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

## TL;DR

A 29-year-old man was diagnosed with IgG4-related disease after presenting with unusual lower limb fasciitis that initially resembled an infection.

## Contribution

This case highlights IgG4-related disease as a rare differential diagnosis for persistent lower limb soft tissue swelling.

## Key findings

- The patient's symptoms did not respond to antibiotics but improved with steroid and immunosuppressant therapy.
- Biopsy confirmed IgG4-related disease with characteristic tissue infiltration.
- IgG4-RD should be considered even in the absence of multi-organ involvement.

## Abstract

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.