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Definitions and Classifications

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A good classification system has 5 components:

1. The definitions involved should be easily understood and unambiguous.
2. It should reflect the complexity of the disease processes involved whilst remaining intrinsically simple.
3. The system should be applicable across population boundaries applying to all ethnic groups.
4. It needs to be clinically relevant.
5. Most importantly it should provide the best assessment of clinical risk for the individual.

Currently there are 3 major classification systems. The first, produced in 1986 by Davey and MacGillivray and subsequently adopted by the ISSHP is now well known. Pre-eclampsia is still defined as hypertension and proteinuria. The system itself is very precisely categorises the other hypertensive disorders. Thus, it produces very pure disease groups and is therefore attractive to researchers in the field. However it is rather cumbersome and not easy to apply in clinical practise. In the year 2000 the National Institute of Health working party in America updated their own classification system. Although it is simpler than the ISSHP system it still mirrors it quite well. The NIH system has 4 categories, a) chronic hypertension, b) pre-eclampsia, c) superimposed pre-eclampsia on chronic hypertension and d) gestational hypertension. Once again pre-eclampsia is diagnosed by the combination of hypertension and proteinuria.

The third major system was produced by the Australasian Society for the Study of Hypertension in Pregnancy (ASSHP) Australasian Society for the Study of Hypertension in Pregnancy (ASSHP) in 1993. This system has 3 categories. Chronic hypertension, pre-eclampsia and pre-eclampsia superimposed on chronic hypertension. Pre-eclampsia itself is subdivided into mild and severe categories. The mild form is defined purely as hypertension in pregnancy after 20 weeks. Severe pre-eclampsia is a combination of hypertension plus one (or more) of several other variables. These include proteinuria, thrombocytopaenia, abnormal liver function, abnormal renal function, neurological abnormalities such as eclampsia and coagulopathies. Any one of these latter variables associated with hypertension warrants the diagnosis of severe PET. The advantage of this system is that it recognises the multi organ system nature of the disease pre-eclampsia.

The 3 systems were compared in a retrospective study of 18,000 pregnancies of which there were 11,000 hypertensive pregnancies. The conclusion of this study was that the NIH system detected more severe disease than the ASSHP with regard to the diagnosis of pre-eclampsia. However, on the other hand the ASSHP classification more accurately identified a lower risk group of women. The advantage of this higher specificity, meant that fewer women would be subjected to unnecessary monitoring and interventions. The researchers then refined the classification systems and produced their own new system. In this system they too have 4 components, however, pre-eclampsia itself is diagnosed in the same way as "severe pre-eclampsia" in the ASSHP system. Thus once again the multi organ system nature of the disease is recognised. In a small retrospective study in the UK this system has been shown to better predict abnormal outcome than any of the others.

The question regarding which classification system should be used in practice is still yet to worked out. It is important for clinicians working in the field to recognise and remember that pre-eclampsia can present in many different ways and that relying upon hypertension and proteinuria to make the diagnosis will result in some patients with pre-eclampsia being ignored until much later in the disease process.

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