WESTERMARK SIGN: CAUSES, SIGNS, FALSE POSITIVES - MEDICINE - 2024

The Westermark sign is a nearly pathognomonic chest radiograph finding for pulmonary embolism without pulmonary infarction. When it appears, it does so early in the stage called acute pulmonary thromboembolism, thus allowing treatment to be started before the lung suffers the heart attack, which improves the prognosis of the clinical picture.

It was first described in 1938 by Dr. Nils Westermark of St. Göran Hospital in Stockholm, Sweden. The sign is still valid to this day since its specificity is very high; however, its absence does not rule out the presence of thromboembolism.

Although it is a very useful radiological finding because it is infrequent, within the framework of the development of new diagnostic technologies the current trend is to rely more on Computed Tomography of the Chest (CT), since it provides more information not only about the state of the lung but on the rest of the thoracic structures.

Radiological findings

The Westermark sign is characterized by a radiolucent area (of lower density than the surrounding tissue), triangular in shape with its apex directed towards the hilum of the lung.

The extension of the area with the sign is variable, and can be very small when the thromboembolism affects only one lung segment, or very large when it affects an entire lobe. It is even possible that it occupies the entire lung in cases of involvement of the main trunk of the pulmonary artery.

Another characteristic of the Westermark sign is the decrease in the vascular network of the lung parenchyma, that is, the network of small pulmonary capillaries is less visible in the radiolucent area.

Causes

The presence of the Westermark sign is due to hypoperfusion of the lung tissue in the area of ​​the thromboembolism.

As the normal amount of blood does not reach the lung parenchyma (due to the infarction), the radiographic density of the tissue decreases and, therefore, it appears blacker on the radiograph (radiolucent) in the area supplied by the affected vessel.

In this sense, since the pulmonary arteries tend to divide into even branches (one artery has two branches, each of which gives two more branches, and so on) it is easy to understand the triangular shape of the radiolucent area.

The vertex corresponds to the point where the compromised artery was obstructed (either main, lobar or segmental) and the base corresponds to the last branches of the same.

Associated signs

When pulmonary embolism occurs in the main pulmonary artery, Westermark's sign is usually accompanied by Fleischner's sign.

The Fleischner sign consists of the enlargement of the proximal pulmonary artery associated with amputation of the same at the point where the thrombus generates the obstruction.

The combination of both signs is practically unequivocal, so the doctor is authorized to start treatment for pulmonary thromboembolism immediately.

Sensitivity and specificity

The Westermark sign appears in only 2% to 6% of cases of pulmonary embolism without infarction; that is, it does not appear frequently, but when it does, it is almost certainly due to the presence of pulmonary thromboembolism.

In the PIOPED study - aimed at determining the diagnostic value of the different radiological findings when comparing them with the diagnostic gold standard (lung scintigraphy) - it was determined that the Westermark sign is very insensitive, since it appears in less than 10% of the cases.

However, when the Westermark sign appears, the diagnostic certainty is close to 90%, which makes it a very specific sign that authorizes starting treatment when it is detected.

Despite the above, the PIOPED study concludes that none of the findings on the chest radiograph (including the Westermark sign) is sufficient for the accurate diagnosis of pulmonary thromboembolism (PE).

In this sense, the identification of any of the signs allows suspecting the diagnosis, although its absence does not exclude it.

Therefore, it is recommended to perform a lung scan (study of choice), or a chest CT or pulmonary angiography (depending on the availability of resources and clinical conditions of the patient), as the diagnostic study of choice in all cases in which it is suspected of TEP.

False positives

While it is true that this is a very specific finding, there is always the possibility of false positive findings; that is, conditions in which the Westermark sign appears (or appears to appear) without the presence of pulmonary thromboembolism.

This is due to certain technical, anatomical or physiological conditions that can generate images similar to the Westermark sign; These conditions include the following:

Technical conditions

- Very penetrated X-ray.

- Poor alignment during X-ray exposure (rotated chest).

- Low resolution radiology equipment.

- X-ray taken with portable equipment (usually the technical conditions for these X-rays are not ideal).

Constitutional factors

In some cases the anatomical and constitutional characteristics of the patient can generate a false positive finding; this can be seen frequently in:

- Patients with prominent breasts that generate a relative increase in lung density in the breast area, which creates the illusion of a radiolucent area in the periphery.

- Asymmetry of the soft tissues of the thorax (as in the cases of patients undergoing unilateral radical mastectomy or agenesis of the pectoralis major muscle), generating an optical effect that can be confused with the Westermark sign.

Pathological conditions

Some medical conditions can present findings very similar to the Westermark sign, thus creating a degree of confusion that could complicate the diagnosis. Such conditions include:

- Focused air trapping (obstruction of a secondary bronchus due to infection or tumor).

- Compensatory hyperinflation (due to contralateral lung disease or surgery).

- Emphysema with the presence of bulls. Depending on the shape and position of a bull, it could be confused with the image of the Westermark sign.

- Congenital heart conditions associated with pulmonary hypoperfusion, as in the case of tetralogy of Fallot, tricuspid atresia, and Ebstein's malformation.

In all these cases, correlation with the clinical findings is essential in order to avoid misdiagnosis.

In this sense, in any patient without risk factors for pulmonary thromboembolism, whose symptoms do not correspond to this entity, the possibility of a false positive should be considered if the chest X-ray shows findings that resemble the Westermark sign.

In any case, chest computed tomography will be very useful to establish both the initial and differential diagnoses, although the clinical finding during the physical examination should always be the cornerstone of the diagnostic process.

References

1. Worsley, DF, Alavi, A., Aronchick, JM, Chen, JT, Greenspan, RH, & Ravin, CE (1993). Chest radiographic findings in patients with acute pulmonary embolism: observations from the PIOPED Study. Radiology, 189 (1), 133-136.
2. Abbas, A., St Joseph, EV, Mansour, OM, & Peebles, CR (2014). Radiographic features of pulmonary embolism: Westermark and Palla signs. Postgraduate medical journal, postgradmedj-2013.
3. Bedard, CK, & Bone, RC (1977). Westermark's sign in the diagnosis of pulmonary emboli in patients with the adult respiratory distress syndrome. Critical care medicine, 5 (3), 137-140.
4. Batallés, SM (2007). Westermark sign. Argentine Journal of Radiology, 71 (1), 93-94.
5. Komissarova, M., Chong, S., Frey, K., & Sundaram, B. (2013). Imaging of acute pulmonary embolism. Emergency radiology, 20 (2), 89-101.