"Megalencephaly is the enlargement of the brain without changes in its outer configuration and with a proportional increase of all its diameters" (Fletscher, von Hansemann). This condition may represent a true hypertrophy, an increase in the size of all the different neurons and neuroglial elements, or it may be a hyperplasia, an increase in the number of neurons and neuroglia. The latter condition, since Virchow, has been subdivided into true hyperplasia—an increase of neurons and neuroglia—and interstitial hyperplasia—an increase of neuroglia alone. If the hyperplasia of glial elements exceeds the normal bounds and spreads diffusely throughout the brain tissues, the interstitial hyperplasia becomes a diffuse glioblastomatosis (pseudohypertrophy, diffuse gliosis). French authors refer to Calmeil's division, "hypertrophies cérébrales simples par hypernutrition" and "processus hypertrophiques pathologiques."
If one reviews the literature on this subject,1 one finds that most of the cases of megalencephaly that have been reported belong to the group