In contrast to gestational hypertension and preeclampsia that occur in the second half of pregnancy, hypertension that is evident before 20 weeks' gestation is diagnosed as chronic hypertension. The Australasian and American classifications are consistent in their definitions of chronic hypertension, blood pressure >140 mmHg systolic and/or >90 mmHg diastolic pre-conception or in the first half of pregnancy (Anonymous, 2000; Brown et al., 2000). Rare exceptions include hydatidiform molar pregnancies, severe renal disease or antiphospholid syndrome and occasional chromosomal anomalies, where true pre-eclampsia can occur before 20 weeks. For women presenting with hypertension for the first time in early pregnancy, it is necessary to confirm that their blood pressure remains elevated at least
3 months after delivery. Chronic hypertension is subdivided into essential and secondary due to underlying causes such as renal disease (e.g. glomerulonephritis, reflux nephropathy, adult polycystic kidney disease), systemic disease with renal complications (e.g. systemic lupus erythema-tosus, diabetes), renal artery stenosis or endocrine disorders (e.g. cushings, phaeochromocytoma) (Brown et al., 2000).
The diagnosis of superimposed pre-eclampsia in the context of chronic hypertension, renal disease or diabetes is difficult. The signs (hypertension and proteinuria) used to make the diagnosis may already be present. Further, new or worsening hypertension or proteinuria can occur alone in these conditions, and do not indicate pre-eclampsia per se. As a consequence of these inherent difficulties, recent definitions do not offer very precise criteria for the diagnosis of superimposed pre-eclampsia (Anonymous, 2000; Brown et al., 2000). Superimposed pre-eclampsia is likely when there is a significant increase in blood pressure (>30/15 mmHg) in the second half of pregnancy associated with either the development of new proteinuria >0.3g24h_1 or a sudden increase (doubling) in prior proteinuria. Other supportive features of multisystem and fetal manifestations (see Table 17.1) are necessary to be secure that this is pre-eclampsia, not just pregnancy-induced aggravation of signs present because of the underlying medical condition (Brown et al., 2000). Given the difficulties diagnosing superimposed pre-eclampsia, and the progressive nature of pre-eclampsia toward severe maternal and fetal morbidity, surveillance should be increased in cases of suspected superimposed pre-eclampsia.
In the last two decades there have been significant advances in understanding pre-eclampsia and with this, better diagnostic classifications. The diagnosis of pre-eclampsia is currently still based on a collection of symptoms, signs and a few laboratory tests. Many of the measures used are imprecise, inaccurate and subject to significant operator variability. The need for large prospective clinical trials to properly evaluate current criteria used to define pre-eclampsia, and to identify subgroups at particular risk of severe sequelae, have been highlighted (Anonymous, 2000). In addition, internationally agreed upon diagnostic criteria for disease subsets, such as early-onset pre-eclampsia or those with a growth restricted fetus, need to be developed (von Dadelszen et al., 2003).
The lack of reliable, objective methods to define and diagnose pre-eclampsia has created the clinical need for a diagnostic test that accurately identifies women with pre-eclampsia, especially those with rapidly progressive disease. Technological advances, such as in proteomics, have created the opportunity to develop such novel diagnostic tests for the syndrome. The objectives of novel diagnostic tests should be clear. Amongst women with pre-eclampsia will the aim be to detect all with hypertension and proteinuria in the second half of pregnancy, or those with multisystem disease or perhaps those who will develop multisystem disease in the near future? In addition, there is a need for a diagnostic test that recognizes superimposed pre-eclampsia when there is underlying maternal disease.
In the future, it is likely that there will be more objective methods of diagnosing and risk-classifying women with pregnancy-induced hypertensive complications. Improved diagnostic/screening criteria for pre-eclampsia would advance obstetric care, with the potential to decrease severe maternal morbidity through timely delivery and to reduce infant morbidity and mortality through unnecessary iatrogenic delivery of the very preterm infant.
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