Molecular Biology
and Lysosomal Disease
Diagnosis Laboratory
Center for Research and Diagnosis of Genetic Diseases
Diagnostico: McArdle Disease

McArdle disease is a rare muscle disorder. In this disease, the muscle cells can’t break down a complex sugar called glycogen. It is part of a group of diseases called glycogen storage diseases. Another name for McArdle disease is glycogen storage disease GSD 5 or GSD V.

Your cells use a simple sugar, called glucose, for energy. When you eat, your digestive system sends a large amount of glucose into your blood. This raises your blood glucose levels. Your body removes this extra glucose from the blood to stabilize the blood glucose level. Your body then converts the extra glucose into glycogen. It stores it in the liver, muscles, and other places in the body. Glycogen is a form of energy storage.

When you haven’t eaten in a while, the glucose level in your blood starts to drop. This tells your body to start using some of the glycogen it saved up earlier. The glycogen gets broken down into the glucose so that your body has a steady supply.

Your muscles need a constant supply of glucose to keep working well. In McArdle disease, your muscles can’t break down the saved up glycogen. That's because an important substance needed for that process is missing from your muscle cells. This means your muscles can’t use the stored glycogen to get the glucose they need. The key missing substance in your muscles is an enzyme called myophosphorylase. An enzyme is a substance that helps speed up chemical reactions in the body.

This may make it hard for you to exercise without becoming tired and having muscle pain. Most of the time, symptoms of this health problem appear by the time a person is age 15.

McArdle disease is an inherited disease. It results from changes (mutations) in the gene for the enzyme muscle phosphorylase. Your muscle cells can’t make this enzyme. So they can’t break down glycogen into glucose. Your muscles need glucose to work their best.

The gene change that causes McArdle disease usually passes down from a parent to a child. You generally have a pair of genes (1 from each parent) for each substance your body makes. In most cases, a person with McArdle disease needs to have 2 copies of a mutated gene. This is recessive inheritance.

A person who has only 1 copy of the mutated gene may still have some symptoms of McArdle disease.

McArdle disease causes muscle symptoms. Generally, it can make it hard for you to exercise without becoming tired. You may find that this does not happen with gentle walking. But you may have trouble with strenuous exercise for more than a few minutes. You may notice that after a brief rest you feel a “second wind” that lets you exercise again. These symptoms can vary in intensity. Some common symptoms of McArdle disease are:

  • Better able to do aerobic exercise after 8 to 10 minutes (second-wind phenomenon)
  • Brownish red urine, especially after periods of activity
  • Easily tiring during activity, with stiffness or weakness soon after starting exercise
  • Muscle cramping
  • Muscle pain
  • Permanent weakness in the thigh or other muscles. This happens in a small portion of people with the condition.

Most of the time, people notice these symptoms before age 15. Often, people assume the symptoms are “growing pains” or due to some other cause. Different people may have symptoms of different severity. You may even notice that the symptoms seem worse or better at different times.

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