A 12-year-old boy with sickle cell disease presents with acute chest pain, fever, and a new pulmonary infiltrate on chest X-ray. What is the most common cause of acute chest syndrome in sickle cell disease?

Explanation

## Most Common Cause of Acute Chest Syndrome in SCD **Key Point:** Pulmonary infarction due to vaso-occlusion is the most common cause of acute chest syndrome (ACS) in sickle cell disease, accounting for approximately 50–70% of cases. ### Pathophysiology of ACS Acute chest syndrome results from: 1. Vaso-occlusion of pulmonary vessels → pulmonary infarction 2. Fat embolism from bone marrow necrosis (second most common) 3. Acute infection (bacterial or viral) 4. In situ thrombosis ### Clinical Features of ACS - Chest pain (pleuritic) - Fever - Dyspnea - New pulmonary infiltrate on imaging - Hypoxemia - Can progress to acute respiratory distress syndrome (ARDS) ### Differential Causes of ACS | Cause | Frequency | Mechanism | Key Feature | |-------|-----------|-----------|-------------| | **Pulmonary infarction** | 50–70% | Vaso-occlusion | Wedge-shaped infiltrate | | **Fat embolism** | 10–15% | Bone marrow necrosis | Petechial rash, thrombocytopenia | | **Infection** | 10–20% | Bacterial/viral pneumonia | Positive culture, focal consolidation | | **In situ thrombosis** | Rare | Endothelial injury | Elevated D-dimer | **High-Yield:** ACS is a medical emergency in SCD and is the second leading cause of death (after stroke) in adult patients with SCD. **Clinical Pearl:** Fat embolism syndrome in SCD typically presents with fever, petechial rash, thrombocytopenia, and neurological symptoms in addition to pulmonary findings—a constellation that helps distinguish it from simple vaso-occlusive pulmonary infarction. **Warning:** Do not confuse ACS with simple vaso-occlusive pain crisis; ACS requires new pulmonary infiltrate and hypoxemia, making it a distinct and more serious complication.