Group 1: Those cases in which a striking proportion of nodules demonstrate pleural or perifissural involvement characterize nodules as predominantly perilymphatic or lymphohematogenous in origin, constituting a separate arm of the algorithm (Table 1). The explanation for this pattern lies in the greater density of lymphatic channels seen in the interlobular septa and subpleural regions, including along the fissures. Once nodules are characterized as predominantly perilymphatic or lymphohematogenous in origin, further assessment requires determining whether or not nodules are distributed diffusely or are patchy or clustered, with particular attention paid to the presence or absence of the extent of axial interstitial involvement. It is recalled that the axial interstitium envelops the main pulmonary vessels and bronchi extending from the hilum outward toward the lung periphery.
If nodules prove to be clustered in a predominantly subpleural/axial distribution, they are deemed to be perilymphatic in distribution (Fig 2). In this category, the main disease to be considered is sarcoidosis (Fig 6, 7). This diagnosis is further suggested by nodules that are typically ill-defined, frequently measuring only a few millimeters in size. Clusters of these nodules often have a “grainy” appearance and when sufficiently profuse may result in an appearance of poorly defined nodules or masses on corresponding chest radiographs (socalled alveolar sarcoid). When coalescent, these may simulate progressive massive fibrosis. Ancillary findings include a predominant upper lobe distribution, focal air-trapping due to bronchiolar obstruction, and diffuse adenopathy, often calcified. Calcified nodules may also be present in later stages of the disease. birthcontroltab.com
Figure 6. Perilymphatic disease: sarcoidosis. An HRCT scan of a 1-mm section at the level of the carina shows innumerable ill-defined small nodules clustered in the mid-portions of both lungs with relative sparing of the anterior aspects of both upper lobes. Note that these preferentially involve the left major fissure (arrow on left lung) as well as the walls of the peripheral airways (curved arrow on right lung).
Figure 7. Perilymphatic disease: sarcoidosis. An HRCT scan of a 1-mm section through the right mid-lung in a different patient than the one in Figure 6 shows evidence of innumerable ill-defined small nodules. Note that these tend to be clustered with relative sparing of the right upper lobe anteriorly and clearly preferentially lie adjacent to the right major fissure (arrow), along pleural surfaces, and along central vascular structures (arrowheads). This distribution of nodules is rarely seen in any other disease.