury in 95 neonates with NE, observing that (1) VUE was increased in neonates with NE developing basal ganglia thalamic (BGT) injury, (2) acute chorioamnionitis was highly prevalent in neonates with NE without brain injury, and (3) low placental weight, a loose correlate of MVM, was protective against brain injury, particularly basal ganglia thalamic injury ²⁴. Wintermark et al. found a significant increase in the combination of acute chorioamnionitis plus prolonged meconium exposure in neonates with NE who developed brain injury ²³. Interestingly, as in the previous study, a protective effect of low placental weight was also observed. Elevated nucleated red blood cells within the placenta were associated with white matter involvement in another study ²⁹. Finally, Barrett et al. showed markedly increased frequencies of fetal vascular malperfusion and VUE, but not acute chorioamnionitis, in neonates with NE and basal ganglia injury ³⁰. The association of severity of placental pathology and neuroimaging/outcome has not yet been reported from a broad population-based study, limiting current generalizability of these findings.

Implications of placental findings for predicting the response to neuroprotective and/or neurorestorative therapies

Current evidence makes it apparent that predicting the response to therapies based on placental findings is complex, may differ by gestational period, and is not yet fully delineated. Two studies have specifically addressed the association between placental findings and the response to neuroprotective therapies. Mir et al. found that an abnormal result after head cooling was significantly associated with high grade VUE, while acute chorioamnionitis with a fetal inflammatory response was borderline significant in predicting a good response ²². In a secondary review of placental reports on 30 neonates with NE with available placental pathology and known outcome from a randomized clinical trial, Wu et al. found that erythropoietin treatment was only protective against subcortical brain injury in the subgroup without placental lesions (maternal vascular malperfusion, fetal vascular malperfusion, and/or VUE) ²⁸. Notably and despite the small numbers, neonates with NE, whose placenta demonstrated VUE, did not derive the same benefit from erythropoietin as the remaining cohort. It is very likely that acute and chronic inflammation will require different postnatal treatment strategies. Future progress depends on improved placental diagnostic reliability and clinical study designs that distinguish different subgroups of neonates with NE.

Additional variables influencing placental findings: Socioeconomic status and thrombophilias as examples

Given the association between placental inflammation, malperfusion and NE, it stands to reason that additional variables that increase the risks of inflammation — such as infection, obesity, diabetes or malperfusion due to thrombophilias — may be associated with NE via their impact on placental function. To date however, few studies have aimed to link clinical conditions that may predispose to NE and specific placental pathologies. It highlights the likely need to subdivide these complex conditions into physiological subsets, based potentially on directly contributing factors, such as inflammatory status rather than weight or glucose levels, to understand the link to placental dysfunction and NE.

The incidence of placental pathology, particularly inflammation, was higher in women from underrepresented minority groups and low socioeconomic status (SES) ³¹. Three studies have highlighted an association of low SES with NE ³² or cerebral palsy ^{33, 34}, although Vik et al.’s study from Canada, which may have fewer barriers to equal medical care, did not find NE to be more frequent in children of women with fewer years of education ¹. Low SES, placental pathology and NE have not yet been jointly studied, but these preliminary associations raise questions about reproductive tract infections and other causes of inflammation, as well as access to care, that could be addressed in large population-based studies including placental assessments.

Studies of maternal thrombophilias, present in about 10% of women, provide the best evidence of a connection between additional maternal conditions, placental pathology and NE, but even here their exact contribution to placental pathology and NE remains ambiguous. Hypercoagulability is a normal feature of pregnancy, presumably evolved to minimize risk of hemorrhage. Placentation places a large burden on the circulatory system. Given the hypothesis surrounding the mechanistic origin of FVM – Virchow’s triad of stasis, hypercoagulability, and endothelial wall damage – it seems likely that inherited or acquired thrombophilias would compound the physiologically hypercoagulable state of pregnancy and present a second hit increasing risk of NE, but evidence of such associations has been mixed. Overall, there has been no strong evidence that an isolated inherited thrombophilia contributes to placental vasculopathy, thrombosis, or NE, but only a few reports of more than a single case in a sibship ^35–37.

Factor V Leiden mutation is the most common inherited thrombophilia and can lead to clot in ~10% of the homozygous population. There is moderate increase in risk of venous thromboembolism in homozygous women, particularly post-partum ³⁸. Despite this moderate risk, there is no clear evidence that factor V Leiden mutation alone increases risk of placental thrombotic lesions and/or NE ^39–42. Prothrombin (PT) mutation G20210A is the second most common thrombophilia and much like factor V Leiden, isolated prothrombin mutations have no demonstrated association with placental thrombotic lesions or NE⁴³. Another common thrombophilic genetic variant, methylenetetrahydrofolate (MTHFR) C677T mutation, increases clot risk by elevating blood homocysteine levels. Homocysteine is physiologically decreased in pregnancy and consumption of folate further decreases homocysteine levels, thus mitigating pregnancy risk. MTHFR mutation association with placental thrombosis and/or NE is weak^{35, 36, 40, 41}. Instead, the contribution of MTHFR mutation may be a contributor in the context of acute placental abruption, a potentially catastrophic sentinel event related to NE ³⁵. Other rare inherited thrombophilias (Antithrombin (AT) III mutation and protein C and S deficiencies) confer greater risk of thrombosis than the more common mutations, but even here there is no strong evidence that an isolated inherited thrombophilia contributes to placental vasculopathy, thrombosis, or NE^35–37.

In contrast, the acquired thrombosis risk seen with antiphospholipid antibodies (APLA), which are both thrombotic and inflammatory ⁴⁴ show a stronger contribution to placental pathology and NE. Antiphospholipid antibodies cross the placenta and the fetal blood brain barrier. Mothers with antiphospholipid antibodies have higher rates of recurrent abortions, preeclampsia, and IUGR ⁴⁵, as well as histopathologic evidence of placental infarctions and other thrombotic lesions⁴⁶. In longitudinal studies of mothers with lupus and antiphospholipid antibodies, children had a small but significant increase in neurodevelopmental disorders including language delay and dyslexia⁴⁷. Unfortunately, only few studies of antiphospholipid antibodies and risk of NE or perinatal stroke have included placental pathology. However, further evaluation of these pregnancies seems warranted given the frequency of this acquired thrombophilia and likely link to NE.

Studies of thrombophilia, placental pathology, and NE have been variable in their methodology. Pregnant women are typically tested only if they have a positive family history, recurrent pregnancy loss, or prior venous thromboembolism, so there is little denominator data. Placental data and long-term neonatal outcomes are generally sparse or lacking altogether. Not all thrombophilias confer similar risk, yet tend to be grouped together in most studies. Most studies to date on NE, thrombophilia, and placental pathology have chiefly included Caucasian women. However, ethnic differences may exist, with some MTHFR polymorphisms more common in non-Hispanic Caucasian women, while antiphospholipid antibodies may be more prevalent in Hispanic and Black women. ⁴⁸ Isolated thrombophilias may compound adverse outcomes in maternal malperfusion syndromes, such as preeclampsia/HELLP syndrome, placental abruption or stillbirth^{35, 49–52}. In these instances, placental pathological analysis has revealed evidence of chronic infarction and spiral artery thrombosis. However, it is malperfusion in the fetal circulation, not in maternal circulation, that predominantly contributes to NE and cerebral palsy. Although maternal thrombophilia is more commonly tested than in the fetal and neonatal period, fetal coagulation disturbances may be an important piece of the puzzle ⁵³. Future studies of NE should focus on fetal cord testing of antiphospholipid antibodies and inherited thrombophilias, maternal antiphospholipid antibodies, and placental pathological analysis – all in diverse populations – to better understand this understudied potential risk. Additionally, more physiologic measure of active coagulation, including thrombin and plasmin generation, thrombin-anti-thrombin complexes, and other markers such as plasminogen activator inhibitor 1 (PAI-1) may be more pertinent and revealing in studies looking at their association with placental vascular injury and NE.

Placental contributions to NE by trimester

The placenta grows, matures, and may become senescent during the course of pregnancy. Each trimester-specific phase must be taken into account if placental function is going to be used to clarify the timing and etiologies of distal and proximal prenatal factors associated with NE. Fetal anomalies must also be taken into consideration not only in terms of genetic background and NE risk, but also because structural anomalies can indirectly alter placental function. For example, congenital heart disease (CHD) is the most common anomaly that can affect brain and placenta ⁵⁴. Syndromic or non-syndromic genetic causes of congenital heart disease adversely affect embryonic and first-trimester cardiac, brain and other organ system development. Maldevelopment of placenta from this shared genetic endowment may contribute to later growth dysfunction with circulatory disturbances ⁵⁵. Abnormal placental blood flow patterns associated with congenital heart disease may cause later fetal brain perfusion injuries. Assessments in each trimester for fetuses with congenital heart disease and other underlying conditions are beginning to reveal the interactions between placenta, fetal brain injury and risk of NE as development progresses.

Pre-conception through first-trimester assessments

Transgenerational familial genetic endowment contributes to placental health or disease. The initial developmental phases require transcriptome-controlled mediators to promote immune tolerance between the mother and fetus for fetal viability after fertilization and first-trimester development of a functional placenta by 8 weeks. Preconception genetic and environmental effects on uterine wall preparation may be critical for proper implantation that is required for future placental function. Early identification of genetic risk for poor placental development may allow early prediction of high-risk pregnancies and intervention⁵⁶. Emerging technologies, such as advanced acoustic ultrasound and new functional magnetic resonance imaging (MRI) techniques, will add to our understanding of early stages of placental function, allowing additional identification of high-risk pregnancies⁵⁷.

Low-lying placenta, placenta praevia and abnormally invasive placenta are implantation disorders⁵⁸. Cord insertion abnormalities (i.e., velamentous and marginal insertions) often accompany these placental abnormalities⁵⁹. These implantation disorders may contribute to NE through impaired placental function or ischemia, and the effects may appear in any trimester of pregnancy (e.g., pre-eclampsia and fetal growth restriction) or with acute intrapartum events (i.e., abruptio placenta and placenta accreta spectrum) ⁶⁰. Additional cord anomalies of length, coiling and morphology have also been cited as associated risks to chronic and/or acute pathophysiologic events⁴.Very early changes in placental function may alter fundamental processes in brain development, such as neuronal/glial proliferation or differentiation, predisposing to NE ⁶¹. However, the earliest standard evaluations between eight and ten weeks, include sonographic studies to screen for anomalies and blood sampling to screen for aneuploidy, blood group incompatibility and congenital infections, but are not highly predictive of later placental disorders. Serum-derived proteomic/genetic markers⁶² are being developed that will complement advances in sonographic imaging to improve first-trimester disease monitoring ⁶³. First-trimester biomarkers such as alpha fetoprotein (AFP) to pregnancy-associated plasma protein A (PAPP-A) ratios may also correlate with placentally-related adverse pregnancy outcomes linked to risk of NE ⁶⁴.

Second trimester assessments

During the second trimester, disorders include ascending infections, maternal systemic diseases, and obesity, as well as other factors including health disparities and adverse lifestyle choices, may contribute to the development of inflammation and malperfusion. It is at this point that malperfusion (i.e., fetal vascular malperfusion) and placental inflammatory processes (i.e., VUE) may have the greatest impact, for example by compromising fetal growth resulting in IUGR. These placental problems currently remain difficult to identify before there is significant growth restriction.

As previously noted, synergism between inflammation and poor perfusion contribute to increased risk for antepartum fetal brain disorders, NE and other neurodevelopmental sequelae. A meta-analysis concluded that the combined diagnostic accuracy of 7 biochemical and ultrasound biomarkers of placental dysfunction-- ultrasound estimated fetal weight, human placental lactogen, estriol, urinary estriol, uric acid, placental growth factor (PlGF) and placental grading-- used after 24 weeks gestation best predicted stillborn or small for gestational age outcomes ⁶⁵. These test results represent proxies for multiple pregnancy disorders, such as maternal hypertensive disorders, prematurity and abruptio placenta that contribute to NE. Improved tests that detect earlier signs of placental dysfunction due to either inflammation or malperfusion might have an impact on understanding the in utero origins of NE and other neurodevelopmental sequalae, such as cerebral palsy.

Third-trimester assessments

A variety of pregnancy disorders may begin or become clinically apparent during the third trimester, but again detection remains limited by current diagnostic tools and biomarker sensitivities. No single or combination of tests reliably predict brain injury. Fetal distress based on abnormal Doppler flows, biophysical scales, or non-reactive fetal heart rate patterns are frequently associated with normal neonatal outcome. ^66–68 Placenta and fetal imaging, particularly with advanced MRI techniques, may improve risk identification. ⁶⁹

Labor/delivery and neonatal resuscitation assessments

Antepartum plus intrapartum risk factors may more accurately predict risk of NE, but continuity of risk may not have been anticipated in seemingly low-risk pregnancies, despite fetal distress documented close to delivery ⁷⁰. Adaptive responses to intrapartum fetal hypoxia avoid brain, cardiac and adrenal injuries in most situations by the protective peripheral chemoreflex, but placental dysfunction may impair this response. The main tool used during delivery, intrapartum fetal heart rate abnormalities, fail to reliably predict the presence, timing or etiologies associated with NE. While intrapartum hypoxic-ischemic (HI) events may contribute to brain injury in selected situations, assuming intrapartum HI is responsible for NE in most cases is often incomplete or erroneous. Review of serial clinical assessments combined with laboratory and imaging results from across gestation more accurately assign timing and etiology to brain injury associated with NE. Subsequent evaluations after starting the hypothermia treatment, including placental and cord pathological assessments, may support alternative diagnoses that require other treatment ⁷¹.

Use of placental information after delivery in guiding management of NE

As described above and in Figure 1, placenta of neonates with NE present with a high percentage of pathological lesions ^{5, 13, 14} (see “Case Histories”). Regular analysis of placenta by pathology still takes a few days and results typically only become available after the first week of life, making it difficult to modify the management of neonates with NE according to their placental findings. In addition, reliability of placental histology diagnosis, especially its finest features on the fetal side, remains dependent on the training and experience of the pathologist, despite standardized definition and protocols ^{5, 72, 73}. Thus, diagnosing these anomalies currently does not allow real-time modification of management of neonates with NE. For example, suspicion of clinical chorioamnionitis without histological diagnosis may be overtreated with antibiotics ⁷⁴. On the other hand, fetal vascular malperfusion may lead to neonatal thrombosis that may not be initially recognized ⁷⁵. The advent of comprehensive automated computer-based analysis of the placenta using artificial intelligence from placental photos ⁷⁶ or from hematoxylin and eosin (H&E) stained placental images ⁷⁷ may markedly shorten the time required for pathological placental assessment and in the near future may provide reliable results within days of birth, allowing refined, individualized treatment in addition to hypothermia.

Future tools for placental assessment: -omics, imaging and functional assessment

In combination with histological assessment of the placenta after delivery, the new available techniques using “-omics” for placental analysis may provide more individualized understanding of placental dysfunction and pregnancy disorders that contribute to NE. Interrogation of placental gene and protein expression (genomics, transcriptomics, proteomics), cellular functions (metabolomics), and methylation or chromatin modifiers of gene expression (epigenomics) are only just starting to produce new insights into placental states that correlate with neonatal outcome. ^{78, 79} To date, such -omics techniques have primarily been applied to placenta after delivery, but in the future, evaluation of placental exosome contents and fetal cell-free DNA in maternal blood may provide biomarkers for elevated NE risk.

New techniques in imaging are also starting to provide insight into impaired placental function that may contribute to NE ^{63, 69, 80, 81}. Currently ultrasound is the primary tool used to assess placental development across pregnancy. Elastography, which uses ultrasound to assess tissue stiffness, may provide new information about early second-trimester tissue changes that reveal potentially compromised pregnancies ⁸², particularly in combination with standard ultrasound techniques and MRI. Advanced MRI methods are being actively studied in pregnancies affected by fetal growth restriction, cardiac anomalies and other fetal pathologies. Real-time, detailed placental perfusion measurements using BOLD (blood-oxygen level-dependent), diffusion-weighted imaging (DWI) or arterial spin labeling (ASL) can also provide early, functional information about placental function. Advance MR imaging using BOLD and maternal hyperoxygenation can even reveal regional placenta oxygen transfer ⁸³. Additional information on placental oxygenation during labor may be provided by use of non-invasive near-infrared-spectroscopy (NIRS). ⁸⁴ Together, these functional measurements may provide more acurate assessment of placental and fetal compromise that can lead to NE.

Conclusion

Direct and sensitive measurements of placental function across pregnancy using advanced techniques in combination with placental examination after delivery should provide more precise predictive correlations with NE and point to specific biological disruptions that may be causative. Early risk prediction provides an opportunity not only to better understand placental factors that contribute to NE, but also a chance to intervene before injury happens.

Continued investigations of the connections between placental dysfunction, histopathological abnormalities and NE risk are urgently needed to better delineate how and when the placenta contributes to NE. Ultrasound documentation of placental health and placental histopathology done after delivery are the current tools used to examine placental function across pregnancy, and while they clearly support a link between placental pathology and NE, neither is highly sensitive or specific. Ideally, rapid, in-depth review of placental information should be available for every neonate, much like a newborn screen. Even neonates without NE may have long-term neurological injury that is linked to placental dysfunction, but this link will only be recognized if the placenta is assessed during and after pregnancy. In the absence of major malformations, future neurologic status can seldom be confidently predicted based on neonatal findings, so studies must be constructed to relate placental and neonatal characteristics to long-term neurologic outcome in the child.

Development of evidence-based treatment strategies to alter placental function and reduce the risk of NE depends on the parallel development of novel tools in addition to the immediate use of the well-defined histopathological tools at hand. Advanced -omics and imaging techniques can potentially provide new insights and novel biomarkers that will guide maternal, neonatal or future pregnancy management, but these tools are only starting to be applied. Newer techniques may yield greater mechanistic insights into the underlying biology, but current tools should be used in well-controlled prospective studies to improve patient stratification and identify adjuvant therapies that can be used in addition to hypothermia in specific populations presenting with NE. New placental biomarkers for NE dervided from -omics, advanced imaging or other novel functional assessments can provide new therapeutic strategies and ultimately reduce the burden of neurological injury resulting from placental dysfunction.

Practice Points.

• Placental pathologies are a frequent finding in the context of NE

• A combination of acute and chronic placental lesions are more likely to be seen in NE

• Defined histopathological criteria using the 2016 Amsterdam consensus document should be used whenever the placenta is assessed

• Efficacy of therapeutic hypothermia may be influenced by antenatal placental factors

Research Directions.

• Large prospective population-based studies are warranted to connect antenatal placental function, postnatal placental histopathology and neonatal outcomes

• Automated image analysis allowing rapid placental assessment in cases of NE should be developed

• New placental molecular and imaging tools for placenta are starting to be applied for NE risk prediction

ABBREVIATIONS:

ACA

acute chorioamnionitis

FVM

fetal vascular malperfusion (FVM)

MRI

magnetic resonance imaging

MTHFR

methylenetetrahydrofolate

MVM

maternal vascular malperfusion

NICU

neonatal intensive care unit

SES

socioeconomic status

VUE

villitis of unknown etiology