Hemimegalencephaly: Understanding One-Sided Brain Overgrowth
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer: What is Hemimegalencephaly?
Hemimegalencephaly is a rare neurological disorder with abnormal enlargement of one half of the brain. The overgrowth often causes severe, recurrent seizures that usually start in early infancy and resist standard medicines. Management focuses on controlling seizures, often with neurosurgery that disconnects or removes the affected hemisphere to protect the child's developing brain.
What the research says
Surgery in infancy can be safe and effective in experienced centers. A retrospective review of 21 infants who had functional hemispherectomy (hemimegalencephaly was the cause in 11) reported a favorable seizure outcome (Engel class I-II) in 95%, with a mean follow-up of about 7.7 years. All received transfusions and 14% developed hydrocephalus; it was a single-center series (Nistal et al., Journal of Neurosurgery: Pediatrics, 2026) [2].
Very early surgery may protect development. In a matched cohort of 23 infants operated on within 6 months (hemimegalencephaly was the most common cause, 13 infants) versus 115 non-surgical controls, 82.6% were seizure-free (Engel Ia) at 1 year and surgery was associated with higher developmental quotient scores. No perioperative deaths were reported (Zhao et al., Epilepsia, 2026) [3].
Alternatives are being studied for the youngest infants. In a single-center comparison, 6 of 8 infants (75%) treated with transarterial embolization as the primary treatment achieved seizure freedom. The study was small and observational (Israel et al., Pediatric Research, 2026) [4].
Understanding the Basics of Hemimegalencephaly
Hemimegalencephaly is a developmental anomaly in which one half of the brain overgrows during gestation [1]. The enlargement can involve part or all of the hemisphere and is often accompanied by structural malformations such as cortical dysplasia, where the brain's outer layer forms incorrectly [1]. These differences disrupt normal electrical activity and lead to persistent, severe seizures.
The condition is often grouped into three types based on how much is involved:
Isolated hemimegalencephaly: involves the cerebral cortex without affecting the skin or other organs [1].
Syndromic hemimegalencephaly: brain enlargement comes with symptoms in the skin and other organs, sometimes with hemihypertrophy, where one side of the body is larger than the other [1].
Total hemimegalencephaly: the most extensive form, with enlargement of the cerebellum and occasionally the brainstem [1].
Key Takeaways for Families
Feature | Details |
|---|---|
Likely Cause | A spontaneous genetic variation in early pregnancy, not inherited [1]. |
Core Symptom | Drug-resistant focal seizures or infantile spasms starting in early infancy [1]. |
Diagnostic Tools | Brain MRI, EEG, and occasionally fetal MRI during pregnancy [1]. |
Primary Treatment | Specialized neurosurgery (hemispherectomy) when medications fail [1]. |
Long-Term Goal | Early seizure control to protect cognitive and motor development [1]. |
Recognizing the Signs and Symptoms
The most common first sign is seizures in a baby, often infantile spasms or focal seizures affecting one side of the body [1]. Because the overgrown tissue disrupts the brain's electrical signals, these seizures are hard to control with standard anticonvulsant medicines.
Other physical and developmental signs may include:
Macrocephaly: an abnormally large or asymmetrical head from the underlying enlargement [1].
Motor impairment: weakness on one side of the body (hemiparesis) and movement difficulties [1].
Developmental delays: speech, movement and cognitive milestones reached later than expected [1].
Vision loss: some children have hemianopia, losing half of the visual field in each eye [1].
Causes and Associated Syndromes
Scientists do not know the exact cause. The leading explanation is a random, spontaneous genetic variation around the third week of pregnancy that affects genes important for brain development [1]. The condition is not inherited, and nothing a parent did during pregnancy is known to cause it [1].
Hemimegalencephaly is often associated with other rare genetic conditions, including:
Diagnostic and Treatment Pathways
Early, accurate diagnosis is essential for planning treatment. A fetal MRI can sometimes detect the condition before birth, but most diagnoses happen after birth through examination, health history, brain MRI, and EEG to map seizure activity [1].
The goal is to stop seizures as quickly as possible to prevent further damage to the developing brain. Treatment usually starts with anti-seizure medicines, which are often ineffective in hemimegalencephaly. In those cases, hemispherectomy, which removes or disconnects one half of the cerebral cortex, is frequently the most effective option [1].
These operations are highly complex and should be done at specialized centers by neurosurgeons experienced in pediatric epilepsy surgery. Challenges include malformed blood vessels, which can cause bleeding, and missing brain landmarks [1].
Long-Term Outlook and Prognosis
Prognosis is a spectrum that depends largely on how well seizures are controlled and how extensive the malformation is [1]. Many children have some intellectual or motor disability, but early surgery can lead to positive outcomes.
In a study cited by Cleveland Clinic, 68% of children were seizure-free after surgery, and 43% had average or mildly impaired cognition [1]. Continued physical, occupational and speech therapy is vital to help each child reach their potential.
Conclusion
Hemimegalencephaly is a challenging diagnosis, but advances in pediatric neurosurgery and early intervention offer real hope. Rapid seizure control and comprehensive developmental support give families the best chance at a good quality of life for their child.
Further resources: Child Neurology Foundation and Epilepsy Foundation.
Frequently Asked Questions
What is the main cause of hemimegalencephaly?
The exact cause is not known. It is thought to result from a random, spontaneous genetic variation during early fetal development, and it is not inherited from parents.
Can hemimegalencephaly be detected during pregnancy?
In some cases a fetal MRI can identify the condition before birth.
Why are seizures so common in this condition?
The overgrown, malformed brain tissue disrupts normal electrical signaling, which leads to epilepsy.
Is hemispherectomy a common treatment?
Because seizures are often resistant to medication, surgery is frequently needed in children with this condition.
What is the difference between functional and anatomical hemispherectomy?
A functional hemispherectomy disconnects the abnormal hemisphere and leaves it in place. An anatomical hemispherectomy removes it.
Will my child have a large head?
An enlarged or asymmetrical head (macrocephaly) is a common physical sign.
Can medication alone control the seizures?
In most cases the seizures are drug-resistant and surgery is needed.
What is hemiparesis?
Weakness or movement difficulty on one side of the body, which is common in children with this condition.
How does the condition affect vision?
Some children lose half of their visual field in each eye, a condition called hemianopia.
Is the condition life-threatening?
The malformation itself is not immediately fatal, but uncontrolled seizures can cause serious harm to the developing brain.
What is the success rate of surgery?
A study cited by Cleveland Clinic found that 68% of children were seizure-free after surgery. Results vary by center and by child.
Can children with this condition walk and talk?
Many can, though some have significant delays or disabilities.
Is hemimegalencephaly the same as megalencephaly?
No. Hemimegalencephaly affects one side of the brain, whereas megalencephaly involves enlargement of the whole brain.
What is cortical dysplasia?
A malformation in which the outer layer of the brain does not form correctly, often seen with this condition.
Are there different types of the condition?
Yes. The article describes isolated, syndromic and total hemimegalencephaly.
What is hemihypertrophy?
Hemihypertrophy is when one side of the whole body is larger than the other, sometimes seen in the syndromic type.
How is the cerebellum involved?
In total hemimegalencephaly, the cerebellum and sometimes the brainstem are also enlarged.
What is an EEG used for?
An EEG records electrical activity in the brain to map and monitor seizures.
Can the condition affect the skin?
Yes. In the syndromic type, skin symptoms may appear months or years after birth.
Is the surgery dangerous?
The surgery is a highly complex brain operation with risks such as bleeding, and it should be done by a very experienced team at a specialized center.
When do symptoms usually start?
Seizures and spasms often begin in early infancy.
What is the corpus callosum?
The corpus callosum is the tissue that connects the two sides of the brain, and it may be malformed in this condition.
Are the ventricles affected?
The fluid-filled cavities (ventricles) can be enlarged on the affected side.
What is McCune-Albright syndrome?
McCune-Albright syndrome is one of the rare genetic conditions that may occur alongside hemimegalencephaly.
Can a child live a normal life?
Outcomes vary. Some children achieve near-typical development when seizures are controlled early.
Is physical therapy necessary?
Physical therapy is an important part of care for motor difficulties and hemiparesis, along with occupational and speech therapy.
How does the condition affect intelligence?
Many children have some level of intellectual disability, ranging from mild to severe. In the study cited by Cleveland Clinic, 43% had average or mildly impaired cognition after surgery.
What is the first sign I should look for?
Infantile spasms or seizures are typically the first noticeable symptom.
Is there a cure for hemimegalencephaly?
The abnormal brain tissue cannot be reversed, but surgery can effectively control seizures in many children.
How long does the surgery take?
Hemispherectomy is a long, multi-hour operation because of its complexity.
Can both sides of the brain be affected?
By definition it affects one side, although malformations are sometimes present on the other side too.
What is homonymous hemianopia?
A loss of the same half of the visual field in both eyes.
Does the condition affect the heart?
Hemimegalencephaly is a neurological condition, but syndromic types can involve the skin and other organs.
How rare is the condition?
Hemimegalencephaly is considered a rare neurological condition.
What is the role of an MRI in diagnosis?
MRI shows the structure of the brain in detail and identifies enlargement and malformations.
Can a child attend regular school?
Depending on cognitive development and seizure control, some children attend regular school and others need specialized schooling.
What is a focal seizure?
A seizure that starts in one specific area or side of the brain.
How often should my child see a neurologist?
Frequent monitoring is needed, especially during infancy and after surgery. Your care team will set the schedule.
Are there support groups for families?
Yes. Many organizations support families dealing with rare pediatric epilepsy and brain malformations.
Can adults have this condition?
Most people are diagnosed in infancy, but individuals live into adulthood with varying levels of support.
What is Proteus syndrome?
A rare condition involving overgrowth of various tissues, sometimes associated with hemimegalencephaly.
Does the enlarged side of the brain work better?
No. The overgrown tissue is usually malformed and does not function normally.
Is there any non-surgical treatment?
Medication is tried first, but surgery is the main treatment for drug-resistant cases.
What is the bleeding risk during surgery?
The bleeding risk during surgery can be significant because malformed blood vessels are common in these patients, which is why experienced centers are recommended.
Can the condition be prevented?
No. Nothing a parent does during pregnancy is known to cause it, and it is not preventable.
What does the cerebellum do?
The cerebellum coordinates movement and balance. Its enlargement marks the total type of the condition.
How do I explain this to other family members?
Explain that it is a rare, non-inherited brain overgrowth that needs specialized care.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional for diagnosis and treatment of neurological conditions.
Related Reading
Hemispherectomy: Types, Procedure, Recovery and Outcomes explains how removing or disconnecting half of the brain stops seizures that medicine cannot control, plus recovery and outcomes.
References
Cleveland Clinic. Hemimegalencephaly: Causes, Symptoms & Treatment. Cleveland Clinic. Last updated August 19, 2024. https://my.clevelandclinic.org/health/diseases/hemimegalencephaly
Nistal D, Buckley R, Boop S, et al. Hemispherectomy in infants: an institutional experience with 21 patients. Journal of Neurosurgery: Pediatrics. 2026;37(3):220-226. PMID 41569724. https://doi.org/10.3171/2025.9.PEDS25241
Zhao HQ, Wang FP, Wang YY, et al. Neurodevelopmental trajectories and clinical outcomes following ultra-early epilepsy surgery in infants: a matched cohort study. Epilepsia. 2026;67(9):4457-4469. PMID 42227679. https://doi.org/10.1002/epi.70315
Israel S, Wolff N, Ruffini L, et al. Transarterial embolization versus hemispherectomy in infants with hemimegalencephaly and drug-resistant epilepsy. Pediatric Research. 2026;99(4):1523-1531. PMID 41028505. https://doi.org/10.1038/s41390-025-04405-2
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