Prematurity and Low Birth Weight Tied to Hyperactivity Disorder

AARHUS, Denmark, June 5 ? Children born prematurely and those born at term but with low birth weights have an increased risk of developing so-called hyperkinetic disorder, according to researchers here.

The syndrome, hyperkinetic disorder, includes a variety of activity and attention disturbances-hyperactivity, poor attention span, impulsive behavior--equivalent to attention deficit hyperactivity disorder (ADHD combined type), according to a report in the Archives of Disease in Childhood. It is the fourth most frequent discharge diagnosis in child psychiatry in Denmark.

The finding emerged from a 14-year birth-record study that included a comparison of 834 children diagnosed with hyperkinetic disorder between ages of two and 18 (median 8.8) and 20,100 controls with no mental disorders.

Data came from four Danish longitudinal registers, while controls were randomly selected from births during the same period, said Karen Linnet, M.D., Ph.D., at Aarhus University Hospital here, and colleagues.

According to the findings, children born close to term at 34 to 36 weeks had a 70% increased risk of developing hyperkinetic disorder (RR 1.7, 95% CI 1.2 to 2.5). Babies born before 34 complete weeks had an almost three-fold increased risk (RR 2.7, CI 1.8-4.1), the researchers reported.

Babies born at term but with low birth weights were also at risk of hyperkinetic disorder. Compared with babies having a birth weight above 2,999 g, term-babies weighing only 1,500 -2,499 g had a 90% increased risk (RR 1.9 CI 1.2-2.9).

Finally, even those with marginally lower birth weights-2,500-2,999 g-- had a 50% increased risk (RR 1.5 CI 1.2-1.8). Results were unchanged even after adjusting for differences in gestational ages, the investigators said.

Single parenthood, social and economic deprivation, and young age of the parents were all risk factors for hyperkinetic disorder, the investigators said. However, the results held even after adjusting for these factors as well as for parents' socioeconomic status, family history of psychiatric disorders, conduct disorders, comorbidity, and maternal smoking during pregnancy.

Of the children with confirmed hyperkinetic disorder, nine out of 10 were boys, although the increased risk was basically the same for both genders, the researchers wrote. However, the small number of girls in the study may have been a factor, they said. It is possible that the central dopamine system matures more lowly in males than in females thereby increasing the period of vulnerability, they speculated.

A strength of the study was its large population, which provided prospective information about each individual, eliminating the risk of parental recall. Adjustment for genetic factors was also an important advantage compared with previous studies, Dr. Linnet said.

The researchers noted that they used birth weight as a proxy measure of intrauterine growth retardation. However, they said, this measure is difficult and is not ideal and that therefore, the results on intrauterine growth retardation should "be taken with caution."

Until now, Dr. Linnet said, most studies of clinically verified ADHD were performed with small samples and children with extremely low gestational ages at birth (under 28 weeks). However, she said, the majority of preterm babies are born at higher gestational ages, and "our findings may therefore have more important public health perspectives."

Detailed knowledge of how the fetal brain is affected by preterm birth and intrauterine growth retardation do not exist, the researchers wrote. Because developing neurons are more vulnerable to cell death during the perinatal period, biological and environmental insults associated with preterm birth may account for some of the anatomical differences found among children with ADHD, the researchers wrote.

Further studies related to the delivery, causes of preterm delivery, and conditions in the perinatal period, are therefore needed, they concluded.