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PA16.1-3 | Sickle Cell Disease & Thalassaemia — Hereditary Haemolytic Anaemias — Part 1
CLINICAL SCENARIO
A 9-year-old child arrives in the emergency ward with severe bone pain in both hands — the fourth such episode this year. His parents say he was diagnosed at birth with a 'blood problem'. His haemoglobin is 7.2 g/dL, MCV 85 fL. The blood film report reads: 'sickle cells, target cells, nucleated RBCs, Howell-Jolly bodies.' What single amino acid substitution turned a child's erythrocytes into molecular roadblocks?
WHY THIS MATTERS
Sickle cell disease (SCD) and thalassaemia are the most common monogenic diseases on earth — an estimated 300,000 severe cases are born annually, the majority in India, sub-Saharan Africa, and the Mediterranean. In India, sickle cell disease prevalence reaches 10–33 % in tribal belt populations (Odisha, Chhattisgarh, Jharkhand). As a clinician you will encounter these diseases in medicine wards, paediatrics, obstetrics, and the blood bank. Understanding the morphological and molecular basis is not optional.
RECALL
Before continuing, recall:
• Normal haemoglobin structure — adult HbA is α2β2; foetal HbF is α2γ2.
• Globin gene locations — α-globin genes on chromosome 16 (4 alleles: 2 per chromosome); β-globin gene on chromosome 11 (2 alleles).
• The iron deficiency vs anaemia of chronic disease distinction from Cluster H3 — you will need it when thalassaemia trait mimics IDA on MCV alone.
• Intravascular vs extravascular haemolysis — the peripheral smear morphology differs fundamentally.
HbS: The Single Amino Acid That Changes Everything
HbS Mutation, Polymerisation, and Sickling
Sickle cell disease arises from a point mutation in the β-globin gene: a GAG→GTG transversion at codon 6 substitutes valine for glutamic acid (β6 Glu→Val). This is an autosomal recessive disorder — full disease requires homozygosity (HbSS); heterozygotes carry sickle cell trait (HbAS).
HbS polymerisation is the central pathophysiology:
1. In the oxygenated state, HbS behaves nearly normally.
2. On deoxygenation, the hydrophobic valine residue on one HbS molecule binds a complementary hydrophobic 'acceptor' pocket on an adjacent HbS molecule.
3. This nucleation event propagates into long, rigid deoxyHbS polymer fibres (tactoids) that distort the RBC membrane into the characteristic sickle shape.
4. Polymerisation is concentration-dependent and reversible — reoxygenation initially reverses sickling. However, repeated cycles damage the membrane irreversibly, producing irreversibly sickled cells (ISCs).
Two key modifiers govern clinical severity:
- HbF concentration — high HbF (as in infancy, or therapeutics like hydroxyurea) inhibits polymerisation because HbF does not participate in HbS tactoid formation.
- Mean corpuscular haemoglobin concentration (MCHC) — higher MCHC accelerates polymerisation.
Consequences of Sickling: Vaso-occlusion and Chronic Haemolysis
Consequences of Sickling: Vaso-occlusion and Chronic Haemolysis
Sickling causes disease through two interconnected mechanisms:
1. Vaso-occlusion
Sickled erythrocytes are rigid, sticky (upregulated adhesion molecules: VCAM-1 interactions), and capable of plugging the microvasculature. This produces:
- Painful vaso-occlusive crises (bones, chest, abdomen) — the most frequent acute complication
- Acute chest syndrome — pulmonary vaso-occlusion ± infection; leading cause of death in adults
- Dactylitis (hand-foot syndrome) — ischaemic infarction of small bones in infants; often the first crisis presentation
- Stroke — large-vessel occlusion, especially in children
- Splenic sequestration crisis — massive acute pooling of blood in the spleen (children <5 years, before autosplenectomy)
2. Chronic haemolysis
Repeatedly sickled RBCs have a lifespan of 10–20 days (normal 120 days), producing a chronic compensated haemolytic anaemia. The bone marrow compensates with erythroid hyperplasia; reticulocyte count is elevated.
Autosplenectomy: Repeated splenic infarctions from vaso-occlusion result in progressive fibrosis and shrinkage — by late childhood the spleen is a tiny fibrocalcific remnant. This leaves patients functionally asplenic and highly susceptible to encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae, Salmonella — the classic osteomyelitis organism in SCD).
INFO: Autosplenectomy is the pathological basis for the Howell-Jolly bodies seen on the blood film — splenectomy (functional or surgical) removes the RBC 'pitting' function that normally eliminates these nuclear remnants.
Blood Film and Haematologic Indices in Sickle Cell Disease
Blood Film and Indices in Sickle Cell Disease
Peripheral blood picture (HbSS):
| Morphology | Significance |
|---|---|
| Sickle cells (drepanocytes) | Hallmark; crescent/elongated forms |
| Target cells (codocytes) | ↑ surface-to-volume ratio, also in thalassaemia |
| Nucleated RBCs | Severe haemolysis + hyperactive marrow |
| Howell-Jolly bodies | Nuclear remnants; post-autosplenectomy |
| Polychromasia / reticulocytes | Compensatory erythroid drive |
| Irreversibly sickled cells (ISCs) | Seen even on oxygenated smears |
Haematologic indices (HbSS, at baseline):
- Haemoglobin: 6–9 g/dL (normocytic normochromic)
- MCV: 80–95 fL (normal — unlike thalassaemia!)
- Reticulocyte count: 5–20 %
- Unconjugated bilirubin: elevated (haemolysis)
- LDH: elevated
- Haptoglobin: reduced/absent
Sickle cell trait (HbAS):
- Clinically silent under normal conditions; sickling occurs only at extreme hypoxia (e.g., high altitude, anaesthesia complications)
- Blood film is essentially normal
- Haemoglobin electrophoresis: ~60 % HbA + ~40 % HbS (HbA predominates because HbA is assembled preferentially)
Diagnostic tests:
- Sickling/solubility test (sodium metabisulphite test): Screens for HbS presence; does NOT distinguish trait from disease
- Haemoglobin electrophoresis / HPLC: Definitive — HbSS shows >80 % HbS + elevated HbF, no HbA; HbAS shows HbA + HbS bands
- Sickle prep: Direct sickling on a slide under reducing conditions
CLINICAL PEARL
MCV in SCD is normal — this distinguishes it from thalassaemia. A student who memorises 'sickle cell = microcytic' has confused two different pathomechanisms. Thalassaemia causes microcytosis because of impaired globin synthesis (less Hb per cell); SCD causes haemolysis of normally-sized cells. The only time SCD becomes microcytic is when it co-exists with iron deficiency or a thalassaemia gene (HbSβ-thalassaemia).
SELF-CHECK
A homozygous HbSS patient's blood film is examined while the patient is in a pain crisis. Which combination of morphological findings is MOST expected?
A. Sickle cells, target cells, Howell-Jolly bodies, polychromasia
B. Hypersegmented neutrophils, macro-ovalocytes, tear-drop cells
C. Microspherocytes, osmotic fragility ↑, polychromasia
D. Schistocytes, thrombocytopenia, low haptoglobin
Reveal Answer
Answer: A. Sickle cells, target cells, Howell-Jolly bodies, polychromasia
Option A describes the classic SCD smear: drepanocytes (sickle cells), codocytes (target cells from altered membrane lipid), Howell-Jolly bodies (post-autosplenectomy nuclear remnants), and polychromasia/reticulocytosis from compensatory marrow drive. Option B describes megaloblastic anaemia. Option C describes hereditary spherocytosis. Option D describes microangiopathic haemolytic anaemia (TTP/HUS).