Health Library

Type V glycogen storage disease

Myophosphorylase deficiency; Muscle glycogen phosphorylase deficiency; PYGM deficiency

Type V (five) glycogen storage disease (GSD V) is a rare inherited condition in which the body is not able to break down glycogen. Glycogen is an important source of energy that is stored in all tissues, especially in the muscles and liver.

GSD V is also called McArdle disease.

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Causes

GSD V is caused by a flaw in the gene that makes an enzyme called muscle glycogen phosphorylase. As a result, the body cannot break down glycogen in the muscles.

GSD V is an autosomal recessive genetic disorder. This means to have the disorder, you must receive a copy of the nonworking gene from both parents. A person who receives a nonworking gene from only one parent usually does not develop this syndrome. A family history of GSD V increases the risk.

Symptoms

Symptoms commonly start during early childhood. But, it may be difficult to separate these symptoms from those of normal childhood. Diagnosis may not occur until a person is over 20 or 30 years old.

• 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

Treatment

There is no specific treatment.

The health care provider may suggest the following to stay active and healthy and prevent symptoms:

• Be aware of your physical limitations.
• Before exercising, warm up gently.
• Avoid exercising too hard or too long.
• Eat enough protein.

Ask your provider if it's a good idea to eat some sugar before exercising. This may help prevent muscle symptoms.

If you need to have surgery, ask your provider if it's OK for you to have general anesthesia.

Support Groups

More information and support for people with type V glycogen storage condition and their families can be found at:

• Association for Glycogen Storage Disease -- www.agsdus.org
• Genetic and Rare Diseases Information Center (GARD) -- rarediseases.info.nih.gov/diseases/6528/glycogen-storage-disease-type-5

Outlook (Prognosis)

People with GSD V can live a normal life by managing their diet and 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 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 GSD V.

Related Information

References

Hijazi G, Kishnani PS. Defects in metabolism of carbohydrates. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 107.

Kishnani PS. Glycogen storage diseases. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 191.

Williams LM, Mcdonald CM. Myopathic disorders. In: Cifu DX, ed. Braddom's Physical Medicine & Rehabilitation. 6th ed. Philadelphia, PA: Elsevier; 2021:chap 42.

Review Date: 12/31/2023  

Reviewed By: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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