Westermark sign explained

Differential:	hypovolemia

In chest radiography, the Westermark sign is a sign that represents a focus of oligemia (hypovolemia) (leading to collapse of vessel) seen distal to a pulmonary embolism (PE).^[1] While the chest x-ray is normal in the majority of PE cases,^[2] the Westermark sign is seen in 2% of patients.^[3]

Essentially, this is a plain X-ray version of a filling defect as seen on computed tomography pulmonary arteriogram.

The sign results from a combination of:

1. the dilation of the pulmonary arteries proximal to the embolus and
2. the collapse of the distal vasculature creating the appearance of a sharp cut off on chest radiography.

Sensitivity and specificity

The Westermark sign, like Hampton's hump (a wedge shaped, pleural based consolidation associated with pulmonary infarction), has a low sensitivity (11%) and high specificity (92%) for the diagnosis of pulmonary embolism.^[4] Put more simply, the Westermark sign is seldom seen in pulmonary embolism. When visible on a chest X-ray, the Positive Predictive Value is only 33%. That is, 33% of the time that Westermark sign is seen on Chest XRay does a pulmonary embolism actually exist https://litfl.com/westermark-sign/.

Etymology

It is named after Nils Westermark, a Swedish radiologist.^[5]

References

1. Ray J . Westermark sign and suspected pulmonary embolism. . Can J Cardiol . 19 . 3 . 317; author reply 317 . 2003 . 12680403.
2. Introduction to Chest Radiography. http://www.med-ed.virginia.edu/courses/rad/cxr/index.html
3. Worsley D, Alavi A, Aronchick J, Chen J, Greenspan R, Ravin C . Chest radiographic findings in patients with acute pulmonary embolism: observations from the PIOPED Study. . Radiology . 189 . 1 . 133–6 . 1993 . 8372182 . 10.1148/radiology.189.1.8372182.
4. Gurney J. CT: Diagnosis of Pulmonary Embolism. chestx-ray.com. Available at: http://www.chestx-ray.com/Lectures/PulmEmbLecture/PulmEmbolus.pdf. Accessed on: November 13, 2006.
5. Book: Dorland's Illustrated Medical Dictionary. 2011-06-09. Elsevier Health Sciences. 978-1455709854. 32. 2080. en.