Papers

Neonatal risk factors for cerebral palsy in very preterm babies: case-control study

BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7078.404 (Published 08 February 1997) Cite this as: BMJ 1997;314:404
  1. Deirdre J Murphy, specialist registrara,
  2. Peter L Hope, consultant neonatologistb,
  3. Ann Johnson, developmental paediatricianc
  1. a Department of Obstetrics and Gynaecology, St Michaels Hospital, Bristol BS2 8EG
  2. b Department of Paediatrics, John Radcliffe Hospital, Oxford OX3 9DU
  3. c National Perinatal Epidemiology Unit, Radcliffe Infirmary NHS Trust, Oxford OX2 6HE
  1. Correspondence to: Dr D J Murphy 22 Manor Park, Redland, Bristol BS6 7HH
  • Accepted 29 November 1996

abstract

Objective: To identify neonatal risk factors for cerebral palsy among very preterm babies and in particular the associations independent of the coexistence of antenatal and intrapartum factors.

Design: Case-control study.

Setting: Oxford health region.

Subjects: Singleton babies born between 1984 and 1990 at less than 32 weeks' gestation who survived to discharge from hospital: 59 with cerebral palsy and 234 randomly selected controls without cerebral palsy.

Main outcome measures: Adverse neonatal factors expressed as odds ratios and 95% confidence intervals.

Results: Factors associated with an increased risk of cerebral palsy after adjustment for gestational age and the presence of previously identified antenatal and intrapartum risk factors were patent ductus arteriosus (odds ratio 2.3; 95% confidence interval 1.2 to 4.5), hypotension (2.3; 1.3 to 4.7), blood transfusion (4.8; 2.5 to 9.3), prolonged ventilation (4.8; 2.5 to 9.0), pneumothorax (3.5; 1.6 to 7.6), sepsis (3.6; 1.8 to 7.4), hyponatraemia (7.9; 2.1 to 29.6) and total parenteral nutrition (5.5;2.8 to 10.5). Seizures were associated with an increased risk of cerebral palsy (10.0; 4.1 to 24.7), as were parenchymal damage (32; 12.4 to 84.4) and appreciable ventricular dilatation (5.4; 3.0 to 9.8) detected by cerebral ultrasound.

Conclusion: A reduction in the rate of cerebral palsy in very preterm babies requires an integrated approach to management throughout the antenatal, intrapartum, and neonatal periods.

Key messages

  • Preterm birth is associated with an increased risk of cerebral palsy

  • Antenatal, intrapartum, and neonatal factors have all been associated with cerebral palsy in preterm babies

  • Neonatal pneumothorax, sepsis, and transfusion are associated with preterm cerebral palsy independently of adverse antenatal factors and mode of delivery

  • Intrauterine infection followed by neonatal sepsis is associated with a very high risk of cerebral palsy among preterm babies

  • The prevention of cerebral palsy among very preterm babies requires an integrated approach throughout the antenatal, intrapartum, and neonatal periods

Introduction

Preterm birth is associated with a clear increase in risk of cerebral palsy.1 2 3 4 5 During the early 1980s there was an increase in the survival of very preterm babies which was accompanied by a sharp increase in the rate of cerebral palsy in this group. The aetiology of the cerebral damage has been the focus of considerable attention, and emphasis has recently shifted from intrapartum and neonatal factors to antenatal and prenatal events.6 7 89 Several hypotheses have been proposed to explain the origins of cerebral palsy in very preterm babies. Firstly, it may be the result of an ischaemic insult in utero leading to both preterm birth and damage to the white matter.1 This damage may be manifest later as cerebral palsy. Secondly, it may be that immature babies who are particularly vulnerable to cerebral haemorrhage and ischaemia sustain injury as a result of intrapartum and neonatal complications.10 A third possibility is that cerebral palsy represents the endpoint of a continuum of adverse events which occur throughout the period when the brain is especially vulnerable to ischaemia. These events may occur before, during, and after birth.

A better understanding of the aetiology of preterm cerebral palsy is necessary for preventive strategies and treatments to be developed. In efforts to understand aetiological factors, however, an attempt must be made to disentangle neonatal factors that are causes of cerebral palsy from those that are consequences of earlier disturbances. In a recent case-control study of antenatal and intrapartum risk factors for cerebral palsy in very preterm babies we found associations between chorioamnionitis, prolonged rupture of membranes, and maternal infection and an increased risk of cerebral palsy.8 We also found associations between pre-eclampsia and delivery without labour and a decreased risk of cerebral palsy. Although adverse antenatal events seem to be important to our understanding of the origins of cerebral palsy, it is likely that these events contribute only to some of the cases of preterm cerebral palsy and that others have their origins in adverse neonatal events or as a result of a continuum of adverse effects throughout antenatal and early neonatal life. To investigate this further we carried out a case-control study on our original study population of singletons born before 32 weeks of gestation that was designed to identify neonatal risk factors for cerebral palsy in very preterm babies and, in particular, the associations independent of the coexistence of previously identified antenatal and intrapartum factors.

Subjects and methods

Selection of subjects

All the selected babies were singletons of less than 32 completed weeks of gestation who survived to hospital discharge, born to mothers resident in Oxfordshire and West Berkshire between 1984 and 1990. Multiple births were excluded from this study as current evidence suggests that the risk factors for cerebral palsy in this group may differ from those in singleton births.11

Gestational age for all groups was estimated by using a combination of menstrual dates and an ultrasound scan performed before 20 weeks' gestation. The scan date was preferred if the menstrual date was uncertain or there was a discrepancy of more than 14 days between the menstrual date and the scan estimate.

Cases–Fifty nine children with cerebral palsy were identified from the Oxford region register of early childhood impairments.12 The definition of cerebral palsy used by the register is that of a permanent impairment of voluntary movement or posture presumed to be due to permanent damage to the immature brain. The register includes children of mothers who were resident within the former Oxford health region at the time of delivery. Multiple sources of ascertainment are used to compile the register, and the condition of the children is determined at the age of 3 and 5 years.

Controls–A total of 474 babies who survived to discharge and did not develop cerebral palsy were identified from two sources to ensure maximum ascertainment: the hospital admission registers and the Oxford record linkage study. On the basis of an audit of preterm birth at the John Radcliffe Hospital (unpublished data) we estimated that an adverse neonatal factor would be present in 25-30% of controls. We predicted that a study population of 59 cases of cerebral palsy with four controls for each case would be large enough to detect for each neonatal factor an odds ratio of 2.5 with 80% power and an α level of 0.05. We wished, therefore, to select approximately half of the potential controls, and 235 controls were thus randomly selected. Controls were selected from the entire geographical population of very preterm babies (<32 completed weeks' gestation) but were not matched with the cases for gestational age. An unmatched case-control study design allowed us to investigate the relation between gestational age and cerebral palsy among very preterm babies.

Data sources

The neonatal notes of babies included in the study were reviewed by a researcher unaware of the children's outcomes. A detailed dataset was completed, encompassing 52 variables including characteristics at birth; cardiovascular, respiratory, systemic, and metabolic complications; neurological sequelae; and cerebral ultrasound findings. Patent ductus arteriosus (clinical diagnosis supported by cardiac ultrasonography, requiring indomethacin or surgical ligation), hypotension (mean blood pressure <30 mm Hg on at least two occasions), blood transfusion for either anaemia or hypotension, prolonged mechanical ventilation (duration of at least seven days), pneumothorax (diagnosis confirmed by chest x ray, requiring insertion of chest drain), and sepsis (clinical diagnosis confirmed with microbiology, requiring antimicrobial therapy) were of special interest. Details of diagnosis, onset, duration, and management were recorded. Birth trauma referred to severe bruising or x ray evidence of a fracture.

Ultrasound data were included if at least two scans were available, the first recorded during the first week of life and the second as near as possible to six weeks after birth. This approach was likely to identify lesions developing in both the early and late neonatal periods. In fact most babies had daily scans for the first week and weekly scans thereafter, with additional scans if clinically indicated. Ultrasound scanners (Advanced Technical Laboratories) used were the ATL 300C until 1988 and the UM4 thereafter with 7.5 MHZ transducer heads. The findings were described by using a classification modified from a data sheet used in a neonatal trial (OSIRIS).13 The right and left cerebral hemispheres were described separately in terms of germinal layer or intraventricular haemorrhage, ventricular dilatation, parenchyma echodensity, and parenchyma cysts. Moderate ventricular dilatation was assigned where the ventricular index was above the 97th centile and hydrocephalus was assigned when the dilatation was more than 4 mm above the 97th centile, using a centile chart from an unpublished study by M Levene et al. Parenchyma cyst was an umbrella term used for any parenchymal echolucency suggesting a cavity. For the purposes of this study parenchymal echodensities and echolucencies were grouped together and termed parenchyma damage.

Antenatal and intrapartum data had been recorded from the obstetric notes of the mothers; these data were available from our earlier study.8 Factors included in the logistic regression model as potentially important confounders were antepartum haemorrhage, maternal infection, chorioamnionitis, prolonged rupture of membranes, pre-eclampsia, and the mode of delivery.

Ethical approval

The approval of the Oxfordshire and West Berkshire ethics committees was obtained before the start of the study.

Statistical methods

The odds ratio associated with a given factor estimates the risk of cerebral palsy given the factor relative to the risk of cerebral palsy without the factor. The 95% confidence intervals for crude odds ratios were calculated with the programme CIA.14 The odds ratios with adjustment for potential confounders were calculated by logistic regression, using the statistical package for the social sciences.15 As the study populations were unmatched for gestational age, in the first instance an odds ratio was calculated for each neonatal factor with adjustment for gestational age. Biologically plausible interactions of neonatal factors were investigated by entering variables into a logistic regression model in a forward conditional fashion. Only variables with a strong association with cerebral palsy remained significant (P<0.05) independent of the other variables. We included these factors in further regression models, looking for relations between neonatal factors and cerebral palsy independent of the presence of adverse antenatal factors and the delivery mode. There were no more than six variables in a logistic regression model at any one time. Trends in the rates of survival and cerebral palsy among survivors by gestational age were tested by χ2 tests for trend.

Results

Of 638 singleton babies born alive at less than 32 completed weeks' gestation to mothers resident in Oxford district and West Berkshire during 1984-90, 105 died before discharge. The survival rate increased with increasing gestational age (P<0.0001; fig 1) and the incidence of cerebral palsy among survivors decreased with increasing gestational age (P<0.0001; fig 2).

Fig 1
Fig 1

Survival of very preterm infants in relation to gestational age. Bars are 95% confidence intervals

Fig 2
Fig 2

Percentage of surviving very preterm infants with cerebral palsy. Bars are 95% confidence intervals

Neonatal notes were available for all 59 children with cerebral palsy and 234 controls, a total of 293 babies. Mean gestational age at birth for children with cerebral palsy was 1.3 (95% confidence interval 0.7 to 1.9) weeks less than for the controls (mean 28.6 (SD 2.3; range 24-32) weeks v 29.9 (1.9; 23-32) weeks; P<0.0001). As this difference in gestational age confounds any comparison between cases and controls, odds ratio estimates were adjusted for gestational age. Further adjustment for birth weight did not affect the results; hence odds ratios are reported without this adjustment.

Neonatal factors

An Apgar score of ≤3 at 5 minutes was significantly associated with an increased risk of cerebral palsy (odds ratio 5.3 (1.4 to 21) (table 1). Otherwise the two groups did not differ significantly in characteristics of the babies or condition at birth.

Table 1

Characteristics at birth in very preterm babies with and without cerebral palsy

View this table:

Cardiovascular and respiratory complications were common among these babies: 232 (79%) had at least one complication. Patent ductus arteriosus, hypotension, transfusion, prolonged ventilation, and pneumothorax were associated with cerebral palsy after gestational age was adjusted for (table 2). On forward conditional logistic regression of the cardiovascular and respiratory factors described, transfusion and pneumothorax were independently associated with cerebral palsy (odds ratios 2.2 (1.1 to 4.7) and 4.8 (2.2 to 10.8) respectively).

Table 2

Cardiovascular and respiratory factors in very preterm babies with and without cerebral palsy

View this table:

Sepsis, total parenteral nutrition, and hyponatraemia were associated with an increased risk of cerebral palsy (table 3). The numbers with hyponatraemia were small, however, and this association could be a chance finding. The association with sepsis was independent of other systemic, cardiovascular, or respiratory complications (odds ratio 3.3 (1.6 to 6.8)). The sequence of antenatal infection and neonatal sepsis was strongly associated with cerebral palsy, but this occurred in only a few subjects (table 4).

Table 3

Systemic and metabolic neonatal factors in very preterm babies with and without cerebral palsy

View this table:
Table 4

Antenatal infection and neonatal sepsis in very preterm babies with and without cerebral palsy

View this table:

Neonatal seizures occurred in 28 (9.6%) babies and were associated with a highly significant increased risk of cerebral palsy (table 5). Cerebral ultrasound scans were available for a total of 239 (82%) babies, with a similar proportion for cases and controls. Isolated intraventricular haemorrhage was not associated with an increased risk of cerebral palsy, but there was a strong association between cerebral palsy and parenchymal lesions and ventricular dilatation. Retinopathy of prematurity (all grades) occurred more frequently among cases than controls, but the difference was not significant at the 5% level.

Table 5

Neurological neonatal factors in very preterm babies with and without cerebral palsy

View this table:

Neonatal factors controlled for antenatal and intrapartum events

Patent ductus arteriosus, hypotension, transfusion, prolonged ventilation, pneumothorax, sepsis, hyponatraemia, and total parenteral nutrition were associated with an increased risk of cerebral palsy after adjustment by logistic regression for gestational age, antenatal complications, and the mode of delivery (table 6). The only antenatal factors of importance in the logistic regression model were chorioamnionitis, any maternal infection, and mode of delivery.

Table 6

Neonatal factors adjusted for gestational age, antenatal factors, and intrapartum factors in very preterm babies with and without cerebral palsy

View this table:

Discussion

Neonatal complications

Several cardiovascular, respiratory, and systemic factors investigated in this study of very preterm babies were associated with an increased risk of cerebral palsy. In earlier studies, hypotension, transfusion, and patent ductus arteriosus have been associated with periventricular leukomalacia,16 17 18 19 an ultrasound finding which predicts later handicap (especially cerebral palsy) more accurately than any other antecedent.20 21 22 23 24 25 26 27 28 29 Pneumothorax and prolonged ventilation have been associated with both periventricular leukomalacia17 19 30 and cerebral palsy.31 32 The findings in this study are consistent with these observations, supporting the hypothesis that cardiovascular and respiratory disturbances have a role in the aetiology of cerebral ischaemia in very preterm babies.

A second hypothesis concerns infection, and several studies have shown associations between neonatal sepsis and both periventricular leukomalacia17 18 19 and cerebral palsy.32 Our results support this hypothesis as neonatal sepsis and cerebral palsy were strongly associated even after other potentially confounding neonatal complications were adjusted for.

These findings suggest a role for several neonatal complications in the aetiology of cerebral palsy in preterm babies. The difficulty in interpreting these findings, however, lies in determining which neonatal factors are causes of cerebral palsy and which are consequences of earlier disturbances in the antenatal and intrapartum periods and already part of the outcome. Some neonatal factors, such as transfusion, may be markers of severity of neonatal illness or may be the consequence of a disabling cerebral haemorrhage. Our previous study of antenatal and intrapartum risk factors for cerebral palsy in very preterm babies found a strong association between maternal infection and, in particular, chorioamnionitis and an increased risk of cerebral palsy. Maternal infection occurred, however, in only 37% of cases and 17% of controls and is likely to explain only a proportion of cases of preterm cerebral palsy. It is possible, therefore, that the origins of cerebral palsy lie in the neonatal period for a large proportion of very preterm babies. In addition, because of the design of a case-control study it is not possible to predict the timing of cerebral damage in relation to the insult and it is possible that the ischaemia associated with chorioamnionitis is not manifest until the neonatal period and may occur only if the baby suffers an additional further insult in the neonatal period. Our finding that the sequence of maternal infection followed by neonatal sepsis was strongly associated with cerebral palsy lends some strength to the theory of a continuum of insults in the pathogenesis of preterm cerebral palsy. However, this sequence of events affected only a small proportion of the study population.

Timing of brain injury

The question of the timing of cerebral damage was addressed in a study of the ultrasound findings and clinical events of preterm babies with cerebral palsy.4 Although antenatal complications were common, only a small proportion of babies (2/18) had evidence of antenatal cerebral damage; most had evidence of parenchymal damage of neonatal onset. These observations suggest a role for neonatal complications in the pathogenesis of preterm cerebral palsy.

Grether et al,9 in a recent study of prenatal and perinatal factors and cerebral palsy in very low birthweight infants, considered the interaction between prenatal and neonatal events and found chorioamnionitis to be associated with cerebral palsy only when seizures occurred in the neonatal period. With our previous study we had a full dataset of antenatal, intrapartum, and neonatal factors and could examine the independent associations between neonatal complications and cerebral palsy by controlling for the presence of antenatal and intrapartum factors. Transfusion, prolonged ventilation, pneumothorax, and sepsis had strong associations with cerebral palsy, adding further support to the hypothesis that cardiorespiratory disturbances and infection contribute to the aetiology of cerebral palsy in at least a proportion of cases and, more particularly, that this contribution can be independent of antenatal and intrapartum disturbances.

Cerebral lesions

As in previous studies of periventricular leukomalacia and cerebral palsy, we found strong associations between neonatal seizures, ultrasonically diagnosed parenchymal damage and ventricular dilatation, and preterm cerebral palsy. It is possible, of course, that cerebral ultrasound lesions arise incidentally as a result of severe physiological disturbances which in themselves cause cerebral palsy. This is unlikely, though, because ultrasound findings are so much more predictive of cerebral palsy than are cardiorespiratory complications. The statistical power of this study was limited for assessment of the complex interaction of these neurological factors and antenatal, intrapartum and neonatal events, and the results of multivariate analyses could be misleading. These interrelationships could be evaluated, however, with combined data from multiple sources.

In conclusion, we suspect that cerebral palsy has multiple risk factors, both causes and modifiers, but that a proportion of cases of cerebral palsy among very preterm singletons have their origins in the neonatal period. It would seem, therefore, that a major reduction in cerebral palsy among very preterm babies will arise only from an integrated approach throughout the antenatal, intrapartum, and neonatal periods to the management of any baby at risk. The possibility that new neonatal interventions may lead to a reduction or an increase in the frequency of cerebral palsy among very preterm babies can be tested by well designed randomised controlled trials.

Acknowledgments

Funding: DJM was funded by Action Research; AJ was funded by the Department of Health.

Conflict of interest: None.

References

  1. 1.
  2. 2.
  3. 3.
  4. 4.
  5. 5.
  6. 6.
  7. 7.
  8. 8.
  9. 9.
  10. 10.
  11. 11.
  12. 12.
  13. 13.
  14. 14.
  15. 15.
  16. 16.
  17. 17.
  18. 18.
  19. 19.
  20. 20.
  21. 21.
  22. 22.
  23. 23.
  24. 24.
  25. 25.
  26. 26.
  27. 27.
  28. 28.
  29. 29.
  30. 30.
  31. 31.
  32. 32.
View Abstract