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Megalencephaly is a condition that results in the development of an abnormally large brain.

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What Is Megalencephaly?

Megalencephaly is a condition that results in the development of an abnormally large brain. The weight of an average adult brain is between 1,300 and 1,400 grams (2.87 pounds and 3.09 pounds). A brain is megalencephalic if it weighs more than 1,600 grams (3.53 pounds).

Megalencephaly may be present at birth. Sometimes, it develops over time. In some cases, a brain may reach twice the normal weight.

Megalencephaly can occur alone and without symptoms. It can also occur with a wide range of neurological problems and/or birth defects. It is sometimes confused with macrocephaly, a condition that describes a head that is large, though not necessarily abnormal.

What Are the Types of Megalencephaly?

This condition has three broad types.

  • primary megalencephaly, or benign familial megalencephaly
  • secondary megalencephaly, which occurs as the result of another disorder
  • unilateral megalencephaly, or hemimegalencephaly, which is the enlargement of one half of the brain and is very rare

What Causes Megalencephaly?

Defects in the way that the brain controls cell production cause megalencephaly. In normal brain cell growth, the correct number of cells is made in the right place at the right time. Megalencephaly occurs when the brain produces too many new cells or cells that are too large. It can also result from an abnormal collection of metabolic byproducts and matter (Menkes, et al., 2000).

Genetic factors can cause the condition. It can also result from genetic conditions of the nervous system. These include:

  • Alexander disease
  • neurofibromatosis
  • tuberous sclerosis
  • overgrowth disorders, like Sotos syndrome and Beckwith-Wiedemann syndrome.
  • chromosomal disorders, like Klinefelter syndrome

Non-genetic causes include disorders of the cerebral spinal fluid.

Sometimes there is no known cause.

Who Is at Risk for Megalencephaly?

Megalencephaly is three to four time more common in males than females. It affects between 10 and 30 percent of patients with macrocephaly (CPN). Asymptomatic cases may not be reported, so the exact incidence is not clear.

What Are the Symptoms of Megalencephaly?

Symptoms can range from mild to severe. They depend on the underlying causes. A patient with benign familial megalencephaly may have normal to advanced intelligence without symptoms.

In some cases, a patient’s facial features have an abnormal size or shape. Mental retardation and seizures can result when the condition is the result of another disorder.

Common neurological symptoms are:

  • delayed development of gross motor milestones (these include the ability to hold the head upright, change position, roll over, sit, and stand)
  • seizures
  • corticospinal dysfunction (this is the inability to send impulses from the brain to the spinal cord)
  • mental retardation
  • delayed speech development
  • defects in muscle tone
  • body asymmetry
  • paralysis
  • inability to coordinate and control movements
  • visual irregularities

How Is Megalencephaly Diagnosed?

A complete physical exam can diagnose megalencephaly. A medical and family history is sometimes needed. A doctor measures the head circumference of the patient and immediate relatives.

Development and neurological exams are sometimes conducted. Magnetic resonance imaging (MRI) and computed tomography (CT) scans can examine the physical appearance and size of the brain. Laboratory tests can identify genetic and chromosomal disorders.

How Is Megalencephaly Treated?

There is no cure for megalencephaly.

Medical treatments can help with symptoms and underlying disorders. Rehabilitative treatment can help associated neurological and physical disabilities.

  • Medications: Anti-epileptic drugs can help to control seizures.
  • Home Care: Physical, speech, and occupational therapy are sometimes advised. This depends on the severity of disabilities.  

What is the Outlook for Megalencephaly?

The outlook for people with megalencephaly depends on the physical and mental problems caused by the condition. Seizures, paralysis, and mental retardation may be severely limiting. Physical, speech, and occupational therapies can help.

Children with these conditions may require special education classes.

Patients with hemimegalencephaly, a rare condition, have a poor prognosis. Mental retardation, severe seizures, and paralysis on one side of the body can occur.

Preventing Megalencephaly

There are no known ways to prevent megalencephaly. Genetic counseling can help patients with a family history of the condition.

Written by: Anna Zernone Giorgi
Edited by:
Medically Reviewed by: George Krucik, MD, MBA
Published: Dec 17, 2013
Published By: Healthline Networks, Inc.
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