GSD type VI (Hers disease)

Pathophysiology & Genetics - The Lazy Liver Enzyme

• Genetic Defect: Autosomal recessive mutation in the PYGL gene.
• Enzyme Deficiency: Results in deficient liver glycogen phosphorylase, the rate-limiting enzyme for hepatic glycogenolysis.
• Pathophysiology: The liver's inability to properly break down glycogen leads to:
  • ↑ Glycogen accumulation → Hepatomegaly
  • ↓ Glucose release during fasting → Mild hypoglycemia

⭐ Exam Pearl: Symptoms often improve with age. The enzyme is deficient or "lazy," not completely absent, leading to a milder phenotype compared to other GSDs. Many patients become asymptomatic by puberty.

📌 Mnemonic: Hers disease = Hepatic Phosphorylase Deficiency & Hepatomegaly.

Clinical Presentation - Big Liver, Mild Trouble

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Diagnosis - Finding the Faulty Fuel-Tap

• Lab Findings:
  • Mild post-absorptive hypoglycemia & ketosis.
  • ↑ Hyperlipidemia.
  • ↑ Serum transaminases (AST, ALT).
• Key Negative Findings (Differentiators):
  • NORMAL blood lactate.
  • NORMAL uric acid.

⭐ Unlike GSD I (von Gierke disease), blood lactate levels remain NORMAL in Hers disease because gluconeogenesis is unaffected.

Management & Prognosis - Starch & Steady Wins

• Primary Goal: Maintain normoglycemia & prevent hypoglycemia, especially during fasting.
• Dietary Therapy:
  • Frequent, small meals rich in complex carbohydrates.
  • High-protein diet to provide substrates for gluconeogenesis.
  • ⚠️ Avoid large boluses of simple sugars.
• Key Intervention: Administration of uncooked cornstarch (a slow-release glucose source), especially nocturnally or before anticipated fasts.
• Prognosis:
  • Generally very good.
  • Hepatomegaly, growth retardation, and hypoglycemia often improve with age and can resolve spontaneously after puberty.

⭐ Hers disease spares the muscle. Unlike McArdle disease (GSD V), patients do not experience exercise intolerance or rhabdomyolysis.

High-Yield Points - ⚡ Biggest Takeaways

• Deficient enzyme: Liver glycogen phosphorylase, leading to impaired glycogenolysis.
• Inheritance: Autosomal recessive.
• Key features: Hepatomegaly, growth retardation, and mild fasting hypoglycemia.
• Labs: Normal blood lactate and uric acid levels, distinguishing it from GSD I.
• Ketosis is present during fasting due to intact fatty acid oxidation.
• Prognosis: Generally mild, with symptoms often improving with age.