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Megalencephaly (also known as macrencephaly) is a condition in which an infant or child has a large, heavy, and potentially malfunctioning brain. By definition, the brain weight is greater than average for the age and biological sex of the child. Head enlargement may be evident at birth or the head may become larger in the early years of life.

Megalencephaly is thought to be related to a disturbance in the regulation of cell production in the brain. Neuron proliferation, or the process through which nerve cells divide to form new generations of cells, is generally regulated so that the correct number of cells is produced in the proper place at the appropriate time. In a megalencephalic brain, too many cells are produced either during development or progressively as part of another disorder, such as one of the neurofibromatoses or leukodystrophies.

Symptoms of megalencephaly include:

  • Delayed development
  • Seizures
  • Corticospinal (brain cortex and spinal cord) dysfunction

Megalencephaly affects males more often than females. Unilateral megalencephaly or hemimegalencephaly is a rare condition that is characterized by the enlargement of one side of the brain. Children with this disorder may have a large, asymmetrical head accompanied by seizures, partial paralysis, and impaired cognitive development. Megalencephaly is different from macrocephaly (also known as megacephaly or megalocephaly), which describes a big head, and which doesn't necessarily indicate abnormality. Large head size can be passed down through the generations in some families.

How can I or my loved one help improve care for people with megalencephaly?

Consider participating in a clinical trial so clinicians and scientists can learn more about megalencephaly and related disorders. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

All types of volunteers are needed—those who are healthy or may have an illness or disease—of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible, and that treatments will be safe and effective for everyone who will use them.

For information about participating in clinical research visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with megalencephaly at

Where can I find more information about megalencephaly?

Information may be available from the following:

March of Dimes
Phone: 888-663-4637


Content source: Accessed June 23, 2023.

The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.

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