McArdle disease is also called glycogen storage disease type 5 or myophosphorylase deficiency. It's a rare muscle disorder. In this disease, muscle cells can't use 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 adults.

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 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.

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. Mutations that cause McArdle disease are passed from both parents to a child.

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

• Feeling tired quickly during physical activity.
• Stiffness or weakness soon after starting exercise.
• Muscle cramping.
• Muscle pain.
• Brownish red urine, especially after periods of activity (myoglobinuria).
• Lasting weakness in the thigh or other muscles (rare).

Symptoms can improve after several minutes with continued exercise. This is called second wind phenomena.

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

Your doctor will ask about your health history and your symptoms. You will also be asked 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:

• Blood or saliva test to check for gene mutations known to cause McArdle disease.
• Blood tests to check for muscle enzymes such as creatine kinase.
• A 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.
• An MRI of muscles.

Your doctor may refer you 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.

The condition has no cure, but symptoms can be controlled. Making dietary changes may help. Examples include having a prescribed amount of sugar before you exercise and eating a high-carbohydrate diet. Talk with your doctor to find out if this may be advised for you. Physical therapy can help with weakness. Genetic counseling is advised if you plan to have children.

Episodes of muscle breakdown (rhabdomyolysis) need emergency medical care. You may need I.V. (intravenous) fluids and medicines to prevent kidney damage from this.