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

Reviewed: October 2022

SUMMARY

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

DESCRIPTION

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.

SIGNS AND SYMPTOMS

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

CAUSES

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:

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

LABORATORY INVESTIGATIONS

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

TREATMENT AND THERAPIES

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:

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 

OUTLOOK

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).  

RELATED DISORDERS

Disorders similar to HME include:  

• Macrocephaly (a large head circumference) 
• Hydrocephalus (high pressure in the head) 
• Ventriculomegaly (enlarged fluid spaces in the brain) 

Some children with HME may have these disorders as part of their HME.

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Research 

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/. 

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