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SICKLE CELL DISEASE — PATHOLOGY REVISION NOTES --- 1. DEFINITION & GENETICS Sickle cell disease (SCD) is an inherited chronic haemolytic anaemia caused by a point mutation in the β-globin gene on chromosome 11 . Mutation: Single nucleotide substitution (GAG → GTG) → Glutamic acid replaced by Valine at position 6 of the β-globin chain (β⁶Glu→Val). Inheritance: Autosomal recessive. Genotypes: - HbSS — Sickle cell anaemia (most severe, most common form) - HbAS — Sickle cell trait (carrier; generally asymptomatic) - HbSC — Milder disease; target cells predominate - HbS/β⁰-thalassaemia — Clinically similar to HbSS (no normal β-globin) - HbS/β⁺-thalassaemia — Milder; some normal HbA present --- 2. PATHOPHYSIOLOGY A. Molecular Basis of Sickling - On deoxygenation , HbS undergoes conformational change → exposes a hydrophobic patch on the βs-globin chain at the site of β6 valine. - This binds a complementary hydrophobic site on another Hb tetramer → triggers polymerization . - Polymers form 21 nm diameter helical fibres (1 inner + 6 peripheral double strands). - Process follows the double nucleation model: - Homogeneous nucleation → tetramers form a nucleus → expands into a fibre - Heterogeneous nucleation → existing fibre surfaces nucleate further fibres - Polymerization has a delay period that is extremely sensitive to deoxy-HbS concentration — even small increases dramatically shorten the delay. B. Factors That Promote Sickling - ↓ Oxygenation (hypoxia) - ↑ Intracellular HbS concentration (dehydration) - Acidosis - ↑ 2,3-DPG (reduces O₂ affinity → more deoxyHbS) - Fever, infection, cold, stress C. Factors That Inhibit Sickling - HbA — dilutes HbS (sickle trait is relatively protected) - HbF — more potent inhibitor; greater amino acid disparity between βs and γ-globin chains → disrupts polymer formation. This is the mechanism exploited by hydroxyurea. D. Effect on Red Blood Cells - Deoxygenation → sickling → reversible initially upon reoxygenation - Repeated/prolonged sickling → irreversible membrane damage → irreversibly sickled cells (ISCs) - Membrane damage → K⁺ and water efflux via Gardos pathway and K⁺-Cl⁻ co-transport → cellular dehydration → ↑ intracellular Hb concentration → shorter delay time → more sickling (vicious cycle) - Phosphatidylserine abnormally appears on outer membrane leaflet (normally inner) → increased coagulability - Red cells become abnormally adherent to endothelium via VCAM-1, thrombospondin, fibronectin Comparison: Feature Normal RBC Sickle RBC --- --- --- Shape Biconcave disc Sickle/elongated Deformability High Rigid Lifespan 120 days ≤20 days --- 3. CONSEQUENCES — HAEMOLYSIS & VASO-OCCLUSION These are the two pillars of SCD pathology. Haemolysis → anaemia (Hb typically 6–9 g/dL in HbSS), jaundice, pigment gallstones, raised reticulocyte count (~10%), raised bilirubin, raised LDH, low haptoglobin. Vaso-occlusion → rigid sickle cells obstruct microvasculature → tissue hypoxia → pain and organ damage. Releases free Hb and haem → scavenges nitric oxide (NO) → endothelial dysfunction → ↑ adhesion molecules, oxidant generation, platelet & coagulant activation, vasculopathy. --- 4. CLINICAL FEATURES Acute Manifestations - Vaso-occlusive (painful) crisis — most common; affects bones, chest, abdomen. Triggered by infection, cold, dehydration, hypoxia. - Dactylitis (hand-foot syndrome) — earliest presentation in infants; painful swelling of hands/feet - Acute Chest Syndrome (ACS) ⚠️ — new pulmonary infiltrate + fever/respiratory symptoms; leading cause of death. Due to infection, fat embolism, in-situ sickling. - Stroke ⚠️ — large vessel occlusion; occurs in ~11% of HbSS patients by age 20 - Splenic Sequestration Crisis ⚠️ — sudden massive splenic enlargement with rapid fall in Hb; mainly in young children - Aplastic Crisis ⚠️ — due to Parvovirus B19 infecting erythroid precursors → sudden drop in Hb + absent reticulocytes - Bacterial Sepsis/Meningitis ⚠️ — Streptococcus pneumoniae , Haemophilus influenzae , Salmonella (osteomyelitis). Due to functional asplenia. - Priapism — prolonged painful erection; can cause erectile dysfunction - Haematuria/Papillary necrosis — due to renal medullary sickling Chronic Manifestations - Anaemia (chronic haemolytic) - Jaundice (unconjugated hyperbilirubinaemia) - Functional asplenia → susceptibility to encapsulated organisms → auto-infarction of spleen by ~5 years in HbSS; Howell-Jolly bodies on blood film - Splenomegaly (early childhood; later atrophies) - Cardiomegaly + functional murmurs (high-output state) - Pulmonary hypertension ⚠️ - Avascular necrosis (AVN) — femoral/humeral head - Proliferative retinopathy — particularly in HbSC - Hyposthenuria (inability to concentrate urine) → enuresis → renal failure - Proteinuria/Nephrotic syndrome - Cholelithiasis (pigment stones — bilirubin) - Leg ulcers (medial malleolus) - Delayed growth and sexual maturation - Restrictive lung disease ⚠️ - Transfusional haemosiderosis ⚠️ (in heavily transfused patients) --- 5. DIAGNOSIS Peripheral Blood Film - HbSS: Normocytic normochromic anaemia; many ISCs, polychromasia, target cells, reticulocytosis (~10%), Howell-Jolly bodies; WBC 12–20 × 10⁹/L; Platelets 300–500 × 10⁹/L - HbSC: Predominant target cells, rare ISCs - S/β-thal: ISCs + target cells + hypochromic microcytic cells - Microcytosis also if coexisting α-thalassaemia or iron deficiency - Anaemia develops by 3–4 months of age as HbF declines Sickling Test - Blood sealed under paraffin wax/petroleum jelly (deoxygenating environment) - Reducing agent (sodium dithionate) hastens sickling - Positive if ISCs appear — does NOT distinguish HbSS from HbAS Solubility Test (Sickledex) - HbS is insoluble in deoxygenated high-molarity phosphate buffer → turbid solution - Positive in both HbSS and HbAS — not diagnostic alone Haemoglobin Electrophoresis Cellulose Acetate (alkaline pH): - HbA migrates fastest toward anode; HbS migrates slower - Pattern: HbSS shows single band at S position; HbAS shows two bands (A and S) HPLC (Gold Standard): - Separates Hb based on ionic interaction with resin - Elution determined by pH and ionic strength of buffer - UV detector quantifies fractions → gives % of each Hb type - Can quantify HbF, HbA2, HbS simultaneously Other methods: Gel electrophoresis, Capillary electrophoresis, Isoelectric focusing Newborn Screening (Heel Prick) Haemoglobin patterns at birth (F = HbF listed first as it is most abundant): - FS → HbSS or HbS/β⁰-thal - FAS → Sickle cell trait - FSA → HbS/β⁺-thal - FSC → HbSC disease Prenatal Diagnosis - Direct detection of GAG→GTG mutation in fetal cells - Via chorionic villus sampling (CVS) or amniocentesis - Preimplantation genetic diagnosis also available --- 6. MANAGEMENT Goals 1. Manage vaso-occlusive crises 2. Manage chronic pain 3. Manage haemolytic anaemia 4. Prevent and treat infections 5. Prevent stroke 6. Manage target organ damage 7. Manage pulmonary hypertension Supportive/General - Folic acid — daily supplementation (compensate for ↑ erythropoiesis) - Penicillin prophylaxis — from 2 months of age until at least age 5 (or lifelong); prevents pneumococcal sepsis in asplenic patients - Vaccinations — pneumococcal, meningococcal, Hib, influenza, hepatitis B - Hydration — crucial to prevent sickling - Analgesics — NSAIDs for mild pain; opioids (morphine) for severe crises - Oxygen if hypoxic - Antiemetics as needed - Iron chelators (desferrioxamine, deferasirox) for transfusional iron overload Hydroxyurea (Hydroxycarbamide) — KEY DRUG ⭐ Mechanism: Stimulates production of HbF → inhibits HbS polymerization; also reduces WBC (↓ adhesion), reduces sickling frequency. Indications: - ≥6 painful crises/year - Acute chest syndrome (recurrent) - Severe vaso-occlusive events - Severe symptomatic anaemia - Severe unremitting chronic pain - History of/high risk for stroke Monitoring: FBC (myelosuppression risk); LFTs; renal function Blood Transfusion - Simple transfusion — for severe anaemia, aplastic crisis, splenic seq