| Group 1: PAH |
| 1.1–1.4. Pulmonary plexogenicarteriopathy |
| Early phase: |
| ● Medical hypertrophy |
| ● Cellular intimal proliferation of muscular pulmonary arteries |
| ● Appearance of muscle in normally nonmuscular arteries |
| Late phase: |
| ● Concentric laminar intimal fibrosis |
| ● Loss of luminal vascular volume |
| ● Dilatation lesions (vein-like branches, angiomatoid lesions) |
| ● Plexiform lesions |
| ● Recanalization of arteries |
| ● Fibrinoid necrosis |
| ● Arteritis |
| Group 1’: PVOD |
| ● Foci of intense congestion of pulmonary parenchyma |
| ● Patchy hemosiderosis associated with areas of congestion |
| ● Encrustation of elastin with iron and calcium salts in congested areas |
| ● Duplication of elastic laminae |
| ● Obliterative fibrosis of small veins and of venules, associated with congested areas |
| ● Abnormalities set against a background of normal or near normal lung tissue |
| ● Prominence of capillaries, associated with increased numbers of capillaries, often blurring the distinction from pulmonary capillary hemangiomatosis (group 1.4.2) |
| Group 1’: PCH |
| ● Marked increase and prominence of capillary vessels in alveolar walls, interlobular septa, bronchovascular bundles, and pleura; masses of capillaries may bulge into lumina of airways and vessels |
| ● Associated features of PVOD in some cases |
| Group 2: Pulmonary hypertension with left heart disease |
| ● Arterialization of large or middle-sized pulmonary veins |
| ● Interstitial edema and fibrosis |
| ● Hemosiderosis |
| ● Medial hypertrophy and adventitial thickening of pulmonary arteries |
| Group 3: Pulmonary hypertension associated with lung disease and/or hypoxemia |
| 3.1 and 3.3–3.5. Hypoxic pulmonary vasculopathy |
| ● Intimal proliferation; adventitial thickening |
| ● Medial hypertrophy of muscular pulmonary arteries and arterioles, especially of smaller branches |
| ● Longitudinally oriented intimal smooth muscle cells |
| ● Slight medial hypertrophy of veins |
| 3.2. Pulmonary vasculopathy associated with interstitial lung disease |
| ● Features of hypoxic pulmonary vasculopathy |
| ● Eccentric intimal fibrosis of arteries and, to a lesser extent, veins |
| Group 4: Pulmonary hypertension due to chronic thrombotic and/or embolic disease |
| ● Thromboembolic obstruction of distal pulmonary arteries |
| Eccentric intimal fibrosis |
| Recanalized organized thrombi forming bands and webs |
| Fresh thrombi very rare |
| Nota bene: lesion may be focal, requiring extensive search in multiple sections |
| ● Nonthrombotic pulmonary embolism |
| Nonthrombotic material or tissue (foreign bodies, bone marrow) |
| Fat embolism: many dilated optically empty blood vessels (down to capillary size) |
| Group 5: Miscellaneous [sarcoidosis, compression of pulmonary vessels (adenopathy), tumor, fibrosingmediastinitis] |
| ● Heterogeneous group of disorders, some showing the features of congestive vasculopathy, some with features of post-thrombotic vasculopathy, some with combinations |
