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

McArdle syndrome results from a mutation in a gene. The gene makes an enzyme called glycogen phosphorylase. As a result of this mutation, glycogen cannot be broken down in the muscles. This may lead to weakness, cramps, and muscle pain.

The disease is inherited as an autosomal recessive genetic disorder. This means that an affected person inherits a copy of the defective gene from both parents. A person who inherits a defective gene from only one parent is usually not affected.

A family history of McArdle syndrome increases the risk.

The symptoms usually begin as a young adult. They may include:

• Muscle pain
• Muscle cramps
• Muscle stiffness
• Muscle weakness
• Intolerance for exercise
• Exercise can produce a burgundy-colored urine (myoglobinuria)

The symptoms can be reduced by avoiding strenuous exercise.

The following tests may be perfomed:

• Lactic acid in blood
• Myoglobin in urine
• Serum creatine kinase
• Muscle biopsy
• Electromyography (EMG)
• MRI
• Genetic testing (enzyme and DNA)

There is no specific treatment, but the symptoms can be managed by controlling exercise and physical activity. For example, avoid excessive or intense exercise.

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

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

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

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

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