Muscle phosphorylase deficiency
Key points
- 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.
- McArdle syndrome is a genetic disorder which results from deficiency in muscle of a specific enzyme called muscle phosphorylase.
- Muscle pain, cramps, and stiffness during exercise are the primary symptoms of McArdle syndrome.
- Brief, intense exercise typically does not lead to an increase in blood lactic acid.
Alternative names
Glycogen storage disease type V; McArdle syndrome (glycogen storage disease type V)
Definition
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.
Symptoms
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.
Treatment
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.
Written and revised by
Gerald Hurowitz, M.D.
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