Hemoglobinopathies

Hemoglobinopathies - The Hb Hiccups

Genetic disorders of hemoglobin structure (qualitative) or synthesis (quantitative).

• Normal Adult Hb:
  • HbA ($\alpha_2\beta_2$): >95%
  • HbA2 ($\alpha_2\delta_2$): 1.5-3.5%
  • HbF ($\alpha_2\gamma_2$): <2% (post 6 months)
• Types:
  • Structural variants: e.g., HbS, HbC, HbE (Qualitative defect)
  • Thalassemias: $\alpha, \beta$ (Quantitative defect: $\downarrow$ globin chain synthesis)
  • Hereditary Persistence of Fetal Hemoglobin (HPFH)

⭐ HbF ($\alpha_2\gamma_2$) is the predominant hemoglobin in fetal life, has higher oxygen affinity than HbA, and its persistence can be protective in certain hemoglobinopathies.

Sickle Cell Disease - Crescent Crisis Crew

• Etiology: AR; β-globin (Glu6Val) → HbS.
• Pathophysiology: Hypoxia → HbS polymerization → sickling → vaso-occlusion, hemolysis.
• Key Crises & Features:
  • Vaso-Occlusive Crisis (VOC): Pain (dactylitis), priapism, stroke.
  • Acute Chest Syndrome (ACS):

    ⭐ Acute chest syndrome, characterized by new pulmonary infiltrate on chest X-ray plus fever and/or respiratory symptoms, is a leading cause of mortality in adult patients with Sickle Cell Disease.

  • Aplastic Crisis: Parvovirus B19.
  • Splenic Sequestration: Splenomegaly, ↓Hb.
  • Chronic Hemolysis: Anemia, jaundice, gallstones.
  • Functional Asplenia: ↑Risk of infection (encapsulated organisms).
• Diagnosis: Hb electrophoresis (HbS, ↑HbF, no HbA in HbSS). Smear: Sickle cells, Howell-Jolly.
• Management Principles:
  • Prophylaxis: Penicillin (children <5 yrs), folic acid.
  • Hydroxyurea: ↑HbF, ↓crises.
  • VOC/ACS: Hydration, analgesia, O2 (SpO2 <92%), antibiotics (ACS), exchange Tx (severe).
  • Cure: Hematopoietic Stem Cell Transplant (HSCT). 📌 CAST: Chronic organ damage, ACS, Stroke, Thromboembolism.

Thalassemias - Chain Defect Dramas

Quantitative defect in globin chain synthesis (α or β) → chain imbalance, ineffective erythropoiesis, hemolysis.

• α-Thalassemia: ↓ α-chain synthesis.
  • Excess γ (fetus) → Hb Bart's ($γ_4$, hydrops fetalis).
  • Excess β (adult) → HbH ($β_4$, HbH disease).
  • Spectrum: Silent carrier → α-trait (mild anemia) → HbH disease (mod-severe anemia, golf-ball cells) → Hb Bart's (lethal). 
• β-Thalassemia: ↓ β-chain synthesis. Excess α-chains precipitate. ↑HbF, ↑HbA2.
  • Minor (Trait): Mild microcytic anemia.

    ⭐ In $\beta$-thalassemia trait (minor), HbA2 levels are characteristically elevated (typically >3.5%), a key diagnostic marker differentiating it from iron deficiency anemia.

  • Intermedia: Moderate anemia.
  • Major (Cooley's): Severe transfusion-dependent anemia, iron overload, "chipmunk facies", "hair-on-end" X-ray.
• Dx: CBC (microcytic, hypochromic, ↑RDW), Hb electrophoresis/HPLC.
• Rx: Genetic counseling; transfusions, iron chelation (major/severe intermedia).

 📌 α: 4 gene deletion = Bart's (hydrops). β-minor: ↑HbA2. β-major: Big Spleen/Bones.

High‑Yield Points - ⚡ Biggest Takeaways

• Sickle Cell Disease (SCD): Autosomal recessive, HbS (β-globin point mutation); causes vaso-occlusive crises. Hydroxyurea ↑ HbF.
• Thalassemias: Quantitative globin synthesis defect. α-thalassemia (gene deletions), β-thalassemia (point mutations).
• β-Thalassemia Major: Severe microcytic anemia, hepatosplenomegaly, requires lifelong transfusions & iron chelation.
• Key Labs: ↑ HbA2 in β-thalassemia trait; ↑ HbF in β-thalassemia major.
• HPLC is gold standard for diagnosis of hemoglobinopathies.
• Aplastic crisis in SCD is commonly triggered by Parvovirus B19 infection.