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.
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.
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.
The condition usually begins before age 15 and is generally not progressive, although weakness between episodes of exercise sometimes develops.
Researchers on metabolic diseases of muscle are making progress on a number of fronts, including: