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Megalencephaly - Types, Diagnosis, and Management

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It is believed that the size of the brain decides intelligence. Read the article to know how an abnormally large brain deteriorates its normal function.

Written by

Dr. Gayathri P

Medically reviewed by

Dr. Kaushal Bhavsar

Published At September 22, 2022
Reviewed AtJanuary 10, 2024


A brain weighs 370 grams in newborns; however, it increases in size till childhood to reach 1500 grams. The main element involved in the physical examination is head circumference measurement. The pediatrician uses tape to measure the prominent area of the forehead (above eyebrows) to the back of the head (occiput). The values are then correlated with the growth charts, height, weight, and gender of the child. The average head circumference is two standard deviations (SD). Any values above or below the standard deviation are considered abnormal.

What Is Megalencephaly?

Megalencephaly refers to the abnormally large and heavy brain that does not function well, and Fletcher first described it. It happens due to abnormal events that disturb the development of the brain. The nerve cells divide to form new cells in exact numbers and places in normal brain development. More cells are formed during brain development or after birth in megalencephaly due to other disorders.

How Does Megalencephaly Differ From Macrocephaly?

1. Megalencephaly:

This refers to a condition where the brain is abnormally large due to an overgrowth of both the brain and the skull. It often starts in the early stages of brain development, usually before birth or during infancy. Megalencephaly can be caused by various factors such as genetic mutations, metabolic disorders, or other underlying conditions. It can be diagnosed through imaging tests like MRI (Magnetic resonance Imaging) or CT (Computed Tomography) scans.

2. Macrocephaly:

This condition involves an abnormally large head circumference that may or may not be due to an enlarged brain. Macrocephaly can be caused by various factors, including increased cerebrospinal fluid, swelling, or thickening of the skull bones. Unlike megalencephaly, macrocephaly might not necessarily involve an actual enlargement of the brain itself.

Macrocephaly or megacephaly: It refers to a large head. The measured head circumference is more than two standard deviations, similar to megalencephaly. However, macrocephaly is not always related to abnormalities and is often inherited from parents. Megalencephaly is more severe than macrocephaly, and it is essential to differentiate both during the examination.

What Is MCAP Syndrome?

MCAP syndrome, also known as Megalencephaly-Capillary Malformation-Polymicrogyria syndrome, is an extremely rare genetic disorder characterized by a combination of features including megalencephaly (enlarged brain), capillary malformations (abnormalities in small blood vessels), and polymicrogyria (an abnormality in the brain's surface characterized by an excessive number of small gyri). This syndrome typically presents with overgrowth of various tissues, particularly the brain, causing developmental delays, intellectual disabilities, seizures, and distinct physical characteristics. MCAP syndrome is often caused by mutations in the PIK3CA gene, which plays a role in controlling cell growth and division.

What Is MCAP?

Megalencephaly-capillary malformation (MCAP) is a rare and complex disorder. It affects the skin, brain, blood vessels, and connective tissue. The characteristic features include an enlarged head, brain abnormalities including the large brain, skin lesions, and other growth abnormalities. It is usually diagnosed at birth and mainly occurs due to the alterations in the PIK3CA gene. The incidence is found to be one in one million newborns.

What Are the Causes of Macrocephaly?

  • Changes in specific genes responsible for regulating brain growth can lead to megalencephaly. Disorders like tuberous sclerosis, PTEN-related disorders, or other genetic conditions can contribute to this enlargement.

  • Certain metabolic conditions, such as disorders of amino acid metabolism or fatty acid oxidation, can cause an accumulation of substances in the brain, leading to megalencephaly.

  • Infections during pregnancy or infancy, such as cytomegalovirus or other prenatal infections, can interfere with normal brain development and result in megalencephaly.

  • Brain injuries, even during early development, can sometimes lead to abnormal brain growth and megalencephaly.

  • Some conditions like neurofibromatosis, Megalencephaly may be linked to specific leukodystrophies or chromosomal abnormalities (like Down syndrome).

What Are the Types of Megalencephaly?

Based on the site of involvement-

Unilateral Megalencephaly: In children, one-half of the brain is enlarged, also known as hemimegalencephaly. The head of the children looks asymmetrical, along with seizures and disturbances in the development of thinking and reasoning abilities.

Bilateral Megalencephaly: Two sides of the brain are enlarged.

Based on the cause

Benign Megalencephaly:

The children have large heads with no other brain abnormalities. The head size gradually increases until 18 months of age and stabilizes after normal brain development. The child's mother or father is presented with a large head.

Metabolic Megalencephaly:

The occurrence of megalencephaly is associated with a metabolic disorder. Metabolic disorders affect the normal process of energy production through food breakdown. It leads to impaired neurological functions and other organs like muscles, eyes, liver, heart, and spleen. The various signs and symptoms of metabolic megalencephaly are evaluated during physical examination: In infants, Irritability, sun-setting signs (the eye appears driven downwards, and the lower eyelid covers the lower pupil), muscle weakness (hypotonia), and large fontanels are observed. In children, development delay, lethargy and seizures occur.

The metabolic disorders linked to megalencephaly are

  • Organic acid defects such as glutaric aciduria.

  • Metabolic encephalopathies such as Canavan disease and Alexander disease.

  • Illnesses related to lysosomal storage, including Tay-Sachs and Sandhoff diseases.

  • Types-of-megalencephaly

  • Anatomic Megalencephaly:

  • Megalencephaly is acquired due to changes (mutation) in single genes.

  • As a result, the children possess an enlarged brain associated with the following disorders. Anatomic-megalencephaly-disorders

What Are the Symptoms of Megalencephaly?

Megalencephaly, characterized by an abnormally enlarged brain, often manifests through an unusually large head size, noticeable especially in early childhood. Alongside this prominent physical feature, individuals might experience developmental delays, struggling to reach milestones like walking or speaking. Seizures can be a part of this condition, varying in intensity and occurrence. Cognitive challenges, including intellectual disabilities, might impact learning abilities and overall cognitive functioning. Some individuals might also exhibit muscle weakness or stiffness, alongside potential behavioral changes such as hyperactivity or attention difficulties. Each person's experience with these symptoms will differ greatly depending on the underlying cause of their megalencephaly and any related illnesses, emphasizing the importance of personalized care and early intervention.

How Is Megalencephaly Diagnosed?

Physical Examination:

The pediatrician examines and measures the head circumference using a non-elastic tape. It is then compared with the growth charts, the age, and the gender of the child.

Magnetic Resonance Imaging (MRI):

It is done to evaluate the abnormalities in the brain. When associated with a specific syndrome, the enlarged brain shows the destruction of nerve cells in the brain.

How Is Megalencephaly Managed?

No particular treatment cures megalencephaly. But, the healthcare specialist provides the following to reduce specific symptoms. It is recommended to use anti-epileptic medications such as carbamazepine and phenytoin to lessen the severity and frequency of seizures. Occupational therapy is given to children with behavioral and cognitive (learning and thinking) disabilities. Physical therapy is provided to improve muscular movements and coordination.


Megalencephaly is a rare condition, and its prognosis depends on the underlying disorders. There is no specific treatment available for megalencephaly. However, the effort of parents and the multi-specialty care given by doctors help alleviate symptoms and improve the quality of life in a few children

Frequently Asked Questions


Are People With Megalencephaly Much Smarter Than Others?

No, there is no evidence of difference or superiority of intelligence in people with megalencephaly. Although, in most cases, conducting IQ tests, the IQ levels of people with megalencephaly are comparatively lower than those with the average brain size.


What Are the Ways to Treat Hemi-Megalencephaly? At What Age Can Hemispherectomy Be Performed?

Although there is no exact cure for treating megalencephaly, there are surgical procedures that are done as palliative measurements. The surgery implemented in the treatment of hemi-megalencephaly is known as hemispherectomy, where it is mainly targeted at controlling the occurrence of seizures. Hemispherectomy is usually performed within the first two years of life.


What Are the Syndromes Causing Macrocephaly? At What Age Is Macrocephaly Diagnosed?

The incidence of macrocephaly can be diagnosed even before birth by normal ultrasound examination. However, the syndromes causing macrocephaly are:
- PTEN hamartoma syndromes.
- Hemimegalencephaly.
- Autism.
- Beckwith - Wiedemann syndrome.


What Are the Factors Causing Macrocephaly?

The factors that cause macrocephaly are: 
- Increase in cerebrospinal fluid.
- Megalencephaly.
- Increased bone thickness.
- increased intracranial pressures.
- Increased blood.
- Increase brain parenchyma, etc.


Is Hemi-Megalencephaly Fatal?

Most of the time, hemi-megalencephaly causes visible complications and symptoms that involve complications like physical or mental disabilities, compromised brain development, seizures, etc, which, on progression, can also result in death.


At Which Age Is Hemi-Megalencephaly Known to Occur?

The seizures associated with hemi-megalencephaly are seen from the first few days of birth up to two and a half years of age. Very rarely, the symptoms of megalencephaly are seen in adulthood.


Which Size Is Considered Megalencephaly?

The size of the head is considered megalencephaly when the size of the head is more than ninety-seven percent of the normal head size or above two-quarters of the normal head levels, where the average size of the head ranges from between fifty- five to fifty-nine centimeters.


What Is Known as Megalencephaly Malformation Syndrome?

Megalencephaly malformation syndrome (MCAP) is a systemic disorder characterized by increased growth of the bodily tissues, mainly characterized by the larger size of the brain, and capillary abnormalities characterized by small blood vessels.


How to Identify If One Is Born With Hemi-Megalencephaly? Can the Size of the Head Show an Increased Stature?

Hemi-megalencephaly is a clinical condition that can be identified by physical examination, where the visible symptoms include:
- Paralysis of the body on one side.
- Seizures.
- Impaired brain development.
- Enlarged head.
- Uncoordinated motor movements.
- Also, there is a significant increase in the size of the body and head ratio and an increase in the length of the head.


How Is Megalencephaly Classified?

Based on the causative factors, megalencephaly can be classified into three types, and they are:
- Metabolic.
- Anatomic.
- Dynamic.


What Are the Syndromes Associated With Hemi-Megalencephaly?

The syndromes associated with hemi-megalencephaly are:
- Epidermal nevus syndrome.
- Proteus syndrome.
- Tuberous sclerosis.
- Linear sebaceous nevus syndrome.
- Neurofibromatosis.
- Sturge-weber syndrome.
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Dr. Kaushal Bhavsar
Dr. Kaushal Bhavsar

Pulmonology (Asthma Doctors)


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