Sarcoidosis is a multisystem granulomatous disorder of unknown etiology characterized by the presence of noncaseating granulomas in several affected organs. It is typically a disease of young adults, with a peak incidence in the third decade. The diagnosis is suggested by a combination of consistent clinical and radiological findings along with the presence of some nonspecific features such as depressed delayed-type hypersensitivity, abnormal immunoglobulins, hypercalcaemia, and elevated serum angiotensin-converting enzyme. It can be confirmed by biopsy evidence of widespread noncaseating granulomas. Pulmonary involvement accounts for most of the morbidity and mortality associated with sarcoidosis. The disease has a worse prognosis in blacks than whites, with a greater tendency for chronicity and multiorgan involvement.
About 90% of patients will have an abnormal chest radiograph at some stage, showing lymphadenopathy, parenchymal opacities, or both. Other findings are uncommon and seldom occur in isolation. Sarcoidosis is traditionally staged according to its appearance on the chest radiograph: stage I, lymphadenopathy; stage II, lymphadenopathy with parenchymal opacity; stage III, parenchymal opacity alone. Low stages at presentation carry a better prognosis than high stages.
Lymphadenopathy is evident on the chest radiograph in about 70–80% of patients at some time during the course of the condition. Sarcoidosis is characterized by bilateral, symmetrical hilar and paratracheal lymphadenopathy. In 90% of patients with lymphadenopathy, nodal enlargement is maximal on the first radiograph and usually disappears within 6–12 months. In about 5%, however, large nodes persist more or less indefinitely, and these can be a source of confusion when found later with other incidental chest pathology. Recurrence of lymphadenopathy is virtually unrecorded. The lymph nodes may calcify, sometimes in a characteristic eggshell fashion. This latter feature is shared by only a few conditions.
About 40% of patients presenting with nodal enlargement will develop parenchymal opacities, usually within a year, and of these, about one third will go on to have persistent (fibrotic) shadowing. Nodal enlargement does not develop after parenchymal opacities have appeared.
Parenchymal changes probably occur histologically in all patients but are only detected on the chest radiograph in 50–70% of cases. They may be classified as reversible, nonreversible (fibrotic), and mixed. Changes may resolve completely (two thirds of cases) or in part, leaving irreversible fibrosis. There are three major patterns of reversible opacity: irregular or rounded 2-4mm nodules, patchy airspace consolidation, or rarely 1-4cm irregular nodules. Characteristically they appear as the nodal enlargement is subsiding, in contrast to lymphoma in which these abnormalities tend to progress in unison.
Parenchymal opacities are well demonstrated on high-resolution CT. High-resolution CT is very sensitive and the findings are frequently highly specific for sarcoidosis. The principal abnormality is 1–5 mm nodules distributed in a perilymphatic fashion, predominantly along the bronchovascular bundles and subpleurally and, to a lesser extent, along interlobular septa.
Other manifestations of sarcoidosis include pleural thickening and effusions. These are unusual though and do not occur in isolation.
Cor pulmonale, mycetoma formation and pneumothorax are all recognized complications of this fibrotic stage.
Reference(s): Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging, 4th Ed., 2001 Churchill Livingstone, Inc.
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