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. 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.