Authors: Ippokratis Sarrisa, Michail Litosa, Susan Bewleya, Iheanyi Okpalab, Paul Seedc, Eugene Oteng-Ntima*.
a Department of Obstetrics and Gynaecology, Guy’s & St Thomas’ NHS Trust, London, UK.
b Department of Haematology, Guy’s & St Thomas’ NHS Trust, London, UK.
c Division of Reproduction and Endocrinology,
King's College London School of Medicine, London, UK.
* Correspondence:
Mr Eugene Oteng-Ntim,
Department of Obstetrics and Gynaecology,
Guy’s & St Thomas’ NHS Trust, London, SE1 7EH, UK.
E-mail: Eugene.oteng-ntim@gstt.nhs.uk
Fax: 0044 (0) 20 7188 6855
In pregnant women with sickle cell disease, the steady state platelet cell count early in pregnancy is higher in those who subsequently develop disease related complications.
Sickle cell disease, pregnancy, complications, platelets.
Recent evidence implicates the immune system and the clotting mechanisms in the pathophysiology of sickle cell disease (SCD). This study investigates the association of steady state platelet count with the severity of SCD in pregnancy. Forty SCD women who were asymptomatic early in pregnancy were studied retrospectively: 14 remained asymptomatic throughout pregnancy and 26 developed at least one SCD-related complication. The early pregnancy platelet count was compared between the two groups using t-test and its ability to predict SCD-related complications in pregnancy was investigated using Receiver-Operator Characteristics (ROC) curve. Compared to asymptomatic patients, women who developed SCD-related complications had significantly higher early-pregnancy platelet count [328x109/L (95%CI: 268-389) versus 210x109/L (146-275), p<0.01]. The area under the ROC curve was 76.4% (95%CI 59.7–93.2). These indicate that the platelet count in early pregnancy is significantly higher in SCD patients who subsequently develop SCD related complications and may be used for screening.
As the life expectancy of patients with sickle cell disease (SCD) rises, a significant number of affected women carry pregnancies. Pregnancy is a well documented precipitating factor of sickle cell crises. These complicate about 30-88% of SCD pregnancies with SS genotype and 30% with SC genotype (Khare & Bewley 2003). Whilst prior to 1970 maternal mortality was 30-40% (Fort et al. 1971), with the advent of health provision this has dropped to 2% (range 0-9%) (Khare & Bewley 2003); this is still significantly higher than that of the general non-SCD pregnant population.
Recent research in SCD has focused on blood cells other than the erythrocyte. Leucocytes have been implicated in the pathogenesis of the vaso-occlusive complications of SCD (Okpala 2004). It has been shown that leucocytosis at steady state is a risk factor for increased incidence of vaso-occlusive episodes (Okpala 2004, West et al. 1992). The majority of studies exclude pregnant patients in order to avoid confounding factors. A study from our group has shown the above to be true for pregnant SCD patients as well (Litos et al. 2006).
The role of platelets in the manifestations of SCD and their role in vaso-occlusive episodes has been a matter of debate. Some researchers suggest that platelet count and volume is increased in SCD during steady state versus non-SCD controls and that both of these measures decrease in vaso-occlusive episodes (Freedman & Karpatkin 1975). Others have reported normal sized platelets with increased count in steady state (Berney et al. 1992). Despite these reports, platelet count at steady state has not been shown to predict subsequent SCD-related morbidity and mortality (Okpala 2002). Once again, pregnant women were excluded.
Pregnancy causes various physiological changes that are relevant to SCD such as increased plasma volume and red cell mass, decreased peripheral resistance, a procoagulant state, increased susceptibility to urinary tract infections (Khare & Bewley 2003) and a drop in platelet number during the last few weeks of pregnancy (Fay et al. 1983). Platelet count in pregnancies complicated by maternal SCD is poorly researched.
The aim of this study was to investigate whether the steady-state platelet count early in pregnancy is associated with the clinical severity of known SCD-related complications later in pregnancy.
The retrospective study was carried at the maternity unit of Guy’s and St Thomas’ NHS Foundation Trust, a London teaching hospital with a multi-ethnic population with high prevalence of SCD. Women with SCD are enrolled into antenatal care late in the 1st trimester or early in the 2nd, and are closely monitored by a multidisciplinary team consisting of obstetricians, haematologists, specialist nurses and midwives, with access to a consultant obstetric physician. There is low threshold for admission in case of SCD-related clinical problems.
Pregnant women with SCD who had no other co-morbidities and were asymptomatic when first seen early in pregnancy were included in the study. None of them where on a prophylactic exchange transfusion programme. Information was obtained from the patients’ files and the hospital’s laboratory electronic records. The total platelet counts early in pregnancy, as well as the gestational age, the use of any medication, the genotype and any potential risk factors were recorded. The notes were scrutinized for any SCD-related complications during pregnancy. Complications associated with SCD were defined as hospital admissions for SCD crisis, chest symptoms, antenatal transfusions (all causes), thromboembolic events, pre-eclampsia and urinary tract infections. Women who subsequently developed complications unrelated to SCD were excluded.
The population was divided into women who remained asymptomatic throughout pregnancy, and those who developed at least one SCD-related complication. The mean platelet count at early pregnancy for these two groups was calculated and compared using t-test. Linear regression analysis was performed in order to investigate if platelet count correlated with the absolute number of complications. The two groups were controlled for SS / SC genotypes and for prophylactic antibiotic cover. Early pregnancy steady-state platelet count was assessed as a potential predictor of SCD complications during pregnancy by calculating the sensitivity and specificity for different values of platelet count to predict the likelihood of developing SCD complications. A Receiver Operator Characteristics (ROC) curve was constructed to plot the sensitivity of the test against the specificity (expressed as 1-specificity) at each value of platelets across the range of platelets values. Stata computer programme (StataCorpLP, Texas, USA) was used for the statistical analysis and statistical significance was defined as p<0.05.
Forty one women fulfilled the inclusion criteria; one was excluded for developing obstetric cholestasis (n=40). The mean age was 29.9 years (range 19-40). 23 patients had HbSS genotype and 17 had HbSC. The mean gestational age at the time of entry into antenatal care was 14.3 weeks (range 6-20). All pregnancies were singleton. All women were on oral Folic Acid 5mg once daily and 18 were on prophylactic oral penicillin V 250mg once daily and remained on it throughout their pregnancies.
Of the 40 women, 26 (65%) developed at least one SCD complication during pregnancy (95%CI 48.3%-79.4%), while the remaining 14 were asymptomatic. Eighteen women had painful crises (total of 32 crises leading to hospital admission), 11 women developed chest symptoms (total of 15 cases), 5 women developed a urinary tract infection (total of 6 infections), 14 women required a transfusion (total of 26 transfusion episodes – none of which were exchange transfusions), 1 woman developed a thromboembolism and 5 developed pre-eclampsia. The two groups were controlled for genotype and use of prophylactic antibiotics; both SC/SS genotype and antibiotic prophylaxis were equally distributed in the two groups and were not confounding the results. Women who developed SCD-related manifestations were found to have significantly higher mean platelet count early in pregnancy than those who remained asymptomatic [328x109/L (95%CI: 268-389) versus 210x109/L (95%CI: 46-275), p<0.01]. The linear regression analysis failed to prove any statistically significant correlation between platelet count and the number of SCD complications during pregnancy.
The area under the ROC curve (figure 1) was 76.4% (95%CI: 59.7–93.2). This value represents the overall probability that SCD-related complications will be correctly predicted by platelet count measured early in pregnancy. If sensitivity is set to >90% in order to minimize the false negatives, a cut-off value for platelets of 190x109/L is obtained. This value gives sensitivity 92.0% (95%CI 74.0-99.0), specificity 57.1% (28.9-82.3), positive predictive value 79.3% (60.3-92) and negative predictive value 80% (44.4-97.5).
Sickle cell disease has traditionally been attributed to red cell dysfunction due to a point mutation in the haemoglobin molecule that causes sickling of the erythrocytes (Steinberg & Rodgers 2001). There is increasing evidence that there is more complex interaction between red blood cells, other blood cells and the vascular endothelium with mediation from different adhesion molecules. This leads to microvascular occlusion and clinical manifestations. Although the contribution of white cells in vaso-occlusive episodes is now well established (Okpala 2004) the role of platelets is still unclear. Thrombogenesis has been implicated in SCD pain episodes (Francis & Johnson 1991). We know that platelets contribute to thrombogenesis at sites of vascular damage through initial attachment and subsequent activation via thromboxane A2 generation, secretion of ADP granules and thrombin cleavage by platelet thrombin receptors (Colman et al 1994). A further theory for the involvement of platelets in the pathogenesis of vaso-occlusion is that platelet adherence causes monocyte activation, leading to decrease of the effective diameter of the lumen of the microvasculature and hence trapping of red cells (Wun et al. 2002). Up until now it has not been demonstrated that thrombocytosis at steady state can predict the severity and frequency of subsequent SCD crises out of pregnancy (Okpala 2002). However, patients with SCD do demonstrate increased platelet activation during clinically quiescent periods and, during crises, platelet activation is amplified even further (Tomer et al. 2001, Wun et al. 1998). Furthermore, it has been shown that the degree of activation at steady state correlates with the subsequent number of crises (Tomer et al. 2001).
During pregnancy platelet count is steady up to 20 weeks gestation and then gradually drops to an average of 280x109/L during the last 8 weeks (Fay et al. 1983, Nelson-Piercy 2006). Furthermore, mean platelet volume significantly rises in the last 4 weeks of pregnancy and platelet volume distribution width rises constantly throughout. Since younger platelets are larger than older ones, these findings indicate that during pregnancy there is hyperdestruction of platelets (Fay et al. 1983). There is little data on the effect that platelet changes have on the clinical course of SCD during pregnancy.
In the present study, 65% of women developed SCD-related complications of the disease during pregnancy. Compared to asymptomatic women, the patients who developed SCD-related clinical problems were found to have significantly higher platelet counts at the time of enrolment into antenatal care. In our definition of SCD-related complications we included pre-eclampsia and urinary tract infection because the risk for these is increased in pregnancies affected by SCD and they are significant causes for poor outcome via prematurity. In addition, in cases where these might have occurred irrespective of the presence of SCD, they constitute predisposing factors to sickling and therefore are useful as a proxy measures of the severity of SCD. Women with SC/SS genotype and women on prophylactic antibiotics were equally distributed in the two groups, so these factors did not confound the results. The ROC curve analysis showed that steady-state platelet count early in pregnancy is related to the prognosis of SCD and this could be an additional useful screening tool, subject to the limitations of the study. The area under the ROC curve (76.4%) was statistically significant although the confidence intervals were wide (59.7%–93.2%). Further studies in larger populations and at more time points during pregnancy could confirm and improve the performance of the test.
The reason that a correlation between high platelet count at steady state and severity of SCD has not been found out of pregnancy might be explained by the physiological changes of pregnancy on the haemopoietic system such as the procoagulant state (Khare & Bewley 2003) and the hyperdestruction of platelets (Fay et al. 1983). When these pregnancy related changes are coupled with a higher steady state platelet count the chance of thrombogenesis, leading to microvascular occlusion and hence a sickle cell crisis, might be amplified. It is also possible that the increased platelet count might not be the cause of the clinical manifestations observed but instead be the result of a separate underlying confounding factor. For example, patients that subsequently develop SCD complications might have an ongoing undiagnosed subclinical infection, as they have also been shown to have high white cell counts (Litos et al. 2006). This could be the cause for both the higher platelet count and the subsequent clinically evident crises. Even so, our current management cannot detect any such factor. There may be many genetic and environmental factors which cause the increased risk of SCD-related complications in pregnancy. For example, the level of HbF can influence the degree of subsequent morbidity during the pregnancy with high levels being protective. This, however, was not routinely measured at our booking visit. The platelet count may be an additional valuable clinical tool for triaging SCD patients early in pregnancy and increasing clinical vigilance in the higher risk group.
In recent years there has been increasing understanding of the multifactorial pathophysiology of SCD. This study observed an association of high steady-state platelet count with the severity of SCD in pregnancy. It is generally recommended that all women with SCD are managed as high risk during pregnancy. Our findings show that a high platelet count early in pregnancy can be added to the predictors of SCD complications and should increase vigilance but a low platelet count cannot be considered as reassuring. Further research may help to improve our understanding and management of SCD in pregnancy.
Berney SI, Ridler CD, Stephens AD, Thomas AE, Kovacs IB. 1992. Enhanced platelet reactivity and hypercoagulability in the steady state of sickle cell anaemia. American Journal of Haematology 40(4): 290-4.
Figure 1. Receiver operator characteristics (ROC) curve of performance of steady state platelet counts (PLT) early in pregnancy as a predictor of sickle cell disease complications during pregnancy. The area under the ROC curve is 76.4% (95%CI: 59.7–93.2). Numbers are PLT in x109/L.
