McArdle disease

McArdle disease is a rare inherited metabolic muscle disorder caused by an enzyme deficiency that prevents muscles from properly breaking down stored glycogen for energy during exercise. This causes exercise intolerance, muscle pain, and cramping, particularly during intense or sustained physical activity.

It's caused by mutations in the PYGM gene, which produces the muscle enzyme myophosphorylase needed to release energy from stored glycogen within muscle cells. It's inherited in an autosomal recessive pattern, meaning both parents typically carry a copy of the faulty gene.

Symptoms include muscle pain, cramping, and fatigue during exercise, particularly at the start of activity or during sustained intense exertion, sometimes with a characteristic 'second wind' phenomenon where symptoms ease after a brief rest. In severe episodes, muscle breakdown (rhabdomyolysis) can occur, which can affect the kidneys.

It's not immediately dangerous with appropriate management, but severe muscle breakdown episodes during excessive exertion can, in rare cases, cause kidney damage from the breakdown products released into the bloodstream. Learning to recognise and respect personal exercise limits significantly reduces this risk.

Yes, it's an inherited genetic condition, and genetic testing can confirm the diagnosis. Genetic counselling is recommended for affected individuals given the autosomal recessive inheritance pattern.

There's no cure, as it's a genetic enzyme deficiency, but management strategies including pacing exercise, warming up gradually, and using the 'second wind' phenomenon to advantage can help many people remain physically active within their individual limits. Some dietary strategies may also help support exercise tolerance.

Management focuses on exercise pacing strategies, including a gradual warm-up and brief rest periods to allow the 'second wind' effect, alongside avoiding sudden intense exertion that could trigger significant muscle breakdown. A specialist familiar with metabolic myopathies can provide tailored exercise and dietary advice.

Diagnosis typically involves blood tests showing elevated muscle enzymes, sometimes combined with a specific exercise test demonstrating the characteristic pattern of exercise intolerance and second wind phenomenon, confirmed with genetic testing or muscle biopsy. A detailed history of exercise-related symptoms is an important part of the diagnostic process.

This refers to a notable phenomenon where, after initial muscle pain and fatigue at the start of exercise, symptoms can ease if the person briefly rests and then resumes activity at a lower intensity, as the muscles switch to using an alternative energy source. Understanding and using this phenomenon can help people with McArdle disease exercise more comfortably within their limits.

Unexplained exercise-related muscle pain and cramping, particularly with a characteristic pattern of improvement after brief rest, should be assessed by a neurologist or metabolic specialist. Private specialist consultations can typically be arranged within about two weeks, considerably faster than standard NHS waiting times.