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.
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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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.
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.
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 can affect any child. It affects at least three times as many males as females.
Bilateral megalencephaly affects both parts of the brain. (Bilateral means “two sides.”)
Hemimegalencephaly means only one side of the brain is enlarged. It’s also called unilateral megalencephaly.
Children with hemimegalencephaly often have:
Healthcare providers classify megalencephaly by how it affects people and whether it occurs with other disorders. The types of megalencephaly are:
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.
A single gene mutation (change) causes this type of megalencephaly. Children have megalencephaly along with other developmental problems. Conditions associated with anatomic megalencephaly include:
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:
Megalencephaly-capillary malformation (MCAP) is a rare condition. In addition to large head size, it involves many body systems, including:
Usually, healthcare providers diagnose MCAP when a baby is born. Children born with MCAP have:
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.
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:
Megalencephaly can occur as a part of many other conditions and syndromes, including:
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.
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.
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.
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.
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:
If your child receives a diagnosis of megalencephaly, ask your healthcare provider:
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.
Last reviewed by a Cleveland Clinic medical professional on 01/24/2022.
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