Nelson Textbook of Pediatrics 22nd Edition โ Glycogen storage diseases affecting muscle: Pompe (GSD II, acid maltase, infantile cardiomyopathy, ERT), McArdle (GSD V, myophosphorylase, exercise intolerance, second wind), Tarui (GSD VII, phosphofructokinase, hemolysis), Cori-Forbes (GSD III, debrancher, hepatomegaly, myopathy).
๐ paeds.online โ Pakistan's Pediatric Platform| GSD | Eponym | Enzyme Defect | Gene | Clinical Features | Diagnosis | Treatment |
|---|---|---|---|---|---|---|
| GSD II | Pompe | Acid maltase (ฮฑ-glucosidase) | GAA (17q25) | Infantile: cardiomegaly, hypotonia, macroglossia, death by age 2. Late-onset: myopathy | GAA enzyme activity (dried blood spot, fibroblasts), muscle biopsy (PAS-positive vacuoles) | Enzyme replacement (alglucosidase alfa, Myozyme) |
| GSD III | Cori-Forbes | Debrancher (amylo-1,6-glucosidase) | AGL (1p21) | Hypoglycemia, hepatomegaly, myopathy (milder, distal weakness) | Liver/muscle biopsy, enzyme assay | High-protein diet, cornstarch, avoid hypoglycemia |
| GSD V | McArdle | Myophosphorylase | PYGM (11q13) | Exercise intolerance, cramps, myoglobinuria, second wind phenomenon | Ischemic forearm test (no lactate rise), muscle biopsy (absent phosphorylase), genetic testing | Pre-exercise carbohydrate (sucrose/glucose), avoid high-intensity anaerobic exercise |
| GSD VII | Tarui | Phosphofructokinase (muscle) | PFKM (12q13) | Similar to McArdle + hemolytic anemia, hyperuricemia | Muscle biopsy (absent PFK), hemolysis labs | Same as McArdle; avoid exercise triggers |
| GSD IV | Andersen | Brancher (amylo-1,4โ1,6-transglucosidase) | GBE1 (3p12) | Cirrhosis, cardiomyopathy, myopathy; polyglucosan bodies | Liver/muscle biopsy, enzyme assay | Supportive; liver transplant |
Procedure: Place BP cuff on upper arm, inflate to systolic pressure. Patient exercises hand (squeezing) for 1 minute. Deflate cuff. Draw venous blood for lactate before, 1 min, and 5 min after exercise.
Normal response: Lactate rises 3-5x (anaerobic glycolysis). McArdle (GSD V): No lactate rise. Tarui (GSD VII): No lactate rise. CPT2/VLCAD: Normal lactate rise.
Second wind phenomenon: In McArdle, after 5-10 minutes of light exercise, heart rate drops, fatigue improves, as fatty acid oxidation provides energy. Pre-exercise carbohydrate (sucrose) can prevent myoglobinuria.