A good classification system
has 5 components:
1. The definitions involved should be easily understood
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.