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. 
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.  
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."