Objective
To determine the incidence of preeclampsia and eclampsia and associated mortality in Australia between 2000 and 2008.
Study design
Analysis of statutorily collected datasets of singleton births in New South Wales using International Classification of Disease coding. Analyzed using cross tabulation, logistic regression, and means testing, where appropriate.
Results
The overall incidence of preeclampsia was 3.3% with a decrease from 4.6% to 2.3%. The overall rate of eclampsia was 8.6/10,000 births or 2.6% of preeclampsia cases, with an increase from 2.3% to 4.2%. The relative risk of eclampsia in preeclamptic women in 2008 was 1.9 (95% confidence interval, 1.28–2.92) when compared with the year 2000. The relative risk of a woman with preeclampsia/eclampsia dying in the first 12 months following birth compared with normotensive women is 5.1 (95% confidence interval, 3.07–8.60).
Conclusion
Falling rates of preeclampsia have not equated to a decline in the incidence of eclampsia. An accurate rate of both preeclampsia and eclampsia is vital considering the considerable contribution that these diseases make to maternal mortality. The identification and treatment of eclampsia should remain a priority in the clinical setting.
Reported rates of preeclampsia and eclampsia in developed countries vary significantly. The preeclampsia rate is commonly quoted in the literature as being anywhere between 5% and 8% of all pregnancies worldwide, although more recently lower rates of 2-8% are being cited. Geographic variation, varying definitions of the disease and the size of data sources used largely account for these variations although seasonal variation at birth and time of conception have also been postulated as explanations of rate variations.
Eclampsia rates also vary significantly in the literature. In Europe rates of 2-3 cases per 10,000 births are quoted although rates in developing countries are between 16-69 per 10,000 births. Knight quotes 2.7/10,000 births in a 12-month surveillance of all maternity units in the United Kingdom in 2005/2006, although an earlier surveillance study in the Unitd Kingdom in 1992 resulted in a rate of 4.9/10,000. Worldwide, 12% of maternal deaths are attributed to eclampsia with a case fatality rate of 3-5% in developing countries. The Australasian Maternity Outcomes Surveillance System (AMOSS) is currently conducting a surveillance of eclampsia in Australia in 2011, although reporting of cases to AMOSS is voluntary and results are to date, not available.
Effective detection and treatment of preeclampsia and eclampsia within any country or health region should at least be influenced by clinician’s knowledge of the incidence of the disease within their area of practice. The aim of this study was to determine the incidence of both preeclampsia and eclampsia in the largest populated area of Australia (which contributes to be over one third of all Australian births), the variations over time periods and maternal mortality subsequent to diagnosis within a 12-month period following birth.
Materials and Methods
Data sources
Birth data including maternal age, parity, delivery type, smoking status, and neonatal outcomes for the period July 1, 2000, till June 30, 2008, of all singleton births was provided by New South Wales (NSW), Australia, Department of Health as recorded in the NSW Midwives Data Collection (MDC). This population based surveillance system contains maternal and infant data on all births of greater than 400 g birthweight or 20 weeks’ gestation and covers over one-third of births that occur in Australia. The NSW MDC contains statistics on 33% of births that occur in Australia annually. This dataset (NSW MDC) was linked to the Admitted Patient Data Collection (APDC) and the NSW Registry of Births, Deaths and Marriages (BDM). The APDC records all admitted patient services provided by NSW Public Hospitals, Public Psychiatric Hospitals, Public Multi-Purpose Services, Private Hospitals, and Private Day Procedures Centres. The BDM is a record of all births, deaths and marriages that occur in NSW. The incidence of preeclampsia and the incidence and timing of event in relation to birth of eclampsia was provided by the APDC using the International Classification of Diseases Coding (ICD-10-Australian Modification) that has been in use in the Australian setting since 1998. The codes O14.0, O14.1, O14.2, and O14.9 were used to identify the cases of preeclampsia and are those that refer to new onset proteinuric hypertension. The codes O15.0, O15.1, O15.2, and O15.9 were used to identify the cases of eclampsia and the timing of the seizure in relation to the delivery of the infant. Cause of death was obtained from ICD-10 codes as recorded on the death certificate and located in the BDM registry. Linkage of the datasets was conducted by the New South Wales Centre for Health Record Linkage (CHeReL). Probabilistic data linkage techniques were used for these purposes and deidentified datasets were provided for analysis. Probabilistic record linkage software works by assigning a “linkage weight” to pairs of records. For example, records that match perfectly or nearly perfectly on first name, surname, date of birth, and address have a high linkage weight, and records that match only on date of birth have a low linkage weight. If the linkage weight is high it is likely that the records truly match, and if the linkage weight is low it is likely that the records are not truly a match. This technique has been shown to have a false positive rate of 0.3% of records. Ethical approval was obtained from the NSW Population and Health Services Research Ethics Committee, protocol no. 2010/12/291.
Subjects
The MDC dataset for this period contains the antenatal, birth, and postnatal details on 691,738 births during this period. The APDC contains >1.7 million admissions for the same women occurring after the index pregnancy. The BDM registry contains death data on 97 women who died within 12 months following delivery.
Data analysis
Incidence and demographic data was calculated. Contingency table analyses, Student t tests and analyses of variance (ANOVA) were used to examine differences between the pregnancies coded as preeclamptic/eclamptic and those not. Binary logistic regression modelling was also undertaken. Significance was determined <0.05 level. All analyses were conducted using IBM SPSS v.20 (IBM, Armonk, NY).
Results
There was an overall preeclampsia rate of 3.3% of singleton births (22,827 cases from 691 738 births) ( Figure 1 for rate variations between year 2000 and 2008). There has been a decrease in the incidence of preeclampsia in this setting of 50% over this period, with a continuous trend of decline occurring (χ 2 , P < .001). In total there were 597 episodes of seizure in 529 pregnancies. Fifty-five women experienced >1 seizure in the 1 pregnancy and 4 women experienced a seizure in 2 pregnancies. This equates to an overall seizure event rate of 8.6/10,000 births. Between the years 2000 and 2008, there has been no change in the incidence of eclampsia (0.1% of all births in 2000 and 2008) ( Figure 1 ). The overall eclampsia rate in women with preeclampsia was 2.6% or 237/10,000 births effected by preeclampsia, with an increase from 2.3% in the year 2000 to 4.2% in 2008 (χ 2 , P = .007). The relative risk of eclampsia in women with preeclampsia in 2008 was 1.9 (95% confidence interval [CI], 1.28–2.92) when compared with the year 2000. Seventy-three percent of seizures occurred in primiparous women (χ 2 , P < .0001) and when an examination of parity and the relationship to seizure occurrence was undertaken (following logistic regression adjustment for age and smoking status) the odds ratio (OR) of a primiparous woman experiencing a seizure when compared with a multiparous woman was 4.5 (95% CI, 3.55–5.63). An analysis of the risk of seizure in 2008 compared with 2000 for all women when adjusted for maternal age, gestational age at birth, smoking, and parity did not show a significant increase in risk over the period of the study (OR, 1.86; 95% CI, 0.52–6.72).
Table 1 displays the time when the event occurred, with the majority of seizures occurring during labor (44.1%). The unadjusted RR of a seizure occurring in all women postnatally following a birth via caesarean section compared with a vaginal birth was 2.9 (95% CI, 2.14–3.99. An analysis of the time from birth to seizure was undertaken and displayed in Figure 2 . The majority of women experienced eclampsia on the day of birth (53%). Median time for eclampsia was zero days (day of birth), range, 0–55 days. When the cases that occurred following the day of birth were examined, the median day of postnatal seizure occurrence was day 4 (range, 1–55).
| When seizure occurred | Percent of all episodes |
|---|---|
| Antenatally | 25.1% |
| During labor | 44.1% |
| Postnatally | 26.3% |
| Not specified to venue | 4.5% |
Maternal characteristics and neonatal outcomes are described in Table 2 . This table demonstrates that women with eclampsia were younger, more likely to be primiparous, delivered at earlier gestations, and their infants suffered higher neonatal mortality.
| Variable | Eclampsia n = 529 | Preeclampsia without eclampsia n = 22,298 | Nonhypertensive n = 668,911 | P value |
|---|---|---|---|---|
| Age a | 28.7 (6.31) | 29.5 (5.86) | 30.2 (5.58) | < .001 |
| Primiparous b | 73.2% | 45.0% | 41.6% | < .001 |
| Smoked during pregnancy b | 13.9% | 8.2% | 14.8% | < .001 |
| Gestation at delivery a | 37.6 (3.27) | 37.9 (2.84) | 39.1 (2.06) | < .01 |
| Vaginal birth b | 49.8% | 40.5% | 73.8% | < .001 |
| Neonatal mortality rate b | 22.3/1000 | 10.7/1000 | 7.9/1000 | < .001 |
a Mean, standard deviation, and ANOVA.
The seasonal variation of eclampsia was explored and when examined in relationship to the number of births that occurred within each season, there was no statistical difference in the incidence of eclampsia based on season of seizure occurrence ( χ 2 , P = .35).
Of all cause maternal deaths that occurred within the 12-month period following delivery (n = 97), 18% (n = 17) had experienced preeclampsia. Of the 30 early deaths (within 42 days of birth) attributed to medical causes (nonviolent deaths), 17% (n = 5) had experienced preeclampsia. The recorded reason (with n = 1 for each cause): surgical misadventure, multiorgan failure, intracranial hemorrhage, ruptured uterus, and aortic dissection. This equates to a RR for women with preeclampsia/eclampsia dying in the first 12 months following birth when compared with normotensive women of 5.1 (95% CI, 3.07–8.60). The Neonatal Mortality Rate for infants of women with eclampsia was 22.3/1000 and 10.7/1000 for infants of women with preeclampsia compared with 7.9/1000 for the normotensive cohort.
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