Myophosphorylase deficiency; Muscle glycogen phosphorylase deficiency; PYGM deficiency
Type V 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 glycogen phosphorylase. As a result, the body cannot break down glycogen in the muscles.
GSD V is an autosomal recessive genetic disorder. This means that 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 usually 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:
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.
The following groups can provide more information and resources:
People with GSD V can live a normal life by managing their diet and physical activity.
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.
Akman HO, Oldfors A, DiMauro S. Glycogen storage diseases of muscle. In: Darras BT, Jones HR, Ryan MM, De Vivo DC, eds. Neuromuscular Disorders of Infancy, Childhood, and Adolescence. 2nd ed. Waltham, MA: Elsevier Academic Press; 2015:chap 39.
Kishnani PS, Chen Y-T. Defects in metabolism of carbohydrates. In: Kliegman RM, Stanton BF, St. Geme JW, Schor NF, eds. Nelson Textbook of Pediatrics. 20th ed. Philadelphia, PA: Elsevier; 2016:chap 87.
Weinstein DA. Glycogen storage diseases. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 207.