Probable Isolated IgG4-Related Rhinosinusitis Presenting as a Unilateral Maxillary Sinus Mass: A Case Report and Literature Review

IgG4-related disease (IgG4-RD) is a systemic fibroinflammatory disorder that can affect multiple organs, while sinonasal involvement remains uncommon and poorly characterized. Its clinical and radiological resemblance to malignancy may delay diagnosis. We report the case of a 41-year-old man presenting with unilateral left exophthalmos, facial pain, ptosis and paraesthesia. Imaging revealed a destructive maxillary sinus mass extending into the pterygopalatine fossa and orbit, raising strong suspicion of malignancy. Endoscopic biopsy demonstrated dense lymphoplasmacytic infiltration, storiform fibrosis, obliterative phlebitis and numerous IgG4-positive plasma cells, suggestive of IgG4-associated chronic rhinosinusitis. Although the lesion fulfilled key histopathological features of IgG4-RD, its isolated maxillary sinus location potentially represents an atypical site of the disease, not included in the current ACR/EULAR classification criteria. Following surgery and corticosteroid therapy, symptoms initially resolved; however, possible local recurrence was detected four months later on PET imaging. Haematological evaluation subsequently recommended clinical surveillance without systemic immunosuppressive therapy. This case highlights the diagnostic challenges of unilateral rhinosinusitis presenting as an isolated tumour-like lesion and displaying dense IgG4 plasma cell infiltrate. IgG4-RD should be considered in the differential diagnosis of destructive sinonasal masses, even without involvement of typical organs. Histopathological evaluation is essential to guide diagnosis and management and avoid unnecessary aggressive surgery.