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 genotype). On examination, he is tachycardic (HR 110/min), tachypneic (RR 28/min), and has decreased air entry in the left lower lobe. Chest X-ray shows a new infiltrate in the left lower lobe. Hemoglobin is 7.2 g/dL (baseline 8.5 g/dL), reticulocyte count is 8%, and LDH is markedly elevated. Blood culture is pending. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Chest Syndrome (ACS) ### Clinical Presentation This patient presents with the classic triad of acute chest syndrome: 1. **Fever** (38.5°C) 2. **Chest pain** with respiratory symptoms 3. **New pulmonary infiltrate** on imaging (CXR) ### Pathophysiology **Key Point:** Acute chest syndrome is a vaso-occlusive crisis affecting the pulmonary vasculature, characterized by sickling of RBCs in lung microvasculature, leading to infarction, inflammation, and secondary infection. ### Diagnostic Features | Feature | ACS | Pneumonia | PE | |---------|-----|-----------|----| | **Fever** | Common (>50%) | Yes | No | | **New infiltrate** | Yes (consolidation) | Yes | No | | **Hemoglobin drop** | Yes (acute) | No | No | | **Reticulocyte count** | Elevated (>5%) | Normal | Normal | | **LDH elevation** | Marked (hemolysis) | Mild | Mild | | **Hypoxemia** | Yes | Yes | Yes | **High-Yield:** ACS is the **second leading cause of death** in sickle cell disease (after stroke) and occurs in ~50% of SCD patients at some point. ### Pathogenesis 1. Sickling of RBCs in pulmonary microvasculature 2. Vaso-occlusion → pulmonary infarction 3. Release of phosphatidylserine → tissue factor activation 4. Secondary infection (common organisms: *Streptococcus pneumoniae*, *Haemophilus influenzae*, atypical organisms like *Mycoplasma*) 5. Fat embolism from bone marrow necrosis (if concurrent bone infarction) ### Clinical Pearl **Key Point:** The acute drop in hemoglobin (7.2 from baseline 8.5) with elevated reticulocytes and LDH indicates **acute hemolysis superimposed on vaso-occlusion**—this is pathognomonic for ACS, not simple pneumonia. ### Management - Supplemental oxygen (target SpO₂ >92%) - Aggressive hydration (avoid over-hydration → ARDS) - Analgesia - Antibiotics (broad-spectrum covering atypical organisms) - Exchange transfusion if severe (Hb drop >1 g/dL, SpO₂ <90%, worsening infiltrates) - Incentive spirometry to prevent atelectasis **Warning:** Incentive spirometry is crucial—many ACS cases begin as atelectasis-triggered sickling events in hypoventilating patients. [cite:Robbins 10e Ch 14]