Megalencephalic leukoencephalopathy with subcortical cysts

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Megalencephalic leukoencephalopathy with subcortical cysts is a progressive condition that affects brain development and function. Individuals with this condition typically have an enlarged brain (megalencephaly) that is evident at birth or within the first year of life. Megalencephaly leads to an increase in the size of the head (macrocephaly). Affected people also have leukoencephalopathy, an abnormality of the brain's white matter. White matter consists of nerve fibers covered by a fatty substance called myelin. Myelin insulates nerve cells (neurons) and promotes the rapid transmission of nerve impulses. In megalencephalic leukoencephalopathy with subcortical cysts, the myelin is swollen and contains numerous fluid-filled pockets (vacuoles). Over time, the swelling decreases and the myelin begins to waste away (atrophy). Individuals affected with this condition may develop cysts in the brain; because these cysts form below an area of the brain called the cerebral cortex, they are called subcortical cysts. These cysts can grow in size and number.

The brain abnormalities in people with megalencephalic leukoencephalopathy with subcortical cysts affect the use of muscles and lead to movement problems. Affected individuals typically experience muscle stiffness (spasticity) and difficulty coordinating movements (ataxia). Walking ability varies greatly among those affected. Some people lose the ability to walk early in life and need wheelchair assistance, while others are able to walk unassisted well into adulthood. Minor head trauma can further impair movements and may lead to coma. Affected individuals may also develop uncontrolled muscle tensing (dystonia), involuntary writhing movements of the limbs (athetosis), difficulty swallowing (dysphagia), and impaired speech (dysarthria). More than half of all people with this condition have recurrent seizures (epilepsy). Despite the widespread brain abnormalities, people with this condition typically have only mild to moderate intellectual disability.

There are three types of megalencephalic leukoencephalopathy with subcortical cysts, which are distinguished by their signs and symptoms and genetic cause. Types 1 and 2A have different genetic causes but are nearly identical in signs and symptoms. Types 2A and 2B have the same genetic cause but the signs and symptoms of type 2B often begin to improve after one year. After improvement, individuals with type 2B usually have macrocephaly and may have intellectual disability.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Mutations in the MLC1 gene cause megalencephalic leukoencephalopathy with subcortical cysts type 1; this type accounts for 75 percent of all cases. The MLC1 gene provides instructions for producing a protein that is made primarily in the brain. The MLC1 protein is found in astroglial cells, which are a specialized form of brain cells called glial cells. Glial cells protect and maintain other neurons. The MLC1 protein functions at junctions that connect neighboring astroglial cells. The role of the MLC1 protein at the cell junction is unknown, but research suggests that it may control the flow of fluids into cells or the strength of cells' attachment to one another (cell adhesion).

Mutations in the HEPACAM gene cause megalencephalic leukoencephalopathy with subcortical cysts types 2A and 2B; together, these types account for 20 percent of all cases. The HEPACAM gene provides instructions for making a protein called GlialCAM. This protein primarily functions in the brain, particularly in glial cells. GlialCAM attaches (binds) to other GlialCAM proteins or to the MLC1 protein and guides them to cell junctions. The function of GlialCAM at the cell junction is unclear.

Most MLC1 gene mutations alter the structure of the MLC1 protein or prevent the cell from producing any of this protein, leading to a lack of functional MLC1 protein at the astroglial cell junctions. HEPACAM gene mutations lead to a protein that is unable to correctly transport GlialCAM and MLC1 proteins to cell junctions. It is unknown how a lack of functional MLC1 or GlialCAM protein at cell junctions in the brain impairs brain development and function, causing the signs and symptoms of megalencephalic leukoencephalopathy with subcortical cysts.

Approximately 5 percent of people with megalencephalic leukoencephalopathy with subcortical cysts do not have identified mutations in the MLC1 or HEPACAM gene. In these individuals, the cause of the disorder is unknown.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

All cases of megalencephalic leukoencephalopathy with subcortical cysts caused by mutations in the MLC1 gene (type 1) and some cases caused by mutations in the HEPACAM gene (type 2A) are inherited in an autosomal recessive pattern. Autosomal recessive inheritance means both copies of a gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Megalencephalic leukoencephalopathy with subcortical cysts type 2B is inherited in an autosomal dominant pattern, which means one copy of the altered HEPACAM gene in each cell is sufficient to cause the disorder. Most cases of type 2B result from new (de novo) mutations in the HEPACAM gene that occur during the formation of reproductive cells (eggs or sperm) or in early embryonic development. These cases occur in people with no history of the disorder in their family.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Megalencephalic leukoencephalopathy with subcortical cysts is a rare condition; its exact prevalence is unknown. More than 150 cases have been reported in the scientific literature.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Ataxia · Frequent (79-30%)

Ataxia refers to impaired coordination of voluntary muscle movement. Cerebellar ataxia refers to ataxia due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits including asynergy (lack of coordination between muscles, limbs and joints), dysmetria (lack of ability to judge distances that can lead to under- or overshoot in grasping movements), and dysdiadochokinesia (inability to perform rapid movements requiring antagonizing muscle groups to be switched on and off repeatedly).

Atypical behavior · Frequent (79-30%)

Atypical behavior is an abnormality in a person's actions that can be controlled or modulated by the will of the individual. While abnormal behaviors can be difficult to control, they are distinct from other abnormal actions that cannot be affected by the individual's will.

Cerebral subcortical cyst · Frequent (79-30%)

A fluid-filled sac (cyst) in the region below the cortex of the cerebrum. In magnetic resonance imaging, the fluid within the cyst has the same appearance as cerebrospinal fluid (CSF).

Mental deterioration · Frequent (79-30%)

Loss of previously present mental abilities, generally in adults.

Motor deterioration · Frequent (79-30%)

Loss of previously present motor (i.e., movement) abilities.

Postnatal macrocephaly · Frequent (79-30%)

The postnatal development of an abnormally large skull (macrocephaly).

Seizure · Frequent (79-30%)

A seizure is an intermittent abnormality of nervous system physiology characterized by a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain.

Spasticity · Frequent (79-30%)

A motor disorder characterized by a velocity-dependent increase in tonic stretch reflexes with increased muscle tone, exaggerated (hyperexcitable) tendon reflexes.

Other findings in the same source

From: Orphanet

Additional reported features include Diffuse swelling of cerebral white matter (Frequent (79-30%)); Unsteady gait (Frequent (79-30%)); Abnormality of extrapyramidal motor function (Occasional (29-5%)); Athetosis (Occasional (29-5%)); Autism (Occasional (29-5%)); Cerebral white matter atrophy (Occasional (29-5%)); Clumsiness (Occasional (29-5%)); Dystonia (Occasional (29-5%)); Intellectual disability (Occasional (29-5%)); Status epilepticus (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Megalencephalic leukoencephalopathy with subcortical cysts is Neurology, with a neurologist as the relevant type of clinician. General physician / Family Medicine; paediatrician for children. Referral depends on symptoms.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Which nervous-system findings help explain the symptoms?
• Would an assessment of walking, communication or daily function be helpful?
• Are rehabilitation or other specialist services relevant?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Megalencephalic leukoencephalopathy with subcortical cysts. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.