Glycogen
On this page 2 sections
Glycogen Storage Diseases
Glycogen storage diseases (GSDs) result from defects in enzymes responsible for glycogen synthesis or breakdown. They commonly affect the liver, skeletal muscle or both.
| Type | Disease | Enzyme defect | High-yield features |
|---|---|---|---|
| I | Von Gierke | Glucose-6-phosphatase system | Severe fasting hypoglycemia, lactic acidosis, hepatomegaly |
| II | Pompe | Lysosomal acid α-glucosidase | Cardiomyopathy, hypotonia and muscle weakness |
| III | Cori/Forbes | Debranching enzyme | Hepatomegaly, fasting hypoglycemia and limit-dextrin accumulation |
| IV | Andersen | Branching enzyme | Abnormal glycogen structure and progressive liver disease |
| V | McArdle | Muscle glycogen phosphorylase | Exercise intolerance, cramps and myoglobinuria |
| VI | Hers | Liver glycogen phosphorylase | Mild fasting hypoglycemia and hepatomegaly |
Clinical distinctions
Von Gierke disease (Type I)
- Severe fasting hypoglycemia.
- Hepatomegaly.
- Lactic acidosis.
- Hyperuricemia and hyperlipidemia may occur.
Pompe disease (Type II)
- Lysosomal glycogen accumulation.
- Infantile disease may produce severe cardiomyopathy and hypotonia.
- Unlike many hepatic GSDs, fasting hypoglycemia is not the defining feature.
Cori disease (Type III)
- Debranching enzyme deficiency.
- Incomplete glycogen breakdown.
- Hepatomegaly and hypoglycemia; muscle involvement may occur.
McArdle disease (Type V)
- Skeletal muscle glycogen cannot be used normally during exercise.
- Exercise-induced cramps and fatigue.
- Myoglobinuria can follow strenuous exercise.
- A second-wind phenomenon may occur as muscles increasingly use circulating fuels.
Exam tip: Match the organ affected, fasting glucose pattern and enzyme defect. These three clues usually distinguish the classic glycogen storage diseases.