Hemimegalencephaly

Overview

Hemimegalencephaly (say: heh-mee-meg-uh-len-SEF-uh-lee) is a rare condition in which one half of the brain — called a hemisphere — grows larger than the other half. It happens while a baby is developing in the womb, when the brain cells on one side grow and divide more than they should. The larger side doesn't work better; in fact, it often works less well, because the brain tissue is not built in the usual organised way. This can lead to seizures, delays in development, and weakness on one side of the body. It is present from birth, not something that develops later in life, and it is not caused by anything a parent did or did not do.

No. Hemimegalencephaly is very rare. It is estimated to affect only a small number of babies each year worldwide, so most doctors will see very few cases in their whole career. Because it is rare, you may be referred to a specialist children's hospital for care.

It is present at birth and is usually noticed in babies and young children, often because of seizures. Boys and girls are affected equally. It is not related to age, lifestyle, diet, or where someone lives. In a small number of children it is linked to a rare genetic condition that also affects other parts of the body, such as one side of the body being larger than the other or certain skin markings.

Symptoms

Common symptoms

• Seizures that may be hard to control, sometimes starting in the first days of life
• Weakness, stiffness, or tight muscles on one side of the body
• Delayed milestones such as rolling, sitting, walking, or talking
• A head that is larger than expected, or grows faster than expected
• One side of the face or body looking slightly larger than the other in some children
• Feeding difficulties, poor weight gain, or being very sleepy or very unsettled as a baby
• Vision problems, such as not tracking objects or a squint

Symptoms in children

• Seizures that look like staring spells, jerking of one arm or leg, or whole-body stiffening
• Using one hand much more than the other, or keeping one hand in a fist
• Walking on tiptoes on one side, or a limp caused by stiffness
• Speech delay, learning difficulties, or slower understanding of instructions
• Behaviour changes, frustration, or tiredness that can be linked to seizure activity or medicines
• Needing more support at nursery or school than other children of the same age

Symptoms in older adults

• Hemimegalencephaly is a condition people are born with, so most adults with it have lived with it since childhood rather than developing it later
• Some adults continue to have seizures and need ongoing specialist care
• Long-term movement difficulties, such as weakness or stiffness on one side, may continue
• Some adults need support with daily living, work, or education, while others live independently
• Adults may also need help with anxiety, low mood, or fatigue linked to living with a long-term condition

Causes

Main causes

• Hemimegalencephaly happens when cells on one side of the developing brain grow and divide more than they should, so that side becomes larger and less organised than the other
• In many children it is linked to changes in genes that control how and when brain cells grow — these changes usually happen by chance, not because of anything either parent did
• It can occur as part of a rare genetic condition that affects other parts of the body too, or on its own with no other affected family members
• It is not caused by infection, diet, stress, medicines, or injury during pregnancy

Risk factors

• There are no lifestyle risk factors you can change — it is not linked to smoking, alcohol, diet, or exercise during pregnancy
• Having a child with a rare genetic growth condition slightly increases the chance that another child could be affected, which is why genetic counselling is sometimes offered
• It is not inherited in most cases, and most families have no history of the condition

When to see a doctor

See a doctor urgently if:

• Your baby has a seizure, or you notice unusual jerking, staring, or stiffening episodes — seek medical help the same day
• A baby's head seems to be growing very fast, or the soft spot on top feels tight or bulging
• Your child loses skills they had before, such as reaching, sitting, or talking
• Feeding becomes difficult, or your baby is not gaining weight
• You are worried about how sleepy, floppy, or stiff your baby is

Book a routine appointment if:

• You have noticed your child clearly using one side of the body less than the other
• Your child is missing developmental milestones, or you want to check whether their progress is typical
• You would like to discuss therapy, school support, or benefits and respite care
• You have questions about medicines, side effects, or what to do in an emergency
• You want to talk about how you are coping as a carer or family member

Diagnosis

Doctors usually start by asking about your child's history and doing a physical and brain-and-nerve examination. They look at head size and shape, movement on each side of the body, and how your child responds. The key test is a detailed scan of the brain, which shows the difference in size and structure between the two halves. If seizures are happening, brain wave testing helps show where they start. Some children also have blood tests to look for a genetic cause. Diagnosis is usually made by a specialist children's brain team, often quite soon after birth when seizures bring a baby to medical attention.

Tests that may be done

• Brain imaging, usually an MRI scan — this gives detailed pictures and is the main way the condition is confirmed
• EEG (electroencephalogram) — sticky pads on the head record the brain's electrical activity and help show where seizures begin
• A careful physical and developmental check, including head measurements over time
• Blood tests, and sometimes a saliva sample, to look for a genetic cause
• Eye and vision checks, since the visual pathways can be affected
• Hearing tests and speech and language assessment as your child grows
• Sometimes video monitoring in hospital to record seizures as they happen

What to expect at your appointment

Some tests need your child to stay very still, so a baby or young child may be gently sedated for a scan — your team will explain this. EEG tests are painless but can take an hour or more, and your child may need to stay overnight for video monitoring. Results often come in stages, so you may be asked to come back for a follow-up appointment. It can help to write down your questions beforehand and take someone with you. Ask the team to explain anything you do not understand — it is always fine to ask again.

Treatment

There is no single cure, but there is a lot that can be done. Treatment aims to control seizures, support development, keep your child comfortable and safe, and help the whole family. Care is usually led by a specialist children's brain team working with your family doctor, therapists, and nursery or school. Which treatments are right depends on your child's age, how severe the seizures are, how development is going, and what matters most to your family.

Self-care at home

• Keep a simple seizure diary — what happened, how long it lasted, and what your child was doing beforehand. This is genuinely useful for the medical team
• Learn seizure first aid: stay calm, protect the head, turn the person onto their side, do not put anything in the mouth, and time the seizure
• Make your home safer where you can — for example, padding sharp corners, using a helmet if your child falls often, and supervising baths
• Follow the agreed sleep routine, since tiredness can trigger seizures in some children
• Ask your pharmacist or doctor before giving any new medicine, including over-the-counter remedies and herbal products, because some can interact with seizure medicines
• Never stop or change seizure medicine suddenly on your own — this can cause serious seizures
• Keep a folder of letters, scan reports, and contact numbers, and share a short one-page summary with nursery, school, or anyone who cares for your child
• Look after yourself too — rest, accept offers of help, and tell your doctor if you are struggling to cope

Medical treatments

Treatment usually combines several approaches. Seizure-controlling medicines are the first step, chosen and adjusted by a specialist; finding the right combination can take time, and the team will watch for side effects. Therapies such as physiotherapy, occupational therapy, and speech and language therapy help with movement, daily skills, and communication. Some children are offered a special medically supervised diet that can reduce seizures; this is only done with a specialist diet team. A child may also need support with feeding, vision, hearing, or behaviour, and educational support is arranged through nursery or school. Regular check-ups track development and adjust the plan as your child grows. Your specialist team will explain the options and the pros and cons for your child specifically.

When is surgery considered?

For some children whose seizures continue despite medicines and who are clearly affected by them, brain surgery may be discussed. The most common operation removes or disconnects the overgrown half of the brain so that seizures cannot spread from it. This is major surgery and is only considered after careful testing by a specialist team. It can greatly reduce or stop seizures for many children, but it also carries risks, including weakness on one side and effects on vision and learning. Families are given time and counselling to weigh this decision, and it is always a shared choice between the family and the medical team.

Living with this condition

Life with a child who has hemimegalencephaly often settles into a rhythm, but it can be demanding. Days may include medicines, therapy appointments, school or nursery meetings, and watching for seizures. Try to keep routines simple and predictable, and build in rest for your child and for you. Many families find it helps to have one notebook or phone note for appointments, questions, and seizures, and to name one person who coordinates information. Celebrate small steps — a new word, a steadier step, a longer stretch between seizures all matter.

Lifestyle tips

• Keep a regular sleep schedule and a calm, predictable bedtime routine
• Plan activities that include your child rather than excluding them — swimming, soft play, music, and sensory play can all be adapted
• Tell close friends, family, and any carers what to do if a seizure happens, and practise it together
• Ask about a medical alert bracelet or card if your child is mobile and may be cared for by others
• Build a small support network of people who can step in for an hour or a day so you can rest
• Keep nursery or school informed and agree a written plan for seizures and emergencies
• Ask your team about financial help, respite care, and disability support available where you live

Diet and exercise

There is no special diet that cures hemimegalencephaly, and no food or supplement replaces medical treatment. A balanced, varied diet with enough calories matters, especially for babies and children who feed with difficulty — a feeding specialist can help with textures, positioning, and growth. Your child should be as active as they safely can be: movement helps strength, balance, mood, and sleep. Physiotherapy can suggest safe activities. If a special seizure-control diet is recommended, it must be planned and monitored by a specialist diet team, never started on your own. Ask your doctor before giving supplements or herbal products.

Mental health and emotional wellbeing

It is very common for parents and carers to feel grief, guilt, anxiety, exhaustion, or anger — none of these mean you are coping badly. Children and young people can also feel frustrated, left out, or anxious, especially around seizures or hospital visits. Talk openly and simply about the condition in words that suit their age, and let them ask questions. If low mood, worry, or sleep problems are affecting you or your child, tell your doctor — support such as counselling is available and can help. If you ever feel you cannot keep yourself or your child safe, contact your local emergency number or a crisis support service straight away.

Prevention

Hemimegalencephaly cannot be prevented. It happens because of changes in how brain cells grow before birth, and nothing a parent does or does not do during pregnancy causes it. Please do not blame yourself. If a genetic cause is found, a genetics team can talk with you about what it might mean for future pregnancies and what options exist.

Vaccines

There is no vaccine for hemimegalencephaly. Children with this condition should still receive their routine childhood vaccinations, which protect them from serious infections. Because some children with seizure conditions may have a fever-related seizure after vaccination, it is sensible to mention your child's condition to your doctor or nurse so you can plan together — but this is a conversation, not a reason to skip protection.

Screening programmes

There is no routine screening test for hemimegalencephaly in pregnancy. It is usually found after birth when a baby has seizures or a rapidly growing head. If a close family member has a rare genetic growth condition, a genetics team may offer counselling and, in some cases, testing.

Complications

If left untreated

• Seizures that continue and become harder to control, which can slow development further
• Ongoing developmental delay and learning difficulties
• Weakness or stiffness on one side of the body, which can affect walking and hand use
• Vision problems, including a reduced field of vision
• Feeding and growth difficulties
• Build-up of fluid in the brain in some children, which needs treating
• Behaviour and sleep difficulties, and increased strain on the whole family
• Rarely, serious complications from prolonged seizures — which is why seizure first aid and an emergency plan matter

Long-term outlook

Every child's path is different, and no one can predict it exactly at the start. What is known is that early treatment of seizures, good therapy, and strong family support can make a meaningful difference. Some children's seizures come under good control with medicines; others benefit greatly from surgery, with some becoming seizure-free. Many children make steady progress in communication, movement, and independence over years, even if they need support along the way. The plan will change as your child grows, and the team will keep adjusting it with you. It is reasonable to hope for good things — and to ask your team what to expect for your child specifically.

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