Table 1.

Comparison of anatomopathological findings in AIP, ARDS, HAPE, and COVID-19.

Disease	Anatomopathological findings
AIP	Lung tissue derangement with alveolar collapse. Hyaline membrane formation. Polymorphonuclear and monocytes infiltration. Pneumocytes characteristics in AIP include epithelial necrosis, erosion, fibrin exudation, Mallory bodies. Pneumocytes Ub+ . Early fibroblastic interstitial fibrosis, septal and para-septal reparative fibrosis.
ARDS	Lung tissue derangement with alveolar collapse. Intra-alveolar and interstitial edema. Necrotic alveolar epithelial cells and exudates of serum proteins from the damaged and leaky capillaries. Alveolar type I cell necrosis, alveolar type II cell proliferation, interstitial proliferation of fibroblast and myofibroblast or organizing interstitial fibrosis. Hyaline tissue formation. Reactive pneumocytes with nuclear atypia and hyperplasia. Mallory like inclusions in type 2 pneumocytes (Ub+). Scattered neutrophilic infiltrates localized to terminal bronchioles and surrounding alveoli with evident confluence of infiltrates between adjacent lobules. Thrombi are common in small/ medium pulmonary arterioles. Abundant fibroblast in the interstitium, fibrosis and interstitial thickening by fibroblasts.
HAPE	Diffuse alveolar edema including red blood cells, polymorphonuclear cells, and macrophages. Widened septa due to interstitial edema. Occasional alveolar hyaline membranes. Type II pneumocytes could be damaged (due to stress failure) and pulmonary surfactant impaired. Congested pulmonary arterioles and capillaries. Intra-alveolar hemorrhage is present in scattered areas of lung parenchyma, particularly in the more severe cases. Thrombi and fibrin clots plugging arterioles and capillaries.
COVID-19	Lung tissue derangement with alveolar collapse. Intra-alveolar hemorrhage. Alveolar rupture. Hyaline tissue formation. Polymorphonuclear and monocytes infiltration. Active viral replication within type 2 pneumocytes Reactive pneumocytes with nuclear atypia and hyperplasia. Mallory like intracytoplasmic inclusions in type 2 pneumocytes. Masson’s bodies. Microthrombi, vasculitis or vascular thrombosis. Early fibroblastic interstitial fibrosis, septal and para-septal reparative fibrosis.

Table Legend: AIP, acute interstitial pneumonia; ARDS, Acute Respiratory Distress Syndrome; COVID-19, Coronavirus disease 2019; HAPE, high-altitude pulmonary edema.