MONDO_0016608 (megalencephaly) can be described as follows. A congenital abnormality in which the occipitofrontal circumference is greater than two standard deviations above the mean for a given age. It is associated with hydrocephalus; subdural effusion; arachnoid cysts; or is part of a genetic condition (e.g., alexander disease; sotos syndrome). Also known as: macroencephaly, megalencephaly (disease).