Table 3.

Biochemical differentiation of inherited metabolic disorders presenting as muscle cramps[3]

Disease	Results of ischemic forearm exercise test	Other biochemical features	Confirmatory test
McArdle disease (GSD V)	Normal pretest lactate and no increase post-test	Elevated CPK myoglobinuria	Deficiency of phosphorylase in muscle
Phosphofructokinase (PFK) deficiency	Excessive increase of ammonium	Myoglobinuria, hyperuricemia, elevated CPK	Deficiency of PFK in muscle
Phosphoglycerate kinase (PGK) deficiency	Normal response	–	Deficiency of PGK in erythrocytes
Phosphoglycerate mutase (PGAM) deficiency	Excessive increase of ammonium	Myoglobinuria, hyperuricemia, elevated CPK	Deficiency of PGAM in muscle
Lactate dehydrogenase (LDH) defect	No lactic acidosis, but marked hyperpyruvic} acidemia during test	Myoglobinuria, hyperuricemia, elevated CPK	Deficiency of LDH-M subunit in erythrocytes
Carnitine palmitoyl transferase II (CPT II) deficiency	Normal lactate and ammonium response, but increased CPK	Increased CPK during fasting, myoglobinuria	Deficiency of CPT II in fibroblasts
Long-chain acyl-CoA dehydrogenase defect (LCAD)	Normal lactate and ammonium response, but increased CPK	Decreased plasma carnitine	Characteristic acylcarnitine profile, deficiency of LCAD in fibroblasts
Short-chain hydroxyacyl-CoA dehydrogenase (SCHAD) defect	Normal response	Myoglobinuria	Characteristic acylcarnitine profile Deficiency of SCHAD in fibroblasts
Myoadenylate deaminase deficiency	Normal lactate response, no increase in ammonium	Elevated CPK in 50%	Deficiency of myoadenylate deaminase in muscle