An unusual case of IgG4-related disease presenting as lower limb fasciitis.

Immunoglobulin G4-related disease (IgG4-RD) is an immune-mediated fibroinflammatory condition that can affect nearly any organ. We report an unusual case of IgG4-RD presenting as gradually progressing bilateral lower limb fasciitis in a 29-year-old male patient. He was initially diagnosed with lower limb cellulitis; however, the symptoms did not improve with long courses of intravenous antibiotics. Autoimmune screen was negative and there were no features to suggest an underlying atypical infection. Subsequent cross-sectional imaging demonstrated deep fasciitis in both lower legs and a deep incision biopsy showed characteristic storiform IgG4 plasma cell tissue infiltration. The patient had an excellent clinical response to steroid and immunosuppressant therapy. IgG4-RD should be considered as a differential diagnosis in a patient presenting with prolonged, unexplained soft tissue swelling, even if there are no other sites of involvement.