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Diseases reference index «McArdle syndrome »

McArdle syndrome is the inability to break down glycogen. Glycogen is an important source of energy that is stored in muscle tissue.


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 non-working gene from both parents. A person who gets a non-working gene from only one parent usually does not develop this syndrome. A family history of McArdle syndrome increases the risk.

The disease usually starts when a person is in their 20s or 30s, but there is a very rare form that begins in infants.


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

Exams 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


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.

Outlook (Prognosis)

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

Possible 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.

When to Contact a Medical Professional

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.

Alternative Names

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