A 2-year-old boy from Mumbai presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly during a 4-hour fast. On examination, he has a "doll-like" face with full cheeks, short stature, and prominent abdomen. Serum lactate is elevated at 8 mmol/L (normal <2), and uric acid is 9.2 mg/dL. A liver biopsy shows excessive glycogen accumulation with normal glycogen structure. Which enzyme deficiency is most likely responsible for this clinical presentation?

Explanation

## Diagnosis: Glycogen Storage Disease Type I (Von Gierke Disease) ### Clinical Recognition **Key Point:** GSD Type I (glucose-6-phosphatase deficiency) is the most severe hepatic glycogenosis, presenting with severe fasting hypoglycemia, lactic acidosis, and hyperuricemia in infants/toddlers. ### Pathophysiology Glucose-6-phosphatase catalyzes the final step of both gluconeogenesis and glycogenolysis: $$\text{Glucose-6-phosphate} \xrightarrow{\text{G6Pase}} \text{Glucose} + \text{Pi}$$ Without this enzyme: 1. **Glucose-6-phosphate accumulates** → shunted into glycolysis → pyruvate → lactate (lactic acidosis) 2. **Pyruvate → acetyl-CoA → fatty acids** → hepatic steatosis 3. **Pyruvate → oxaloacetate → purines** → hyperuricemia and gout 4. **No free glucose release** → severe fasting hypoglycemia within 3–4 hours ### Clinical Features of GSD Type I | Feature | Mechanism | |---------|----------| | **Severe fasting hypoglycemia** | No glucose release from liver | | **Hepatomegaly (massive)** | Glycogen + fat accumulation | | **Lactic acidosis** | Shunting of G6P into glycolysis | | **Hyperuricemia / gout** | Increased purine degradation | | **Growth retardation** | Chronic metabolic derangement | | **"Doll-like" face** | Fat deposition in cheeks | | **Normal glycogen structure** | Enzyme defect is NOT in branching/debranching | | **Bleeding tendency** | Platelet dysfunction (secondary) | **High-Yield:** The **normal glycogen structure** rules out branching enzyme (Type IV) and debranching enzyme (Type III) deficiencies, which produce abnormal glycogen. ### Differential Diagnosis | GSD Type | Enzyme Defect | Key Distinguishing Feature | |----------|---------------|---------------------------| | **Type I** | Glucose-6-phosphatase | Severe hypoglycemia, lactic acidosis, hyperuricemia, normal glycogen structure | | **Type II** | Lysosomal acid maltase | Infantile cardiomyopathy, muscle weakness, normal blood glucose | | **Type III** | Debranching enzyme | Hepatomegaly + myopathy, abnormal glycogen (short outer branches) | | **Type IV** | Branching enzyme | Hepatic cirrhosis, abnormal glycogen (few branches, polyglucosan) | | **Type V** | Muscle phosphorylase | Exercise intolerance, myalgia, normal hepatomegaly | **Clinical Pearl:** The combination of **severe fasting hypoglycemia + lactic acidosis + hyperuricemia** is pathognomonic for GSD Type I. ### Management Principles 1. **Frequent feeds** (every 2–3 hours) with complex carbohydrates 2. **Cornstarch** (provides slow glucose release over 4–6 hours) 3. **Avoid fasting** (even overnight) 4. **Allopurinol** for hyperuricemia 5. **Monitoring:** regular glucose, lactate, uric acid, liver function **Mnemonic: "G6P LACTIC"** - **G**lucose-6-phosphatase deficiency - **6**-phosphate accumulates - **P**yruvate shunting - **L**actic acidosis - **A**cute hypoglycemia - **T**iny glucose output - **I**ncreased uric acid - **C**arbohydrate intolerance - **S**evere hepatomegaly [cite:Robbins 10e Ch 5]