McArdle disease

A genetic disorder affecting muscle metabolism

McArdle disease
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Synonyms	Glycogen storage disease type V (GSD-V)
Pronounce	N/A
Specialty	N/A
Symptoms	Muscle pain, cramps, fatigue, myoglobinuria
Complications	N/A
Onset	Childhood or early adulthood
Duration	Lifelong
Types	N/A
Causes	Mutations in the PYGM gene
Risks	N/A
Diagnosis	Genetic testing, Muscle biopsy
Differential diagnosis	N/A
Prevention	N/A
Treatment	Exercise management, dietary modifications
Medication	N/A
Prognosis	N/A
Frequency	Rare
Deaths	N/A

McArdle disease, also known as glycogen storage disease type V (GSD-V), is a rare genetic disorder that affects the metabolism of glycogen in muscle cells. It is caused by mutations in the PYGM gene, which encodes the enzyme myophosphorylase. This enzyme is crucial for the breakdown of glycogen into glucose-1-phosphate, a necessary step for energy production in muscle cells during exercise.

Pathophysiology

In individuals with McArdle disease, the deficiency of myophosphorylase leads to an inability to properly break down glycogen stored in muscle tissue. As a result, during physical activity, affected individuals experience a lack of available glucose for energy, leading to symptoms such as muscle pain, cramps, and fatigue. The accumulation of glycogen in muscle cells can also cause damage, leading to muscle weakness and, in some cases, myoglobinuria, which is the presence of myoglobin in the urine due to muscle breakdown.

Clinical Presentation

Symptoms of McArdle disease typically present in childhood or early adulthood. Common symptoms include:

• Exercise intolerance
• Muscle pain and cramps during physical activity
• Early fatigue
• "Second wind" phenomenon, where symptoms improve after a brief rest during exercise
• Myoglobinuria, especially after intense exercise

Diagnosis

Diagnosis of McArdle disease is often based on clinical symptoms and confirmed through genetic testing to identify mutations in the PYGM gene. A muscle biopsy may also be performed, revealing an excess of glycogen and a deficiency of myophosphorylase activity.

Management

There is currently no cure for McArdle disease, but symptoms can be managed through lifestyle modifications. Patients are often advised to:

• Engage in regular, moderate exercise to improve muscle conditioning
• Avoid strenuous activities that trigger symptoms
• Follow a diet that includes adequate carbohydrates to maintain energy levels
• Consider creatine supplementation, which may help improve muscle function

Prognosis

The prognosis for individuals with McArdle disease is generally good, with most patients able to lead normal lives with appropriate management of symptoms. However, severe episodes of muscle breakdown can lead to complications such as acute kidney injury.

Also see

• Glycogen storage disease
• Metabolic myopathy
• Muscle biopsy
• Genetic testing