Muscle phosphorylase deficiency, also known as McArdle disease or glycogen storage disease type V, is a rare genetic disorder characterized by the deficiency or absence of the enzyme muscle phosphorylase. This enzyme is responsible for breaking down glycogen, a stored form of glucose, into glucose-1-phosphate. Glucose-1-phosphate is then utilized by muscles as an energy source during physical activity.
In individuals with muscle phosphorylase deficiency, the inability to break down glycogen results in a lack of glucose-1-phosphate availability, leading to severe muscle cramps, muscle pain, and muscle fatigue during exercise. These symptoms typically occur shortly after initiating physical activity and can persist for hours, resulting in reduced stamina and exercise intolerance. Additionally, myoglobin, a muscle protein, may be released into the bloodstream, leading to dark-colored urine known as myoglobinuria.
Muscle phosphorylase deficiency is an inherited condition caused by mutations in the PYGM gene, which provides instructions for making the muscle phosphorylase enzyme. This disorder follows an autosomal recessive inheritance pattern, meaning that affected individuals inherit two defective copies of the gene, one from each parent.
Diagnosis of muscle phosphorylase deficiency is commonly achieved through a combination of medical history evaluation, physical examination, exercise testing, and genetic testing. Treatment mainly focuses on symptom management, and individuals are advised to avoid intense physical activities that could trigger symptoms. Maintaining a balanced diet, staying well-hydrated, and adopting gradual exercise programs are also recommended. Although the condition is typically chronic and there is no cure, individuals with muscle phosphorylase deficiency can lead fulfilling lives with proper management and support.
