Hemimegalencephaly

What is hemimegalencephaly?

Hemimegalencephaly is a rare neurological condition in which one side (cerebral hemisphere) of your child’s brain is larger than the other. It happens because the affected half of their brain overgrows. This can happen to part of the affected hemisphere or the entire half.

Kids with hemimegalencephaly might also have other differences in their brain anatomy, including:

• Malformed neurons. Types of malformed neurons include cytoarchitecture (how their neurons are stacked) and cortical dysplasia (when the top layer of their brain doesn’t form correctly).
• Missing or poorly formed corpus callosum (the part of their brain that connects the two sides).
• Enlarged ventricles (fluid-filled cavities in their brain) on the affected side.

Brain regions not formed normally tend to cause recurrent seizures, also called epilepsy. It’s hard to stop these seizures with antiseizure medications. When medications don’t effectively manage the seizures, epilepsy surgery may be an option in many children.

Types of hemimegalencephaly

There are several types of hemimegalencephaly, including:

• Isolated hemimegalencephaly: The condition only affects the outermost layer of your child’s brain, their cerebral cortex, with no involvement of the skin or other organs.
• Syndromic hemimegalencephaly: The condition involves their brain, as well as their skin and some other organs. They might not have symptoms in their skin for months or years after birth. They may also have hemihypertrophy, where one-half of their body is larger than the other.
• Total hemimegalencephaly: The condition includes the enlargement of their cerebellum and sometimes, their brainstem. Total hemimegalencephaly may be isolated or syndromic.

What are the symptoms of hemimegalencephaly?

The first symptom you may notice is infantile spasms or seizures in your baby. They’re typically focal seizures, affecting one side of your baby’s body. Other hemimegalencephaly symptoms may include:

• Macrocephaly (large head).
• Asymmetrical head shape.
• Movement difficulties.
• Weakness on one side of their body.
• Developmental delays.
• Vision problems, like loss of half the vision in each eye (hemianopia).

What can cause hemimegalencephaly?

Scientists don’t know what exactly causes the condition. Some believe it’s genetic (caused by a variation in your child’s DNA) but not inherited — meaning that you didn’t have the condition and didn’t pass it on to your child. A random (spontaneous) genetic variation that occurs in one of the many genes important for brain development at about the third week of pregnancy causes hemimegalencephaly. Nothing you did during your pregnancy caused the condition.

What are the risk factors for this condition?

Hemimegalencephaly may occur with other genetic conditions or syndromes, including:

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

How is hemimegalencephaly diagnosed?

To diagnose hemimegalencephaly, your child’s healthcare provider will perform a physical exam and ask about their health history. Your child will likely get the following tests to diagnose the condition:

• Brain MRI.
• Electroencephalogram (EEG).

With advances in imaging, pregnancy care providers can sometimes diagnose hemimegalencephaly during pregnancy. This is done by fetal magnetic resonance imaging (fetal MRI) on the fetus’s brain.

How is hemimegalencephaly treated?

The main goal of hemimegalencephaly treatment is controlling seizures. Seizures can start in very early infancy and are often drug-resistant.

There are many different kinds of seizures associated with hemimegalencephaly. It’s very important to note that seizures in childhood, especially if they start in early infancy, can be devastating to a child’s development. Uncontrolled seizures can lead to severe developmental and intellectual impairment and can damage a child’s developing brain. Stopping seizures as soon as possible is vital.

Seizure treatment starts with anti-seizure medication. Some children with hemimegalencephaly may be able to control their seizures throughout their lives, but most will have drug-resistant epilepsy that requires a hemispherectomy. This is a type of brain surgery where one-half of your child’s cerebral cortex is removed or disconnected from the rest of their brain.

The surgical procedure may involve “functional hemispherectomy” in which a surgeon severs the tissue and nerves connecting one side of your child’s brain to the other, but they leave the abnormal hemisphere inside your child’s skull. Anatomic or complete hemispherectomy involves disconnecting one side of your child’s brain from the other and removing the abnormal hemisphere. A neurosurgeon trained in epilepsy surgery should perform this surgery.

How difficult is a hemispherectomy for hemimegalencephaly?

Hemispherectomy is a tremendously difficult brain surgery. Hemispherectomy for hemimegalencephaly is especially difficult for many reasons. First, the blood vessels are frequently malformed, making them hard to locate and cut during surgery. This can sometimes cause excessive bleeding. In addition, your child’s overall brain malformation makes surgery hard because the familiar landmarks a surgeon uses to identify various structures in the brain are often missing. In addition, infants and young children can have substantial blood loss during this surgery that needs to be carefully monitored and replaced as necessary.

Be sure that your child’s care team is skilled and experienced with this procedure. It’s safer for your child to get this procedure in a center with a lot of experience performing hemispherectomy on infants with hemimegalencephaly.

Can hemimegalencephaly be prevented?

Researchers don’t know why exactly hemimegalencephaly occurs, so you can’t prevent it.

What is the prognosis for someone with hemimegalencephaly?

Your child’s outcome is linked closely to seizure control and whether their brain malformation is present on one or both sides of their brain.

Hemimegalencephaly is a spectrum condition, which means it has a wide range of outcomes among children. Most children with hemimegalencephaly will have some amount of intellectual disability. Some children with controlled seizures can have near-typical intellectual development. But others may have severe motor and intellectual disabilities.

All children with hemimegalencephaly have a type of cerebral palsy called hemiparesis that affects one side of their body. This may range from mild to severe. Some children may have homonymous hemianopsia, which is a visual impairment that affects half their visual field. Most children can walk and talk, but not all. Some may need to receive nutrition while others can eat typically.

In a recent study, researchers found that after surgery:

• 68% of children were seizure-free.
• 43% of children had average or mildly impaired cognition.
• 26% of children could speak age appropriately.
• 21% of children had acceptable reading skills.

How can I care for my child with hemimegalencephaly?

If your child has hemimegalencephaly, they’ll have a robust care team. With your child’s healthcare team, you’ll develop a plan to suit your child’s needs. Your child’s care team may include:

• Pediatricians.
• Neurologists.
• Neurosurgeons.
• Developmental specialists.
• Speech, occupational and physical therapists.
• Other healthcare providers.

What questions should I ask my child’s healthcare provider?

If your child receives a diagnosis of hemimegalencephaly, you may want to ask their healthcare provider:

• What type of hemimegalencephaly does my child have?
• Does my child have other conditions?
• What kind of treatment will work best?
• What’s the outlook for my child?
• How can we improve their symptoms?