Purpose: To determine whether nonreferred children with idiopathic megalencephaly show evidence of specific neurodevelopmental dysfunction compared with sibling control subjects and age-matched control subjects.

Design: Cross-sectional, case-control study in a large, suburban pediatric practice. Subjects included 20 children between the ages of 6 and 15 years with a head circumference at greater than the 98th percentile, 19 siblings of these children with normal-size heads, and 16 age-matched control subjects. Standardized tests of language, academic achievement, visuomotor integration, motor function, and neurodevelopmental function were administered.

Results: Multivariate analysis, with control for age, showed a main effect for the presence of megalencephaly ( F =3 2; p < 0.05). Follow-up univariate analyses, with control for age, showed that children with megalencephaly had poorer performance on tasks of upper limb speed, visuomotor control, running speed, bilateral coordination, visuomotor integration, naming fluency, and minor neurologic indicators.

Conclusion: The relationship between idiopathic megalencephaly and external hydrocephalus in infants is discussed. Results show that so-called "benign" idiopathic megalencephaly in nonreferred school-age children appears to be a clinical entity associated with subtle motor problems and neurodevelopmental dysfunction. (J Pediatr 1997;131;320-4)