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.
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 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)
- Exercise intolerance, poor stamina
- Muscle cramps
- Muscle pain
- Muscle stiffness
- Muscle weakness
Exams and Tests
The following tests may be performed:
- Genetic testing
Lactic acid in blood MRI Muscle biopsy Myoglobin in urine
- Plasma ammonia
Serum creatine kinase
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.
More information and support for people with type V glycogen storage condition and their families can be found at:
- Association for Glycogen Storage Disease --
- National Organization for Rare Disease Disorders --
People with GSD V can live a normal life by managing their diet and physical activity.
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.
Brandow AM. Enzymatic defects. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 490.
Weinstein DA. Glycogen storage diseases. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 196.
Williams LM, Mcdonald CM. Myopathic disorders. In: Cifu DX, ed. Braddom's Physical Medicine & Rehabilitation. 6th ed. Philadelphia, PA: Elsevier; 2021:chap 42.
Last reviewed on: 11/1/2021
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 Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.