IgG4-related arthritis revealed by synovial biopsy: a case-based review.

introductionIgG4-related disease (IgG4-RD) is a systemic autoimmune condition characterized by fibroinflammatory lesions, potentially occurring at any anatomical site. Typical histopathological features include dense lymphoplasmacytic infiltrates rich in IgG4 + plasma cells, storiform-pattern fibrosis, and obliterative phlebitis. Diagnosing IgG4-RD remains challenging due to its heterogeneous features. Here, we report a rare case of IgG4-RD diagnosed by a synovial biopsy of the wrist. CASE REPORT: An 80-year-old man presented with relapsing right wrist arthritis. Laboratory tests showed elevated erythrocyte sedimentation rate and C-reactive protein, while immunological profiles remained negative. Imaging showed severe erosive arthritis of the carpal bones. A synovial biopsy revealed low-grade synovitis (grade 2 according to Krenn score), with histological examination showing significant infiltration of IgG4 + plasma cells (up to 40 per high-power field, with an IgG4/IgG ratio of 40%). 18-Fluorodeoxyglucose-positron emission tomography (PET) showed increased uptake in lymph nodes, with excisional biopsy revealing reactive changes along with a focal increase in IgG4-positive plasma cells (up to 70 per high-power field) and an IgG4/IgG ratio of 50%. Consequently, IgG4-RD was diagnosed, and corticosteroid therapy was initiated, resulting in clinical improvement and reduced IgG4 levels. DISCUSSION: This case illustrates a rare case of IgG4-RD involving joints and lymph nodes, diagnosed by synovial biopsy. The presence of IgG4-positive plasma cells in the joint and lymph nodes strongly supported the diagnosis of IgG4-RD. Synovial biopsy can play a significant role in the diagnostic work-up of arthritis, particularly in cases that are difficult to categorize. Further studies are needed to better characterize IgG4-related arthritis.