Muscle phosphorylase deficiency

Alternative names 
Glycogen storage disease type V; McArdle syndrome (glycogen storage disease type V)


McArdle syndrome is a metabolic disease characterized by an inability to break down glycogen, an important source of energy that is stored in muscle tissue. Without glycogen, muscle strength and effectiveness are diminished.

Causes, incidence, and risk factors

McArdle syndrome is a genetic disorder which results from deficiency in muscle of a specific enzyme called muscle phosphorylase. The disease is recessive, which means that a person must inherit the defective gene from both parents in order to be affected. A parent who carries only one copy of the gene will not be affected.

Without muscle phosphorylase, glycogen cannot be broken down. Weakness, cramps and pain can result.


Muscle pain, cramps, and stiffness during exercise are the primary symptoms of McArdle syndrome. Other symptoms include the following:

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

Symptoms usually do not begin until the affected person has reached adulthood. Avoiding strenuous exercise decreases the severity of these symptoms and of the disease.

Signs and tests

Brief, intense exercise typically does not lead to an increase in blood lactic acid.

Tests may reveal the presence of myoglobin in urine and increased serum creatine kinase (indicating rhabdomyolysis), especially after exercise.

Muscle biopsy shows stored glycogen and decreased muscle phosphorylase activity. Specialized centers may be able to show abnormal muscle metabolism using an MRI machine in a test called spectroscopy.

The ischemic forearm test (brief application of a tourniquet to cut off blood supply to the forearm, and then blood lactate is measured) can be helpful in making the diagnosis.

In certain cases, genetic testing may be recommended.


The most important treatment is avoiding strenuous exercise. In particular, brief, intense exercise such as carrying a heavy load and prolonged exercise such as climbing long hills must be avoided.

Rarely, severe exercise-induced muscle ache associated with burgundy urine (rhabdomyolysis) may need hospital-based treatments such as intravenous fluids.

Support Groups

Participating in a support group can often help with the stress of dealing with illness, and support groups members can often provide information on the latest advances and treatments.

Association for Glycogen Storage Disease:

Expectations (prognosis)
Although most patients are not diagnosed with this disease until they reach their 20s or 30s, many report having had poor tolerance for exercise in childhood. Aside from avoiding strenuous exercise, McArdle syndrome patients can live a normal life.


Strong exercise may produce muscle pain or even breakdown of skeletal muscle (rhabdomyolysis). This is associated with a burgundy-colored urine and an increased risk for kidney failure (if severe).

Calling your health care provider

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.

Contact your health care provider if you have a family history of McArdle syndrome and you are planning a pregnancy.

Johns Hopkins patient information

Last revised: December 7, 2012
by Mamikon Bozoyan, M.D.

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