Megalencephaly: definition and classification.

The various definitions and classifications of megalencephaly are reviewed, and numerous diseases and syndromes associated with megalencephaly are listed. A new definition of megalencephaly based on quantitative radiographic features is proposed. We define megalencephaly as a brain volume which exceeds the mean by more than twice the standard deviation. Furthermore, a modified etiopathogenic classification of megalencephaly results in three main groups, viz anatomic, metabolic and dynamic megalencephaly. The clinical pictures in these main groups of megalencephaly, and the largest subgroup of anatomic megalencephaly, familial anatomic megalencephaly, appear to be quite different.