Rare Immunology News

Disease Profile


Prevalence estimates on Rare Medical Network websites are calculated based on data available from numerous sources, including US and European government statistics, the NIH, Orphanet, and published epidemiologic studies. Rare disease population data is recognized to be highly variable, and based on a wide variety of source data and methodologies, so the prevalence data on this site should be assumed to be estimated and cannot be considered to be absolutely correct.


US Estimated

Europe Estimated

Age of onset






Autosomal dominant A pathogenic variant in only one gene copy in each cell is sufficient to cause an autosomal dominant disease.


Autosomal recessive Pathogenic variants in both copies of each gene of the chromosome are needed to cause an autosomal recessive disease and observe the mutant phenotype.


dominant X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome.


recessive Pathogenic variants in both copies of a gene on the X chromosome cause an X-linked recessive disorder.


Mitochondrial or multigenic Mitochondrial genetic disorders can be caused by changes (mutations) in either the mitochondrial DNA or nuclear DNA that lead to dysfunction of the mitochondria and inadequate production of energy.


Multigenic or multifactor Inheritance involving many factors, of which at least one is genetic but none is of overwhelming importance, as in the causation of a disease by multiple genetic and environmental factors.


Not applicable


Other names (AKA)

Macrencephaly; Unilateral Megalencephaly


Congenital and Genetic Diseases; Nervous System Diseases


Hemimegalencephaly is a rare malformation involving one side of the brain.[1][2] It may occur alone or in association with other syndromes such as Proteus syndrome, tuberous sclerosis, linear sebaceous nevus syndrome, neurofibromatosis, Sturge-Weber syndrome, or Klippel-Trenaunay syndrome.[1] Children with this disorder may have a large, asymmetrical head accompanied by seizures, partial paralysis, and impaired cognitive development.[2][3] Because the seizures associated with hemimegalencephaly are difficult to treat with anticonvulsant medications, a surgery called hemispherectomy is often the most successful treatment.[1][3] The cause of hemimegalencephaly is not fully understood, but involves a disturbance of cells early in development and likely involves genes involved in patterning and symmetry.[1][2]


Because the seizures associated with hemimegalencephaly are difficult to control with anti-epileptic medications, most patients undergo surgery to separate one hemisphere of the brain from the other (hemispherectomy). There are several surgical options to consider. One is functional hemispherectomy which involves severing the nerves and tissue connecting one side of the brain to the other, leaving the brain within the skull. Another is a complete or anatomic hemispherectomy in which the affected side of the brain is surgically removed. These surgeries are usually performed by a neurosurgeon with experience treating epilepsy syndromes.[1][3] Over time, the remaining side of the brain may take over the functions lost.[3]


Support and advocacy groups can help you connect with other patients and families, and they can provide valuable services. Many develop patient-centered information and are the driving force behind research for better treatments and possible cures. They can direct you to research, resources, and services. Many organizations also have experts who serve as medical advisors or provide lists of doctors/clinics. Visit the group’s website or contact them to learn about the services they offer. Inclusion on this list is not an endorsement by GARD.

Learn more

These resources provide more information about this condition or associated symptoms. The in-depth resources contain medical and scientific language that may be hard to understand. You may want to review these resources with a medical professional.

Where to Start

  • Information on cephalic disorders and megalencephaly can be found on the National Institute of Neurological Disorders and Stroke (NINDS) Web site. To view the information pages click on the links above.
  • The Johns Hopkins Medicine Web site provides information on hemimegalencephaly. Click on the link above to view this information page.
  • The National Organization for Rare Disorders (NORD) has a report for patients and families about this condition. NORD is a patient advocacy organization for individuals with rare diseases and the organizations that serve them.

    In-Depth Information

    • MeSH® (Medical Subject Headings) is a terminology tool used by the National Library of Medicine. Click on the link to view information on this topic.
    • The Monarch Initiative brings together data about this condition from humans and other species to help physicians and biomedical researchers. Monarch’s tools are designed to make it easier to compare the signs and symptoms (phenotypes) of different diseases and discover common features. This initiative is a collaboration between several academic institutions across the world and is funded by the National Institutes of Health. Visit the website to explore the biology of this condition.
    • Orphanet is a European reference portal for information on rare diseases and orphan drugs. Access to this database is free of charge.
    • PubMed is a searchable database of medical literature and lists journal articles that discuss Hemimegalencephaly. Click on the link to view a sample search on this topic.


      1. Crino P. Hemimegalencephaly. National Organization for Rare Disorders (NORD). 2012; https://rarediseases.org/rare-diseases/hemimegalencephaly/. Accessed 11/27/2015.
      2. NINDS Megalencephaly Information Page. National Institute of Neurological Disorders and Stroke (NINDS). June 30, 2015; https://www.ninds.nih.gov/disorders/megalencephaly/megalencephaly.htm. Accessed 11/27/2015.
      3. Hemimegalencephaly. Johns Hopkins Medicine. https://www.hopkinsmedicine.org/neurology_neurosurgery/centers_clinics/epilepsy/seizures/causes/hemimegalencephaly.html. Accessed 11/27/2015.

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