Metabolic Myopathies

Phosphorylase deficiency (McArdle disease)

What is phosphorylase deficiency (McArdle disease, myophosphorylase deficiency, glycogenosis type 5)?

This disease is one of a group of metabolic muscle diseases that interferes with the processing of food (in this case, carbohydrates) for energy production.

What are the symptoms of phosphorylase deficiency?

Phosphorylase deficiency causes exercise intolerance, such as cramps, muscle pain and weakness, shortly after beginning exercise.

A person with this disorder may tolerate light-to-moderate exercise such as walking on level ground, but strenuous exercise usually will bring on symptoms quickly. Resting may lead to a “second wind,” in which activity is then better tolerated. Isometric exercises that require strength, such as lifting heavy objects, squatting or standing on tiptoe, also may cause muscle damage.

Symptoms of this disease vary in severity among people and even within the same person from day to day. Symptoms usually don’t persist between attacks, although fixed weakness later in life is possible.

To learn more about the effect of diet in this disease, see What Not to Eat: Some consensus, much controversy about diet in three metabolic diseases.

What causes phosphorylase deficiency?

The disease is caused by a genetic defect in the phosphorylase (also known as myophosphorylase) enzyme, which affects the breakdown of glocogen, the stored form of glucose (sugar). For more, see Causes/Inheritance.

What is the progression of phosphorylase deficiency?

The condition usually begins before age 15 and is generally not progressive, although weakness between episodes of exercise sometimes develops.

What is the status of research on phosphorylase deficiency?

Researchers on metabolic diseases of muscle are making progress on a number of fronts, including:

• better diagnosis to allow for earlier identification of at-risk individuals and earlier treatment;
• continued examination of the role of exercise and diet in metabolic diseases;
• development of animal models of metabolic diseases, both to improve understanding of the diseases and to test possible treatments;
• development of enzyme replacement therapies; and
• development of gene therapies.