Overview:

• Synonyms: Muscle Phosphorylase Deficiency, McArdle Disease, Myophosphorylase Deficiency
• Cause: Mutations in the PGYM gene result in deficiency of muscle phosphorylase enzyme
• Effect: Inability to break down glycogen in skeletal muscle results in an energy shortage during anaerobic activity and muscle damage

Symptoms:

• Muscle pain, cramps, fatigue during exercise
• Myoglobinuria (dark red/brown urine from muscle breakdown)
• Rhabdomyolysis can occur
• Elevated creatine kinase levels
• No liver enlargement or hypoglycemia; normal physical exam
• Clinical clues: Exercise intolerance especially with anaerobic activity with “second wind” after cessation of exercise

Diagnosis:

• Genetic testing: Identifies mutations in the PGYM gene
• Muscle biopsy: Shows excess glycogen and lack of phosphorylase enzyme although rarely required
• Blood tests: Elevated CK; no metabolic abnormalities
• Clinical clues: Exercise intolerance, especially with anaerobic activity

Management:

• No cure; focus on lifestyle adaptation
• Aerobic exercise: Safe after reaching “second wind” (~10 minutes into activity)
• Avoid anaerobic exertion: Prevents rhabdomyolysis and kidney damage
• Monitor for long-term muscle complications (myopathies)
• A high protein diet may be beneficial

Prognosis:

• Generally good with proper exercise management
• Muscle symptoms may worsen with age
• Kidney injury possible if myoglobinuria is severe