Westermark Sign
Westermark sign is a finding that can indicate a life-threatening pulmonary embolism (PE) on a chest X-ray. While CT pulmonary angiography (CTPA) is the gold standard for diagnosing PE, recognizing the Westermark sign on an initial chest X-ray can prompt urgent testing. This article explains how Westermark sign appears on imaging, its significance, and why radiologists pay close attention to it.
What Is the Westermark Sign?
Westermark sign refers to a localized region of decreased lung markings on a chest X-ray, caused by reduced blood flow to the lungs. It is named after Nils Westermark, a Swedish radiologist who first described it.
This sign occurs due to:
Blockage of a pulmonary artery by a clot (pulmonary embolism)
Decreased blood flow beyond the obstruction, leading to fewer visible lung vessels
Compensatory dilation of the central pulmonary arteries, which can sometimes be seen as an enlarged pulmonary vessel near the blockage
While not always present in pulmonary embolism cases, its detection is a red flag to investigate further with more sensitive imaging techniques like pulmonary CTA.
Chest X-Ray Appearance of Westermark Sign
A standard chest X-ray may not always show obvious signs of pulmonary embolism, but when the Westermark sign is present, it appears as:
A sharply defined area in the lung with fewer blood vessels than expected
A wedge-shaped region of increased radiolucency (darker area on the X-ray)
Enlargement of a nearby pulmonary artery due to blood pooling behind the obstruction
Since chest X-rays have limited sensitivity for PE, the absence of Westermark sign does not rule out the condition. However, its presence should raise suspicion for a clot in the pulmonary arteries.
CT Pulmonary Angiography: The Best Test for PE
When the Westermark sign is suspected on an X-ray, doctors typically order a CT pulmonary angiography (CTPA) to confirm or rule out a pulmonary embolism. This imaging test uses contrast dye to highlight blood vessels in the lungs, providing a detailed view of the pulmonary arteries.
On CTPA, pulmonary embolism appears as:
A filling defect in the contrast-filled pulmonary arteries, indicating a blood clot
Areas of decreased blood flow that correspond with the X-ray findings
Potential lung infarction (lung tissue damage) in severe cases
CTPA has high sensitivity and specificity, making it the preferred test for diagnosing pulmonary embolism. The Westermark sign, when present, helps prioritize patients for urgent CTPA scanning.
Other Imaging Signs of Pulmonary Embolism
Besides the Westermark sign, radiologists look for other imaging clues of pulmonary embolism, including:
Hampton’s Hump: A wedge-shaped opacity near the lung periphery, representing lung infarction.
Fleischner Sign: Enlargement of a pulmonary artery due to clot obstruction.
Palla Sign: Enlargement of the right descending pulmonary artery.
These findings, combined with clinical symptoms like sudden shortness of breath and chest pain, guide doctors in testing for pulmonary embolism.
Limitations of the Westermark Sign
While the Westermark sign is a classic radiologic finding, it is not always reliable for diagnosing PE. Some limitations include:
It is only visible in a small percentage of PE cases.
Other lung conditions, like chronic obstructive pulmonary disease (COPD) or previous lung surgery, can mimic its appearance.
A normal chest X-ray does not exclude PE, as many cases show no abnormalities.
Because of these factors, additional imaging especially with CT pulmonary angiography is essential for confirming a diagnosis.
Importance of Early Detection
Pulmonary embolism is a medical emergency that requires rapid diagnosis and treatment. Left untreated, it can lead to:
Severe oxygen deprivation
Heart strain, particularly on the right ventricle
Risk of sudden death in large or multiple emboli
By identifying the Westermark sign early, radiologists help guide prompt medical intervention, including anticoagulant therapy, thrombolytics, or even surgical procedures in severe cases.
Conclusion
The Westermark sign is a significant but uncommon radiologic clue that suggests pulmonary embolism. While chest X-rays alone are not definitive for diagnosing PE, this sign can prompt further testing with CT pulmonary angiography. Recognizing such findings is important in emergency settings, as early detection and treatment of PE can prevent serious complications.
References
1.https://www.ahajournals.org/doi/10.1161/circulationaha.106.665422
2.https://pmc.ncbi.nlm.nih.gov/articles/PMC8143826/
3.https://radiopaedia.org/articles/westermark-sign-1?case_id=pulmonary-embolism-westermark-and-fleischner-signs