The Westermark sign is a radiographic finding associated with acute pulmonary embolism (PE). It appears on a standard posteroanterior (PA) chest radiograph as a focal area of decreased vascular markings (regional oligemia) distal to a pulmonary artery obstruction. The sign reflects diminished perfusion of the affected lung segment or lobe due to the embolic blockage of the pulmonary arterial tree.
Clinical Context
- The Westermark sign is one of several imaging indicators used to raise suspicion for PE, alongside other signs such as the Fleischner sign (enlarged pulmonary artery) and the Hampton hump (wedge‑shaped peripheral opacity).
- While its presence is highly specific for PE, the sign is relatively uncommon, being identified in only 2–4 % of confirmed PE cases on routine chest radiographs. Consequently, a normal chest X‑ray does not exclude the diagnosis.
Pathophysiology
- An embolus lodged in a segmental or lobar pulmonary artery obstructs blood flow, leading to reduced arterial pressure downstream. The resultant hypoperfusion diminishes the caliber of distal pulmonary vessels, which manifests as a paucity of vascular markings on the radiograph.
Diagnostic Utility
- Sensitivity: Low (approximately 2–4 %).
- Specificity: High when present, though false‑positive interpretations can occur due to technical factors or other causes of regional oligemia (e.g., lung hypoplasia, emphysema).
- The sign is primarily a supportive finding; definitive diagnosis of PE relies on computed tomography pulmonary angiography (CTPA), ventilation‑perfusion (V/Q) scanning, or pulmonary angiography.
Historical Note
- The sign is named after Swedish radiologist Nils Westermark, who described the phenomenon in the mid‑20th century while studying chest radiographs of patients with pulmonary embolism.
Limitations
- The Westermark sign is often subtle and may be missed without careful inspection. Variability in radiographic technique, patient positioning, and underlying lung pathology can affect its visibility.
See Also
- Pulmonary embolism
- Chest radiography
- Fleischner sign
- Hampton hump
References
- Stein PD, et al. "Diagnostic accuracy of chest radiography for pulmonary embolism." JAMA. 2006.
- Westermark N. "The pulmonary embolus: a radiologic and pathologic study." Acta Radiol. 1948.