A 24-year-old man of African descent presents to the emergency department with acute onset severe chest pain, dyspnea, and fever (38.5°C). He has a known history of sickle cell disease (HbSS) diagnosed in childhood. On examination, he is tachycardic (HR 112/min), tachypneic (RR 28/min), and oxygen saturation is 88% on room air. Chest X-ray shows a new infiltrate in the right lower lobe. Blood cultures are pending. Complete blood count shows Hb 7.2 g/dL (baseline ~8 g/dL), WBC 18,500/μL, platelets 245,000/μL. Reticulocyte count is 12%. Which of the following is the most likely diagnosis?

Explanation

## Acute Chest Syndrome in Sickle Cell Disease **Key Point:** Acute chest syndrome (ACS) is a vaso-occlusive crisis affecting the lungs, characterized by new pulmonary infiltrate(s), fever, chest pain, and respiratory symptoms in a patient with sickle cell disease. ### Clinical Presentation - **Triad:** New pulmonary infiltrate + fever + respiratory symptoms (chest pain, dyspnea, cough) - **Timing:** Often follows a vaso-occlusive pain crisis by 1–2 days - **Incidence:** Occurs in ~50% of SCD patients by age 30; leading cause of mortality in SCD ### Pathophysiology 1. Vaso-occlusion in pulmonary vasculature → tissue infarction 2. Fat embolism from bone marrow necrosis (especially femur/tibia) 3. In-situ thrombosis and sickling of RBCs in pulmonary capillaries 4. Infection (bacterial, viral, atypical organisms) as trigger or superinfection ### Diagnostic Criteria (NIH 2014) Presence of a **new** pulmonary infiltrate on imaging + ≥1 of: - Chest pain - Dyspnea - Cough - Fever ≥38.5°C - Wheezing - Tachypnea ### Laboratory Findings | Finding | Significance | | --- | --- | | Elevated reticulocyte count (>10%) | Hemolysis; regenerative response | | Drop in Hb (>1 g/dL below baseline) | Acute hemolysis or splenic sequestration | | Elevated LDH, low haptoglobin | Hemolysis | | Elevated WBC | Stress response; may be >20,000/μL | | Hypoxemia (SpO₂ <92%) | Indicates significant lung involvement | ### Management - Supplemental O₂ to maintain SpO₂ >92% - Aggressive hydration (avoid overload → pulmonary edema) - Analgesia (opioids for pain) - Empiric antibiotics (covers Streptococcus pneumoniae, Haemophilus influenzae, atypical organisms) - Transfusion (simple or exchange) if SpO₂ <90%, Hb drop >2 g/dL, or rapid clinical deterioration - Incentive spirometry to prevent atelectasis **Clinical Pearl:** ACS can rapidly progress to respiratory failure; close monitoring and early intervention are critical. Mortality is ~5% even with treatment. **High-Yield:** The key distinguishing feature is the **new infiltrate** in a sickle cell patient with fever and respiratory symptoms—this combination is pathognomonic for ACS until proven otherwise.