GSD type VII (Tarui disease)

Pathophysiology - The Energy Choke

• Genetic Defect: Autosomal recessive deficiency of the muscle (M) subunit of Phosphofructokinase-1 (PFK-1).
• Metabolic Block: PFK-1 is a rate-limiting enzyme in glycolysis. Its absence halts the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, choking cellular energy production.

• Consequences:
  • ↓ ATP in Muscle: Impairs energy supply during anaerobic exercise, causing premature fatigue, pain, and cramps.
  • Paradoxical Effect: Glucose infusion worsens symptoms by increasing upstream substrates ($Glucose-6-P$, $Fructose-6-P$), intensifying the metabolic traffic jam.

⭐ A hallmark finding is a flat venous lactate curve with exercise, as glycolysis cannot proceed to produce pyruvate and subsequently lactate.

Clinical Presentation - Pain, No Gain

• Exercise Intolerance & Myalgia: Manifests as premature fatigue and painful muscle cramps, typically triggered by brief, strenuous activities like sprinting. Onset is in childhood or adolescence, not infancy.
• Rhabdomyolysis & Myoglobinuria: Strenuous exercise can induce muscle breakdown (rhabdomyolysis), leading to dark, reddish-brown urine.
• Key Associated Findings:
  • Compensated Hemolytic Anemia: RBCs also have PFK deficiency, leading to non-spherocytic hemolysis and sometimes jaundice.
  • Hyperuricemia: Caused by increased nucleotide degradation in muscle during exercise.

⭐ Absence of "Second Wind": Unlike McArdle disease, patients do not experience a "second wind" (improvement in symptoms with continued aerobic activity), as both glycogenolysis and glycolysis are blocked.

Diagnosis - The Lactate Flatline

• Lab Findings:
  • ↑ Serum Creatine Kinase (CK), which spikes dramatically with exercise.
  • Hyperuricemia.
  • Signs of hemolysis: ↑ reticulocytes, ↑ unconjugated bilirubin.
• Ischemic Forearm Exercise Test:
  • Key finding: A flat or absent rise in venous lactate post-exercise.
  • Ammonia levels, in contrast, show a normal or exaggerated rise.
• Muscle Biopsy:
  • Histology: Reveals glycogen accumulation, staining positive with PAS.
  • Definitive diagnosis: Assay shows deficient PFK-1 enzyme activity.
• Genetic Testing:
  • Identifies mutations in the PFKM gene.

⭐ High-Yield: Tarui disease is unique among GSDs for its dual presentation of metabolic myopathy and compensated hemolysis, as the PFK enzyme is deficient in both muscle and red blood cells.

Management - Fueling Carefully

• Primary Strategy: Strict avoidance of triggers like intense exercise and high-carbohydrate meals, particularly before physical activity.
• Dietary Therapy: A ketogenic (high-fat, protein-rich) diet can supply alternative muscle fuel (ketones, fatty acids), bypassing the enzymatic block.
• Symptomatic Care: Manage myoglobinuria with vigorous hydration and urine alkalinization to prevent renal failure.
• Genetic Counseling: Advised for patients and families.

⭐ A pre-exercise glucose load paradoxically worsens exercise tolerance by inhibiting lipolysis and ketogenesis, the muscle's only alternative energy pathways.

High‑Yield Points - ⚡ Biggest Takeaways

• Enzyme deficiency: Muscle Phosphofructokinase-1 (PFK-1), which impairs glycolysis.
• Classic presentation: Exercise intolerance, painful muscle cramps, and myoglobinuria upon strenuous activity.
• Key diagnostic finding: No rise in blood lactate levels after an ischemic exercise test.
• Associated condition: Mild, compensated hemolytic anemia is often present, as RBCs are also affected.
• Inheritance pattern: Autosomal recessive.
• Metabolic block: Leads to accumulation of G6P and F6P.