McArdle

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

Symptoms and Signs

Individuals with McArdle's Disease typically experience exercise intolerance, manifesting as fatigue, muscle pains, and cramps during physical activities. Unlike in other conditions, the second wind phenomenon—marked improvement in exercise tolerance after a few minutes of rest—can be observed in McArdle's patients. Other symptoms may include myoglobinuria (the presence of myoglobin in urine, which can be a sign of muscle damage) following strenuous activity, and in severe cases, this can lead to renal failure. Muscle weakness and wasting can occur over time, but the severity varies among individuals.

Diagnosis

Diagnosis of McArdle's Disease involves a combination of clinical evaluation, laboratory tests, and genetic testing. The forearm exercise test, which measures lactate and ammonia levels in the blood before and after mild exercise, is a common initial test. In McArdle's Disease, lactate levels do not rise as expected due to the block in glycogen breakdown. Muscle biopsy can confirm the diagnosis by demonstrating the absence of myophosphorylase activity. Genetic testing can identify mutations in the PYGM gene, which provides instructions for making myophosphorylase.

Treatment

There is no cure for McArdle's Disease, but management strategies can help alleviate symptoms and improve quality of life. These include tailored exercise programs to increase muscle stamina without causing damage, dietary modifications like consuming simple carbohydrates before exercise to provide an alternative energy source, and in some cases, Creatine supplements may be beneficial. Avoiding strenuous activity is crucial to prevent muscle damage and myoglobinuria.

Epidemiology

McArdle's Disease is a rare condition, with an estimated incidence of 1 in 100,000 to 1 in 140,000 people worldwide. It affects both males and females equally and can present at any age, although symptoms typically begin in childhood or adolescence.

History

The disease is named after Brian McArdle, who first described the condition in 1951. McArdle identified the absence of muscle phosphorylase activity in a patient with muscle cramps and weakness, establishing the link between the enzyme deficiency and the disease symptoms.

See Also

• Glycogen storage disease
• Metabolic myopathy
• Exercise intolerance
• Muscle biopsy