Glycolysis (Embden-Meyerhof pathway) is the cytosolic sequence of enzymatic reactions converting glucose into pyruvate to generate free energy in the form of Adenosine Triphosphate (ATP) and NADH.
Erythrocytes rely exclusively on glycolysis for ATP to maintain membrane ion pumps and shape; pathway failure leads to hemolysis.
Skeletal muscle utilizes glycolysis as a major ATP source during intense anaerobic exercise; failure results in myopathy, muscle cramps, and rhabdomyolysis.
The brain is highly dependent on glucose metabolism, and glycolytic defects can cause severe encephalopathy.
Classification
Most glycolytic defects are rare Autosomal Recessive disorders, with the exception of Phosphoglycerate Kinase deficiency, which is X-linked Recessive. They are broadly categorized by their predominant clinical presentation.
Phosphofructokinase (PFK) Deficiency (GSD Type VII / Tarui Disease). Phosphoglycerate Mutase Deficiency (GSD Type X). Lactate Dehydrogenase (LDH) Deficiency (GSD Type XI). Aldolase A Deficiency (GSD Type XII). Phosphoglycerate Kinase (PGK) Deficiency (X-linked; causes combined myopathy, hemolysis, and CNS symptoms).
Specific Clinical Syndromes
Pyruvate Kinase (PK) Deficiency
Caused by a defect in the final step of glycolysis, leading to ATP depletion, erythrocyte membrane rigidity, and splenic destruction.
Characterized by chronic nonspherocytic hemolytic anemia, neonatal jaundice, splenomegaly, and pigment gallstones.
Elevated 2,3-Bisphosphoglycerate (2,3-BPG) causes a right-shift in the oxygen-dissociation curve, improving oxygen delivery and making the anemia better tolerated.
Caused by a defect in muscle Phosphofructokinase (PFK-M), the rate-limiting step of glycolysis.
Presents with early-onset fatigue, severe muscle cramps, and myoglobinuria after vigorous exercise.
Exhibits the "Out-of-Wind" phenomenon, where symptoms worsen with activity and lack the "Second Wind" phenomenon seen in McArdle disease.
Associated with compensated hemolytic anemia and myogenic hyperuricemia.
Symptoms paradoxically worsen after a high-carbohydrate meal due to glucose-mediated inhibition of lipolysis.
Triosephosphate Isomerase (TPI) Deficiency
The most severe glycolytic defect, frequently lethal in early childhood.
Presents with severe hemolytic anemia, progressive neuromuscular dysfunction (dystonia, spasticity, tremor), and cardiomyopathy.
Lactate Dehydrogenase (LDH) Deficiency
Caused by a defect in the M-subunit of Lactate Dehydrogenase (LDH-A).
Presents with exercise intolerance, myoglobinuria, and occasionally a characteristic erythematous skin rash.
Diagnostic Investigations
Investigation Type
Findings
Screening Tests
Complete Blood Count shows anemia and reticulocytosis in hemolytic forms. Creatine Kinase (CK) is elevated at rest or massively elevated post-exercise in myopathic forms. Urinalysis reveals myoglobinuria after exercise.
Ischemic Forearm Exercise Test
Normal response is a simultaneous rise in Lactate and Ammonia. Glycolytic defects (GSD V, VII, X, XI) show a flat Lactate curve (no rise) with a normal Ammonia rise. In LDH deficiency specifically, venous pyruvate rises significantly, but lactate does not.
Confirmatory Tests
Molecular genetics via gene sequencing or Next-Generation Sequencing (NGS) panels is the gold standard. Enzyme assays can be performed on erythrocytes, leukocytes, or muscle biopsy tissue.
Management
General And Myopathic Forms
Strict avoidance of precipitating factors, particularly strenuous anaerobic exercise.
Aggressive hydration during episodes of myoglobinuria to prevent Acute Kidney Injury.
High-protein diets may provide alternative metabolic substrates.
Glucose or sucrose supplementation before exercise is strictly contraindicated in Tarui Disease (GSD VII), as it inhibits alternative fatty acid fuel sources.
Hemolytic Forms
Folic acid supplementation.
Blood transfusions as required for severe anemia.
Splenectomy is beneficial in PK deficiency and GPI deficiency to reduce erythrocyte destruction, though it does not completely arrest hemolysis.