GSD type II (Pompe disease)

Pathophysiology - Lysosome's Sugar Jam

• Enzyme Defect: Deficiency of lysosomal acid α-glucosidase (GAA), also known as acid maltase.
  • This enzyme is crucial for breaking down glycogen within the lysosome.
• Cellular Pathology: Without GAA, glycogen progressively accumulates inside lysosomes, causing them to swell.
  • Engorged lysosomes disrupt cellular architecture and function, leading to cell death.
  • Primarily affects high-metabolism tissues: cardiac, skeletal, and smooth muscle.
• Result: Progressive muscle damage and organ dysfunction.

⭐ Unlike other GSDs, Pompe disease is a lysosomal storage disease. Blood glucose levels are typically normal as cytosolic glycogen metabolism is unaffected.

Clinical Presentation - The Floppy Baby Saga

• Infantile-Onset (Classic): Presents within the first few months of life.
  • Profound Hypotonia & Muscle Weakness: The hallmark "floppy baby." Generalized weakness, head lag, and inability to meet motor milestones.
  • Cardiomyopathy: Massive cardiomegaly, almost always hypertrophic, is characteristic.
  • Feeding & Breathing: Poor feeding leads to failure to thrive. Respiratory distress from diaphragm weakness.
  • Other Signs: Macroglossia (enlarged tongue) and mild hepatomegaly.
• Late-Onset (Juvenile/Adult): Milder, progressive proximal muscle weakness, often mimicking muscular dystrophies. Spares the heart initially.

📌 Mnemonic: Pompe hurts the Pump (heart), Oesophagus (feeding), Muscles, and Pulmonary system.

⭐ Exam Favorite: The combination of profound hypotonia and massive cardiomegaly on chest X-ray is virtually pathognomonic for infantile-onset Pompe disease.

Diagnosis - Spotting Sugar Buildup

• Screening: Dried blood spot (DBS) test for ↓ acid alpha-glucosidase (GAA) enzyme activity.
• Confirmatory Testing:
  • Definitive GAA enzyme assay in fibroblasts or leukocytes.
  • GAA gene sequencing to identify mutations.

• Supportive Labs:
  • ↑ Serum Creatine Kinase (CK), AST, ALT.
  • ↑ Urinary glucose tetrasaccharides ($Glc_4$).

⭐ Muscle biopsy, though less common now, classically shows lysosomes filled with periodic acid-Schiff (PAS)-positive material that is resistant to diastase digestion.

Management - Enzyme Power-Up

• Enzyme Replacement Therapy (ERT): Cornerstone of treatment.
  • Drug: Alglucosidase alfa (Myozyme®, Lumizyme®), a recombinant human acid α-glucosidase (rhGAA).
  • Mechanism: IV infusion replaces the deficient enzyme, reducing glycogen accumulation in lysosomes, particularly in skeletal and cardiac muscle.
  • Goal: Improve muscle function, reduce ventilator dependence, and enhance survival.

⭐ ERT significantly improves cardiomyopathy and motor outcomes, but immune reactions (infusion-associated reactions) can occur. Antibody formation against the replacement enzyme may limit efficacy.

• Supportive Care: Multidisciplinary approach is crucial.
  • Respiratory: Non-invasive ventilation (BiPAP) or mechanical ventilation.
  • Nutrition: Caloric support, feeding tube if needed.
  • Rehabilitation: Physical, occupational, and speech therapy.

High‑Yield Points - ⚡ Biggest Takeaways

• The only lysosomal storage disease among glycogen storage disorders, due to deficient lysosomal α-1,4-glucosidase (acid maltase).
• Autosomal recessive inheritance.
• Classic infantile form presents with profound hypotonia, massive cardiomegaly, and macroglossia, leading to death by age 2.
• Unlike other GSDs, it does not cause hypoglycemia; blood glucose levels are normal.
• Adult-onset form mimics muscular dystrophy, primarily affecting skeletal muscles.