Westermark sign

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] This means that the sign is only present in 11% of pulmonary emboli, but that when the sign is present, it correctly indicates PE 92% of the time. Put more simply, it is not often seen in PE, but if it is seen, it is very likely PE.

Name

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

References

1. ↑ Ray J (2003). "Westermark sign and suspected pulmonary embolism.". Can J Cardiol. 19 (3): 317; author reply 317. PMID 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 (1993). "Chest radiographic findings in patients with acute pulmonary embolism: observations from the PIOPED Study.". Radiology. 189 (1): 133–6. PMID 8372182. doi: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. ↑ Dorland's Illustrated Medical Dictionary (32 ed.). Elsevier Health Sciences. 2011-06-09. p. 2080. ISBN 1455709859.