Risk factors for neurologic impairment among very low-birth-weight infants

The curse of the perinatal epidemiologist: inferring causation amidst selection

Purpose of review

Human reproduction is a common process and one that unfolds over a relatively short time, but pregnancy and birth processes are challenging to study. Selection occurs at every step of this process (e.g., infertility, early pregnancy loss, and stillbirth), adding substantial bias to estimated exposure-outcome associations. Here we focus on selection in perinatal epidemiology, specifically, how it affects research question formulation, feasible study designs, and interpretation of results.

Recent findings

Approaches have recently been proposed to address selection issues in perinatal epidemiology. One such approach is the ongoing pregnancies denominator for gestation-stratified analyses of infant outcomes. Similarly, bias resulting from left truncation has recently been termed "live birth bias," and a proposed solution is to control for common causes of selection variables (e.g., fecundity, fetal loss) and birth outcomes. However, these approaches have theoretical shortcomings, conflicting with the foundational epidemiologic concept of populations at risk for a given outcome.

Summary

We engage with epidemiologic theory and employ thought experiments to demonstrate the problems of using denominators that include units not "at risk" of the outcome. Fundamental (and commonsense) concerns of outcome definition and analysis (e.g., ensuring that all study participants are at risk for the outcome) should take precedence in formulating questions and analysis approach, as should choosing questions that stakeholders care about. Selection and resulting biases in human reproductive processes complicate estimation of unbiased exposure- outcome associations, but we should not focus solely (or even mostly) on minimizing such biases.

Causal inference in studies of preterm babies: a simulation study

OBJECTIVE

Using a simple simulation, we illustrate why associations estimated from studies restricted to preterm births cannot be interpreted causally.

DESIGN, SETTING AND POPULATION

Data simulation involving a hypothetical cohort of fetuses who may be healthy or have one or more of four pathological factors (termed A through D, increasing in severity) with known effects on gestational length and risk of mortality. We focus on babies born at ≤32 weeks of gestation.

METHODS

We visually represent the simulated population and compare the association between A (which may represent pre-eclampsia) and neonatal death. We then repeat the exercise with D (standing in for chorioamnionitis) as the exposure of interest.

MAIN OUTCOME MEASURES

Odds ratios of neonatal death in the simulated data.

RESULTS

In most weeks, and for both A and D, the calculated odds ratios are substantially biased and underestimate the true risk of neonatal death associated with each pathology. For example, factor A has a true causal odds ratio of 1.50, yet it appears protective among births ≤32 weeks (estimated crude odds ratio 0.39; gestational age-adjusted odds ratio 0.71).

CONCLUSIONS

Among very preterm births, virtually all babies are born with pathologies that increase the risk of adverse outcomes. Hence, babies exposed to one factor (e.g. pre-eclampsia) are compared with babies who have a mix of other pathologies. Such selection bias affects studies carried out among very preterm births (e.g. where pre-eclampsia appears to reduce risk of adverse neonatal outcomes).

TWEETABLE ABSTRACT

Selection bias affects studies of preterm births, complicating interpretation.

Patterned feeding experience for preterm infants: study protocol for a randomized controlled trial

BACKGROUND

Neurobehavioral disabilities occur in 5-15% of preterm infants with an estimated 50-70% of very low birth weight preterm infants experiencing later dysfunction, including cognitive, behavioral, and social delays that often persist into adulthood. Factors implicated in poor neurobehavioral and developmental outcomes are hospitalization in the neonatal intensive care unit (NICU) and inconsistent caregiving patterns. Although much underlying brain damage occurs in utero or shortly after birth, neuroprotective strategies can stop lesions from progressing, particularly when these strategies are used during the most sensitive periods of neural plasticity occurring months before term age. The purpose of this randomized trial is to test the effect of a patterned feeding experience on preterm infants' neurobehavioral organization and development, cognitive function, and clinical outcomes.

METHODS

This trial uses an experimental, longitudinal, 2-group design with 120 preterm infants. Infants are enrolled within the first week of life and randomized to an experimental group receiving a patterned feeding experience from the first gavage feeding through discharge or to a control group receiving usual feeding care experience. The intervention involves a continuity of tactile experiences associated with feeding to train and build neuronal networks supportive of normal infant feeding experience. Primary outcomes are neurobehavioral organization as measured by Neurobehavioral Assessment of the Preterm Infant at 3 time points: the transition to oral feedings, NICU discharge, and 2 months corrected age. Secondary aims are cognitive function measured using the Bayley Scales of Infant and Toddler Development, Third Edition at 6 months corrected age, neurobehavioral development (sucking organization, feeding performance, and heart rate variability), and clinical outcomes (length of NICU stay and time to full oral feeding). The potential effects of demographic and biobehavioral factors (perinatal events and conditions of maternal or fetal/newborn origin and immunologic and genetic biomarkers) on the outcome variables will also be considered.

DISCUSSION

Theoretically, the intervention provided at a critical time in neurologic system development and associated with a recurring event (feeding) should enhance neural connections that may be important for later development, particularly language and other cognitive and neurobehavioral organization skills.

TRIAL REGISTRATION

NCT01577615 11 April 2012.

A model of neurodevelopmental risk and protection for preterm infants

The purpose of this article is to introduce a model of neurodevelopmental risk and protection that may explain some of the relationships among biobehavioral risks, environmental risks, and caregiving behaviors that potentially contribute to neurobehavioral and cognitive outcomes. Infants born before 30 weeks of gestation have the poorest developmental prognosis of all infants. These infants have lengthy hospitalization periods in the neonatal intensive care unit (NICU,) an environment that is not always supportive of brain development and long-term developmental needs. The model supports the premise that interventions focused on neuroprotection during the neonatal period have the potential to positively affect long-term developmental outcomes for vulnerable very preterm infants. Finding ways to better understand the complex relationships among NICU-based interventions and long-term outcomes are important to guiding caregiving practices in the NICU.

Mediators of the association between pre-eclampsia and cerebral palsy: population based cohort study

OBJECTIVE

To test the hypothesis that pre-eclampsia is a risk factor for cerebral palsy mediated through preterm birth and being born small for gestational age.

DESIGN

Population based cohort study.

SETTING

Clinical data from the Norwegian Cerebral Palsy Registry were linked with perinatal data prospectively recorded by the Medical Birth Registry of Norway.

PARTICIPANTS

All singleton babies who survived the neonatal period during 1996-2006 (849 children with cerebral palsy and 616,658 control children).

MAIN OUTCOME MEASURES

Cerebral palsy and cerebral palsy subtypes.

RESULTS

Children exposed to pre-eclampsia had an excess risk of cerebral palsy (unadjusted odds ratio 2.5, 95% confidence interval 2.0 to 3.2) compared with unexposed children. Among children born at term (≥ 37 weeks), exposure to pre-eclampsia was not associated with an excess risk of cerebral palsy in babies not born small for gestational age (1.2, 0.7 to 2.0), whereas children exposed to pre-eclampsia and born small for gestational age had a significantly increased risk of cerebral palsy (3.2, 1.5 to 6.7). Non-small for gestational age babies born very preterm (<32 weeks) and exposed to pre-eclampsia had a reduced risk of cerebral palsy compared with unexposed children born at the same gestational age (0.5, 0.3 to 0.8), although the risk was not statistically significantly reduced among children exposed to pre-eclampsia and born small for gestational age (0.7, 0.4 to 1.3). Exposure to pre-eclampsia was not associated with a specific cerebral palsy subtype.

CONCLUSIONS

Exposure to pre-eclampsia was associated with an increased risk of cerebral palsy, and this association was mediated through the children being born preterm or small for gestational age, or both. Among children born at term, pre-eclampsia was a risk factor for cerebral palsy only when the children were small for gestational age.

Glial fibrillary acidic protein as a biomarker for periventricular white matter injury

OBJECTIVE

Periventricular white matter injury (PWMI), a precursor of cerebral palsy, traditionally is not diagnosed until 6 weeks of life by head ultrasound scanning. We sought to determine whether early neonatal glial fibrillary acidic protein (GFAP) levels could identify PWMI in low birthweight (<2500 g) infants.

STUDY DESIGN

Each case with PWMI on head ultrasound scanning at 6 weeks of life from April 2009 to April 2011 was matched by gestational age and mode of delivery to 2 subsequent neonates with a normal head ultrasound scan. GFAP was measured in cord blood at birth, at neonatal intensive care unit admission, and on days 1-4 of life.

RESULTS

During this 2-year period, 21 cases with PWMI with gestational age 27.4 ± 3.3 weeks were compared with 42 control infants. The incidence of cesarean delivery was 61.9% in both groups. GFAP was not significantly different in cord blood or at neonatal intensive care unit admission but was significantly elevated on day 1 (median, 5-95%; 0, 0-0.98 ng/mL cases; 0, 0-0.06 ng/mL control infants; P = .03), day 2 (0, 0-1.21 ng/mL; 0, 0-0.05 ng/mL, respectively; P = .02), day 3 (0.05, 0-0.33 ng/mL; 0, 0-0.04 ng/mL, respectively; P = .004), and day 4 (0.02, 0-1.03 ng/mL; 0, 0-0.05 ng/mL, respectively; P < .001). The odds of the development of PWMI significantly increased with increasing levels of GFAP from day 1-4 of life when adjustment was made for preeclampsia, antenatal steroid administration, and neonatal chronic lung disease.

CONCLUSION

The ability to predict PWMI with a blood test for GFAP shortly after birth opens the possibility for rapid identification of infants for early intervention and provides a benchmark for the qualification of new therapies to improve neurodevelopmental outcomes.

Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix

OBJECTIVE

To assess the diagnostic performance of transabdominal sonographic measurement of cervical length in identifying patients with a short cervix.

METHODS

Cervical length was measured in 220 pregnant women using transabdominal and transvaginal ultrasound (US). Reproducibility and agreement between and within both methods were assessed. The diagnostic accuracy of transabdominal US for identifying cases with a cervical length <25 mm was evaluated.

RESULTS

Twenty-one out of 220 cases (9.5%) had a cervical length <25 mm by transvaginal US. Only 43% (n = 9) of patients with a short cervix were correctly identified by transabdominal US. In patients with a cervical length of <25 mm by transvaginal US, transabdominal measurement of the cervix overestimated this parameter by an average of 8 mm (95% LOAs, -26.4 to 10.5 mm). Among women without a short cervix, transabdominal US underestimated cervical length on average (LOA) by 1.1 mm (95% LOAs, -11.0 to 13.2 mm). Transvaginal US was also more reproducible (intraclass correlation coefficient: (ICC) (0.96; 95% CI, 0.94 to 0.97) based on comparisons between 2D images and immediately acquired 3D volume datasets relative to transabdominal US (ICC: 0.71; 95% CI, 0.57 to 0.84). Transvaginal US detected 13 cases with funneling and six cases with sludge whereas only three cases of funneling and one of sludge were detected by transabdominal US.

CONCLUSION

Transabdominal measurement overestimated cervical LOA by 8 mm among women with a short cervix and resulted in the underdiagnosis of 57% of cases.

Uncovering the complex relationship between pre-eclampsia, preterm birth and cerebral palsy

Pre-eclampsia is a leading cause of preterm birth, which is strongly associated with cerebral palsy (CP). However, there is controversy about whether pre-eclampsia is associated with increased risk of CP. We evaluated the association between pre-eclampsia and CP in 122,476 mother-child pairs insured by the South Carolina Medicaid programme, with births between 1996 and 2002. Prenatal billing records were linked to the children's Medicaid billing records after birth until December 2008. The odds of CP were modelled using logistic regression with generalised estimating equations. There were 337 children (0.28%) diagnosed with CP by at least two different health care providers, and 4226 (3.5%) women were diagnosed with pre-eclampsia at least twice during pregnancy. Children whose mothers had pre-eclampsia were almost twice as likely to have CP compared with children of mothers without pre-eclampsia [odds ratio (OR)=1.94, 95% confidence interval (CI) 1.25, 2.97]. The association was only significant for pre-eclampsia diagnosed prior to 37 weeks' gestation. Full term (gestational age ≥ 37 weeks) infants whose mothers were diagnosed with pre-eclampsia prior to 37 weeks had increased odds of CP compared with full term children whose mothers did not have pre-eclampsia (OR=3.41, 95% CI 1.40, 8.31). Preterm infants whose mothers had pre-eclampsia were at significantly increased risk of CP compared with full term infants whose mothers did not have pre-eclampsia (OR=5.88, 95% CI 3.40, 10.17). The greatest risk for CP was in preterm infants whose mothers did not have pre-eclampsia (OR=8.12, 95% CI 6.49, 10.17 compared with full term infants without exposure to pre-eclampsia). We conclude that pre-eclampsia with onset before 37 weeks' gestation is a significant risk factor for CP. Some of the association is probably attributable to high risk of preterm birth because of early pre-eclampsia, while a 'direct' effect of pre-eclampsia on fetal brain development also seems likely.

A model of neurodevelopmental risk and protection for preterm infants

The purpose of this article is to introduce a model of neurodevelopmental risk and protection that may explain some of the relationships among biobehavioral risks, environmental risks, and caregiving behaviors that potentially contribute to neurobehavioral and cognitive outcomes. Infants born before 30 weeks of gestation have the poorest developmental prognosis of all infants. These infants have lengthy hospitalization periods in the neonatal intensive care unit (NICU,) an environment that is not always supportive of brain development and long-term developmental needs. The model supports the premise that interventions focused on neuroprotection during the neonatal period have the potential to positively affect long-term developmental outcomes for vulnerable very preterm infants. Finding ways to better understand the complex relationships among NICU-based interventions and long-term outcomes are important to guiding caregiving practices in the NICU.

Perinatal factors associated with the neurologic impairment of children born preterm

OBJECTIVE

To determine the perinatal risk factors of long-term neurologic impairment for preterm infants.

METHODS

A case-control study was conducted with 60 neurologically impaired and 60 healthy children, all born prematurely.

RESULTS

There was no relation between neurologic impairment and maternal pregnancy complications or prenatal steroid administration, bacteriologic content of cervical smear, fetal presentation, fetal heart rate, or mode of delivery. Cerebral palsy was associated with early neurologic signs, perinatal asphyxia, neonatal septicemia, abnormal brain ultrasound findings, prolonged interval between rupture of membranes and delivery, and multiple placental lesions. Children with minimal cerebral dysfunction were more frequently first born. Multiple placental lesions, neonatal septicemia, abnormal brain ultrasound findings, and perinatal asphyxia were independently correlated with long-term neurologic impairment.

CONCLUSION

Perinatal infection, perinatal asphyxia, and abnormal brain ultrasound findings seem to be risk factors for cerebral palsy whereas primigravidity seems to be correlated with minimal cerebral dysfunction.

Neonatal antecedents for cerebral palsy in extremely preterm babies and interaction with maternal factors

BACKGROUND

Preterm delivery is associated with an increased risk of cerebral palsy (CP). The greatest risk is for infants born <28 weeks' gestation.

AIMS

To identify significant neonatal risk factors for CP and explore the interactions between antenatal and neonatal risk factors, among extremely preterm infants of 27 weeks' gestation or less.

STUDY DESIGN

Nested case control design.

METHODS

Infants born between 1989 and 1996, at 24-27 weeks' gestation, were evaluated: 30 with CP at 2 years corrected age and 120 control infants matched for gestation age. Neonatal variables were compared using matched analyses with the interaction between antenatal and neonatal factors being examined using logistic regression analyses.

RESULTS

Risk factors for CP on matched analyses included patent ductus arteriosus requiring surgical ligation, peri-intraventricular haemorrhage, moderate to severe ventricular dilatation, periventricular leukomalacia (PVL) and need for home oxygen. Independent neonatal predictors were ventricular dilatation (OR 7.3; 95% CI 1.6, 32.3), PVL (OR 29.8; 95% CI 5.6, 159.1) and home oxygen use (OR 3.4; 95% CI 1.2, 9.4). No interaction terms in the logistic models were significant between the previously identified pregnancy risk factors of absence of antenatal steroids and intrauterine growth restriction and the neonatal risk factors.

CONCLUSIONS

PVL is the most powerful independent predictor of CP in extremely preterm infants of 27 weeks' gestation or less and appears to be uninfluenced by antenatal factors.

Bronchopulmonary dysplasia is not associated with ultrasound-defined cerebral white matter damage in preterm newborns

Bronchopulmonary dysplasia (BPD) and cerebral white matter damage (WMD) are neonatal disorders that occur most commonly in those who are born much before term. In a large multicenter database, we sought to determine whether the two disorders occur together more frequently than expected and whether BPD and other neonatal respiratory characteristics are more common among infants who develop ultrasound-defined WMD than among those who do not. In a sample of 904 infants who were born before the 30th week of gestation and survived until 36 wk postmenstrual age, we did not find a co-occurrence of BPD and WMD above what would be expected by chance. Confounding does not seem to account for this lack of association between WMD and BPD. In conclusion, our findings do not support the hypothesis that BPD contributes to the occurrence of sonographically defined WMD.