Westermark Sign
The Westermark sign is a radiographic finding on chest X-ray characterized by focal or regional oligemia (decreased pulmonary vascular markings) distal to a pulmonary embolus, indicating reduced blood flow to that area of the lung.
Definition and Appearance
The Westermark sign represents an area of increased lucency (darker appearance) on chest radiography due to diminished pulmonary vascularity downstream from an obstructing pulmonary embolus 1. This occurs because the thrombus blocks blood flow, resulting in fewer visible vascular markings in the affected lung region compared to normal areas 2, 3.
Clinical Significance
While the Westermark sign is specific for pulmonary embolism (PE), it is not sensitive 1. This means:
- When present, it strongly suggests PE and should prompt immediate further investigation with CT pulmonary angiography (CTPA)
- When absent, it does not exclude PE - the sign appears in only a minority of PE cases
In the landmark PIOPED study of 1,063 patients with suspected PE, oligemia (Westermark sign) was found to be a poor predictor of PE due to its low sensitivity, despite its specificity 1. The chest radiograph was normal in only 12% of patients with confirmed PE 1.
Diagnostic Context
The Westermark sign is one of several classic but insensitive chest X-ray findings in PE, alongside:
- Fleischner sign (prominent central pulmonary artery)
- Hampton hump (pleural-based wedge-shaped opacity)
The primary value of chest radiography in suspected PE is to exclude alternative diagnoses that mimic PE clinically, not to diagnose PE itself 1. When the Westermark sign is identified, it serves as a red flag requiring definitive imaging with CTPA 2.
Special Considerations
The "wet Westermark" sign describes accentuation of regional oligemia when mild heart failure is present - the interstitial markings in normally perfused areas become more prominent, creating greater contrast with the oligemic region 4. This can paradoxically make the sign more apparent in patients with volume overload.
In severe cases, a complete hemithorax Westermark sign may occur with near-complete pulmonary artery trunk occlusion 3.