Sickle Cell Disease & Thalassemias
A boards-focused walkthrough of the two hemoglobinopathy mechanisms — qualitative (sickle cell) versus quantitative (thalassemias) — mapping pathophysiology to smear and electrophoresis findings, classic vignette buzzwords, and next-best-step management for STEP 1 and STEP 2 CK.
Two ways hemoglobin fails
Normal adult hemoglobin (HbA) is α2β2. Boards split the hemoglobinopathies into two mechanisms:
- Qualitative (structural) defect — sickle cell disease: a normal amount of an abnormal β-globin.
- Quantitative defect — thalassemias: normal structure but reduced amount of α- or β-globin, so the unpaired chain precipitates.
Sickle cell disease (SCD): autosomal recessive point mutation in HBB (chromosome 11) — glutamic acid → valine at codon 6 (Glu6Val; GAG→GTG). Deoxygenated HbS polymerizes → rigid sickled RBCs → vaso-occlusion + hemolysis.
Thalassemia: α-thal from gene deletions (HBA, chromosome 16, four α genes); β-thal from point mutations (HBB, chromosome 11) that reduce (β+) or abolish (β0) β-chain. Excess unpaired chains precipitate → ineffective erythropoiesis + extravascular hemolysis.
HbF (α2γ2) is protective, so β-globin disorders (SCD, β-thal major) declare themselves at ~6 months as fetal hemoglobin wanes.
- Genetics: AR; Glu→Val at β-codon 6. HbSS = disease; HbAS = trait (usually asymptomatic; protective vs P. falciparum malaria).
- Sickling triggers: hypoxia, dehydration, acidosis, infection, cold, high altitude.
- Vaso-occlusion: painful crises; dactylitis (hand–foot swelling) — often the first sign at ~6 mo; acute chest syndrome (fever, chest pain, new infiltrate — leading cause of death in adults); priapism; avascular necrosis of femoral head; stroke; renal papillary necrosis (hematuria, isosthenuria); leg ulcers.
- Hemolysis: normocytic anemia, ↑reticulocytes, ↑LDH, ↑indirect bilirubin, ↓haptoglobin; pigment gallstones.
- Spleen: acute splenic sequestration in young kids (rapid splenomegaly + Hb drop with HIGH reticulocytes); repeated infarction → autosplenectomy → functional asplenia → encapsulated-organism sepsis and Salmonella osteomyelitis.
- Aplastic crisis: Parvovirus B19 (sudden Hb drop with LOW reticulocytes).
- Smear: sickle cells, target cells, Howell–Jolly bodies (asplenia).
Case: A 9-month-old presents with irritability and symmetric painful swelling of the hands and feet. Hb 8 g/dL, normal MCV, reticulocytes elevated; smear shows sickled cells and Howell–Jolly bodies.
- Diagnosis: SCD presenting as dactylitis (hand–foot syndrome).
- Confirm: hemoglobin electrophoresis / HPLC — HbS predominant, no HbA in SS (newborn screen usually already positive).
- Chronic next steps: hydroxyurea (↑HbF), folic acid, penicillin prophylaxis until age 5, and pneumococcal/meningococcal/Hib vaccination.
Twist vignettes:
- Sudden pallor + Hb drop with LOW reticulocytes after a viral illness → Parvovirus B19 aplastic crisis.
- Rapidly enlarging spleen + Hb drop with HIGH reticulocytes in a toddler → splenic sequestration crisis (hypovolemic emergency).
- Fever, hypoxia, new pulmonary infiltrate → acute chest syndrome → O2, antibiotics (cover atypicals + pneumococcus), analgesia, and exchange transfusion.
α-thalassemia (chr 16; deletions of 4 α genes):
- 1 deleted: silent carrier.
- 2 deleted: α-thal trait — mild microcytic anemia; cis (--/αα, Asian) raises offspring risk more than trans (-α/-α, African) because cis carriers can produce --/-- offspring.
- 3 deleted: HbH disease (β4 tetramers) — moderate hemolytic anemia.
- 4 deleted: Hb Barts (γ4) — hydrops fetalis, death in utero/neonatal.
β-thalassemia (chr 11; usually point mutations):
- Minor/trait: one β gene affected; mild microcytic anemia often mistaken for iron deficiency; ↑HbA2 (>3.5%) is the marker.
- Major (Cooley anemia): β0/β0; severe transfusion-dependent anemia from ~6 mo; ↑↑HbF, ↑HbA2; marrow expansion → "crew-cut" skull X-ray, chipmunk facies, hepatosplenomegaly (extramedullary hematopoiesis).
- Iron studies are NORMAL in thalassemia TRAIT — do not give iron reflexively. (β-thal major instead develops iron overload from chronic transfusions + increased GI iron absorption.)
Case: A 25-year-old of Mediterranean descent has mild fatigue. CBC: Hb 11 g/dL, MCV 68 (very low), RBC count high-normal, RDW normal; iron studies normal; symptoms/labs unchanged after a trial of oral iron.
- Clue: microcytosis out of proportion to mild anemia + normal/high RBC count + normal RDW + no response to iron → thalassemia, not iron deficiency (where RBC is low and RDW high).
- Mentzer index (MCV ÷ RBC): <13 → thalassemia; >13 → iron deficiency.
- Next best step: hemoglobin electrophoresis → ↑HbA2 confirms β-thalassemia minor.
- Management: reassurance + genetic counseling; no iron unless true deficiency is proven.
- α-thal trait: electrophoresis is normal (no ↑HbA2) — diagnosis of exclusion / genetic testing.
Comparison — anemia, smear, markers
| Condition | Anemia / MCV | Peripheral smear | Electrophoresis / marker | Classic clue |
|---|---|---|---|---|
| Sickle cell (HbSS) | Normocytic, hemolytic | Sickle cells, target cells, Howell–Jolly | ↑HbS, ↑HbF, no HbA | Dactylitis, acute chest, Salmonella osteo |
| β-thal minor | Mild, very low MCV | Target cells, basophilic stippling | ↑HbA2 (>3.5%), slight ↑HbF | Microcytosis ≫ anemia, normal RDW, normal iron |
| β-thal major | Severe, microcytic | Target cells, nucleated RBCs, teardrops, stippling | ↑↑HbF, ↑HbA2, low/no HbA | Crew-cut skull, chipmunk facies, transfusion-dependent |
| α-thal trait | Mild, low MCV | Target cells | Normal electrophoresis | Diagnosis of exclusion / genetic testing |
| HbH disease | Moderate, hemolytic | HbH inclusions (β4, "golf-ball" cells on supravital stain), target cells | HbH (β4) band | 3-gene deletion |
- Encapsulated organisms (deadly in SCD functional asplenia) — "SHiNE SKiS": S. pneumoniae, H. influenzae type b, N. meningitidis, E. coli, Salmonella, Klebsiella, group B Strep. (Pneumococcus = #1 cause of sepsis in SCD children → hence penicillin prophylaxis + vaccines.)
- Mentzer index: MCV ÷ RBC — *"Mentzer < 13 = Microcytosis of thalassemia."*
- HbF is protective → β-globin diseases (SCD, β-thal major) present at ~6 months as fetal Hb falls.
- Sickling triggers (the stressors): hypoxia, dehydration, acidosis, infection, cold.
- Vaso-occlusive pain crisis: prompt analgesia (opioids) + hydration; supplemental O2 only if hypoxic; treat the precipitant. Avoid routine/unnecessary transfusion.
- Acute chest syndrome: O2, antibiotics (atypical + pneumococcal cover), analgesia, and simple or exchange transfusion; low threshold to admit — leading cause of death in adults.
- Hydroxyurea: ↑HbF → fewer pain crises and fewer acute-chest episodes; first-line disease-modifier.
- Prophylaxis: penicillin until age 5, full vaccination vs encapsulated organisms, folic acid.
- Curative: allogeneic HSCT; gene therapy (CRISPR-based exa-cel / Casgevy) now FDA-approved.
- β-thal major: chronic transfusions + iron chelation (deferasirox/deferoxamine); HSCT curative.
- Golden rule: never treat thalassemia trait with iron reflexively — confirm true deficiency first.
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