Question: What is megalencephaly, and how do I choose a diagnosis code for the condition? Connecticut Subscriber Answer: Coding for the condition is easy enough. Report Q04.5 (Megalencephaly). Condition description: Megalencephaly, which is also sometimes referred to as macrencephaly, is a condition where a patient has an abnormally large brain specifically due to an increased number of neurons and glial cells. This condition could be associated with a number of genetic and neurological disorders.