Westermark sign


In chest radiography, the Westermark sign is a sign that represents a focus of oligemia seen distal to a pulmonary embolism. 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:
  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, has a low sensitivity and high specificity for the diagnosis of pulmonary embolism. 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 .

Etymology

It is named after Nils Westermark, a Swedish radiologist.