There was a significant progressive increase in both systolic and diastolic blood pressure in normal pregnancy (P=.001 and P<.001, respectively). Compared with baseline, there was a nonsignificant increase in diastolic blood pressure at 20–24 weeks and 27–32 weeks of gestation. However, the subsequent changes were significant at 34–38 weeks (P<.001) and at 5–15 weeks postpartum (P<.001) (Table 3; Fig. 1A and B).
Compared with baseline at 11–16 weeks of gestation, the increase in systolic and diastolic blood pressure was nonsignificant at all visits in women who developed preeclampsia (Table 3; Fig. 1A and B). It is important to stress here that the blood pressure changes analyzed are those taken at the fixed predetermined visits (27–32 weeks and 34–38 weeks of gestation), which do not reflect the higher readings when preeclampsia was actually diagnosed, because then these women did not attend for their designated visits. This introduces the problem of missing data and analysis of blood pressure taken either before the onset of preeclampsia, or in the puerperium when the blood pressure had normalized. This explains the apparent lack of a significant rise in systolic and diastolic blood pressure in a group of women who nevertheless developed preeclampsia.
Compared with the baseline visit at 11–16 weeks of gestation, the earliest statistically significant fall in mean basal capillary density occurred at 34–38 weeks of gestation (P=.009). At the postpartum visit, the changes were not statistically significant. The earliest statistically significant decrease in maximal capillary density occurred at 34–38 weeks of gestation (P=.017). At the 5–15 weeks postpartum visit, the mean decrease was not statistically significant. (Table 3; Fig. 2A and B).
The mean onset of preeclampsia was at 35.6±4.8 weeks of gestation (range. 27.0–41.5 weeks). The changes in basal capillary density were not significant throughout pregnancy or in the postpartum visit. However, when compared with the baseline visit, there was a significant reduction in maximal capillary density (ie, structural rarefaction) at the 20- to 24-week visit (mean change 7.0 capillaries per field; 95% confidence interval −12.6 to −1.4; P=.015), 7–21 weeks before the onset of preeclampsia. There was further significant rarefaction at 27–32 weeks of gestation (P=.009) and at 34–38 weeks of gestation (P<.001) (Fig. 2B). In contrast to normal pregnancy, significant reduction in maximal capillary density persisted in the postpartum visit.
The changes in basal capillary density in women who went on to develop preeclampsia were not significant compared with women who had a normal pregnancy. We could not rule out the possibility that we have not seen significant differences because of the small number of participants with preeclampsia. However, maximal capillary density was significantly lower at 27–32 weeks of gestation (P=.003), at 34–38 weeks of gestation (P=.03), and at the postpartum visit (P=.02).
In normal pregnancy, serum VEGF receptor 2 levels increased significantly at 27–32 weeks of gestation (P=.02) and then reduced at 34–38 weeks of gestation but remained significantly higher compared with the first visit (P=.04). In preeclampsia, the increases from baseline in VEGF receptor 2 levels at 27–32 weeks of gestation and at 34–38 weeks of gestation were not statistically significant (Table 3).
The increase from baseline in serum fms-like tyrosine kinase 1 at 27–32 weeks and 34–38 weeks of gestation was not significant when compared with the first visit in normal pregnancy and in women who developed preeclampsia. In normal pregnancy, serum soluble Endoglin did not change significantly from baseline to 27–32 weeks of gestation but increased significantly at 34–38 weeks of gestation (P=.04). The changes in soluble Endoglin in pre-eclamptic pregnancies were not significant. Again we could not rule out the possibility that we have not seen significant differences at other visits because of the small number of participants who developed preeclampsia. There was no significant difference in VEGF receptor 2, fms-like tyrosine kinase 1, and soluble Endoglin levels between the two groups at 11–16 weeks and 27–32 weeks of gestation. At 34–38 weeks of gestation, VEGF receptor 2 levels were significantly lower (P=.05), whereas fms-like tyrosine kinase 1 and soluble Endoglin levels were significantly higher (P<.001 and P=.004, respectively) in women who went on to develop preeclampsia. The change in soluble Endoglin from 11–16 weeks to 27–32 weeks of gestation was significantly related with the change in maximal capillary density in the same period (Pearson correlation coefficient −0.45, P=.02), indicating that the higher the increase in soluble Endoglin, the greater the loss of structural capillary density. There was no correlation between levels of fms-like tyrosine kinase 1 or VEGF receptor 2 and other capillary measurements (P=.154 and P=.108, respectively).
This study shows that significant structural capillary rarefaction precedes the onset of preeclampsia. Women who went on to develop preeclampsia had a significant (P=.015) 9% reduction in their structural capillary density by 20–24 weeks of gestation, which progressed to 17% by 27–32 weeks of gestation and reached a nadir of 20% by 34–38 weeks of gestation. Our observations suggest that the earlier and more pronounced capillary rarefaction may play a role in the pathophysiology of preeclampsia. Our findings therefore not only corroborate our previous report that capillary rarefaction is associated with preeclampsia,4 but have advanced our knowledge in establishing the temporal relationship that structural capillary rarefaction precedes preeclampsia. These findings further support the concept of widespread maternal microcirculatory abnormalities that precede the onset of preeclampsia.11 We further substantiate previous studies which suggest that preeclampsia is an exaggerated antiangiogenic state12 with the structural capillary rarefaction and the resultant loss of endothelial surface and reduction of nitric oxide.13
Although the pathophysiological basis of preeclampsia remains uncertain, the consistent histopathologic lesion is defective and inadequate cytotrophoblastic invasion of spiral arterioles.14 In response to the hypoxia exaggerated by capillary rarefaction, the placenta secretes soluble factors into the maternal vasculature, which in turn induce widespread endothelial dysfunction and the clinical features of preeclampsia.15 One of the factors thought to be important is the naturally occurring antiangiogenic molecule soluble fms-like tyrosine kinase-1, also known as soluble VEGF receptor 1,16 which has been shown to be produced in large amounts by placental trophoblasts in preeclampsia17 and released into the maternal circulation.18,19 It acts as a potent antiangiogenic molecule by binding circulating VEGF and placental growth factor.20 Increased antiangiogenic activity in response to placental ischemia may seem contradictory, because teleologically one would expect increased angiogenic activity not least because VEGF, a proangiogenic molecule, is also upregulated by hypoxia.21 Cytotrophoblasts, in contrast to endothelial cells, respond in opposite ways to hypoxia in terms of angiogenic balance because they induce excess production of fms-like tyrosine kinase 1 and a deficiency of VEGF.17 However, the situation is undoubtedly complex and other factors may well play a role too. For instance, we have previously reported that placental catecholamine release is increased in preeclampsia,22 and we postulated that in response to hypoxia, the placenta releases a signal (catecholamine) recognizable by the maternal vasculature to raise blood pressure and improve perfusion of the fetoplacental unit and that what starts as a physiological response becomes pathologic because the rise in blood pressure fails to correct the hypoxia.22
An additional remarkable finding in our study was that although the reduction in capillary density recovered to baseline levels within the puerperium in women who remained normotensive, in those who developed preeclampsia, there was no resolution of the capillary rarefaction. This observation appears to be consistent with the reports of long-term risk of endothelial dysfunction, hypertension, and cardiovascular disease in women who develop preeclampsia23,24 and the observations that capillary rarefaction is a hallmark of essential hypertension.25,26 The strong familial predisposition to preeclampsia, essential hypertension, and atherosclerosis suggests that there may be common risk factors, pathways, or both for these disorders.27 The mechanisms and consequences of microvascular rarefaction are not fully understood but could possibly increase peripheral vascular resistance either directly through the reduction of the total cross-sectional area of the terminal vasculature or indirectly through inducing renal ischemia and stimulation of the renin–angiotensin system. Microvascular rarefaction is not confined to the skin but is a widespread phenomenon affecting several tissues in hypertensive individuals including the myocardium, the kidneys, and the brain.10,28,29 In support for a possible role of microvascular rarefaction in increasing blood pressure is the current realization that angiogenesis inhibitor drugs induce hypertension in a significant proportion of patients and this increase in blood pressure can be related to or even be explained by capillary rarefaction and alteration in endothelial function in the whole systemic vascular network.30 More recently it has been shown in preclinical studies that these drugs can cause a preeclampsia-like syndrome further supporting a role for the capillary microcirculation in the pathogenesis of this syndrome.31
We acknowledge the limitations of our study that include the relatively small number of women who developed preeclampsia and the fewer numbers of observations as a prospective study progresses. However, the small number of women who developed preeclampsia represented 4% of our study population, a reflection of the prevalence of preeclampsia (3–7%) in populations worldwide. A longitudinal study design overcomes the issue of small numbers because the variability between individuals, which would require large numbers to compensate in a cross-sectional study, is minimized by repeated measurements in a given individual. In addition, this longitudinal study design is arguably the only effective way of examining temporal relationships in observational studies.
In conclusion, significant structural capillary rarefaction precedes the onset of preeclampsia and may therefore play a role in the pathogenesis of this disease. Our results substantiate the concept of widespread maternal microcirculatory abnormalities that precede the onset of preeclampsia.
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