IgG4‐related lung disease progressing to respiratory failure

Immunoglobulin G4 (IgG4)‐related disease (IgG4‐RD) is a systemic immune‐mediated, fibroinflammatory disorder categorized as proliferative or fibrotic depending on the responsiveness of corticosteroid treatment. IgG4‐related lung disease (IgG4‐RLD) accounts for 13–14% of IgG4‐RD cases, but respiratory failure is quite rare. A 71‐year‐old man diagnosed with interstitial lung disease was referred to our department after a 10‐month observational period. He presented with respiratory failure at the first visit, with significant elevations in serum IgG4 levels and histopathological findings meeting the criteria of IgG4‐positive plasma cells and IgG4/IgG‐positive plasma cell ratio in transbronchial lung biopsy and inguinal lymph node biopsy, resulting in a diagnosis of IgG4‐RD. Systemic corticosteroid treatment promptly ameliorated the respiratory failure. (18)F‐fluorodeoxyglucose (FDG) positron emission tomography/computed tomography showed significant FDG accumulation in the lung fields, indicating the proliferative and reversible status of IgG4‐RLD, which responded well to corticosteroid treatment. The patient recovered from respiratory failure even after a 10‐month observational period.