Understanding McArdle Disease

McArdle disease is a rare muscle disorder. Muscle cells can’t store energy well. Energy in muscles is stored as glycogen. This condition is one of a group of diseases called glycogen storage diseases. The symptoms often appear in childhood. But some people are not diagnosed until later as an adult.

How energy is stored and used in the body

Your body’s 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 extra glucose from the blood. This lowers the blood glucose to a healthy level. Your body then converts the extra glucose into glycogen. Glycogen is then stored 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 because a 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 missing substance in your muscles is an enzyme called muscle phosphorylase. An enzyme is a substance that helps speed up chemical reactions in the body.

What causes McArdle disease?

McArdle disease is a genetic disease. It is caused by changes (mutations) in the gene for the enzyme called muscle phosphorylase. Your muscle cells can’t make this enzyme and then can’t break down glycogen into glucose. Your muscles need glucose to work well. The mutation that causes McArdle disease often passes down from a parent to a child.

Symptoms of McArdle disease

Symptoms can range from mild to severe. They can depend on your level of activity. Some common symptoms include:

  • Feeling tired quickly during activity

  • Stiffness or weakness soon after starting exercise

  • Feeling less tired after a brief rest

  • Muscle cramping

  • Muscle pain

  • Brownish red urine, especially after periods of activity

  • Permanent weakness in the thigh or other muscles (rare)

In many cases, people notice these symptoms before age 15. Often people assume the symptoms are growing pains or have some other cause.

Diagnosing McArdle disease

Your healthcare provider will ask about your health history and your symptoms. He or she will also ask about your family’s health history. You will have a physical exam. This will include tests of your muscle strength. You may also have tests to see how well you can keep exercising. This is called an endurance test. You may have other tests such as:

  • Forearm exercise test

  • Blood test to check for gene mutations known to cause McArdle disease

  • Blood tests to check for muscle enzymes such as creatine kinase

  • Urine test to check for myoglobin

  • Electromyography, to measure the electrical activity of the muscles

  • Muscle biopsy, to look at muscle cells for glycogen buildup

  • MRI of muscles

You may first see your main healthcare provider. You may then be referred to a specialist, such as a neurologist. An exercise specialist may help you come up with ways to pace yourself. This can help you to exercise at a moderate level without getting any symptoms.