Sickle Cell Disease in Children

Sickle cell disease (SCD) is the commonest clinically significant haemoglobinopathy worldwide and a recurring NEET PG favourite in Paediatrics and Haematology. The single point mutation in the β-globin gene produces a haemoglobin that polymerises on deoxygenation, sickles the red cell, and drives both chronic haemolysis and recurrent vaso-occlusion. This note builds the topic from the molecule up to the bedside crisis.

Definition and classification

Sickle cell disease is an autosomal recessive disorder caused by a point mutation in the β-globin gene (chromosome 11): GAG → GTG at codon 6, replacing glutamic acid (Glu) with valine (Val) at position 6 of the β-chain. This produces haemoglobin S (HbS, α₂β₂^6Glu→Val).

"Sickle cell disease" is an umbrella term for all genotypes that produce a sickling clinical phenotype, whereas "sickle cell anaemia" refers specifically to the homozygous HbSS state.

Genotype	Name	Severity	Key feature
HbSS	Sickle cell anaemia	Most severe	No HbA; highest crisis frequency
HbSC	Haemoglobin SC disease	Moderate	Target cells, more retinopathy & avascular necrosis
HbS/β⁰-thalassaemia	Sickle β⁰-thal	Severe (≈ HbSS)	No HbA produced
HbS/β⁺-thalassaemia	Sickle β⁺-thal	Mild–moderate	Some HbA present
HbAS	Sickle cell trait	Usually asymptomatic	Carrier; protective against falciparum malaria

High-yield: Sickle cell trait (HbAS) is NOT a disease. It is asymptomatic except under extreme hypoxia/dehydration, classically causing renal papillary necrosis and isosthenuria (loss of urine-concentrating ability) with painless haematuria. Splenic infarction at high altitude and exercise-related sudden death are described.

Etiology and pathophysiology

The disease flows from one biochemical event into a cascade of cellular and vascular consequences.

Deoxygenation → HbS polymerisation → RBC sickling → vaso-occlusion + haemolysis → tissue ischaemia & chronic anaemia.

1. Polymerisation. When HbS gives up oxygen, the exposed hydrophobic valine binds a complementary pocket on an adjacent β-chain, forming long insoluble polymers (tactoids) that distort the cell into the classic sickle shape.
2. Determinants of sickling. Polymerisation depends on (a) degree of deoxygenation, (b) intracellular HbS concentration (worsened by dehydration), (c) presence of other haemoglobins — HbF strongly inhibits polymerisation, which is the entire rationale for hydroxyurea, and (d) acidosis (Bohr effect shifts curve right, promoting deoxy-Hb).
3. Reversible → irreversible. Early sickling is reversible on re-oxygenation, but repeated cycles damage the membrane (calcium influx, potassium/water efflux via the Gardos channel) producing dense, dehydrated, irreversibly sickled cells (ISCs).
4. Haemolysis. Rigid sickled cells are removed extravascularly (spleen, liver) and also lyse intravascularly. Free haemoglobin scavenges nitric oxide (NO), producing endothelial dysfunction — central to pulmonary hypertension, priapism and leg ulcers.
5. Vaso-occlusion. Sickled cells, activated endothelium, leucocytes and platelets adhere within the microcirculation, causing ischaemia–reperfusion injury and a chronic inflammatory, hypercoagulable state.

High-yield: HbF (fetal haemoglobin) levels determine why infants are protected. Symptoms appear only after ~6 months of age, as HbF is physiologically replaced by HbS. This is why newborn screening detects the genotype before symptoms and why hereditary persistence of HbF ameliorates disease.

Clinical features

Age-based presentation

• Infancy (first presentation): Dactylitis (hand-foot syndrome) — symmetrical painful swelling of the dorsa of hands and feet from ischaemia of small bones/marrow. Frequently the first clinical manifestation, typically at 6 months–2 years.
• Childhood: recurrent painful (vaso-occlusive) crises, anaemia, splenomegaly then autosplenectomy, growth delay, frequent infections.
• Later childhood/adolescence: delayed puberty, gallstones, leg ulcers, retinopathy, priapism, chronic organ damage.

The crises (very high-yield)

Crisis	Mechanism	Hallmark	Management pearl
Vaso-occlusive (painful) crisis	Microvascular occlusion	Bone/back/abdominal pain	Hydration + analgesia (opioids); O₂ if hypoxic
Acute chest syndrome (ACS)	Pulmonary vaso-occlusion/infection/fat embolism	New infiltrate + fever/respiratory signs	Antibiotics, O₂, exchange transfusion; leading cause of death
Aplastic crisis	Parvovirus B19 infects erythroid precursors	Sudden ↓Hb with low reticulocytes	Transfusion; self-limited
Splenic sequestration	Sudden pooling of blood in spleen	Rapidly enlarging spleen + shock + ↓Hb, high reticulocytes	Volume resuscitation, transfusion; splenectomy if recurrent
Hyperhaemolytic crisis	Accelerated haemolysis	Falling Hb with high reticulocytes, jaundice	Supportive ± transfusion

High-yield: Distinguish aplastic vs sequestration by reticulocyte count — aplastic = LOW reticulocytes (marrow shutdown by parvovirus B19); sequestration = HIGH reticulocytes (active haemolysis, blood trapped in spleen).

Functional asplenia and infection

Repeated splenic infarction causes autosplenectomy (usually by 5 years in HbSS), leaving the child functionally asplenic and at risk of overwhelming sepsis from encapsulated organisms.

High-yield: The leading cause of death in young children with SCD is infection — Streptococcus pneumoniae sepsis above all. Salmonella is the classic cause of osteomyelitis in SCD (though Staph aureus is overall more common in the general paediatric population, Salmonella is the SCD-associated answer).

Chronic organ involvement

• Bone: avascular necrosis of femoral head, vertebral "fish-mouth/codfish" deformity, osteomyelitis.
• Renal: isosthenuria, papillary necrosis, hyposthenuria, later sickle nephropathy/CKD.
• CNS: ischaemic stroke (children) — screened by transcranial Doppler.
• Eye: proliferative retinopathy (commoner in HbSC).
• Hepatobiliary: pigment gallstones from chronic haemolysis.
• Genitourinary: priapism (a urological emergency).
• Cardiopulmonary: pulmonary hypertension, cardiomegaly.
• Skin: chronic leg ulcers (malleolar).

Diagnosis and investigation of choice

• Newborn screening: HPLC (high-performance liquid chromatography) or isoelectric focusing on a heel-prick blood spot. The HbSS newborn pattern is "FS" (HbF predominant, HbS present, no HbA). India's National Sickle Cell Anaemia Elimination Mission (target 2047) uses point-of-care solubility tests for community screening, confirmed by HPLC.
• Confirmatory / definitive test: Haemoglobin electrophoresis (or HPLC) — HbSS shows mostly HbS, raised HbF, absent HbA. Distinguishes SS from S-trait (which retains HbA, normally HbA > HbS).
• Sickling/solubility test (sodium metabisulphite): screening only — positive in both disease and trait, so it cannot differentiate; always confirm with electrophoresis/HPLC.
• Peripheral smear: sickle cells, target cells, Howell–Jolly bodies (indicate hyposplenism/autosplenectomy), nucleated RBCs, polychromasia.
• CBC: normocytic normochromic anaemia (Hb ~6–9 g/dL in SS), reticulocytosis.
• Stroke risk: Transcranial Doppler (TCD) — elevated cerebral artery velocity (≥ 200 cm/s) predicts stroke and prompts chronic transfusion.
• Antenatal diagnosis: chorionic villus sampling with DNA analysis (the GTG mutation can be detected by restriction enzyme/PCR).

High-yield: A positive sickling test does not diagnose SCD — it is positive in trait too. The investigation of choice for diagnosis is Hb electrophoresis/HPLC; the presence or absence of HbA separates disease (SS, absent HbA) from trait (AS, HbA present and dominant).

Management and drug of choice

Comprehensive preventive care

• Penicillin prophylaxis: oral penicillin V started by 2 months of age and continued at least until age 5 — proven to reduce pneumococcal sepsis (landmark PROPS trial).
• Immunisation: routine schedule plus the pneumococcal vaccines (PCV conjugate + PPSV23 polysaccharide), Haemophilus influenzae type b (Hib), meningococcal and annual influenza vaccines — because of functional asplenia.
• Folic acid supplementation for chronic haemolysis.
• Early/aggressive treatment of fever as a medical emergency.

High-yield: Two interventions that most reduce early mortality — prophylactic penicillin from 2 months and pneumococcal vaccination (plus newborn screening that allows them to start before the first crisis).

Disease-modifying drug of choice

• Hydroxyurea (hydroxycarbamide) is the drug of choice for disease modification. Mechanism: induces HbF, which inhibits HbS polymerisation; it also lowers neutrophils, reduces adhesion and is an NO donor.
  • Indications: ≥ 3 vaso-occlusive crises/year, recurrent or any acute chest syndrome, severe symptomatic anaemia, recurrent dactylitis. Increasingly offered to all HbSS/Sβ⁰ children from ~9 months irrespective of severity.
  • Effects: ↓ pain crises, ↓ ACS, ↓ transfusions, ↓ hospitalisation, improved survival.
  • Monitor: myelosuppression (CBC), titrate to maximum tolerated dose.
• L-glutamine, crizanlizumab (anti–P-selectin), voxelotor (HbS polymerisation inhibitor) are newer agents (board awareness).

Acute crisis management

Vaso-occlusive crisis flow: Assess & remove triggers → hydration (oral/IV) → analgesia (paracetamol/NSAID; escalate to opioids for severe pain) → oxygen if hypoxic → treat infection → watch for evolving ACS.

• Acute chest syndrome: oxygen, broad-spectrum antibiotics (cover atypicals), analgesia, incentive spirometry, and transfusion — simple or exchange transfusion for severe/hypoxic cases.
• Splenic sequestration / aplastic crisis / severe symptomatic anaemia: blood transfusion.
• Stroke or high TCD velocity: chronic (regular) transfusion programme, with iron chelation to manage overload.
• Priapism: hydration, analgesia, urological intervention (aspiration/irrigation).

Curative therapy

• Allogeneic haematopoietic stem cell transplant (HSCT) from an HLA-matched sibling is the only established cure; best results in children with a matched donor.
• Gene therapy / gene editing (e.g. exagamglogene autotemcel — CRISPR-based BCL11A editing to reactivate HbF) is now approved for severe disease — high-yield "recently approved cure" fact.

Triggers to memorise (mnemonic "DICHE"): Dehydration, Infection, Cold/acidosis, Hypoxia, Exertion/Emotional/physical stress — all precipitate sickling.

Complications

• Acute chest syndrome — leading cause of death overall.
• Stroke (ischaemic in children, haemorrhagic in adults) and silent cerebral infarcts.
• Overwhelming sepsis from encapsulated organisms (pneumococcus).
• Splenic sequestration and eventual autosplenectomy.
• Aplastic crisis (parvovirus B19).
• Avascular necrosis of femoral/humeral head.
• Pigment gallstones, cholecystitis.
• Sickle nephropathy, papillary necrosis, eventual CKD.
• Pulmonary hypertension, cardiomegaly.
• Proliferative retinopathy (HbSC), leg ulcers, priapism, growth and pubertal delay.
• Transfusion-related: iron overload, alloimmunisation, infection.

Key differentials

Condition	Differentiating feature
β-thalassaemia major	Microcytic hypochromic, marked HbF/HbA₂; transfusion-dependent from infancy, no sickling, no HbS on electrophoresis
Hereditary spherocytosis	Spherocytes, ↑ osmotic fragility, negative sickling, splenomegaly with haemolysis; family history dominant
G6PD deficiency	Episodic haemolysis after oxidant drugs/fava beans; bite cells, Heinz bodies; enzyme assay
Sickle cell trait (HbAS)	Asymptomatic; HbA > HbS on electrophoresis; only haematuria/isosthenuria
Acute abdomen (surgical)	Crisis abdominal pain can mimic appendicitis/cholecystitis — image before operating
Osteomyelitis vs bone infarction	Both cause bone pain/fever in SCD; MRI and culture (Salmonella) help distinguish

High-yield: In a child of African/Indian tribal descent with dactylitis, recurrent bone pain, anaemia and Howell–Jolly bodies on smear, the answer is sickle cell disease — confirm with HPLC/electrophoresis, not the sickling test alone.

Recently asked / exam angle

• First manifestation in infancy = dactylitis (hand-foot syndrome). Repeatedly asked single-best-answer.
• Mutation: Glu→Val at position 6 of β-globin (GAG→GTG); β-chain, chromosome 11 — direct one-liner.
• Aplastic crisis cause = Parvovirus B19 with low reticulocyte count — classic distractor against sequestration.
• Osteomyelitis organism in SCD = Salmonella.
• Drug that increases HbF = hydroxyurea (drug of choice for disease modification); know its indications.
• Penicillin prophylaxis from 2 months to 5 years + pneumococcal vaccine — preventive package question.
• Investigation of choice = Hb electrophoresis/HPLC; sickling test is screening only and positive in trait.
• Leading causes of death: infection (young children) and acute chest syndrome.
• India-specific: National Sickle Cell Anaemia Elimination Mission (target 2047), high prevalence in tribal belts of central India (Madhya Pradesh, Chhattisgarh, Maharashtra, Odisha, Gujarat).
• Curative options: matched-sibling HSCT and newly approved CRISPR gene therapy reactivating HbF via BCL11A.
• Trait protects against falciparum malaria (balanced polymorphism / heterozygote advantage).
• Howell–Jolly bodies indicate autosplenectomy/functional asplenia.

Rapid revision

1. SCD = autosomal recessive; Glu→Val at β6 (GAG→GTG) producing HbS.
2. Symptoms begin after 6 months as protective HbF falls.
3. Dactylitis is the first presentation in infancy.
4. HbF inhibits polymerisation — basis of hydroxyurea therapy.
5. Aplastic crisis = Parvovirus B19 = LOW reticulocytes; sequestration = enlarging spleen + shock + HIGH reticulocytes.
6. Acute chest syndrome is the leading overall cause of death — treat with O₂, antibiotics, exchange transfusion.
7. Salmonella osteomyelitis; pneumococcus sepsis (functional asplenia).
8. Penicillin prophylaxis 2 months–5 years + pneumococcal/Hib/meningococcal vaccines + folic acid.
9. Diagnosis = Hb electrophoresis/HPLC; newborn pattern "FS"; sickling test cannot separate disease from trait.
10. Howell–Jolly bodies + target cells + sickle cells on smear; autosplenectomy by ~5 years.
11. Trait (HbAS) — asymptomatic, isosthenuria/painless haematuria, protects against falciparum malaria.
12. Cure = matched-sibling HSCT or CRISPR gene therapy (BCL11A → ↑HbF); TCD screens for stroke risk.