Myophosphorylase Deficiency: Overview and More

Myophosphorylase deficiency, also known as McArdle disease or glycogen storage disease type V (GSDV), is a condition that occurs when muscle cells can't break down glycogen or stored sugars for fuel. This rare genetic condition causes fatigue, stiffness, and weakness during exercise and affects around 1 in every 100,000 people.

This article discusses myophosphorylase deficiency, including what it is, the causes of the condition, how it is diagnosed, and its treatments.

What Causes Myophosphorylase Deficiency?

Glucose is a simple sugar that supplies energy to cells in the body. Glucose comes from carbohydrates in the diet. Excess glucose is stored in the muscles and liver as glycogen. During exercise, the body takes stored glycogen and breaks it down into glucose to be used as fuel.

Myophosphorylase deficiency is a condition caused by a mutation in the gene responsible for glycogen phosphorylase, an enzyme that initiates the breakdown of glycogen in the muscle during physical activity.

Symptoms of Myophosphorylase Deficiency

Myophosphorylase deficiency symptoms appear in childhood and begin after a few minutes of high-intensity physical activity or exercise.

Symptoms during exercise include:

• Poor tolerance to exercise (excessively increased heart rate)
• Muscle cramps or pain
• Weakness
• Fatigue

These symptoms typically resolve after a few minutes of rest and are referred to as the "second-wind phenomenon."

Myoglobinuria (proteins that supply oxygen to the muscles) in the urine can occur with myophosphorylase deficiency. A condition called rhabdomyolysis (breakdown of muscles) occurs in about half of people with myophosphorylase deficiency when damaged muscle releases electrolytes and proteins into the blood.

Symptoms of rhabdomyolysis include:

• Severe muscle pain or cramping
• Fatigue
• Muscle weakness
• Dark-colored urine

In the most severe cases, myophosphorylase deficiency can lead to kidney failure.

How Is Myophosphorylase Deficiency Diagnosed?

Myophosphorylase deficiency is diagnosed through blood and urine tests, muscle biopsy, genetic testing, electromyography (which tests electric signals in your muscles), and the forearm ischemic exercise test (FIET).

• Blood tests look for higher levels of creatine kinase (an enzyme that helps with muscle function), indicating muscle damage.
• Levels of lactic acid typically increase during exercise. However, levels stay low with myophosphorylase deficiency. Urine tests look for myoglobin, the protein that transports and stores oxygen in muscle cells.
• A muscle biopsy is performed to help diagnose myophosphorylase deficiency. Muscle cells are examined under a microscope to look for the buildup of glycogen.
• Genetic testing can help diagnose myophosphorylase deficiency by looking for a mutation of the muscle glycogen phosphorylase gene.
• Electromyography (EMG) records electrical signals in the muscle through small needle electrodes. In people with myophosphorylase deficiency, EMG can be used to detect abnormal muscle activity.
• The forearm ischemic exercise test (FIET) can help diagnose myophosphorylase deficiency by taking blood samples at rest, exercising the arm while wearing an inflated blood pressure cuff, then taking additional blood samples to compare chemical levels.
  

How Is Myophosphorylase Deficiency Treated?

There is no cure for myophosphorylase deficiency. Symptoms are managed through diet and exercise modifications. It is important to work with a healthcare provider to tailor a diet and exercise plan to your needs.

Examples of diet and exercise treatments include:

• Consuming sucrose before exercise
• Vitamin B6 supplements
• Creatine supplements
• Low to moderate aerobic exercise
• ACE inhibitor medications

Summary

Myophosphorylase deficiency is a rare genetic disorder that occurs when the body cannot break down glycogen in muscle to be used for fuel during exercise. Symptoms of this condition include muscle pain and cramping, weakness, and fatigue. Mysophosphorylase deficiency is diagnosed through a variety of testing. There is no cure for this condition, but diet and exercise modifications can help manage symptoms.

A Word From Verywell

While myophosphorylase deficiency can be a frustrating diagnosis, you can likely manage your symptoms with lifestyle changes and by adjusting your expectations during exercise. Always follow your healthcare provider's instructions. Continuing to push through your symptoms during exercise can lead to permanent muscle damage and kidney issues.