Megalencephaly and chromosomal anomaly.

A 6-year-old mentally retarded boy died after cardiac surgery for Fallot's tetralogy. The brain was of enormous size, weighing 2230 grams and showing widespread polymicrogyria and dysdifferentiation to regular large pyramidal neurons without increased DNA content. Nuclear Barr bodies indicated a XXY-chromosomal (Klinefelter) syndrome, which has not been previously reported in megalencephaly. True megalencephaly (excluding the symptomatic forms) should remain a pathological diagnosis and restricted to cases with an abnormally high brain weight in addition to morphological alterations. There is evidence for a genetic base in many cases of megalencephaly, and chromosomal studies may yield more information in this respect