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Authors: Bianca C. Kapoor-Heaphy, MD; Sonika Agarwal, MBBS, MD 
Children’s Hospital of Philadelphia 

Reviewed: October 2022


Hemimegalencephaly (HME) is a rare birth defect. It occurs when one half of the brain is larger than the other. The affected half may also form abnormally.  

Sometimes HME is an isolated abnormality. Other times, it occurs with other problems. It can be part of a genetic or syndromic diagnosis.  

Nearly all children will have seizures. The majority of cases are difficult to fully control with medication. Some children may have:  

  • Developmental delays.  
  • Weakness on one side of the body. This typically affects the side opposite the abnormal brain hemisphere.


Disorder Overview


Typically, the two halves of the brain are the same size. HME means that one half of the brain is larger than the other.  

HME occurs because the affected half of the brain overgrows. This can happen to part of the affected side or the entire half. Sometimes the affected half is also malformed.  

Children with HME present at a young age. They typically present with seizures or weakness on one side of the body.  

With advances in imaging, HME has also been diagnosed in pregnancy. This is done by fetal magnetic resonance imaging (fetal MRI). A fetal MRI is performed when the baby’s brain is developing in the womb.

cerebral ALD 1


HME is diagnosed with brain imaging either after birth or during pregnancy. An MRI is usually performed. Seizures are diagnosed through a medical history and physical exam. A brain wave test called an electroencephalogram (EEG) is also often performed.  

Common symptoms include:  

  • Seizures, including focal seizures (affecting one side of the body) and infantile spasms 
  • Macrocephaly or a large head  
  • Movement difficulties 
  • Weakness on one side of the body 
  • Developmental delays 
  • Vision problems (hemianopia or loss of half the vision in each eye) 

When HME is diagnosed, other syndromes should be considered. These syndromes include: 

  • Epidermal nevus syndrome 
  • Klippel-Trenaunay syndrome 
  • McCune-Albright syndrome 
  • Neurofibromatosis 1 (NF1) 
  • CLOVES syndrome 
  • Proteus syndrome 
  • Hypomelanosis of Ito


We do not yet fully understand why HME occurs. Many researchers think there may be a genetic cause. Genetic causes may be:  

  • Inherited from parents  
  • Due to a new change in a child’s genes  

These result in overgrowth of one brain hemisphere: 

  • Defective organization  
  • Migration of neuronal cells 

The abnormal hemisphere may have abnormalities including cortical malformation (CNF Neuronal Migration Link). Cortical malformation is when the outer layer of the brain develops abnormally.  

Others believe HME is due to damage to the brain during pregnancy. It is possible a cause for HME will never be found.  

Sometimes HME occurs in a syndrome, or a group of symptoms that occur together. HME can occur in:

Genetic syndromes.

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Neurocutaneous syndromes.

There are many types of these disorders that affect the brain and skin.

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If a child is found to have one of these syndromes, it may explain why they have HME.

Spinal Muscular Atrophy 1


There are no laboratory tests to diagnose HME. Sometimes laboratory tests may be done to:

Look for a syndrome that includes HME.

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Monitor treatment.

For example, antiseizure medication may be used.

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Perinatal Stroke 1


Children with HME should be followed by a pediatric neurologist.  

Treatment for HME most often includes treating seizures. Seizure treatment starts with anti-seizure medication. However, seizures are often difficult to control. They may require other treatments. These include:

Special diets.

A ketogenic diet may be recommended.

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Neuromodulation treatments.

For example, vagus nerve stimulation may be used.

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Some patients may undergo surgery. This is to separate the two halves of the brain or remove the abnormal hemisphere (hemispherectomy). This surgery is done by a trained neurosurgeon. Surgery can control seizures when other treatments cannot.  

Several tests are done as part of surgical planning. This includes imaging. Before surgery, a team of specialists meets to decide the best surgical treatments. This team includes: 

  • Pediatric neurologists (epilepsy surgery specialists)  
  • Neurosurgeons  
  • Other specialists 

Some children may have:  

  • Movement difficulties 
  • Weakness 
  • Developmental delays  

These can help: 

  • Occupational therapy 
  • Physical therapy 
  • Speech therapy 
  • Special instruction and educational support 
  • Vision therapy 
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Children with HME cannot outgrow it. Most children also have developmental delays. These include differences with:  

  • Movement 
  • Communication 
  • Socialization 
  • Learning 
  • Vision 

These delays are often severe. However, they can be mild in some children. In school, some children may receive help with an individualized education plan (IEP).  

Most children will have seizures. Treating seizures is important. Earlier seizure control may improve development. Often seizures fail to respond to medication. They may require surgery.  

A pediatric neurologist will help treat seizures. People that care for a child with HME may need to treat prolonged seizures or clusters of seizures. They should have a seizure rescue plan (also called a seizure action plan).  


Hemimegalencephaly Family Support Network 

Hemimegalencephaly Family Support Network (HFSN) is a web-based HoME of information, hope, support, encouragement and connections created for families of children who have been diagnosed with Hemimegalencephaly (HME). The HFSN strives to be a place where affected families can find not only relevant and essential information, but, just as important, all the hope, support, encouragement and connections that make the daily journey a little easier, a lot less scary, and not nearly as lonely. HFSN hosts a private Facebook group for families of children (or the children/adults themselves) diagnosed with HME. 

Pediatric Epilepsy Surgery Alliance
The Pediatric Epilepsy Surgery Alliance (formerly known as The Brain Recovery Project) enhances the lives of children who need neurosurgery to treat medication-resistant epilepsy. They empower families with research, support services, and impactful programs before, during, and after surgery. PESA’s programs include research-based, reliable information to help parents and caregivers understand when a child’s seizures are drug-resistant; the risks and dangers of seizures; the pros and cons of the various neurosurgeries to treat epilepsy; the medical, cognitive, and behavioral challenges a child may have throughout life; school, financial aid, and life care issues. PESA’s resources include a comprehensive website with downloadable guides, pre-recorded webinars, and virtual workshops; an informative YouTube channel with comprehensive information about epilepsy surgery and its effects; a private Facebook group (Education After Pediatric Epilepsy Surgery) with over 300 members; Power Hour (bi-monthly open forums and live virtual workshops on various topics); and free school training to help your child’s education team understand the impact of their epilepsy surgery in school. Their Peer Support Program will connect you with a parent who has been there. The Pediatric Epilepsy Surgery Alliance also hosts biennial family conferences and regional events that allow families to learn from experts, connect with other families, and form lifelong friendships. They also provide a travel scholarship of up to $1,000 to families in need to fund travel to a level 4 epilepsy center for a surgical evaluation.

In addition, PESA has resources for medical professionals to assist in helping clinicians help the parents of their patients find the resources they need after surgery. Educators and therapists will also find helpful resources and information, including videos, guides, and relevant research. Patients who have undergone surgery are encouraged to register with the Global Pediatric Epilepsy Surgery Registry to help set future research priorities.



JCN: NICU Series — Cortical malformations: Polymicrogyria, pachygyria, lissencephaly, heterotopia

Podcast from SAGE Neuroscience and Neurology/Journal of Child Neurology (JCN). Dr. Sonika Agarwal of Children’s Hospital of Philadelphia talks about Cortical malformations: Polymicrogyria, pachygyria, lissencephaly, heterotopia.

Childhood Stroke 1

Child Neurology Foundation (CNF) solicits resources from the community to be included on this webpage through an application process. CNF reserves the right to remove entities at any time if information is deemed inappropriate or inconsistent with the mission, vision, and values of CNF. 


ClinicalTrials.gov for Hemimegalencephaly (birth to 17 years).

These are clinical trials that are recruiting or will be recruiting. Updates are made daily, so you are encouraged to check back frequently. 

ClinicalTrials.gov is a database of privately and publicly funded clinical studies conducted around the world. This is a resource provided by the U.S. National Library of Medicine (NLM), which is an institute within the National Institutes of Health (NIH). Listing a study does not mean it has been evaluated by the U.S. Federal Government. Please read the NLM disclaimer for details.    

Before participating in a study, you are encouraged to talk to your health care provider and learn about the risks and potential benefits.

The information in the CNF Child Neurology Disorder Directory is not intended to provide diagnosis, treatment, or medical advice and should not be considered a substitute for advice from a healthcare professional. Content provided is for informational purposes only.  CNF is not responsible for actions taken based on the information included on this webpage. Please consult with a physician or other healthcare professional regarding any medical or health related diagnosis or treatment options. 


Sencen L. Hemimegalencephaly [Internet]. Danbury, CT: NORD (National Organization for Rare Disorders). 2018 [cited 2022 Jul 27]. Available from: https://rarediseases.org/rare-diseases/hemimegalencephaly/. 

Hemimegalencephaly [Internet]. Los Angeles: The Brain Recovery Project. 2021 [cited 2022 Jul 27]. Available from: https://www.brainrecoveryproject.org/about-epilepsy-surgery/when-to-ask-for-a-surgical-evaluation/your-child-has-a-likely-drug-resistant-condition/hemimegalencephaly/. 

Wu N, Borlot F, Ali A, Krings T, Andrade DM. Hemimegalencephaly: what happens when children get older? Dev Med Child Neurol. 2014 Sep;56(9):905-9. https://doi.org/10.1111/dmcn.12390. Epub 2014 Feb 5. PMID: 24494819. 

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