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Glycogen storage disease type V
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Conditions

Glycogen Storage Disease Type V

Glycogen storage disease type V (McArdle disease) is a rare inherited disorder that impairs the body's ability to break down glycogen in muscle cells, leading to exercise intolerance, muscle pain, and potential kidney complications.

Overview

Glycogen storage disease type V (also known as GSDV or McArdle disease) is an inherited disorder caused by an inability to break down a complex sugar called glycogen in muscle cells. A lack of glycogen breakdown interferes with the function of muscle cells, leading to the primary symptoms of the condition.

Symptoms

The signs and symptoms of GSDV can vary significantly among affected individuals. While features typically begin in a person's teens or twenties, they can appear anytime from infancy to adulthood. In most people with GSDV, muscle weakness worsens over time; however, in about one-third of affected individuals, the muscle weakness remains stable. Some people experience mild symptoms such as poor stamina, while others do not experience any symptoms at all.

Exercise Intolerance and the "Second Wind"

People with GSDV typically experience fatigue, muscle pain, and cramps during the first few minutes of exercise. Activities such as weight lifting or jogging usually trigger these symptoms. The discomfort is generally alleviated with rest. If individuals rest after brief exercise and wait for their pain to go away, they can usually resume exercising with little or no discomfort, a characteristic phenomenon known as the "second wind."

Rhabdomyolysis and Kidney Damage

Prolonged or intense exercise can cause muscle damage in people with GSDV. About half of people with GSDV experience breakdown of muscle tissue (rhabdomyolysis). In severe episodes, the destruction of muscle tissue releases a protein called myoglobin, which is released in the urine (myoglobinuria). Myoglobin causes the urine to be red or brown. This protein can also damage the kidneys, and it is estimated that half of those individuals with GSDV who have myoglobinuria will develop life-threatening kidney failure.

Causes & Risk Factors

Mutations in the PYGM gene cause GSDV. The PYGM gene provides instructions for making an enzyme called myophosphorylase. This enzyme is found only in muscle cells, where it breaks down glycogen into a simpler sugar called glucose-1-phosphate. Additional steps convert glucose-1-phosphate into glucose, a simple sugar that is the main energy source for most cells.

PYGM gene mutations prevent myophosphorylase from breaking down glycogen effectively. As a result, muscle cells cannot produce enough energy, so muscles become easily fatigued. Reduced energy production in muscle cells leads to the major features of GSDV.

Inheritance

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Frequency

GSDV is a rare disorder; however, its overall prevalence is unknown. In the Dallas-Fort Worth area of Texas, where the prevalence of GSDV has been studied, the condition is estimated to affect 1 in 100,000 individuals.

Other Names for This Condition

  • Glycogen storage disease type 5
  • Glycogenosis 5
  • GSD type V
  • GSD V
  • McArdle disease
  • McArdle syndrome
  • McArdle type glycogen storage disease
  • McArdle's disease
  • Muscle glycogen phosphorylase deficiency
  • Muscle phosphorylase deficiency
  • Myophosphorylase deficiency
  • PYGM deficiency

Frequently Asked Questions

The “second wind” appears after about 6–10 minutes of low‑intensity activity when heart rate and breathing stabilize and muscle pain eases. To use it safely, start with a brief, gentle warm‑up (e.g., slow walking), stop at the first sign of cramping, rest for a few minutes, then resume at a slower pace. Once the second wind is felt, you can continue the activity at a moderate intensity without pain, but avoid sudden bursts of high effort.
high‑carbohydrate, low‑fat diet with frequent small meals helps maintain blood glucose for muscle use. Consuming a carbohydrate snack (e.g., fruit juice or a glucose gel) 15–30 minutes before exercise can provide an immediate fuel source. Staying well‑hydrated (≥2 L/day) and limiting alcohol also lessen the chance of muscle breakdown. Some patients benefit from a modest increase in protein and a balanced intake of electrolytes, especially potassium and magnesium.
Seek emergency care if you experience severe, persistent muscle pain lasting more than an hour, dark‑colored (tea‑colored) urine, rapid swelling of a limb, fever, or weakness that does not improve with rest. These signs suggest myoglobin release that can damage the kidneys; prompt intravenous fluids and monitoring are essential.
Creatine monohydrate has shown modest benefit in some patients by increasing phosphocreatine stores, but it should be tried under medical supervision. Oral ribose, a precursor for ATP, has limited evidence and is not routinely recommended. No drug can replace the missing myophosphorylase enzyme, so management focuses on lifestyle, diet, and avoiding triggers rather than pharmacologic cure.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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