Macrencephaly is a condition present at or soon after birth in which the brain is abnormally large. This can be due to certain diseases that make the brain larger, such as Alexander disease, later phases of Tay-Sachs disease, and spongy degeneration of infancy. Alexander disease is a slowly progressive and deadly disease caused by the loss of the structure and function of nerve cells. Tay-Sachs disease is a genetic disorder that causes progressive deterioration of mental and physical abilities. Spongy degeneration of infancy is a rare and deadly brain disease in infancy characterized by nerve cell death and loss of myelin (a fatty covering of some nerve cells) that gives the brain a spongy appearance (due to round spaces) and can also cause paralysis (loss of movement and/or sensation) and blindness.

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The large growth of the brain is macrencephaly is usually not due to the formation of tumors, which are abnormal tissues that grow faster than other tissues. Macrencephaly is associated with an abnormal enlargement of the head, which is known as macrocephaly.
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Macrencephaly is also known as macrencephalia and macroencephaly. Macrencephaly comes from the Greek word "makros" meaning "large," and the Greek word "enkephalos" meaning "brain." Put the to words together and you get "large brain."