📚 Hematology — L3: Hemolytic Anemia

📚 Lecture Overview

This lecture covers hemolytic anemias, defined as disorders featuring premature red blood cell destruction resulting in a shortened lifespan of under 120 days. It details classifications into inherited and acquired causes, alongside their specific pathogenesis, clinical findings, laboratory features, and management protocols. Mastering these pathways is clinically vital for diagnosing and treating various forms of hemolytic states.

🎯 Key Concepts & Definitions

• Hemolysis: Premature destruction of red blood cells leading to a shortened lifespan of less than 120 days.
• Spherocyte: A round, less deformable red blood cell with a decreased surface-area-to-volume ratio, easily trapped and destroyed in the spleen.
• Ineffective Erythropoiesis: The destruction of erythroblasts and red cells directly within the bone marrow before they enter circulation.
• Osmotic Fragility Test: A diagnostic test measuring red cell susceptibility to hemolysis in decreasing concentrations of hypotonic saline.
• Direct Antiglobulin Test (DAT / Coombs Test): A test used to demonstrate antibodies or complement attached to red cells in vivo.
• Indirect Antiglobulin Test (IAT): A test used to detect free antibodies or complement circulating in the serum.

📖 Main Content

Hereditary Spherocytosis (HS)

• Congenital hemolytic disorder caused by an inherited defect in the red cell membrane cytoskeleton.
• Pathogenesis: Deficiency in membrane linkages (ankyrin interacting with spectrin and band 3; or glycophorin C with protein 4.1) weakens contact between the lipid bilayer and skeleton, causing decreased red cell surface area and spherocyte formation. Spherocytes are trapped and destroyed in the spleen.
• Most common inheritance is autosomal dominant (with some autosomal recessive cases).
• Clinical features: Mild to moderate anemia, intermittent jaundice, splenomegaly, gallstones, and chronic leg ulcers.
• Laboratory: Reticulocytosis, blood smear showing spherocytosis, and bone marrow showing erythroid hyperplasia.
• Management: Splenectomy for severe disease; folate supplementation to prevent megaloblastic anemia.

The Thalassaemias

• A heterogeneous group of inherited hemoglobin disorders characterized by reduced or absent synthesis of specific globin chains ($\alpha$ or $\beta$).
• $\beta$-Thalassaemia Major (Cooley's Anemia):
• Severe anemia requiring regular blood transfusions.
• Unbound excess $\alpha$ chains precipitate inside erythroblasts, causing ineffective erythropoiesis and marrow expansion.
• Complications include iron overload (damaging liver, pancreas, heart), skeletal deformities, and hypersplenism.
• Peripheral blood shows microcytic hypochromic red cells, anisopoikilocytosis, target cells, and basophilic stippling. Hemoglobin electrophoresis shows markedly elevated HbF (10-98%).
• $\beta$-Thalassaemia Minor: Heterozygous carrier state, mostly asymptomatic with microcytic red cells, normal or mildly decreased hemoglobin ($≥ 9$ g/dl), and increased red cell count.
• $\alpha$-Thalassaemias (Four $\alpha$ genes on chromosome 16):
• Hb Bart's Hydrops Foetalis ($\alpha^0$ homozygous): $\gamma$-tetramers ($\gamma_4$) form due to total $\alpha$ chain absence; results in stillbirth or neonatal death with massive anasarca.
• Hb H Disease (3-$\alpha$ gene deletion): $\beta$-tetramers form ($\beta_4$ or HbH), causing moderate anemia, icterus, and splenomegaly.
• $\alpha^0$ Trait & Silent State: Asymptomatic or mild microcytic hypochromic indices.
• Therapy Principles: Regular transfusions to maintain $Hb > 9.5--10 g/dl$, iron chelation therapy using desferrioxamine (DF), deferiprone, or deferasirox, and hematopoietic stem cell transplantation as the only cure.

Sickle Cell Diseases (SCD)

• Sickle Cell Anemia (SCA): Homozygous state for HbSS. Caused by a single nucleotide mutation ($A\rightarrowT$) in the 6th codon of the $\beta$-globin gene, substituting valine for glutamic acid ($\beta^6 Glu\rightarrowVal$).
• Pathogenesis: Intracellular polymerization of HbS under conditions of decreased oxygen tension, cellular dehydration, acidosis, or cold leads to rigid sickle cells and splenic destruction.
• Clinical Complications: Vaso-occlusive crises (severe pain), aplastic crises triggered by parvovirus infection, splenic sequestration crises, autosplenectomy (fibrotic, small spleen in later life), and fulminant infections due to loss of splenic function.
• Management: Pneumococcal/influenza vaccines, penicillin prophylaxis, analgesics/hydration for crises, hydroxyurea (increases HbF to prevent polymerization), and stem cell transplantation.

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

• Pathogenesis: G6PD catalyzes the first step of the HMP shunt, generating NADPH. NADPH maintains reduced glutathione (GSH), which detoxifies harmful hydrogen peroxide ($H_2O_2$). G6PD deficiency leads to $H_2O_2$ accumulation, globin denaturation, and Heinz body / bite cell formation.
• X-linked disorder, occurring exclusively in males.
• Hemolysis is triggered by oxidant stress (drugs, infections) or ingestion of fava beans (favism).
• Laboratory: Fragmented cells, bite cells, hemoglobinuria, unconjugated hyperbilirubinemia, and definitive diagnosis via quantitative assay or fluorescent spot test.

Immune Hemolytic Anemias (AIHA)

• Warm AIHA: Autoantibodies (mostly IgG) are maximally active at $37°C$. IgG-coated red cells are sequestered and destroyed in the spleen. Treated with corticosteroids, splenectomy, immunosuppressants, or rituximab (anti-CD20).
• Cold AIHA: Autoantibodies (mostly IgM) are maximally active at $0--4°C$. Causes red cell autoagglutination. Treated by avoiding cold, cytotoxic therapy, and plasmapheresis; corticosteroids and splenectomy are unhelpful.

📊 Visual Learning

💡 Important Points to Remember

• Lifespan benchmark: Hemolysis is defined by a shortened red cell life span of $< 120$ days.
• Genetic mutation in Sickle Cell: Substitution of valine for glutamic acid at position 6 of the $\beta$-polypeptide chain ($\beta^6 Glu\rightarrowVal$).
• Thalassaemia Major Electrophoresis: Marked elevation of HbF (10-98%) with microcytic hypochromic indices.
• G6PD Inheritance: X-linked disorder, manifesting almost exclusively in males following oxidant stress or fava beans.
• Spherocyte site: Trapped and destroyed primarily in the spleen; corrected via splenectomy in severe Hereditary Spherocytosis.
• Aplastic crisis trigger: Classically caused by parvovirus infection targeting bone marrow erythroblasts in sickle cell disease.
• Iron overload chelation: Treated with desferrioxamine (DF) via subcutaneous infusion pump, deferiprone, or deferasirox.
• Warm vs. Cold AIHA antibodies: Warm AIHA is mediated by IgG ($37°C$), whereas Cold AIHA is mediated by IgM ($0--4°C$).
• Hydroxyurea mechanism: Increases production of HbF, which does not participate in HbS polymerization.
• Coombs test utility: Direct Antiglobulin Test (DAT) detects in-vivo coating of RBCs by antibodies or complement.

⚠️ Common Exam Questions

• MCQ Trick: Examiners frequently swap the amino acid substitution in sickle cell anemia (saying glutamic acid is substituted for valine, instead of valine substituted for glutamic acid). Remember: Normal has Glutamic acid, Mutant has Valine.
• AIHA Management Trap: Testing whether corticosteroids or splenectomy work in Cold AIHA. Crucial fact: Corticosteroids and splenectomy are not helpful in cold-reacting antibody AIHA.
• Splenectomy Age Warning: Splenectomy should be avoided before 5-6 years of age in thalassaemia and hereditary spherocytosis due to overwhelming post-splenectomy sepsis risks.
• G6PD Inheritance Trap: Watch out for questions implying female homozygosity or inheritance patterns other than X-linked for G6PD deficiency.

📝 Quick Review Checklist

I can explain the basic definition and lifespan criteria of hemolytic anemia
I understand the membrane defect and laboratory features of Hereditary Spherocytosis
I can classify $\alpha$ and $\beta$-thalassaemias and explain $\beta$-thalassaemia major pathogenesis
I know the precise molecular mutation causing Sickle Cell Anemia
I understand the biochemical role of G6PD, NADPH, and glutathione in preventing oxidative hemolysis
I can differentiate between Warm-reactive (IgG) and Cold-reactive (IgM) Autoimmune Hemolytic Anemia
I know the indications and side effects of iron chelation therapy (Desferrioxamine)
I can distinguish between Direct and Indirect Antiglobulin (Coombs) tests