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# McArdle disease (glycogen storage disease type V)

In McArdle disease muscle cannot use its glycogen: exercise brings on cramps, pain and sometimes dark urine.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Vastus lateralis](https://durgical.com/en/anatomy/vastus-lateralis-muscle/)
- [Gastrocnemius (inner head)](https://durgical.com/en/anatomy/medial-head-of-gastrocnemius/)
- [Rectus femoris](https://durgical.com/en/anatomy/rectus-femoris-muscle/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## In biochemistry

- [Glycogen metabolism: Step 6 · Glycogen phosphorylase](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/#paso-6)
- [Glycogen phosphorylase](https://durgical.com/en/biochemistry/enzymes/glycogen-phosphorylase/)
- [Glycogen](https://durgical.com/en/biochemistry/molecules/glycogen/)
- [Creatine kinase (CK) and CK-MB as lab tests](https://durgical.com/en/biochemistry/topics/creatine-kinase-and-ck-mb-as-lab-tests/)
- [Purine nucleotide cycle (muscle)](https://durgical.com/en/biochemistry/topics/purine-nucleotide-cycle/)
- [Exercise metabolism](https://durgical.com/en/biochemistry/topics/exercise-metabolism/)

## In physiology

- [Fast and slow fibers, energy and fatigue](https://durgical.com/en/physiology/fast-and-slow-fibers-energy-and-fatigue/)

## What it is

Muscle glycogen storage disease caused by lack of myophosphorylase (PYGM gene). It is the commonest muscle glycogen storage disease.

## Who gets it

About 1 in 100,000 people according to US studies; probably underdiagnosed. No Colombian figures of its own.

## Causes

- Autosomal recessive inheritance: PYGM gene variants (the commonest in Europe, R50X)

## Risk factors

- Carrier parents or affected siblings
- Intense or isometric exercise (trigger)

## Pathophysiology

Without muscle phosphorylase the fibre cannot break down its glycogen at the start of intense effort: ATP runs short, cramps and contractures appear and the fibre breaks down (rhabdomyolysis, myoglobinuria). After about 8–10 minutes blood glucose and fatty acids reach the fibre and tolerance improves (second-wind phenomenon). Lactate does not rise with exercise; ammonia does.

## Symptoms and signs

- Exercise intolerance from childhood or adolescence
- Cramps and muscle pain in the first minutes of effort
- Improvement on slowing down and carrying on (second wind)
- Dark urine (myoglobinuria) after intense effort

## Red flags

- Dark urine with little urine output after exercise (risk of acute [kidney](https://durgical.com/en/anatomy/kidney/) injury)
- Severe pain and tension in a muscle compartment

## Diagnosis

**Clinical**
Cramps in the first minutes of exercise, myoglobinuria and second-wind phenomenon.

**Tests**
High CK even at rest. Forearm exercise test: lactate does not rise while ammonia does. PYGM gene testing; muscle biopsy (absent phosphorylase) is rarely needed.

**Imaging**
Imaging is not needed for diagnosis; muscle MRI may show affected muscles in adults.

## Treatment

**Lifestyle**
Regular moderate aerobic exercise with gradual warm-up; avoid isometric or maximal efforts

**Lifestyle**
A sugary drink before planned effort may improve tolerance, as advised by the care team

**Acute**
Urgent intravenous fluids in rhabdomyolysis

**Follow-up**
Review of anaesthesia and muscle-toxic drugs before surgery

## Complications

- Rhabdomyolysis with acute kidney injury
- Compartment syndrome
- Mild proximal weakness in adulthood

## Prognosis

Life expectancy is normal; with adapted exercise most lead an active life.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the second-wind phenomenon?

Improvement after a few minutes of exercise, when blood glucose and fatty acids replace muscle glycogen.

## What does the forearm exercise test show in McArdle disease?

Lactate does not rise while ammonia does.

## Why is there no hypoglycaemia in McArdle disease?

Liver phosphorylase is a different enzyme (PYGL) and works normally.

## What is the serious complication of myoglobinuria?

Acute kidney injury from rhabdomyolysis.

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## Related conditions

- [Tarui disease (muscle phosphofructokinase deficiency)](https://durgical.com/en/conditions/tarui-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Acute compartment syndrome](https://durgical.com/en/conditions/acute-compartment-syndrome/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Glycogen storage disease type V: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/glycogen-storage-disease-type-v/) (MedlinePlus, US National Library of Medicine, 2026); [PYGM gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/pygm/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Source: https://durgical.com/en/conditions/mcardle-disease/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/mcardle-disease/