Type I: Von Gierke Disease (Glucose-6-Phosphatase Deficiency)
- Let’s break down Von Gierke’s! This is an autosomal recessive disorder where hepatocytes lack the crucial enzyme Glucose-6-Phosphatase. This enzyme is the final common gateway for both glycogenolysis (breaking down glycogen) and gluconeogenesis (synthesizing new glucose!) to release free glucose into the blood.
- Because the liver cannot release glucose, patients suffer from profound, severe fasting hypoglycemia. The trapped G6P is shunted down glycolysis, generating excess pyruvate which gets converted to massive amounts of lactic acid (causing lactic acidosis).
- Other hallmarks include a soft, enlarged liver (hepatomegaly) stuffed with glycogen, doll-like faces from fat deposits, hyperlipidemia, and gout due to excess purine synthesis. It’s treated with regular feeds of uncooked cornstarch to feed the body slow-release glucose.
Type II: Pompe Disease (Lysosomal Acid Alpha-Glucosidase Deficiency)
- Pompe Disease is a lysosomal storage disease resulting from a deficiency of Acid Alpha-1,4-Glucosidase (commonly called acid maltase). This enzyme normally chews up glycogen trapped inside lysosomes during normal cellular remodeling.
- When missing, glycogen compiles progressively within lysosomes, destroying the architecture of cardiac, skeletal, and smooth muscle cells. Unlike other GSDs, there is no hypoglycemia because cytoplasmic glycogenolysis is 100% normal—the problem is purely inside the lysosomes!
- Remember the classic student phrase: "Pompe trashes the Pump (heart)!" It presents in infants with heavy generalized hypotonia ("floppy infant" posture), massive cardiomegaly, macroglossia (enlarged tongue), and early cardiorespiratory failure.
Type V: McArdle Disease (Myophosphorylase Deficiency)
- McArdle’s is a muscle-specific glycogen storage disease caused by a deficiency of skeletal muscle Glycogen Phosphorylase (myophosphorylase). The liver is perfectly fine, so blood glucose levels remain normal!
- Under intense exercise, muscle cells cannot break down their internal glycogen stores, leaving them depleted of anaerobic energy. This leads to classic painful cramps, fatigue, and muscle breakdown (rhabdomyolysis), which can turn the urine cherry-red (myoglobinuria) and threaten the kidneys.
- Students must watch for the "second-wind phenomenon": after about 10 minutes of light exercise, the patient’s symptoms improve as the body dilates local vessels and mobilizes alternative fuel sources like fatty acids to feed the working muscles.
💡 Memory Mnemonic Aid:
V-P-M: Von Gierke, Pompe, McArdle (Types 1, 2, 5)
Type I (Von Gierke - liver glycogen), Type II (Pompe - heart lysosomal), Type V (McArdle - skeletal muscle) are the core high-yield GSDs.
🌟 High-Yield Boards Summary Indicator:
To easily distinguish Von Gierke (Type I) from Cori (Type III, Debranching enzyme) in clinical scenarios: blood lactate rises severely after fasting in Von Gierke, but remains completely normal in Cori disease because its gluconeogenesis pathway is intact.