Year : 2001 | Volume
: 11 | Issue : 4 | Page : 191--198
Thoracic sarcoidosis : The spectrum of roentgenologic appearances
D Deepak, A Shah
Department of Respiratory Medicine, Vallabhbhai Patel Chest Institute, University of Delhi, Delhi-110007, India
Dept of Respiratory Medicine, Vallabhbhai Patel Chest Institute, University of Delhi, P.O Box 2101, Delhi 110 007
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Deepak D, Shah A. Thoracic sarcoidosis : The spectrum of roentgenologic appearances.Indian J Radiol Imaging 2001;11:191-198
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Deepak D, Shah A. Thoracic sarcoidosis : The spectrum of roentgenologic appearances. Indian J Radiol Imaging [serial online] 2001 [cited 2020 Jul 5 ];11:191-198
Available from: http://www.ijri.org/text.asp?2001/11/4/191/28400
Sarcoidosis, a multisystem disorder of unknown etiology, is rarely reported in India , . Although, often mistaken for tuberculosis in our country, it is now beginning to emerge from obscurity [Figure1]  . Histolologic manifestations have been documented in many organs but it predominantly affects the lungs
In the West, up to 50% of patients with Sarcoidosis are asymptomatic. These patients are diagnosed incidentally when chest radiographs are performed for other reasons and abnormalities are detected  . Radiographic findings in thoracic sarcoidosis are diverse and for descriptive purposes have been categorised into five stages or types [Table1]  . Not only do imaging techniques help to establish the diagnosis, but also help to monitor the progress of the disease. The spectrum of roentgenologic findings seen in thoracic sarcoidosis is presented.
Plain Roentgenologic Appearances The chest radiograph is abnormal in 90-95% of patients with sarcoidosis  . The most common abnormalities encountered are lymph node enlargement and parenchymal lung involvement [Figure 2]  .
Radiological evidence of hilar or mediastinal lymphadenopathy is seen in two-thirds of patients at the time of diagnosis  . Nodal enlargement is commonly localized to the hilar, tracheobronchial and paratracheal groups. It is usually symmetrical with bilateral hilar lymph node enlargement being the most characteristic finding [Figure 3]  . This occurs in 75-85% patients and is the most common pattern of lymph node involvement , , . The enlargement when striking is termed as 'potato nodes' [Figure 4] A & B ,  . However, asymmetrical enlargement may also be seen [Figure 5]. Unilateral hilar lymph node enlargement has been seen in only 3-5% of patients in the West , but in India, it has been reported in up to 43% of patients  . Paratracheal adenopathy usually occurs with hilar adenopathy and has been reported in up to 71% of patients  . The combination of right paratracheal and bilateral hilar lymph node enlargement is often considered a reliable sign of sarcoidosis and has been called "1-2-3 sign"  or "pawnbroker's sign" [Figure 6]  . Left paratracheal adenopathy is seen less commonly on a plain radiograph but is usually visible on CT of the thorax. This fallacy occurs because the left paratracheal lymph nodes are located more posteriorly, are smaller in size and fewer in number as compared to the right side. They are also obscured by the superimposed aorta and brachiocephalic vessels on the plain radiograph ,  . The contour of the outer borders of enlarged hila, particularly on the right side, is usually lobulated [Figure 5]. Enlargement of right paratracheal nodes causes a loss of visibility of the right paratracheal stripe and creates a smooth or slightly lobulated contour. The nodes tend to maintain shape and remain discrete  . Calcification [Figure 7]B of hilar lymph nodes occurs in 5-20% of patients and appears to be related to the length of observation, probably indicative of burnt out hyalinized lymph node , ,  . A pattern of peripheral calcification, similar to that observed in silicosis called 'eggshell calcification' is sometimes seen, but is rare  . Nodal calcification is usually a late manifestation and has been reported to be associated with advanced disease and corticosteroid therapy ,  . Statistics show that 70-80% of patients with hilar lymph node enlargement, and with or without visible pulmonary involvement, regress and eventually show complete radiograph resolution without residua ,  . The regressed nodes can sometimes show enlargement again , . It has been suggested that most patients in India show radiological progression, which may occur in seven to nine years or even more ,, . However, hilar and paratracheal node enlargement can persist unchanged for 15 or more years as reported in the Western literature ,  . Diminution in the size of the lymph nodes or at least cessation of growth [Figure 7]A,B has been commonly observed with the onset of parenchymal disease.
Pulmonary parenchymal infiltrates are present in 25-50% patients, with or without hilar lymphadenopathy  . In India, one study reported that most of the radiographic abnormalities belonged to Stages I and II  but another study found that Stages II and III were more common  . Parenchymal infiltrates are often symmetrical bilaterally and preferentially involve the central, rather than peripheral regions. The upper lobes, particularly the posterior and apical segments are affected, especially in the fibrotic stage [Figure 8]  . Unilateral disease is a rarity [Figure 8]  .
Parenchymal nodules are the most frequent radiological observation [Figure 9],[Figure 10]  . These are caused by accumulation of many granulomas, which are a reaction to the initial lesion of alveolitis  . The nodular lesions may be large and dense. These may simulate a metastatic neoplasm  . Rarely, a miliary pattern (1%) may also be seen [Figure 9]a,b. Nodules commonly occur in combination with reticular pattern though infiltration may vary from purely reticular to purely nodular  . Reticulations are observed in 46% of patients and may form a very fine or a very coarse network [Figure 11]  . Linear infiltrates are common and may extend fanwise from the hilum  . Mottled opacities have been found to have a better prognosis than streaky opacities in Indian studies ,  .
Although sarcoidosis is chiefly an interstitial disease, an alveolar filling or acinar pattern is also seen infrequently [Figure 1],[Figure 12]. This pattern consists of indistinctly defined opacities measuring up to 6 or 7 mm in diameter and these may be discrete or coalescent. Sometimes confluence of acinar shadows can produce large areas of segmental consolidation [Figure 2] or scattered, hazy areas of consolidation with irregular borders [Figure 13]a,b. This may be seen as peripheral non-segmental airspace consolidation. The consolidation may also show an air-bronchogram  . In one study, this pattern was seen in as high as 20% of the patients and was frequently associated with a reticulonodular pattern  . With the progress of the disease, distortion of pulmonary architecture may lead to bronchiectasis, cysts or bullae formation [Figure 13]c,d. All other causes of cavitation should be ruled out before accepting sarcoidosis as an underlying condition  . Cavitation typically occurs in upper lobes and may contain a freely mobile fungus ball  . New pleural thickening [Figure 11] adjacent to a known cystic space usually precedes the appearance of a fungus ball or 'air-crescent sign'  . Atelectasis may also occur which is caused either due to extrinsic compression by enlarged lymph nodes or by endobronchial inflammation but may also be due to a combination of both the processes. Atelectasis and cavitation are the least common parenchymal findings in sarcoidosis  .
Advanced bullous emphysema  , honeycombed lung with volume loss, signs of pulmonary hypertension and cor pulmonale are the evidences of advanced disease [Figure 7]b. Sixty percent of pulmonary opacities show spontaneous clearing. Radiological clearing occurs within two years in 80 % of these patients with the average time being 11 months [Figure 13]a,c . When pulmonary changes have been present for more than two years, resolution appears to be an exception  . Occurrence of hilar lymphadenopathy precedes parenchymal disease but it has not been reported to develop subsequent to parenchymal involvement.
Pleural involvement, in sarcoidosis, is infrequent and can be seen only in 1-12% of patients  . This may manifest as pleural effusion, pleural thickening or spontaneous pneumothorax. The presence of pleural effusion was previously thought to exclude sarcoidosis, but is now reported in 0.7-7% of the cases  . In India, it has been seen in 6.6% of patients  . It is seen with equal frequency on either side and is bilateral in a third of the cases  . It is usually mild to moderate and tends to clear in 4-8 weeks  . Pleural thickening and fibrothorax are often seen at thoracotomy and autopsy but can also be seen on plain chest radiograph [Figure 11],[Figure 13]c. Spontaneous pneumothorax is rare with an estimated incidence of 1-2% [Figure 14] , .
Computed Tomography Appearances
HRCT is superior to chest radiography for assessing the presence and extent of thoracic abnormalities in patients with sarcoidosis  . CT can reveal enlargement of lymph nodes in locations not visible on conventional radiograph. Paratracheal involvement is mostly bilateral on CT unlike plain radiograph, which more commonly shows a unilateral lymphadenopathy on the right side. Paratracheal node enlargement seldom occurs without concomitant enlargement of hilar nodes  . Enlargement of lymph nodes in the aortopulmonary window is seen in 33-75% patients and in approximately 21% patients in the subcarinal region  . Enlargement of other lymph node groups like anterior mediastinal, posterior mediastinal, axillary, internal mammary and infradiaphragmatic may also be found on CT. However, these nodes are also usually accompanied by hilar lymphadenopathy ,  . In the neck, the posterior triangle nodes are more commonly affected than nodes in anterior triangle  .
The most common CT manifestation of pulmonary sarcoidosis are nodular opacities [Figure 9]b,[Figure 10]  . The nodules vary in size from 2 mm to 1 cm and have irregular margins. Less commonly, they may be larger and may have cavities. Nodules tend to be related to pulmonary vessels, interlobular septa and pleura leading to nodular thickening of interlobular septa, beaded appearance of bronchovascular bundles and fissural nodularity. A combination of beaded appearance of bronchovascular bundles, perihilar concentration of abnormalities and associated lobular distortion is typical of sarcoidosis [Figure 15]a,b  .
Involvement of the interstitium is typically axial in distribution  . It may appear as ground glass opacities [Figure 9]c,[Figure 16]a, which are now thought to be due to the presence of numerous sarcoid granulomas below the resolution of HRCT rather than active alveolitis  . The ground glass opacities may coexist with honeycombing [Figure 16]a,b.
Large alveolar consolidation with air bronchograms may be seen on CT [Figure 13]b. Linear opacities are less common and less profuse than nodules. These may appear as septal lines or less commonly as polygonal structures. Long irregular linear opacities are situated mainly along bronchovascular bundles, and represent early manifestations of fibrosis  . HRCT is superior to conventional CT in distinguishing alveolitis from fibrosis  . Changes suggesting fibrosis on HRCT are posterior displacement of main or upper lobe bronchus, distortion of interlobar fissures and crowding of central bronchovascular structures  . Conglomerate fibrosis may give rise to large parahilar masses. Traction bronchiectasis and bronchiolectasis also reflect fibrosis [Figure 13]D  . Honeycombing cysts or bullae formation and air trapping may sometimes be seen [Figure 16]b,[Figure 15]b, ,  . Air trapping occurs at the level of secondary pulmonary lobules. This is due to narrowing of the small airways caused by deposition of the granulomas in the peribronchial locations.
In conclusion, although not pathognomonic, many radiological features are highly suggestive of sarcoidosis, especially when they occur concurrently. The remarkable radiological similarity of sarcoidosis with pulmonary tuberculosis has important clinical implications in our country as these patients are often prescribed antituberculous therapy repeatedly while lung damage continues to progress.
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