A nonischemic forearm exercise test for McArdle disease

Ischemic forearm exercise invariably causes muscle cramps and pain in patients with glycolytic defects. We investigated an alternative diagnostic exercise test that may be better tolerated. Nine patients with McArdle disease, one with the partial glycolytic defect phosphoglycerate mutase deficiency, and nine matched, healthy subjects performed the classic ischemic forearm protocol and an identical protocol without ischemia. Blood was sampled in the median cubital vein of the exercised arm. Plasma lactate level increased similarly in healthy subjects during ischemic (Δ5.1 ± 0.7mmol L−1) and non‐ischemic (Δ4.4 ± 0.3) tests and decreased similarly in McArdle patients (Δ−0.10 ± 0.02 vs Δ−0.40 ± 0.10mmol L−1). Postexercise peak lactate to ammonia ratios clearly separated patients and healthy controls in ischemic (McArdle, 4 ± 2 [range, 1–12]; partial glycolytic defect phosphoglycerate mutase deficiency, 6; healthy, 33 ± 4 [range, 17–56]) and non‐ischemic (McArdle, 5 ± 1 [range, 1–10]; partial glycolytic defect phosphoglycerate mutase deficiency, 5; healthy, 42 ± 3 [range, 35–56]) protocols. Similar differences in lactate to ammonia ratio between patients and healthy subjects were observed in two other work protocols using intermittent handgrip contraction at 50% and static handgrip exercise at 30% of maximal voluntary contraction force. All patients developed pain and cramps during the ischemic test, and four had to abort the test prematurely. No patient experienced cramps in the non‐ischemic test, and all completed the test. The findings indicate that the diagnostic ischemic forearm test for glycolytic disorders should be replaced by an aerobic forearm test.