Microscopic polyangiitis-associated interstitial lung disease with acute exacerbation after SARS-CoV-2 infection: a case report and literature review.

Microscopic polyangiitis (MPA) is a type of systemic inflammatory small vessel vasculitis that is frequently accompanied by interstitial lung disease (ILD). Acute exacerbations (AE) of ILD are fatal complications in patients with MPA and can be triggered by various factors, including infections. We report a case of AE of MPA-ILD following severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) infection. An 81-year-old male with MPA during remission maintenance therapy was hospitalised for Coronavirus disease 2019 pneumonia. Treatment for SARS-CoV-2 infection improved his respiratory symptoms and radiological findings, but his respiratory condition suddenly deteriorated with a high fever on hospital Day 52. Chest high-resolution computed tomography showed diffuse bilateral ground glass opacity, and we diagnosed AE of MPA-ILD. He was refractory to steroid pulse therapy and died on the second day after the onset of AE. Pathologic autopsy revealed hyaline membrane formation over the entire bilateral lobes, consistent with the exudative phase of the diffuse alveolar damage pattern. Our case suggests that persistent immune activation during the post-acute phase of SARS-CoV-2 infection may contribute to delayed AE of MPA-ILD with a diffuse alveolar damage pattern through macrophage-driven lung injury, given that ILD exacerbation occurred approximately 2 months after the onset of Coronavirus disease 2019 pneumonia.