## Distinguishing Type I from Type III Glycogen Storage Disorders ### Pathophysiology Comparison | Feature | Type I (Von Gierke) | Type III (Cori) | |---------|-------------------|------------------| | **Enzyme Defect** | Glucose-6-phosphatase | Debranching enzyme | | **Substrate Accumulation** | Glucose-6-phosphate → lactate, pyruvate, uric acid | Limit dextrin (branched glycogen) | | **Fasting Glucose** | Severely low (<40 mg/dL) | Mildly low | | **Lactic Acidosis** | Marked (lactate >> glucose) | Absent or mild | | **Muscle Involvement** | Spared (no myopathy) | Present (progressive) | | **Hepatic Fibrosis** | Early onset (by age 5–10) | Late or absent | ### Key Point: **Severe fasting hypoglycemia with lactic acidosis is pathognomonic for Type I.** In Type I, glucose-6-phosphate cannot be converted to free glucose, forcing shunting through glycolysis and gluconeogenesis, generating massive lactate. Type III patients retain some glucose homeostasis via debranching and have normal lactate metabolism. ### High-Yield: - **Type I:** "Hypoglycemia + Lactic Acidosis + Hepatomegaly" (the "metabolic triad") - **Type III:** "Hepatomegaly + Myopathy" (muscle involvement distinguishes it from Type I) ### Clinical Pearl: Type I patients present with "doll-like" facies, protuberant abdomen (massive hepatomegaly), and severe growth retardation. Type III patients may have delayed-onset muscle weakness in adolescence, which Type I patients do not develop. ### Warning: ~~Both cause hepatomegaly~~ — hepatomegaly is common to many GSDs. The discriminator is the **metabolic signature**: lactate elevation in Type I is disproportionate to glucose, whereas Type III maintains near-normal lactate. [cite:Robbins 10e Ch 5]
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