Megalencephaly (Macrencephaly)

In megalencephaly, or macrencephaly, a child has an unusually enlarged brain. Often, the child has neurological and developmental problems as well. Megalencephaly can affect both parts of the brain or just one part, called hemimegalencephaly. While there isn’t a cure for megalencephaly, treatment aims to improve symptoms and quality of life.


What is megalencephaly?

Megalencephaly, sometimes called MEG, is a problem with a baby or child’s brain. Their brain is too large, too heavy and doesn’t function as it should. Doctors used to call this condition macrencephaly.


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When do parents or providers usually notice megalencephaly?

Sometimes, parents and healthcare providers notice that a baby has an enlarged head at birth or within the first few months of life. Other times, the brain becomes unusually large within the child’s early years.

In a rare type of megalencephaly called hemimegalencephaly, only one side of the child’s brain becomes enlarged. If a child has this type, healthcare providers may notice it on an ultrasound during pregnancy.

What’s the difference between megalencephaly and macrocephaly?

Macrocephaly, which is also called megacephaly or megalocephaly, also describes a problem with head size. In macrocephaly, a person has a large head but not necessarily any related problems or abnormalities.


Megalencephaly can cause macrocephaly. But macrocephaly can also develop because of fluid buildup in your brain (hydrocephalus) or increased pressure within your skull.

How common is megalencephaly?

Megalencephaly isn’t common. It affects between 2% and 6% of children, although many cases don’t cause significant problems. In people with macrocephaly, 10% to 30% also have megalencephaly.

The subtypes are rarer still:

  • Megalencephaly-capillary malformation (MCAP) occurs in fewer than 1 in 1 million births.
  • Megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH) occurs even less frequently.

Who is at risk for megalencephaly?

Megalencephaly can affect any child. It affects at least three times as many males as females.

What is bilateral megalencephaly?

Bilateral megalencephaly affects both parts of the brain. (Bilateral means “two sides.”)

What is hemimegalencephaly?

Hemimegalencephaly means only one side of the brain is enlarged. It’s also called unilateral megalencephaly.

Children with hemimegalencephaly often have:

  • Large, asymmetrical heads.
  • Seizures.
  • Partial paralysis.
  • Problems with cognitive development.

What are the types of megalencephaly?

Healthcare providers classify megalencephaly by how it affects people and whether it occurs with other disorders. The types of megalencephaly are:

Benign familial megalencephaly or idiopathic megalencephaly

Children have an unusually large head but don’t have other symptoms or neurological problems. Usually, in these cases, one or both parents also have an increased head size. The child’s head size often increases until they’re about 18 months. Then it stabilizes.

Anatomic megalencephaly

A single gene mutation (change) causes this type of megalencephaly. Children have megalencephaly along with other developmental problems. Conditions associated with anatomic megalencephaly include:

  • Achondroplasia, a type of skeletal dysplasia (dwarfism).
  • Bannayan-Riley-Ruvalcaba syndrome (BRRS), a genetic condition where people grow both benign (noncancerous) and cancerous tumors.
  • Megalencephaly-capillary malformation (MCAP), a combination of megalencephaly and blood vessel problems.
  • Megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH), a condition that involves multiple birth defects.
  • Pretzel syndrome, a disorder that combines skeletal problems, brain malformation and problems with the heart and other organs.
  • Simpson-Golabi-Behmel syndrome (SGBS), an overgrowth syndrome that mostly affects males.
  • Sotos syndrome, a genetic growth disorder.
  • Weaver syndrome, a condition involving tall stature.

Metabolic megalencephaly

Some people have megalencephaly combined with metabolic errors — problems with how their body breaks down and uses energy. Children with metabolic megalencephaly usually have neurological problems and developmental delays, along with metabolic conditions.

Metabolic disorders associated with megalencephaly include:

What is megalencephaly-capillary malformation (MCAP)?

Megalencephaly-capillary malformation (MCAP) is a rare condition. In addition to large head size, it involves many body systems, including:

  • Skin.
  • Blood vessels.
  • Connective tissue.
  • Brain.

Usually, healthcare providers diagnose MCAP when a baby is born. Children born with MCAP have:

  • Heads too large for their bodies, including an enlarged brain (megalencephaly).
  • Problems with the capillaries, tiny blood vessels.
  • Other neurological problems.

Symptoms and Causes

What causes megalencephaly?

Researchers think megalencephaly is a result of an error in how the brain produces cells. Neurons, or nerve cells, divide to form new cells. That process is called neuron proliferation.

Typically, your body regulates that process to produce the right number of cells at the right time. But babies and children with megalencephaly produce too many cells. This may happen as they develop or as part of another disorder they have.

What are the symptoms of megalencephaly?

Sometimes, megalencephaly happens on its own, without any symptoms. Other times, it comes along with other neurological problems and birth defects. In these cases, the other syndrome or condition may cause megalencephaly.

Symptoms of megalencephaly may include:

  • Developmental delays.
  • Seizures (epilepsy).
  • Neurological problems (abnormalities with their brain and spinal cord).

What other conditions are associated with megalencephaly?

Megalencephaly can occur as a part of many other conditions and syndromes, including:

Diagnosis and Tests

How is megalencephaly diagnosed?

Your healthcare provider will do a physical exam. They’ll measure your child’s head using a measuring tape. Your healthcare provider checks the measurement against a growth chart according to typical measurements for your child’s age, sex and height. Your healthcare provider might also measure your (the parents') head for comparison.

Your child may also have imaging exams, including an MRI.

Management and Treatment

What are treatment options for megalencephaly?

Currently, there aren’t any treatments for megalencephaly. Healthcare providers develop a treatment plan based on related disorders and conditions. Your child’s treatment plan will consider their symptoms and needs.

For example, your child’s care team will aim to control epilepsy, as children with megalencephaly often have many seizures. Speech, language and occupational therapy can support children who have developmental delays, too.


Can megalencephaly be prevented?

There’s no known way to prevent megalencephaly. If you suspect a family history of megalencephaly, you can speak with a genetic counselor. They can help you assess your risk of passing megalencephaly to your child.

Outlook / Prognosis

What’s the outlook for children with megalencephaly?

Your child’s outlook depends on what other conditions they have. The underlying cause of megalencephaly also plays a role. Many children who have a large brain size don’t have any problematic symptoms.

For those who do have symptoms, children with bilateral megalencephaly fare better than children with hemimegalencephaly. But because these conditions are uncommon, each child may have a different outlook.

Living With

How can I care for my child with megalencephaly?

Your child will have a robust care team. Together, they’ll develop a plan to suit your child’s needs. Your child’s care team may include:

  • Pediatrician.
  • Neurologist.
  • Neurosurgeon.
  • Developmental specialists.
  • Speech, occupational and physical therapists.
  • Other healthcare professionals.

What should I ask my healthcare provider about megalencephaly?

If your child receives a diagnosis of megalencephaly, ask your healthcare provider:

  • What type of megalencephaly 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?

A note from Cleveland Clinic

Megalencephaly (MEG) was once known as macrencephaly. In this condition, a baby or child has an unusually enlarged brain and head size. The condition may happen by itself or with another condition. Children with MEG often have neurological and developmental problems. There’s no cure for megalencephaly, but treatment can help control seizures and improve other symptoms. If you notice your child has an unusual head size or other worrying signs, talk to your child’s healthcare provider.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 01/24/2022.

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