Home > Encyclopedia of Neurological Disorders > Megalencephaly

Definition

Megalencephaly (also called macrencephaly) describes an enlarged brain whose weight exceeds the mean (the average weight for that age and sex) by at least 2.5 standard deviations (a statistical measure of variation). Megalencephaly may also be defined in terms of volume rather than weight. Hemimegalencephaly (or unilateral megalencephaly) is a related condition in which brain enlargement occurs in one hemisphere (half) of the brain.

Description

A person with megalencephaly has a large, heavy brain. In general, a brain that weighs more than 1600 grams (about 3.5 pounds) is considered megalencephalic. The heaviest brain on record weighed 2850 grams (about 6.3 pounds). Macrocephaly, a related condition, refers to an abnormally large head. Macrocephaly may be due to megalencephaly or other causes such as hydrocephalus (an...