"We believe that this is very important and interesting information - not only for the scientific community, but for the parents of preterm infants," says lead investigator Laura Ment, M.D. of Yale University School of Medicine in New Haven, Connecticut. The study was funded in part by the National Institute of Neurological Disorders and Stroke (NINDS) and appears in the February 12, 2003, issue of the Journal of the American Medical Association.*
Very low birthweight infants are defined as those weighing less than 1500 grams (about 3 pounds, 5 ounces) at birth, while most premature or low-birthweight babies weigh between 1500 and 2500 grams. About 1.4 percent - more than 50,000 - of the babies born in the United States each year have very low birthweight. Previous studies have found that, depending on birthweight and the year of birth, up to 50 percent of children with very low birthweight require special assistance in school and 20 percent require special education. However, other recent reports have found that almost 75 percent graduate from high school and more than 40 percent enter college. Taken together, these data suggest that cognitive functions improve over time in children born prematurely. The researchers set out to test that idea.
In the new study, Dr. Ment and her colleagues examined 296 children who weighed 600 to 1250 grams at birth. The children were part of a larger, long-term study that tested effects of the anti-inflammatory drug indomethacin for preventing intraventricular brain hemorrhage in premature infants, and they had been examined using brain scans and other tests since they were 6 hours old. The researchers tested these children using the Peabody Picture Vocabulary Test-Revised (a test of verbal comprehension) and Wechsler intelligence scales when they reached 3, 4 ½, 6, and 8 years of age.
Most of the children improved significantly on both verbal comprehension and intelligence tests between their 3rd and 8th birthdays, the researchers found. Median scores on the verbal comprehension test increased from 88 to 99 points during this time frame, while median intelligence scores increased from 90 to 95 points (100 points is the average score on both tests).
Forty-five percent of the children had increases of 10 points or more on the verbal comprehension test between 3 and 8 years of age. Seventy-one percent of those who scored in the borderline range (70-80) on this test at age 3 were in the normal range by age 8, and almost half of those who scored in the mental retardation range (below 70) at age 3 had scores in the normal range by age 8. Only children with both early onset intraventricular hemorrhage and evidence of significant brain damage had consistent decreases in scores. Overall, twenty-three percent of the children had scores that decreased with time.
The researchers examined a number of environmental and biological factors to see if they influenced the results of the cognitive tests. They found that increasing age, more years of maternal education, residence in a two-parent household, not needing special services, and absence of significant brain injury were all linked to higher scores on both tests. However, children whose mothers had less than a high school education scored higher if they used special services. Birthweight, indomethacin treatment, and other factors did not significantly impact children's scores on these tests.
"These results are the first indication that the brain may recover from injury over time in these children," Dr. Ment says. Verbal comprehension scores increased by more than twice as much in this group of children as would be expected in the normal population, she adds. Many experiments have suggested that animals can recover from early brain injury, but similar results have not previously been shown in prematurely born children.
Several factors may explain the difference between the new results and those of previous studies, Dr. Ment says. The new study began at about the same time that a drug known as surfactant became widely available to treat respiratory distress syndrome, which is a common, life-threatening problem in premature infants. Because of this and other advances in medical care, about 85 percent of very low birthweight infants now survive - many more than in previous years. Also, previous studies did not evaluate the same groups of children with the same tests each time, she adds.
This study would not have been possible without the commitment of the parents who brought their children to be tested repeatedly during the course of the study, Dr. Ment says. She and her colleagues are continuing to study this group of children. A previous study has shown that these children's brains are smaller than those of other children at age 8, so the researchers are giving the children brain scans at age 12 to determine if the brain size differences persist or decrease over time.
They are also repeating the cognitive tests at that time. Ultimately, they hope to study the children through the end of high school to see how they mature.
The NINDS is a component of the National Institutes of Health in Bethesda, Maryland, part of the U.S. Department of Health and Human Services, and is the nation's primary supporter of biomedical research on the brain and nervous system.
*Ment LR, Vohr B, Allan W, Katz KH, Schneider KC, Westerveld M, Duncan CC, Makuch RW. "Change in Cognitive Function Over Time in Very Low-Birth-Weight Infants." Journal of the American Medical Association, February 12, 2003, Vol. 289, No. 6, pp. 705-711.