Abnormalities of coagulation and platelet function in preeclampsia

INTRODUCTION — Hypertension is a common complication of pregnancy, occurring with a frequency of 10 to 15 percent. Its development is due to a number of different and distinct etiological and pathophysiological mechanisms, many of which are poorly understood. Preeclampsia complicates about 5 to 10 percent of hypertensive pregnancies and remains a major cause of maternal and neonatal morbidity and mortality .  

Normal pregnancy is associated with a transient hypercoagulable state, which most likely evolved in response to the dangers of post-partum hemorrhage. This prothrombotic state is balanced by adaptations in the fibrinolytic pathway. Activation of these systems may be more marked in the uteroplacental circulation than in the systemic circulation in both normotensive and preeclamptic pregnancies .

When a major pathological crisis occurs in pregnancy, such as eclampsia and preeclampsia, this delicate balance is disturbed. In addition to hypertension, there appears to be excess platelet activation, as well as endothelial damage and dysfunction, with the consequences of thrombosis, low birth weight, fetal loss, and maternal morbidity and mortality.

The aim of this topic review is to summarize the changes in hemostasis through a normal pregnancy and puerperium and to discuss the abnormalities in coagulation and platelets during disorders of hypertension in pregnancy. The prevention, screening, and clinical manifestations of preeclampsia, including discussions of endothelial dysfunction, are discussed separately.  

CHANGES IN HEMOSTASIS DURING NORMAL PREGNANCY — The processes involved in hemostasis include complex interactions that occur between its three main components: platelets, the endothelium, and the coagulation cascade [3]. The fibrinolytic system has a complementary role, and is responsible for the removal of fibrin and clot dissolution. These processes interact to ensure that the circulating blood flows freely in the vascular bed and that bleeding is quickly arrested following trauma.  

In pregnancy, however, the demands made on the hemostatic and fibrinolytic systems differ markedly from the non-pregnant state. There is a marked increase in some of the coagulation factors, as well as changes in endothelial function, platelet activity, and fibrinolysis, which result in a hypercoagulable state. During delivery, placental separation presents a serious hazard of hemorrhage; adaptations are needed to combat this danger. However, as a result of these adaptations, there is also an increased risk of intravascular clotting during pregnancy, which is expressed as an increased risk of thromboembolism.  

Coagulation and fibrinolysis — Activation of the coagulation cascade results in the formation of an insoluble fibrin clot from soluble fibrinogen (figure 1).

The activity of the coagulation cascade is inhibited by naturally occurring anticoagulants (eg, antithrombin, protein S, and protein C), and is balanced by the fibrinolytic system, in which fibrin and fibrinogen are degraded by plasmin, an enzyme formed from the inactive plasminogen  

Fibrinogen is the final component of the common coagulation pathway. Elevated fibrinogen levels are seen in normal pregnancy in spite of the increased plasma volume .  Indeed, the amount of fibrinogen available to the pregnant woman at delivery is often double the amount present in the non-pregnant state. There are also changes in other clotting factors, with significant increases in von Willebrand factor, factors VII, VIII, X, XII, and prothrombin , which can be detected from the third month of gestation.  

In one study of 100 women with uneventful pregnancies, an increase in activated factors XII and VII was noted, suggesting that both the intrinsic and extrinsic coagulation pathways were activated . Further evidence that the coagulation system is activated in pregnancy is provided by the presence of increased levels of cleavage products. For example, prothrombin is cleaved by prothrombinase into thrombin and prothrombin fragments 1 and 2. The thrombin thus generated cleaves fibrinogen into fibrin monomers and fibrinopeptides A and B. Prothrombin fragments 1 and 2 and fibrinopeptides A and B are increased in normal pregnancy. During the course of a normal pregnancy, thrombin-antithrombin [TAT] complex levels also increase significantly, providing further evidence of thrombin activation .

In contrast to the increases in coagulation factors and an increased ability to form thrombin, there appears to be little, if any, evidence for consistent increases in the levels of physiological anticoagulants. For example, one study noted a significant fall in the activity of proteins C and S during pregnancy . By contrast, a significant fall only in free protein S was seen in a second study  , while others have found an increase in both these factors, with a concurrent fall in antithrombin levels .

The changes seen in hemostatic factors are nevertheless accompanied by changes in the fibrinolytic pathway during normal pregnancies. For example, plasminogen and tissue-type plasminogen activator antigen levels rise, resulting in an increased potential to generate plasmin, which cleaves the polymerized fibrin strand at multiple sites and releases fibrin degradation products (FDPs)  

Alpha-2-antiplasmin levels also rise during pregnancy, partly due to a placental-derived plasminogen activator inhibitor, thus potentially limiting plasmin generation and contributing to the prothrombotic state during pregnancy.

Fibrin D-dimer — In one study of 108 normal pregnancies, a significant elevation in fibrin D-dimer, an indicator of fibrin turnover and fibrinolytic activity, was found in the second and third trimesters . This increase in fibrin D-dimer has been widely confirmed  . Accordingly, the utility of a normal level of D-Dimer for helping to rule out VTE appears to fall progressively throughout the course of pregnancy.  

Platelets — The platelet count during pregnancy has been variably described as decreased  , unchanged  , or increased  . In a longitudinal study of changes in platelet count during normal pregnancy, the mean platelet count decreased, although there was a very wide variation, with increases in some individuals and no change in others  

This study also reported that the mean platelet volume increased during the course of pregnancy but, again, this finding was variable, and of uncertain significance.  

It has been suggested that platelet survival time may be decreased during normal pregnancy  . Plasma beta thromboglobulin and platelet factor 4 levels, both reflecting platelet activation, were significantly increased during normal pregnancy, indicating an increase in platelet activation, and supporting the hypothesis that there is an increased turnover of platelets during the progression of normal pregnancy . A second study confirmed the presence of platelet activation during the progression of a normal pregnancy, but did not find differences between those with a normal pregnancy versus those with preeclampsia  

Endothelium — The endothelium produces substances responsible for the regulation of vascular tone as well as products that influence hemostasis and platelet function. For example, prostacyclin [PGI2] is a potent vasodilator and inhibitor of platelet aggregation, whose effects are balanced by the vasoconstrictor and platelet aggregator, thromboxane. The latter is also produced by platelets, and in the uteroplacental circulation of normal pregnancy, it is believed that the production of prostacyclin is increased. The endothelium also produces nitric oxide, another potent vasodilator, whose action is countered by the vasoconstricting hormone, endothelin.  

The pathophysiology of preeclampsia likely involves both maternal and fetal/placental factors. Abnormalities in the development of placental vasculature early in pregnancy may result in relative placental underperfusion/hypoxia/ischemia, which then leads to release of antiangiogenic factors into the maternal circulation that alter maternal systemic endothelial function and cause hypertension and other manifestations of the disease. This subject is discussed in depth separately.

In a guinea pig model of pregnancy, the in vitro release of endothelium derived relaxing factor (EDRF, or nitric oxide) is increased. Treatment with 17-beta-estradiol increases endothelium-dependent relaxation in response to acetylcholine, a stimulator of nitric oxide synthase. This response was not affected by treatment with indomethacin, an inhibitor of prostaglandin production.

In the spontaneously hypertensive rat, an animal model of preeclampsia, pregnancy is associated with vasodilation and a profound decrease in blood pressure. This is due to nitric oxide release rather than prostacyclin production . Thus, nitric oxide, together with prostacyclin, plays an important role in the development and maintenance of the uteroplacental circulation as a low resistance, high flow system. It has also been suggested that reduced levels of inhibitors of the L-arginine nitric oxide pathway may play a role in the reduction of vascular tone in normal pregnancy .

In preeclampsia and pregnancy-induced hypertension (PIH), a cross-sectional study reported that women with PIH had significantly higher levels of plasma von Willebrand factor (VWF, 19 percent), E-selectin (40 percent) and thrombomodulin (61 percent) than normotensive women  . Only thrombomodulin was independently associated with PIH. This molecule could be useful in monitoring and in providing clues for etiology and pathophysiology, and may have implications for the clinical complications associated with PIH.

HEMOSTATIC CHANGES AFTER HYPERTENSION IN PREGNANCY — Alterations in coagulation, fibrinolysis, and platelet and vascular endothelial function are believed to play an important role in the pathogenesis of preeclampsia, which is characterized by placental hypoperfusion and infarction, fibrin deposition, consumptive thrombocytopenia, and coagulopathy  

Coagulation — In pregnancies complicated by preeclampsia, compared with normal pregnancy, there is a significant increase in thrombin-antithrombin [TAT] complex, in vitro thrombin generation, prothrombin fragment 1+2, and fibrin D-dimer levels, whereas antithrombin and fibrinogen levels were reduced . Increases in TAT-complex were also found in a longitudinal study of 36 primigravida, in which four subjects subsequently developed preeclampsia and 12 patients had established preeclampsia  . Such findings are of relevance, since TAT levels reflect the amount of thrombin formed in vivo and fibrin D-dimer levels reflect the presence of fibrinolysis.

Accompanying this increase in coagulation in preeclampsia, there is also a reduction in physiologic anticoagulants. In two studies, no variation in the level of protein C was seen during normal pregnancy ; a reduction in protein C levels in preeclampsia was seen in one study  , but not in the second .

Antithrombin levels do not appear to change significantly in normal pregnancy, but may be slightly reduced in preeclampsia, while protein S levels decreased in both normal and preeclamptic pregnancies. Other investigators have reported similar reductions of antithrombin levels in preeclampsia, compared with normal pregnancy and chronic hypertension in pregnancy  .

Abnormalities in tissue factor pathway inhibitor (TFPI) have also been reported in pregnant women, including those who subsequently develop preeclampsia . Other studies suggest that histidine-rich glycoprotein (HRG) and fibrinogen might be involved in the hypercoagulability and angiogenic imbalance seen in early-onset preeclampsia . Given that preeclampsia has been linked to angiogenic abnormalities, the presence of coagulation abnormalities could contribute to the pathophysiology of preeclampsia.

Fibrinolysis — The activity of the fibrinolytic pathway is also altered in preeclampsia  . In a study of 42 patients with preeclampsia, significant increases in tissue-type plasminogen activator were found, while levels of urokinase-type plasminogen activator were unchanged .

The two plasminogen activator inhibitors PAI-1 and PAI-2 increase progressively in normal pregnancy ; total PAI levels are found to be greater in preeclampsia than in normal pregnancies. This increase in PAI seems due to the PAI-1 fraction, as the levels of PAI-2 are decreased in pregnancies complicated by preeclampsia, particularly in those with a poor fetal outcome  . However, in one study PAI-2 levels were reduced in preeclamptic patients with extensive placental infarction but not in those without infarction . The increase in PAI-1 levels may be a reflection of endothelial damage or of platelet activation, both of which are believed to occur in preeclampsia, whereas the decrease in PAI-2 levels may reflect impaired placental function.

Platelet abnormalities — One of the most characteristic coagulation abnormalities in severe preeclampsia is a fall in the platelet count, probably due to consumption during low-grade intravascular coagulation  . In addition to changes in absolute platelet numbers, the platelets also appear to circulate in a more activated state  . One study compared platelets from 10 preeclamptic third trimester pregnant women with those from 10 normotensive third trimester pregnant women, using flow cytometry to determine platelet surface antigen expression  . Platelets from all the preeclamptic patients showed more extensive activation, as indicated by increased expression of P-selectin, CD63, and platelet endothelial cell adhesion molecule (PECAM)-1.  

A number of other studies support the hypothesis of increased platelet activation in preeclampsia:

  • In a longitudinal study, increased expression of platelet CD63 during the first trimester was a predictor of the subsequent development of preeclampsia  .
  • Soluble P-selectin levels increase in preeclampsia .
  • In one study, levels of beta-thromboglobulin were elevated in preeclampsia, which preceded the clinical manifestation of the disease  .
  • A cross-sectional study reported that women with pregnancy-induced hypertension had significantly raised plasma soluble P-selectin, platelet P-selectin, and beta-thromboglobulin when compared with normotensive pregnant women and controls  . Using a novel adhesion assay, platelet adhesion in patients with PIH was similar to that in the normotensive pregnancies, but both were higher than the normal controls. There was no difference among the three groups with respect to platelet mass or platelet volume. Platelet P-selectin and platelet adhesion correlated significantly with gestational age and with systolic and diastolic blood pressure.
  • Platelet abnormalities were also clearly evident in a comprehensive cross-sectional study in which activated platelets, platelet-monocyte/neutrophil aggregates, platelet microparticles (measured by flow cytometry), and four markers of thrombin generation capacity (endogenous thrombin potential (ETP), peak height, lag time and time to peak, using the Calibrated Automated Thrombogram system) were studied, showing marked evidence of both platelet and coagulation activation in women with preeclampsia .

Thus, increased platelet activation and adhesion develop during normal pregnancy, with some indices being further altered in pregnancy-induced hypertension.

CLINICAL IMPLICATIONS — Pregnancy is associated with changes in endothelial function, platelets, and the coagulation cascade that confer a transient hypercoagulable state. These changes normally represent a physiologic response for controlling the hemorrhage that occurs during placental separation at delivery. However, sometimes this physiologic response prog resses to a pathologic state, and results in an increased risk of thromboembolic phenomena during pregnancy.

In addition, hypertension is a common complication of pregnancy, and in its more severe forms (eg, preeclampsia and eclampsia), abnormalities of clotting and platelet function are pathologically manifest. Widespread damage to the endothelium in preeclampsia and eclampsia results in vasoconstriction and promotes platelet adhesion and aggregation as well as the activation of coagulation factors, resulting in further hypoxic damage to the endothelium. A clearer understanding of these conditions will allow appropriate use of antithrombotic therapy, if appropriate, and enable more effective treatments to be developed.

SUMMARY

  • Normal pregnancy is associated with a transient hypercoagulable state, balanced by adaptations in the fibrinolytic pathway.  
  • When eclampsia or preeclampsia occur, this balance is disturbed, along with platelet activation and endothelial damage and dysfunction  
  • Increase in markers of coagulation along with a reduction in physiologic anticoagulants.
  • The activity of the fibrinolytic pathway and endothelial function are altered, although in a complex manner.
  • The platelet count is decreased along with increased platelet activation.

 

  • The clinical consequences of the hemostatic changes seen in preeclampsia and eclampsia include thrombosis, low birth weight, fetal loss, and maternal morbidity and mortality.