On routine chest radiography, a 58-year-old male with chronic obstructive pulmonary disease presents with acute dyspnea. CXR shows a homogeneous opacity in the left upper lobe with loss of volume, ipsilateral mediastinal shift, and absence of air bronchograms. What is the most common cause of this radiological pattern?

Explanation

## Collapse (Atelectasis): Recognition and Common Etiologies **Key Point:** The combination of volume loss, mediastinal shift, and **absence of air bronchograms** is diagnostic of collapse/atelectasis, not consolidation. In a COPD patient with acute onset, mucus plugging is the most common reversible cause. ### Radiological Features Distinguishing Collapse from Consolidation | Feature | Collapse | Consolidation | |---------|---|---| | **Volume loss** | Present (lobe smaller) | Absent | | **Mediastinal shift** | Toward collapsed lobe | No shift | | **Air bronchograms** | Absent | Present | | **Silhouette sign** | Often present | May be present | | **Mechanism** | Airway obstruction or resorption | Alveolar filling | **High-Yield:** Mucus plugging is the most common acute cause of lobar collapse in hospitalized and COPD patients. It is reversible with aggressive chest physiotherapy, suctioning, and bronchodilators. ### Causes of Atelectasis (Collapse) ```mermaid flowchart TD A[Atelectasis/Collapse]:::outcome --> B{Mechanism?}:::decision B -->|Obstruction| C[Airway obstruction]:::action B -->|Resorption| D[Pleural pressure effect]:::action C --> E[Mucus plug - MOST COMMON ACUTE]:::action C --> F[Tumor/foreign body]:::action D --> G[Pneumothorax]:::action D --> H[Pleural effusion]:::action E --> I[Reversible with therapy]:::outcome F --> J[Requires bronchoscopy]:::outcome ``` **Mnemonic for atelectasis causes:** **COLLAPSE** - **C**ircumstantial (postoperative, immobility) - **O**bstruction (mucus, tumor, foreign body) ← Most common acute - **L**oss of surfactant (ARDS, atelectasis of prematurity) - **L**ateral pressure (effusion, pneumothorax) - **A**dhesions (pleural thickening) - **P**neumonia (resorption atelectasis) - **S**urgery (postoperative) - **E**xudation (post-radiation) **Clinical Pearl:** In COPD patients, acute lobar collapse with mucus plugging is a medical emergency. The absence of air bronchograms helps differentiate it from pneumonia, which would show air bronchograms and preserved volume. **Tip:** Look for the **silhouette sign** in collapse—loss of normal cardiac/mediastinal borders due to adjacent consolidation or collapse. This is more common in collapse than in simple consolidation.