A Collection of Conditions Affecting the Lungs

Large Pulmonary Embolus With Unilateral Hyperlucent Lung

Authors:
Min Qiao, MD; Vishaal Gupta; Olivia Onwodi; and Rajat Mukherji, MD
Kingsbrook Jewish Medical Center, Brooklyn, New York

Citation:
Qiao M, Gupta V, Onwodi O, Mukherji R. Large pulmonary embolus with unilateral hyperlucent lung. Consultant. 2017;57(10):596-597.

A 91-year-old woman presented to the hospital with acute onset of dyspnea and pain and swelling of the left leg.

Venous duplex ultrasonography of the left leg confirmed the presence of deep-vein thrombosis. Computed tomography angiography (CTA) of the chest showed a large embolus in the right main pulmonary artery, almost completely occluding the lumen of the vessel (Figure 1). A few peripheral emboli were present in the left lung, as well. Perfusion to the entire right lung was markedly diminished, and there was a compensatory increase in the perfusion of the left lung (Figures 2 and 3). The radiographic findings were consistent with the Westermark sign.

Figure 1: Axial CTA of the chest showed a large embolus (arrowhead) in the right main pulmonary artery.

Figure 2: Axial CTA of the chest also showed severe hypovascularity of the right lung with a compensatory increase in the vascularity of the left lung (the Westermark sign).

Figure 3: Coronal CTA of the chest showed markedly reduced vascularity of the right lung and compensatory hypervascularity of the left lung.

The patient was hemodynamically stable and was treated successfully with enoxaparin and later switched to warfarin.

Discussion. The Westermark sign is a relatively rare radiographic finding that is seen in approximately 2% of patients with confirmed pulmonary embolism (PE).¹ The responsible embolus usually lodges in a main or lobar branch of the pulmonary artery. Chest radiographs or chest CT scans show a paucity of vascular markings (oligemia) in the lung parenchyma distal to the embolic obstruction. The affected area looks darker (hyperlucent) compared with the rest of the lung.

The radiographic findings of localized or unilateral hyperlucency may mimic localized bullous disease or unilateral emphysema (Swyer-James syndrome [SJS]). The latter is characterized by unilateral hyperlucency caused by postinfectious hypoplasia of the vasculature and parenchyma of 1 lung.² The process begins in childhood as bronchiolitis obliterans and manifests later as emphysematous change in the affected lung.

An acute exacerbation of SJS may mimic acute PE, with clinical manifestations such as acute-onset dyspnea with hypoxemia. In such patients, when chest radiographs reveal unilateral hyperlucency, an incorrect diagnosis of acute PE with the Westermark sign may be made.³ CTA of the chest would establish the correct diagnosis.

Apart from SJS, acquired unilateral emphysema may be the result of previous necrotizing pneumonia, wherein the lung parenchyma is disrupted by the infectious process.⁴

Unilateral hyperlucent lung is also seen in rare instances of an endobronchial lesion, such as a foreign body or tumor.⁵ In this circumstance, the lesion in the bronchus works in a check-valve mechanism, allowing air to enter during inspiration but retarding adequate deflation during expiration. The clue to this diagnosis is on chest radiographs. When the radiograph is taken during inspiration, both lungs are equally expanded. During expiration, the affected lung with the endobronchial lesion remains inflated and looks relatively hyperlucent compared with the opposite lung, which deflates and looks smaller.

In our patient, the embolus almost completely obstructed the right main pulmonary artery, causing much of the output from the right ventricle to be diverted from the right lung into the left lung. This resulted in a relatively dark lung field on the right and a highly congested lung on the left. Oligemia in the embolized lung is largely due to anatomic obstruction of blood flow by the embolus. However, another contributory factor is the physiologic effect of hypoxic vasoconstriction, which further reduces perfusion to the embolized lung.⁶ 

REFERENCES:

1. Worsley DF, Alavi A, Aronchick JM, Chen JT, Greenspan RH, Ravin CE. Chest radiographic findings in patients with acute pulmonary embolism: observations from the PIOPED Study. Radiology. 1993;189(1):133-136.
2. Mori J, Kaneda D, Fujiki A, Isoda K, Kotani T, Ushijima Y. Swyer-James syndrome in a 7-year-old female. Pediatr Rep. 2016;8(3):6643.
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5. Vimala LR, Sathya RKBS, Lionel AP, Kishore JS, Navamani K. Unilateral obstructive emphysema in infancy due to mediastinal bronchogenic cyst—diagnostic challenge and management. J Clin Diagn Res. 2015;9(5):TD03-TD05.
6. Burrowes KS, Clark AR, Wilsher ML, Milne DG, Tawhai MH. Hypoxic pulmonary vasoconstriction as a contributor to response in acute pulmonary embolism. Ann Biomed Eng. 2014;42(8):1631-1643.

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