Early screening for preeclampsia: A myth or a reality?

Preeclampsia (PE) is a hypertensive disorder in pregnancy affecting 2-8% of pregnancies worldwide and remains one of the leading causes of maternal and neonatal morbidity and mortality. PE is characterized by hypertension and end-organ damage secondary to oxidative stress, endothelial dysfunction, and vasoconstriction. The pathophysiology of the disease being elusive, early screening, diagnosis, and effective management of PE is challenging. Traditionally several predictive models including clinical and biochemical parameters have been used to stratify the risk of developing PE in pregnancy. Poor sensitivity and specificity of these models have been a limitation and has inspired the clinical world to explore more tools to screen and diagnose PE prior to the onset of clinical syndrome. This review attempts to highlight the recent developments in the newly developed screening tools and explores the future direction of preeclampsia screening research.


Introduction
Preeclampsia (PE) is a pregnancy specific hypertensive disorder associated with endothelial dysfunction, vasoconstriction, and end-organ damage.PE affects 2-8% of pregnancies worldwide and is responsible for 14% of maternal deaths annually 1,2 .In Sri Lanka, hypertensive disorders in pregnancy accounted for 4.5% maternal deaths in 2020 3 .PE results in variable short-and long-term maternal and fetal adverse outcomes.These include renal failure, liver failure and seizures (eclampsia; 2-3 vs 40-90 cases/10,000 live births in Europe vs developing countries, respectively) in short-term and 3.7 times higher risk of developing hypertension; 2.2 times higher risk of developing coronary heart disease; and 1.8 times higher risk of developing stroke in long-term in mother.Fetal complications include growth restriction, placental abruption, fetal death and complications of prematurity in surviving babies [4][5][6] .Low middle income countries are the most severely affected by the disease and financial burden.
International Society for the Study of Hypertension in Pregnancy (ISSHP) defines pre-eclampsia as de novo hypertension after 20 weeks' gestation accompanied by one or more of the following features: (i) proteinuria; (ii) other maternal organ dysfunction (acute kidney injury, liver dysfunction, neurological complications, haematological complications); and/or (iii) uteroplacental dysfunction 1 .
The underlying pathophysiology of PE remains poorly understood and that limits the ability to (i) effectively screen for the disease prior to the onset of clinical syndrome; and (ii) to treat the disease with minimal morbidity and mortality.It is known that (i) PE occurs in trophoblastic tumours (without the presence of a fetus) 7 ; (ii) PE is more prevalent in multiple pregnancies (with greater placental mass) 8 ; (iii) PE occurs in ectopic pregnancies (excluding the involvement of uterine tissue) 9 ; and (iv) by far the only definitive treatment for PE remains the delivery of placenta (irrespective of gestational age).Therefore, the placenta is believed to be the primary site to the pathogenesis of preeclampsia.
Recently the placental origin of pathophysiology of PE has been challenged.Follow up studies of women with PE has shown ample evidence for short-term and longterm cardiovascular changes suggesting maladaptation of the cardiovascular physiological changes in pregnancy as a possible etiology.Women developing PE demonstrate impaired cardiac output, an increase in systemic vascular resistance and left ventricular diastolic dysfunction especially in early-onset PE.There are persistent long term cardiovascular alterations and life-long increased risk of cardiovascular complications 10 .
Extensive epidemiological studies have identified various maternal and paternal risk factors to stratify a pregnant woman into high, moderate, or low risk of developing PE, thereby facilitating the prioritization of resources.A pregnant woman has increased risk of PE if the partner was a product of preeclamptic pregnancy (OR-2.1;95% CI: 1.0-4.3) 11.Donor insemination increases the risk of PE compared to natural insemination (OR-1.6;95% CI: 1.4-1.8) 12and change of male partner increases the risk of developing PE by 1.6% in the female partner in the subsequent pregnancy 13 .Table 1 summarizes previously described risk factors 14 .
Currently, PE is a clinical diagnosis based on a criterion adopted from the ISSHP definition (Figure 1).Early screening and diagnosis of preeclampsia (prior to the onset of clinical syndrome) has been challenging due to the limitations of existing predictive models (which lack good positive predictive value).

Risk factor
Mean RR 95% CI High/Moderate risk 15 History of PE in the past 7.19

Screening for preeclampsia
In thentext, screening refers to the utilization of a procedure or test on individuals, who do not exhibit any symptoms of a specific disease, with the aim of assessing the probability of them being affected by the disease.The screening procedure itself is not diagnostic of the illness.Patients who have a positive screening test result will need further evaluation with subsequent gold standard diagnostic test or procedure to confirm or rule out the diagnosis.
In 1968, Wilson and Jungner published "Principles and Practice of Screening for Disease", that presented 10 principles that govern the decision of screening for a disease (Figure 2) 16 .
As previously highlighted, PE poses significant health burden causing short-and long-term adverse outcomes in mother and the fetus.PE is diagnosed based on clinical criteria and once the clinical syndrome has set in, the diagnosis is straight forward.Once PE is diagnosed, there are well established guidelines for managing the disease.The pathogenesis of PE is believed to start early in pregnancy and thus leaves a considerable latent period prior to the onset of clinical syndrome.Currently, the screening for PE in Sri Lanka is based on clinical risk factors and are checked via history and examination, and these clinical methods are widely acceptable to the Sri Lankan population.
Even though the pathophysiology of the disease is not well understood to date, it is evident that the placental pathology begins early in first trimester followed by a latent phase until the mid-second trimester when the clinical syndrome becomes apparent (>20 weeks).The well-established clinical diagnostic criteria clearly differentiate preeclamptic women from normal pregnancy.Case finding for PE in Sri Lanka involves dayto-day clinical work and it is a continuous process.Thus, PE fits the principles of screening, and by employing an effective screening strategy, the resources could be prioritized to the high-risk pregnancies as opposed to low-risk pregnancies.
An effective screening test should have high sensitivity and specificity.Sensitivity measures how accurate the screening test is in identifying disease in people who truly have the disease and specificity focuses on the accuracy of the screening test in correctly classifying truly non-diseased people.Positive predictive value measures the probability of a subject with a positive screening test to truly have the disease and negative predictive value measures the probability of a subject with a negative screening test to truly not have the disease (Figure 3).

AND
Proteinuria: ≥ +1 in dipstick analysis initially, followed by protein/creatinine ratio ≥ 30mg/mmol in spot urine.1.The condition should be and important health problem.

OR
2. There should be an accepted treatment for patients with recognized disease.
3. Facilities for diagnosis and treatment should be available.
4. There should be a recognizable latent or early symptomatic phase.
5. There should be a suitabble test or examination.
6.The test should be acceptable to the population.
7. The natural history of the condition, including development from latent to declared disease, should be adequately understood.
8.There should be an agreed policy on whom to treat as patients.
9. The cost of case-finding (including a diagnosis and treatment of patients diagnosed) should be economically balanced in relation to possible expenditure on medical care as a whole.
10. Case-finding should be a continuous process and not a "Once and for all" project.

Recent developments in screening and diagnosis of preeclampsia
In the current context, risk stratification for preeclampsia involves predictive tools based on clinical and biochemical features and measurement of soluble factors in maternal blood that are proven to be differentially expressed in preeclamptic women.

PE risk calculator
Fetal Medicine Foundation (FMF) has compiled an online PE risk calculator, which calculates risk of a pregnant woman in her first trimester to develop PE based on clinical and biochemical parameters.Internal validation of FMF risk calculator showed a detection rate of 90% and 75% for the prediction of PE cases <34 weeks and <37 weeks respectively 17 .PRSEIDE trial independently validated the calculator to predict 77.4% and 66.8% of cases with PE cases <34 weeks and <37 weeks respectively 19 .The risk calculator could be accessed online via: https://fetalmedicine.org/research/assess/preeclampsia/first-trimester

Measurement of soluble factors
Recent studies have found that soluble factors such as placental growth factor (PIGF), soluble fms-like tyrosine kinase-1 (sFlt-1) and endoglin play an important role in maternal physiology and pathology.
Placental growth factor is part of the vascular endothelial growth factor family and is responsible for angiogenesis.PlGF is predominantly released by placental tissue.Even though PlGF is expressed in other tissues such as heart, lung, thyroid gland, liver, skeletal muscle and bone, the relative expression was comparatively low compared to placental expression.Circulating level of PIGF was found to be reduced before the onset of symptoms in preeclampsia and was considered as an early screening biochemical test 20 .
Soluble fms-like tyrosine kinase-1 is an alternatively spliced and truncated version of vascular endothelial growth factor receptor-1 (VEGFR-1).VEGFR-1 has (i) an extracellular ligand binding site for the proangiogenic factors-vascular endothelial growth factor (VEGF) and PlGF; and (ii) a transmembrane and cytoplasmic domain that regulates intracellular changes when the respective ligands bind the receptor.In contrast, sFlt-1 contains the extracellular ligand binding domain but lacks the transmembrane and cytoplasmic portions.Therefore, sFlt-1 can bind and reduce the circulating levels of both VEGF and PlGF.
In PE, increased level of sFlt-1 is associated with decreased levels of free VEGF an PlGF resulting in a net anti-angiogenic state causing endothelial dysfunction 8,[23][24][25] .Soluble fms-like tyrosine kinase-1 (sFlt-1) is found to be elevated and levels of free circulating VEGF and PlGF were found to be lower several weeks prior to the onset of clinical features in PE 26 .
A multicenter case control study in Europe measured maternal serum concentrations of sFlt-1 and PlGF and reported that the concentrations of the soluble factors clearly differentiated women with PE from controls.The sFlt-1/PlGF ratio performed best (AUC 0.95) in the diagnosis of all PE compared to the sole measurement of sFlt-1 (AUC 0.91) or PlGF (AUC 0.92) (p<0.05).A cut off value of 85 resulted in the best sensitivity (82%) and specificity (95%) for the diagnosis of all PE and for the diagnosis of early-onset PE (89% and 97%, respectively) 27 .On the other hand, a sFlt-1/PlGF ratio of 38 or lower was able to rule out PE in the following 7 days with a negative predictive value (NPV) of 99.3% (95% CI 97.9-99.9)with 80% sensitivity (95% CI, 51.9-95.7)and 78.3% specificity (95% CI, 74.8-81.7) 28.The ability of high NPV of the lower ratio to effectively rule out the risk of developing PE in a suspected woman with preeclampsia, enables the clinician to prioritize resources while simultaneously reassuring the patient.
Soluble endoglin (sEng) is a coreceptor for transforming growth factor -beta 1 (TGFβ1) and -beta 3 (TGFβ3), which is highly expressed on the cell membranes of vascular endothelial cells and syncytiotrophoblasts 29 .Soluble endoglin inhibits TGFβ1 signaling in vasculature and thus is anti-angiogenic.Placental expression of endoglin is up regulated in PE releasing more sEng into the maternal circulation 30 .
Levine et al reported that circulating sEng levels markedly increased 2 to 3 months prior to the onset of preeclampsia 31 .Recently, it was reported that serum level of sEng was significantly elevated in PE compared to normal pregnancy (P<0.001) and a cut off value of 20.4 effectively distinguished PE from normal pregnancies (sensitivity of 92.1% and specificity of 90%) 32 .Increased level of sEng was usually accompanied by an increased ratio of sFlt-1:PlGF and the risk of developing PE was greatest among women with highest quartile of the control distributions for both biomarkers 31 .
Leptin, pappalysin A1 (PAPPA-A1), human chorionic gonadotrophin-beta (β-hCG), alfa feto protein (AFP) and uric acid are few other circulating factors that are being investigated as potential markers for early screening and diagnosis of preeclampsia.Large scale studies and trials are needed to validate the above markers as effective screening tools.

Future direction in screening for preeclampsia
Extracellular vesicles (EVs) are membrane bound biomolecules released by various cells, and they carry various proteins and genetic material as its cargo.The syncytiotrophoblast layer of the placenta releases EVs (STB-EVs) into the maternal circulation from early first trimester and the amount of EV release increases as the pregnancy advances 33 .Preeclamptic placenta is known to release more STB-EVs compared to normal pregnancy 34 .STB-EVs in preeclampsia have been shown to regulate distant cells (such as endothelial cells) by depositing the contents of their cargoes causing vascular dysfunction, platelet activation, vasoconstriction, and endothelial activation [35][36][37][38] .
Recently, the differential expression of cargoes carried within STB-EVs in PE and normal pregnancy have been explored and it could be the future of early screening, diagnosis, and monitoring disease prognosis 39,40 .EVs in general have been already used in the diagnosis of breast, pancreatic, and colorectal cancers [41][42][43][44] .

Intervention
A pregnant woman deemed high risk for developing PE based on clinical risk factors is started on low dose aspirin (75mg-150mg) every night from 12 weeks until delivery of the baby.ASPRE trial confirmed that administration of aspirin reduced the incidence of preterm preeclampsia by 62% if started before 16 weeks, when compared with placebo 45,46 .
The World Health Organization recommended daily dosage 1.5g to 2.0g calcium supplementation to prevent preeclampsia.Sri Lankan guidelines recommended daily intake of 1g of calcium, considering that routine calcium supplementation during pregnancy in Sri Lanka is 600 mg 18 .
Vitamin D supplementation in prevention of preeclampsia is an area of controversy.In 2017, Sasan, et al. showed that vitamin D supplementation in first trimester contributed in preventing recurrence of preeclampsia (p<0.036) 47.But this evidence was not strong enough to be generalized to the general population and warrants further research.

Conclusion
Sri Lanka is in par with most of the developed countries when it comes to diagnosing and managing preeclampsia.The recent advances in screening and ruling out possibility of developing preeclampsia in a woman suspected with symptoms suggestive of preeclampsia is an area that we need to explore and invest.Incorporating predictive tool or biochemical investigations in conjunction with the existing clinical risk categorization to effectively stratify the risk will help the clinicians to prioritize resources to the neediest patients while reassuring the low-risk women.

Figure 1 .
Figure 1.Diagnostic criteria for preeclampsia based on ISSHP guidelines.