Palla and Westermark Signs

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Appearance: The Palla sign refers to enlargement of the right descending pulmonary artery proximal to a cutoff of the pulmonary artery due to acute pulmonary embolism,1 whereas the Westermark sign refers to areas of regional oligemia.2
Explanation: The Palla sign is demonstrated as enlargement of the right descending pulmonary artery (Fig. 1A).1 On the magnified chest radiograph (Fig. 1B) and the coronal computed tomographic (CT) image (Fig. 1C), the enlargement proximal to the cutoff of the pulmonary artery is more conspicuous. Its diameter is measured at the superior venous angle and the subsequent distal points at 10, 20, and 30 mm.1 The cutoff for the Palla sign at the superior venous angle is 16 mm.2 The chest radiograph demonstrates enlargement of the right descending pulmonary artery at the superior venous angle measuring 2.27 cm in diameter (Fig. 1B). The CT image shows bilateral extensive filling defects in the main and lobar pulmonary arteries (Fig. 1C). It also shows dilated superior vena cava and azygos veins, which indicate elevated central venous pressure (Table). The chest radiograph also shows focal oligemia on the right side of the chest, indicative of Westermark sign (Fig. 1A).
Discussion: In 1983, Palla et al1 noticed that in approximately one-fourth of confirmed pulmonary embolism cases studied the right descending pulmonary artery was enlarged and assumed a “sausage” appearance on chest radiographs. Although rare, the Palla sign can be seen in combination with other signs such as the Hampton hump, the Westermark sign, and the Fleischner sign, and together they can aid in the diagnosis of pulmonary embolism.3–6 In 1934, Neil Westermark was the first to describe regional oligemia distal to occlusive pulmonary embolus seen as region of increased translucency on chest radiograph and which now the sign bears his name.4–6 The Polo mint sign and railway track sign are partial filling defects seen on axial and longitudinal imaging of vessels on CT scans, respectively, and may also support the diagnosis.5,7 The Palla sign with Westermark sign suggests an embolic occlusion of a lobar or segmental pulmonary artery or widespread occlusion in small arteries.

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