Hemimegalencephaly: Causes, Symptoms, and Treatment
Medically reviewed by Harsha Moole, MD, Internal Medicine. Last reviewed: October 23, 2026.
What kind of doctor treats Hemimegalencephaly?
A neurologist, a doctor who treats brain and nerve problems, usually guides care for hemimegalencephaly. When surgery is needed, it is typically done by a neurosurgeon, a brain surgeon, who is trained in epilepsy surgery. A direct primary care or concierge doctor can be a first stop for an evaluation and can refer you to a specialist when needed.
What is hemimegalencephaly?
Hemimegalencephaly is a rare brain condition. One side of the brain is larger than the other. The brain on the affected side may look very different from normal, or it may show only small changes.
The enlarged brain tissue causes frequent seizures. A seizure is a burst of unusual electrical activity in the brain. The condition is often linked to learning and behavior problems. Hemimegalencephaly may occur on its own, or it may come with other syndromes. A syndrome is a group of signs that appear together.
Symptoms
Signs of hemimegalencephaly often show up in a baby. They can include:
- Seizures, which often begin in early infancy. Some babies have infantile spasms, a type of seizure that starts in the first year of life.
- Seizures that usually do not get better with seizure medicine.
- A large head or a head with an uneven shape.
- Weakness or trouble moving on the side of the body opposite the enlarged brain.
- Delays in thinking skills (cognitive delays) and behavior problems.
- Intellectual disability, meaning trouble with learning and daily tasks.
Some children also have skin color changes. These are seen when the condition comes with a syndrome that affects the skin.
Causes and who gets it
The exact cause is not well understood. Cells in one half of the brain grow much faster than cells in the other half. Many experts believe changes in one or more genes play a part.
Some of these gene changes are found in the PIK3CA gene. They happen early in development and are found in only some of the body's cells. Some doctors also think that damage to the baby's brain early in pregnancy may play a role.
Hemimegalencephaly can occur with other syndromes. These include Proteus syndrome, tuberous sclerosis complex, neurofibromatosis, and Sturge-Weber syndrome. It is very rare, and no estimate of how many people have it is available.
How doctors diagnose it
A neurologist may suspect the condition when a child has seizures, a large head, or skin color changes. Doctors then order a brain MRI, a scan that makes detailed pictures of the brain. An MRI is usually enough to confirm hemimegalencephaly.
An EEG, a test that records the brain's electrical activity, is used to find and define seizures. Some cases are found before birth. This can happen with ultrasound or MRI of the unborn baby. When doctors find the condition, they also look for other syndromes that may be involved.
Treatment
Seizure medicines seldom bring these seizures under control. Because of this, most patients have surgery to separate one half of the brain from the other.
There are two kinds of surgery. In a functional hemispherectomy, the nerves and tissue that connect the two sides are cut. The abnormal side stays in the skull. In an anatomic hemispherectomy, the abnormal half of the brain is removed. A neurosurgeon trained in epilepsy surgery usually does these operations. After either surgery, the remaining side of the brain may slowly take over the jobs of the affected side.
When to see a doctor
Seek medical care for any baby or child who has seizures, a large or unevenly shaped head, or weakness on one side. A child with skin color changes and seizures should also be checked.
Call 911 or go to the emergency room if a child has a seizure that lasts longer than 5 minutes, many seizures without waking up in between, trouble or abnormal breathing, bluish or grey skin, unconsciousness, or a significant change in mental status such as confusion.
How a direct primary care or concierge doctor fits in
A direct primary care doctor charges a flat monthly fee instead of billing insurance, while a concierge doctor usually charges more and often works alongside insurance. Either can do an exam, keep track of your child's care, and help coordinate visits with a neurologist and a surgeon.
Common questions
Can medicine control seizures in hemimegalencephaly?
Often it cannot. Seizure medicines seldom bring these seizures under control. This is why most patients have surgery to separate the two halves of the brain.
What is a hemispherectomy?
A hemispherectomy is brain surgery that removes or disconnects the affected half of the brain. The remaining side of the brain may slowly take over the jobs of the affected side.
Can hemimegalencephaly be found before birth?
Yes, sometimes. Ultrasound or MRI of the unborn baby has found a number of cases before birth.
Is hemimegalencephaly linked to other conditions?
Yes, it can be. It may occur with other syndromes, such as Proteus syndrome, tuberous sclerosis complex, neurofibromatosis, and Sturge-Weber syndrome.
Sources
- National Organization for Rare Disorders (NORD) - Hemimegalencephaly
- MedlinePlus - Seizures
- MedlinePlus Genetics - PIK3CA gene
Medical Disclaimer
This content is for general educational and informational purposes only and is not medical advice. It is not a substitute for professional medical advice, diagnosis, or treatment, and reading it does not create a doctor-patient relationship. Always seek the advice of your physician or another qualified health provider with any questions about a medical condition, and never disregard or delay seeking professional advice because of something you read here. If you think you may have a medical emergency, call 911 (or your local emergency number) immediately.
