Definition

McArdle syndrome is the inability to break down glycogen. Glycogen is an important source of energy that is stored in all tissues, but especially in the muscles and liver.

Alternative Names

Glycogen storage disease type V (GSDV); Myophosphorylase deficiency; Muscle glycogen phosphorylase deficiency; PYGM deficiency

Causes, incidence, and risk factors

McArdle syndrome is caused by a defect in a gene that makes an enzyme called glycogen phosphorylase. As a result, the body cannot break down glycogen in the muscles.

The disease is an autosomal recessive genetic disorder. This means that you must get a copy of the nonworking gene from both parents. A person who gets a nonworking gene from only one parent usually does not develop this syndrome. A family history of McArdle syndrome increases the risk.

Symptoms

Symptoms usually start during early childhood. However, it may be difficult to separate these symptoms from those of normal childhood, and diagnosis may not occur until a person's 20s or 30s.

• Burgundy-colored urine (myoglobinuria)
• Fatigue
• Exercise intolerance, poor stamina
• Muscle cramps
• Muscle pain
• Muscle stiffness
• Muscle weakness

Signs and tests

The following tests may be performed:

• Electromyography (EMG)
• Genetic testing
• Lactic acid in blood
• MRI
• Muscle biopsy
• Myoglobin in urine
• Plasma ammonia
• Serum creatine kinase

Treatment

There is no specific treatment, but you can manage the symptoms by improving exercise tolerance and controlling physical activity. Avoid excessive or intense exercise.

Dietary management should include having a supply of glucose (candy, for example) on hand if you need to replace energy.

Avoid general anesthesia.

Support Groups

For additional information and resources, visit the Association for Glycogen Storage Disease at www.agsdus.org.

Expectations (prognosis)

People with McArdle syndrome can live a normal life by managing their physical activity.

Complications

Exercise may produce muscle pain, or even a breakdown of skeletal muscle (rhabdomyolysis). This condition is associated with burgundy-colored urine and a risk for kidney failure, if it is severe.

Calling your health care provider

Contact your health care provider if you have repeated episodes of sore or cramped muscles after exercise, especially if you also have burgundy or pink urine.

Consider genetic counseling if you have a family history of McArdle disease.

References

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