Sickle Cell Disease — Crisis, Acute Chest Syndrome & Hydroxyurea

Creator: Progress Note
Published: 2026-07-22
Keywords: sickle cell disease crisis, sickle cell pain crisis treatment, acute chest syndrome sickle cell, hydroxyurea sickle cell, HbS pathophysiology

The 60-second version

HbS: glutamate→valine at beta-globin position 6; polymerizes under hypoxia/dehydration
Functional asplenia by age 5 → susceptibility to encapsulated bacteria → vaccinate + penicillin prophylaxis
Acute chest syndrome: new CXR infiltrate + respiratory symptoms = leading cause of death; treat with exchange transfusion
Hydroxyurea increases HbF → inhibits HbS polymerization → fewer crises
VOC treatment: fluids + NSAIDs/opioids + O2 + incentive spirometry

Pathophysiology

HbS (beta-S/beta-S): glutamate replaced by valine at position 6 of the beta-globin chain. Under deoxygenation (low O2, dehydration, cold, acidosis, infection), HbS polymerizes into rigid fibers that distort RBCs into sickle shape. Consequences: (1) Vascular occlusion: sickled RBCs are rigid and sticky → occlude small vessels → ischemia. (2) Hemolytic anemia: sickled cells are fragile, lifespan 10–20 days (normal 120 days) → normocytic anemia, elevated LDH, indirect bilirubin, low haptoglobin, reticulocytosis. (3) Functional asplenia: repeated splenic infarcts → autosplenectomy by age 5 → susceptibility to encapsulated organisms. Sickle trait (HbAS): one normal + one sickle beta-globin allele — generally asymptomatic; sickling only under extreme hypoxia; associated with renal medullary carcinoma, hematuria, and exercise-associated sudden death.

Vaso-occlusive Crisis (VOC)

Most common complication: acute pain from bone/soft tissue ischemia. Precipitants: dehydration, infection, cold, hypoxia, stress, menses. Common sites: long bones (bone marrow), chest, abdomen, back, CNS. Treatment: IV fluids (hydration reverses sickling), NSAIDs (ketorolac) + opioids (IV morphine) for pain, incentive spirometry (prevents atelectasis → ACS), supplemental O2 if hypoxic. Hand-foot syndrome (dactylitis): symmetric swelling/pain of hands and feet in infants — often first presentation of SCD. Avascular necrosis: repeated infarcts of femoral/humeral heads → joint destruction — MRI is most sensitive early.

Acute Chest Syndrome (ACS)

ACS = new pulmonary infiltrate on CXR + respiratory symptoms (chest pain, cough, fever, hypoxia) in a SCD patient. Leading cause of death in SCD patients. Causes: fat embolism from infarcted bone marrow (most common in adults), infection (Mycoplasma, Chlamydia, S. pneumoniae — most common in children), pulmonary infarction, atelectasis. Treatment: exchange transfusion (reduces % HbS immediately) + antibiotics (azithromycin + ceftriaxone) + O2 + pain control. Critical: simple transfusion can worsen hyperviscosity if Hb is already high — exchange transfusion is preferred. Prevention of ACS: incentive spirometry during VOC, hydroxyurea for recurrent episodes.

Long-term Complications and Hydroxyurea

Functional asplenia: vaccinate against encapsulated bacteria (pneumococcal, meningococcal, H. influenzae vaccines) + daily penicillin prophylaxis in children until age 5. Stroke: 10% of children with SCD have overt stroke (ischemic from large vessel occlusion); transcranial Doppler screening annually — high velocity → exchange transfusion to prevent stroke. Priapism: sustained painful erection from vascular stasis — treat with aspiration, phenylephrine, exchange transfusion. Splenic sequestration: acute massive splenomegaly (young children before autosplenectomy) → acute anemia + hypovolemic shock → emergency transfusion. Hydroxyurea: increases fetal hemoglobin (HbF) production → HbF inhibits HbS polymerization. Reduces VOC frequency, ACS episodes, and transfusion requirements. Side effects: myelosuppression — monitor CBC.

Test yourself

Quick check — 3 questions

1. A 22-year-old with SCD is hospitalized for vaso-occlusive crisis. On day 2, he develops fever 38.8°C, chest pain, and new bilateral lower lobe infiltrates on CXR with O2 sat 91%. The most appropriate immediate intervention is:

2. A 3-year-old with sickle cell disease presents with sudden pallor, lethargy, and massive splenomegaly. Hb is 3.2 g/dL (baseline 8). Diagnosis?

3. Which vaccine is most critical to administer to a child with sickle cell disease?

Frequently asked questions

Why are SCD patients susceptible to encapsulated bacteria?

Repeated splenic infarcts from sickling cause progressive autosplenectomy (functional asplenia) by age 5. The spleen is the primary organ that filters and opsonizes encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis) that lack effective T-cell-independent antibody responses. Without splenic function, these bacteria cause overwhelming sepsis. Prevention: vaccinate against all three organisms + penicillin prophylaxis (PCN VK 125–250 mg BID) starting at 2 months until age 5.

What is the mechanism of hydroxyurea in sickle cell disease?

Hydroxyurea inhibits ribonucleotide reductase, causing stress erythropoiesis that mimics fetal conditions — resulting in increased production of fetal hemoglobin (HbF, alpha2-gamma2). HbF has a higher O2 affinity and, crucially, does NOT interact with HbS to form polymers. By diluting HbS with HbF, hydroxyurea reduces intracellular HbS concentration, decreasing polymerization and sickling. Reduces VOC by ~50%, decreases ACS, reduces transfusion requirements, and prolongs survival. HbF >20% correlates with clinical benefit.

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