Chest ImagingSarcoidosis
systemic disorder of unknown cause. Pathologically, the most characteristic feature of
sarcoidosis is the present of noncaseating
granulomas in a lymphatic or perilymphatic distribution. The granulomas are well formed, with histiocytes centrally, surrounded by a collar of lymphocytes and mononuclear cells. These may resolve spontaneously or progress to
fibrosis.
Sarcoidosis may involve almost any organ, but most morbidity and mortality is the result of
pulmonary disease.
Pulmonary manifestations are present in 90% of patients, 20-25% of whom have permanent functional impairment.
Approximately 60-70% of patients with sarcoidosis have characteristic radiological findings. These consist of symmetric bilateral hilar and paratracheal lymphadenopathy, with or without concomitant parenchymal abnormalities. In 25-30% of cases, however, the radiological findings are nonspecific or atypical, and in 5-10% of patients, the chest radiograph is normal.
Sarcoid granulomas, which are the hallmark of this disease, are distributed primarily along the lymphatics in the peribronchovascular interstitial space (both in the parahilar regions and lobular core) and, to a lesser extent, in the subpleural interstitial space and along the interlobular septa.
This characteristic "perilymphatic" distribution of sarcoid granulomas is difficult to recognize on plain radiographs, but is clearly seen on HRCT. The most characteristic HRCT abnormality in patients with sarcoidosis consists of small nodules (1) in the peribronchovascular regions, adjacent to the parahilar vessels and bronchi or in a centrilobular location, (2) adjacent to the major fissures, and (3) within interlobular septa (Fig.1). Nodules visible on HRCT can appear as small as a few mm in diameter; they tend to be sharply defined despite their small size. In most cases, these nodules represent coalescent groups of microscopic granulomas, although nodules visible on HRCT can also represent nodular areas of fibrosis. In up to 50% of patients, nodules may be scanty or focal, localized to small areas in one or both lungs. An upper lobe predominance is common.
Confluence of granulomas may result in large opacities with ill-defined contours, or areas of frank consolidation. Patients with sarcoidosis sometimes show patchy areas of ground glass opacity on HRCT, which may be superimposed on a background of interstitial nodules or fibrosis. In patients with sarcoidosis, who have been followed using HRCT, areas of nodularity, consolidation and ground-glass opacity tend to decrease over time. Although fibrosis need not occur with healing of granulomatous lesions, findings of fibrosis tend to become more prominent over time. As fibrosis develops, irregular reticular opacities, including irregular septal thickening, often become a prominent feature. The most common early HRCT finding of fibrosis with lung distortion is posterior displacement of the main and upper lobe bronchi. Progressive fibrosis also leads to abnormal central conglomeration of parahilar bronchi and vessels, associated with masses of fibrous tissue, typically most marked in the upper lobes.
As on chest radiographs, enlargement of hilar and/or mediastinal lymph nodes is commonly visible on CT. Lymph node enlargement is typically symmetrical.
RW