13 Hemolytic Anemia: Membrane Abnormalities & Enzymopathies
- Nonimmune hemolysis → consider hereditary hemolytic anemia; RBC morphology guides diagnosis
- HS: most common congenital hemolytic anemia (Northern European, ~1:2,000)
- Defects in ANK1, SLC4A1, SPTA1, EPB42 → ↓ surface-to-volume ratio
- HE: RBC cytoskeleton protein defects (horizontal interactions); qualitative spectrin abnormalities common
- Genetic testing recommended pre-splenectomy (efficacy & risks vary)
- G6PD deficiency: episodic hemolysis w/ oxidative exposures; enzyme level falsely normal during acute episode
- Glycolytic ATP maintains RBC membrane integrity; NADPH via HMP shunt maintains antioxidant capacity
- Management: supportive care, folic acid, transfusions, iron chelation, splenectomy consideration
- Mitapivat: oral PK activator available for PK deficiency
13.1 Introduction
- Hemolysis: ↑ RBC destruction w/ compensatory ↑ erythropoiesis
- Intrinsic causes: hemoglobin structure, RBC metabolism, membrane structure/permeability
- RBC membrane structure
- Lipid bilayer w/ transmembrane protein complexes
- α & β-spectrin hexagonal lattice
- Transverse & junction complexes (actin, protein 4.1)
- Unique flexibility & deformability under stress
- Clinical spectrum: asymptomatic (compensated) → symptomatic chronic hemolysis w/ transfusion dependence
- Diagnosis critical: monitoring, supportive care, splenectomy efficacy/risks, targeted therapy availability vary
13.2 Monitoring Recommendations
| Parameter | Frequency |
|---|---|
| Growth & development | Baseline; q3-6 months year 1; then annual |
| CBC, retic, RBC indices | Annual; ↑ frequency w/ accelerated hemolysis |
| Hemolysis markers (bili, LDH, hapto) | q3-6 months if transfused; annual if not |
| Ferritin, TSAT, MRI T2* iron | q3-6 months if transfused; annual if not |
| Bone health (25-OH vitamin D) | Annual; consider late adolescence baseline |
| DEXA scan | Consider late adolescence baseline |
| Echocardiogram | Annual if regularly transfused or ↑ iron overload |
| Viral screening (HIV, HBV, HCV) | Annual if regularly transfused |
| Abdominal ultrasound | q2-3 years for gallstone disease (esp w/ Gilbert syndrome) |
| Echocardiogram | Age >50 yr, pre-pregnancy, or pulmonary HTN concern |
13.3 Red Cell Membrane Disorders
13.3.1 Hereditary Spherocytosis
Pathophysiology - Prevalence: most common congenital hemolytic anemia in Northern European descent (~1:2,000) - Defect: vertical interactions (membrane ↔︎ cytoskeleton) - ANK1 (ankyrin), SLC4A1 (band 3), SPTA1 (α-spectrin), EPB42 (protein 4.2) - ↓ membrane lipid & surface area via microvesiculation - Pathophysiology - ↓ surface-to-volume ratio → limited deformability - Splenic conditioning → progressive spherocytic changes → hemolysis - RBCs damaged traversing capillaries
Laboratory Evaluation - EMA binding assay: preferred test for membranopathies - Better sensitivity & specificity than osmotic fragility - Widely available; comparable to ektacytometry - Osmotic fragility test: 24-hr incubation at 37°C enhances sensitivity (inferior to EMA) - Osmotic gradient ektacytometry: laser diffraction; measures RBC deformability vs osmolality - Differentiates HS from HE & RBC cation permeability defects - Genetics: use w/ specialized RBC testing, especially no family history - Coinheritance of >1 variant common - Recommended pre-splenectomy (efficacy/risk differences) - RBC morphology: spherocytes on smear, ↑ MCHC
| Severity | Hgb (g/dL) | Retic (%) | Bili (mg/dL) | Features |
|---|---|---|---|---|
| Mild | 11–15 | 3–8 | 1–2 | Asymptomatic or few symptoms |
| Moderate | 8–13 | 8–10 | 2–3 | Mild-moderate; rare transfusion |
| Severe | 6–8 | >10 | >3 | Regular transfusions |
Clinical Manifestations - Spectrum: asymptomatic (compensated) → severe hemolytic anemia w/ jaundice & splenomegaly - Complications - Gallstones: common in childhood/adolescence - Pigmented stones, early satiety, LUQ pain - Risk ↑ w/ Gilbert syndrome - Aplastic crisis: parvovirus → selective RBC suppression, reticulocytopenia - Megaloblastic anemia: rare; 2° to acquired folate deficiency (pregnancy, high demand) - Gilbert syndrome: present in majority of HS patients; ↑ unconjugated hyperbilirubinemia
Management - Folic acid supplementation: especially during ↑ erythropoiesis (pregnancy) or limited intake - Supportive care: monitoring, patient education on crisis signs/symptoms - Splenectomy: markedly improves hemolysis in almost all HS - Does NOT correct underlying RBC defect - Rule out RBC cation permeability defects pre-op (hemolysis risk if severe) - Severe/autosomal recessive HS: partial splenectomy data limited - Vaccinations: consider pre-splenectomy (encapsulated organisms)
13.3.2 Hereditary Elliptocytosis
Pathophysiology - Defect: RBC cytoskeleton protein interactions (horizontal interactions) - Qualitative spectrin abnormalities most common - SPTA1, SLC4A1, others - Coinheritance: >1 RBC gene variant not uncommon - RBC morphology: elliptocytes, may see mushroom-shaped RBCs (protein-specific)
Laboratory Evaluation - RBC morphology: guides initial diagnosis of hereditary hemolytic anemia - Specialized testing: morphologic abnormalities specific for causative protein defect - Note: Enzymopathies show nonspecific RBC morphology → differential w/ nonspherocytic hemolytic anemia
Clinical Manifestations & Management - Spectrum: variable severity; some w/ severe hemolysis requiring repeated transfusions - Splenectomy: markedly diminishes hemolysis but does NOT fully correct anemia in most
13.4 Red Cell Enzymopathies
13.4.1 Glycolytic Pathway & ATP Production
- ATP generation: maintains RBC membrane integrity & oxygen affinity
- Hexose monophosphate (HMP) shunt: produces NADPH
- Maintains antioxidative capacity via glutathione system
- Protects vs oxidative damage
13.4.2 Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency
Overview - Prevalence: most common enzymatic RBC disorder globally - Genetics: X-linked; G6PD synthesized 4-fold excess vs β-spectrin - Clinical spectrum: asymptomatic → hemolytic crises w/ oxidative stress
Clinical Manifestations - Asymptomatic until oxidative exposures occur - Hemolytic episodes: dark/cola-colored urine, jaundice - Triggers: oxidative stressors - Medications: antimalarials, sulfonamides, aspirin, NSAIDs, dapsone, nitrofurantoin - Infections: bacterial & viral (can precipitate crises) - Dietary: fava beans - Lab note: enzyme level may be falsely normal during acute episode - Test during non-hemolytic periods for accurate diagnosis
Management - Supportive: avoidance of trigger factors; transfusions if needed acutely - Education: medication triggers, dietary precautions critical - Monitoring: complications, folic acid supplementation - Family counseling: X-linked inheritance pattern
13.4.3 Pyruvate Kinase Deficiency
Pathophysiology - Enzyme role: glycolytic pathway ATP generation (essential) - Genetics: PKLR mutations → ↓ enzyme activity & ↓ ATP - Result: ↓ RBC membrane deformability → hemolysis
Management - Supportive: folic acid, transfusions if needed - Targeted therapy: mitapivat (oral PK activator) - Available for adults w/ PK deficiency - Advances beyond supportive care alone
13.5 Laboratory Evaluation Strategy
Initial workup: extravascular hemolysis pattern - ↑ LDH, ↑ indirect bilirubin, reticulocytosis, ↓ haptoglobin - Family history, RBC morphology review essential
Specialized testing order: 1. EMA binding assay: preferred for membranopathies - Sensitivity & specificity superior to osmotic fragility - Widely available 2. Osmotic gradient ektacytometry: if EMA unavailable - Differentiates HS from HE & cation permeability defects 3. Osmotic fragility test: 24-hr incubation at 37°C (enhanced sensitivity) 4. Genetic testing: w/ specialized RBC testing - Particularly if no family history of HS - Recommended pre-splenectomy
Sample handling - Transport quickly to lab w/ hot packs - Keep warm (cooling → agglutination & failed analysis) - Use Styrofoam container w/ hot packs
Splenectomy improves hemolysis in nearly all HS patients, but does NOT correct underlying RBC defect. Rule out RBC cation permeability defects pre-op due to hemolysis risk in severe cases.
Quantitative G6PD measurement may be falsely normal during acute hemolytic episode. Test during non-hemolytic periods or use alternative diagnostics when clinical suspicion high.
Mitapivat (oral PK activator) now available for adults w/ PK deficiency & represents targeted therapy beyond supportive care.
13.6 Key Genes in RBC Membrane & Enzyme Disorders
| Gene | Protein | Function | Disorder |
|---|---|---|---|
| ANK1 | Ankyrin-1 | Vertical interactions; membrane-cytoskeleton linkage | HS |
| SLC4A1 | Band 3 (Cl−/HCO3− exchanger) | Vertical & horizontal interactions | HS, some HE |
| SPTA1 | α-Spectrin | Horizontal interactions; RBC skeleton | HS, HE |
| EPB42 | Protein 4.2 | Membrane scaffolding; ankyrin binding | HS |
| PKLR | Pyruvate kinase | Glycolytic ATP production | PK deficiency |
| G6PD | Glucose-6-phosphate dehydrogenase | Pentose phosphate pathway; antioxidant | G6PD deficiency |