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Megalencephaly

What is megalencephaly?

Megalencephaly is a condition in which your brain is abnormally large. An average adult brain weighs between 1,300 and 1,400 grams, or between 2.87 pounds and 3.09 pounds. According to The Gale Encyclopedia of Neurological Disorders, the brain of an adult with megalencephaly weighs more than 1,600 grams, or 3.53 pounds.

Megalencephaly may be present at birth. It can also develop over time. In some cases, your brain may reach twice the normal weight.

Megalencephaly can occur alone. It may not occur with any symptoms. It can also occur with a wide range of neurological problems or birth defects. It’s sometimes confused with macrocephaly. This is a condition in which your head is large but not necessarily abnormal.

Three broad types of megalencephaly exist:

  • primary megalencephaly, or benign familial megalencephaly
  • secondary megalencephaly, which occurs as the result of another disorder
  • unilateral megalencephaly, or hemimegalencephaly, which occurs when half of your brain is enlarged

What are the symptoms of megalencephaly?

If you have megalencephaly, your symptoms can range from mild to severe. They depend on the underlying cause of your condition. If you have benign familial megalencephaly, you may have no symptoms. You may have normal to advanced intelligence. In other cases, your facial features may have an abnormal size or shape. If another disorder causes megalencephaly, you may have cognitive impairment, seizures, or other symptoms.

Common neurological symptoms include:

  • delayed development of your gross motor skills, including your ability to hold your head upright, change position, roll over, sit, and stand
  • delayed speech development
  • corticospinal dysfunction, in which your brain doesn’t send impulses to your spinal cord properly
  • intellectual disability
  • defects in your muscle tone
  • body asymmetry
  • paralysis
  • an inability to coordinate and control your movements
  • seizures
  • visual irregularities

What causes megalencephaly?

Defects in the way that your brain controls cell production cause megalencephaly. In normal brain cell growth, your body produces the correct number of brains cells. It produces them in the right place at the right time. Megalencephaly occurs when your brain produces too many new brain cells or cells that are too large. It can also occur when metabolic byproducts and matter build up in your brain.

Genetic factors and genetic disorders can cause megalencephaly. For example, the condition can result from:

  • Alexander disease
  • neurofibromatosis
  • tuberous sclerosis
  • overgrowth disorders, such as Sotos syndrome and Beckwith-Wiedemann syndrome
  • chromosomal disorders, such as Klinefelter’s syndrome

Non-genetic causes can also lead to megalencephaly. For example, disorders affecting your cerebral spinal fluid can cause it.

Sometimes, the doctor can’t identify the cause.

Who is at risk of megalencephaly?

Megalencephaly is three to four time more common in males than females, reports The Gale Encyclopedia of Neurological Disorders. It affects between 10 and 30 percent of patients with macrocephaly. Asymptomatic cases may not be reported, so the incidence is unknown.

How is megalencephaly diagnosed?

Your doctor will perform a complete physical exam to diagnose megalencephaly. They’ll measure the circumference of your head. They may also measure the heads of your immediate relatives. They may also take your personal and family medical history.

In some cases, your doctor might perform developmental and neurological exams. For example, they may order MRI or CT scans. This can help them examine the appearance and size of your brain. They may also order laboratory tests. This can help them check for genetic and chromosomal disorders.

How is megalencephaly treated?

No cure for megalencephaly is available. However, your doctor may prescribe treatments for your symptoms, underlying disorders, or associated disabilities.

For example, your doctor may prescribe medications. Anti-epileptic drugs can help control seizures. Physical, speech, and occupational therapy may help you manage physical or neurological disabilities.  

What is the outlook for megalencephaly?

If you have megalencephaly, your long-term outlook depends on the severity of your condition.

In some cases, your symptoms may be very mild. They may require little to no treatment. They may have little impact on your life.

In other cases, your symptoms may be severe. For example, seizures, paralysis, and cognitive impairments can be limiting. If you have these symptoms, you may need physical, speech, or occupational therapies. Children with these symptoms may also require special education classes.

If you have hemimegalencephaly, the long-term outlook is poor. This condition is rare. It can lead to cognitive impairment, severe seizures, and paralysis on one side of your body.

Ask your doctor for more information about your specific diagnosis and long-term outlook.

Preventing megalencephaly

Megalencephaly isn’t preventable. If you have a family history of the condition, talk to your doctor about genetic counseling. This can help you assess your risk of developing the condition. It can also help you assess the risk for your children or future children.


Content licensed from:

Written by: Anna Zernone Giorgi
Published on: Dec 17, 2013
Medically reviewed on: May 26, 2016: Judi Marcin, MD

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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