Sickle cell disease Study Guide

📖 Core Concepts Sickle cell disease (SCD) – an autosomal‑recessive hemoglobinopathy caused by a single‑base substitution (GAG → GTG) in the HBB gene → glutamate → valine at β‑globin position 6, producing HbS. HbS polymerisation – deoxygenated HbS exposes a hydrophobic patch, sticks together, and forms rigid polymers; this deforms red cells into a “sickle”. Vaso‑occlusion – rigid sickled cells block small capillaries → tissue ischemia, pain, organ damage. Hemolysis – sickled cells survive only 10‑20 days (vs. 90‑120 days); rapid destruction releases free heme → oxidative injury & inflammation. Fetal hemoglobin (HbF) – inhibits HbS polymerisation; higher HbF → milder disease. Autosomal recessive inheritance – both parents must transmit the mutant β‑globin allele for disease (25 % risk per pregnancy when both are carriers). --- 📌 Must Remember Genotype‑Phenotype: HbSS = sickle cell anemia (most severe). HbSC / HbS‑β⁰ / HbS‑β⁺ = milder variants (not detailed here). Carrier risk: 2 × carrier parents → 25 % disease, 50 % carrier, 25 % unaffected. Key labs: Peripheral smear → sickled cells (disease only). Solubility test → positive in disease & trait (cannot differentiate). Hemoglobin electrophoresis/DNA PCR → definitive. Major complications: infection (especially S. pneumoniae), stroke, acute chest syndrome (ACS), splenic sequestration, aplastic crisis (Parvovirus B19), avascular necrosis, renal disease, pulmonary hypertension. Hydroxyurea effect: ↑ HbF, ↓ adhesion molecules, ↓ vaso‑occlusive crises. Transcranial Doppler (TCD) threshold: cerebral blood flow velocity > 200 cm/s → high stroke risk → start chronic transfusion program. Vaccination/Prophylaxis: daily penicillin (infancy‑early childhood) + pneumococcal, meningococcal, Hib vaccines. --- 🔄 Key Processes HbS Polymerisation & Sickling ↓ O₂ → HbS deoxygenates → hydrophobic patch exposed → polymerises → rigid HbS fibers → RBC adopts sickle shape (seconds). Vaso‑occlusive Crisis (VOC) Sickled RBCs → mechanical blockage of capillaries → ischemia → pain (bones, chest, back). Triggers: dehydration, cold, infection, stress, high altitude. Hemolysis Cascade Sickled RBCs → membrane damage → premature destruction → free heme released → oxidative stress → endothelial activation → further VOC. Splenic Sequestration Sickled cells trapped in spleen → sudden splenomegaly + rapid Hb drop → hypovolemia → shock risk. Aplastic Crisis Parvovirus B19 infects erythroid precursors → abrupt halt in RBC production → severe anemia, low reticulocyte count. --- 🔍 Key Comparisons Sickle cell disease vs. Trait Disease: two HbS alleles → chronic hemolysis, VOC, organ damage. Trait: one HbS allele → usually asymptomatic; symptoms only under extreme hypoxia/dehydration. Simple transfusion vs. Exchange transfusion Simple: adds donor RBCs → raises Hb temporarily; used for mild‑moderate anemia. Exchange: removes sickled cells, replaces with donor cells → lowers HbS % rapidly; used in ACS, stroke, severe VOC, pre‑op. Hydroxyurea vs. Voxelotor Hydroxyurea: ↑ HbF production (indirectly reduces sickling). Voxelotor: binds HbS, ↑ O₂ affinity → directly prevents polymerisation. --- ⚠️ Common Misunderstandings “Carriers never have problems.” Carriers can develop pain or hematuria under severe hypoxia/dehydration. “All sickle cell patients need lifelong transfusions.” Only high‑risk groups (e.g., abnormal TCD, ACS) receive chronic transfusion; most are managed with hydroxyurea and supportive care. “Aplastic crisis is caused by sickling.” It is caused by Parvovirus B19 infection, not by sickling itself. --- 🧠 Mental Models / Intuition “Polymer‑once‑deoxy → sickle‑once‑re‑oxy → repeat damage.” Imagine a rubber band that stiffens when cold (deoxy) and snaps back when warmed (re‑oxy) – each snap weakens the band (RBC membrane). “Two‑hit model of VOC: 1️⃣ Sickling (intrinsic) → rigid cells. 2️⃣ Adhesion (extrinsic) → endothelial activation → cells stick → occlusion. --- 🚩 Exceptions & Edge Cases High HbF individuals (e.g., hereditary persistence of fetal hemoglobin) may have markedly milder disease despite HbSS genotype. Patients with functional asplenia may still have a small residual spleen early in life; splenic sequestration can occur up to age 5. Gene‑editing therapy (Casgevy) – approved in the UK, but long‑term safety/efficacy data are still emerging. --- 📍 When to Use Which Hydroxyurea → first‑line disease‑modifying therapy for ≥2 pain crises/year or recurrent ACS. Voxelotor → consider when hydroxyurea insufficient and patient has chronic anemia with low Hb despite transfusion avoidance. Crizanlizumab → add if VOC frequency remains high despite optimal hydroxyurea dose. Simple transfusion → acute symptomatic anemia, mild ACS, or preparation for surgery when Hb < 7‑8 g/dL. Exchange transfusion → severe ACS (PaO₂ < 70 mmHg), stroke, or life‑threatening sequestration. Chronic transfusion program → abnormal TCD velocities, history of stroke, or recurrent silent infarcts. --- 👀 Patterns to Recognize Pain + recent dehydration → VOC (look for triggers). Fever + new infiltrate on CXR → ACS (differentiate from pneumonia). Sudden Hb drop + splenomegaly → splenic sequestration (especially in toddlers). Pallor + severe anemia + retic count ≈ 0 → aplastic crisis (think Parvovirus B19). Recurrent infections + absent spleen → functional asplenia → ensure prophylaxis. --- 🗂️ Exam Traps “Hb electrophoresis distinguishes trait from disease.” Trap: Both disease and trait show HbS; only disease shows absent HbA. “All patients with SCD need lifelong penicillin.” Trap: Prophylaxis is recommended only through early childhood (usually until age 5) once splenic function is lost; adult management focuses on vaccination. “Hydroxyurea works by increasing HbS affinity for O₂.” Trap: It works by ↑ HbF, not by changing HbS O₂ affinity (that’s voxelotor’s mechanism). “Exchange transfusion is always safer than simple transfusion.” Trap: Exchange carries higher procedural risk and is reserved for severe scenarios; simple transfusion is sufficient for mild anemia. ---