Glycogen storage disease type V (McArdle disease)
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A teenager experiences muscle cramps and fatigue during exercise. He notices dark urine after intense activity. He describes a "second wind" phenomenon where exercise becomes easier after initial fatigue.
AR; PYGM (muscle glycogen phosphorylase)
- Exercise intolerance with muscle cramps
- "Second wind" phenomenon (symptoms improve after ~10 min of exercise)
- Myoglobinuria after intense exercise (risk of renal failure)
- Elevated CK (especially after exercise)
- No rise in lactate during ischemic forearm exercise test
- Fixed weakness may develop in older adults
- Not on newborn screening; suspect from exercise intolerance, the "second wind" phenomenon, and recurrent myoglobinuria
- Persistently elevated baseline CK; non-ischemic (or ischemic) forearm exercise test shows a flat venous lactate with a normal ammonia rise
- Absent muscle glycogen phosphorylase activity and excess glycogen on muscle biopsy
- Confirmatory PYGM molecular testing (preferred first-line, avoids biopsy)
- Avoid intense exercise, pre-exercise glucose/sucrose, moderate aerobic training
GSD Type V = muscle only: "no pain, no muscle gain." Glycogen is trapped in muscle (unlike von Gierke, where it is in liver and kidney). No liver involvement, no fasting hypoglycemia.
Flat venous lactate with exercise: the ischemic forearm exercise test shows no rise in lactate (because muscle glycogen phosphorylase is deficient) but a normal rise in ammonia.
Vit B6 (5 + 1 = 6): pyridoxal phosphate (vitamin B6) is a cofactor for muscle glycogen phosphorylase. McArdle patients in their 3rd-4th decade of life are the oldest-presenting GSD patients.