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). While the chest x-ray is normal in the majority of PE cases, the Westermark sign is seen in 2% of patients.
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:
- the dilation of the pulmonary arteries proximal to the embolus and
- 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 [1].
Etymology
It is named after Nils Westermark, a Swedish radiologist.5
References
Ray J (2003). "Westermark sign and suspected pulmonary embolism". Can J Cardiol. 19 (3): 317, author reply 317. PMID 12680403. /wiki/PMID_(identifier) ↩
Introduction to Chest Radiography. http://www.med-ed.virginia.edu/courses/rad/cxr/index.html http://www.med-ed.virginia.edu/courses/rad/cxr/index.html ↩
Worsley D, Alavi A, Aronchick J, Chen J, Greenspan R, Ravin C (1993). "Chest radiographic findings in patients with acute pulmonary embolism: observations from the PIOPED Study". Radiology. 189 (1): 133–6. doi:10.1148/radiology.189.1.8372182. PMID 8372182. /wiki/Doi_(identifier) ↩
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. http://www.chestx-ray.com/Lectures/PulmEmbLecture/PulmEmbolus.pdf ↩
Dorland's Illustrated Medical Dictionary (32 ed.). Elsevier Health Sciences. 2011-06-09. p. 2080. ISBN 978-1455709854. 978-1455709854 ↩