GSD type V

Glycogen Storage Disease Type V (GSD V), also known as McArdle's Disease, is a metabolic disorder characterized by a deficiency of the enzyme muscle phosphorylase. This enzyme is crucial for the breakdown of glycogen into glucose during intense muscle activity, providing energy for muscle cells. The absence or malfunctioning of this enzyme in individuals with GSD V leads to a range of symptoms primarily affecting muscle function.

Symptoms and Diagnosis

The hallmark symptom of GSD V is exercise intolerance, which includes fatigue, muscle cramps, and pain during activities that require quick bursts of energy, such as running or lifting heavy objects. Additionally, patients may experience myoglobinuria (the presence of myoglobin in urine) following strenuous exercise, which can lead to kidney damage in severe cases.

Diagnosis of GSD V typically involves a combination of clinical evaluation, family history, and laboratory tests. The forearm exercise test, which measures lactate and ammonia levels in the blood before and after muscle activity, is a common diagnostic tool. A definitive diagnosis, however, is usually obtained through a muscle biopsy, which shows the absence of muscle phosphorylase activity. Genetic testing can also identify mutations in the PYGM gene, responsible for encoding the muscle phosphorylase enzyme.

Treatment and Management

There is no cure for GSD V, and treatment focuses on managing symptoms and preventing complications. Strategies include:

• Avoiding strenuous exercise that triggers symptoms.
• Adopting a high-protein and high-complex-carbohydrate diet to help maintain energy levels.
• Supplementing with oral creatine and other nutrients to improve muscle energy metabolism.
• Engaging in regular, moderate-intensity aerobic exercise to improve stamina and muscle function.

Epidemiology

GSD V is a rare condition, with an estimated incidence of 1 in 100,000 to 1 in 140,000 individuals worldwide. It affects both males and females equally.

History

GSD V was first described in 1951 by Dr. Brian McArdle, after whom the disease is named. His research identified the lack of muscle phosphorylase activity as the underlying cause of the disease, marking a significant advancement in the understanding of metabolic muscle disorders.

See Also

• Glycogen storage disease
• Metabolic disorder
• Exercise intolerance
• Myoglobinuria