Trends in Mortality and Cerebral Palsy in a Geographically Based Cohort of Very Low Birth Weight Neonates Born Between 1982 to 1994, by T. Michael O'Shea, John S. Preisser, Kurt L. Klinepeter, and Robert G. Dillard, Pediatrics, 1998;101:642–647.
The authors of this study examined the relationship between the survival of infants born with very low birth weight and the prevalence of cerebral palsy. The mortality of infants born prematurely is decreasing, and the authors note that previous studies have not clearly shown whether these infants have an increased risk of developing cerebral palsy. Surfactant treatment has been used routinely since 1990 for infants that have a birth weight of less than 1350 g and require mechanical ventilation. The authors also analyzed the impact of this treatment on mortality and the prevalence of cerebral palsy.
The subjects were a cohort of 2067 infants that had birth weights between 500 and 1500 g, had no major congenital anomalies, were born between 1982 and 1994, and lived in northwest North Carolina. Many infants from the study also took part in a placebo-controlled trial of surfactant between July 1989 and August 1990.
Data obtained for this study included the infant's birth weight, gestational age, gender, age, race, and hospital of birth. Treatments and diagnoses from the neonatal intensive care unit were also recorded, and these included the use of surfactant treatment as well as the results of neonatal ultrasonographic examinations.
Of the 2076 infants, 508 died in the first year of life. Of the 1369 that were examined, 113 were found, at 1 year of corrected age, to have cerebral palsy. A pediatrician conducted a neurological examination on each child, keeping in mind important aspects of their neonatal history. Subjects were examined by both a pediatrician and a pediatric physical therapist for cerebral palsy, and the child was diagnosed with the condition if both agreed that the child demonstrated abnormal posture, and delayed motor development due to impaired neuromuscular function. Three full-time physical therapists were used for this purpose.
Chi-square tests, Cochran-Armitage tests, and logistic regression models were used to examine the changes in mortality and cerebral palsy prevalence from 1982 to 1994. Infants were categorized into two-year epochs according to date of birth to analyze trends across time. The effects of surfactant replacement therapy on mortality and cerebral palsy were analyzed using logistic regression models.
The Cochran-Armitage test was significant (p < 0.001) for a decrease in mortality during the time frame of the study. The rate of cerebral palsy from 1982 to 1988 was constant, fell slightly between 1988 and 1990, and decreased further between 1990 and 1994. After adjusting for confounding factors, the rate of cerebral palsy for children born between 1990 and 1992 was significantly lower than the rate for children born between 1988 and 1990.
The results of the analysis of the surfactant treatment suggested that the decline in the rate of mortality between 1990 and 1994 was due to the use of the surfactant treatment. When surfactant treatment was not included in the regression models, infants born after 1990 were at a lower risk of dying. However, when surfactant treatment was included, date of birth was no longer a significant predictor of mortality. A similar analysis suggested that surfactant treatment was not responsible for the decreased prevalence of cerebral palsy.
The results of the study show that the decline in mortality for infants born with very low birth weight in the 1990s has not led to an increase in the number of cases of cerebral palsy. The authors note that many studies in the 1970s and 1980s demonstrated an increase in the number of cases of cerebral palsy with a decrease in infant mortality for those with very low birth weight, which may have resulted from new neonatal intensive care unit techniques. However, this study suggests that this trend has not persisted into the 1990s.
The authors note several limitations of the study. They acknowledge that some infants may have died before they could be included in the study. Also, bias may have resulted from children with cerebral palsy failing to return for the one-year follow-up examination. Variations in certain aspects of the surfactant treatment, such as the number of treatments or the type administered, may have affected the results. Finally, the criteria used to diagnose the cases of cerebral palsy may have changed during the course of the study.
The authors state that future studies are needed to determine exactly what factors are responsible for the decrease in the prevalence of cerebral palsy for infants with very low birth weight. Studies already completed suggest prenatal factors may cause the downward trend, but more research is needed to identify these factors.
Limitations and Implications
The authors reviewed studies that found a varying prevalence of cerebral palsy for infants with very low birth weight, demonstrating a need for more research in this area. Studies that showed a decrease in the mortality of these infants in the 1970s and 1980s are referenced, but there is no mention of the intensive care unit procedures used during this time, and what effect these may have had on the risk of developing cerebral palsy. The introduction of surfactant replacement therapy is discussed in this article, but the authors do not indicate how this new procedure differs from previous treatments.
Some details of the methods were not reported, making reproduction of this study difficult. The exclusion criteria for this study state that infants with major congenital anomalies were not included, but no examples of these are offered. Also, the authors state that both the pediatrician and a pediatric physical therapist had to agree on the diagnosis of cerebral palsy for each subject, but no specific criteria were given. In the Discussion, the authors state that these criteria may have changed during the course of the study, but they offer no examples of how this may have occurred.
Generalizability of the results of the study is limited due to the small geographical area represented by subjects of the study. Also, the authors failed to clearly demonstrate that the risk of developing cerebral palsy for infants with very low birth weight has fallen, although the results do suggest that the risk of developing cerebral palsy for these infants has not risen. These results must be compared with studies in other areas of the country, and more research is needed to determine the effectiveness of the surfactant replacement therapy.
This study supports the theory that the mortality of infants with very low birth weight has declined with improved neonatal intensive care unit treatment techniques, but the study fails to show whether these techniques have significantly changed the prevalence of cerebral palsy for these infants. Surfactant replacement therapy appears to be behind the decline in mortality since becoming routinely used in 1990, but other factors need to be studied to determine their effects on outcomes such as those reported in this article.