Organizing Pneumonia as a Histopathological Term

Chronic interstitial pneumonia with features of organizing pneumonia in an adult horse

“…In EMPF, nodules with myofibroblasts and collagen are located predominantly in the pulmonary interstitium, diffusely thickening the alveolar septa, 28 whereas in cases of OP, the nodules with myofibroblasts and connective tissue extend and obliterate alveolar lumens (Masson bodies), frequently extending into the lumens of bronchioles. 1,3,10 The term bronchiolitis obliterans (BO) has different interpretations in human medicine and veterinary medicine. In general, the term BO has been used by human pathologists to refer to the presence of polypoid projections of granulation or fibrous tissue into the lumen of bronchioles, with narrowing and obliteration of lumens.…”

“…9,13 In human medicine, luminal granulation tissue polyps extending from the adjacent parenchyma with a restrictive effect, which are not associated with a fibrosing lung disease, are regarded as part of OP (previously known as BOOP). 1,3,10 The presence of dense granulation tissue between the bronchiolar epithelium and muscular layer, without extending into the adjacent parenchyma, is now regarded as constrictive bronchiolitis (previously known as "obliterative bronchiolitis"). 9 In constrictive bronchiolitis, granulation tissue is much denser (frequently collagenized) than seen in OP, and typically becomes re-epithelialized 9,10 ; fibrosis of the bronchiolar wall causes external compression and reduces the diameter of the lumen, a lesion not commonly identified in animals.…”

“…OP is characterized by multifocal areas of alveolar thickening with fibromyxoid connective tissue, and extension of this tissue into bronchiolar lumens, generating lesions resembling bronchiolitis obliterans. 1,10 We describe herein a case of chronic interstitial pneumonia in a horse, with findings compatible with the cicatricial form of OP.…”

“…10 Today, it is called OP to avoid confusion. 1,10 The distinguishing histologic lesion of OP is the proliferation of fibromyxoid connective tissue containing fibroblasts and myofibroblasts (Masson bodies) 11,12 within alveolar ducts and sacs, with extension to bronchiolar lumens. These findings are similar in all cases, independent of the radiographic pattern of the cases.…”

“…6,22,25 Among the most common causes of secondary OP are autoimmune diseases that affect collagen, adverse inflammatory reaction to certain drugs, infections, tumors, and radiation. 1,6,24 It is believed that the involved mechanisms have in common diffuse alveolar epithelial damage. 6 Reactive OP has minor clinical significance and accompanies other preexisting pulmonary diseases, such as neoplastic foci or granulomas in the pulmonary parenchyma.…”