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What is megalencephaly?

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.

Learn About Clinical Trials
Clinical trials are studies that allow us to learn more about disorders and improve care. They can help connect patients with new and upcoming treatment options.

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


Order publications from the NINDS Catalog
The NINDS Publication Catalog offers printed materials on neurological disorders for patients, health professionals, and the general public. All materials are free of charge, and a downloadable PDF version is also available for most publications.