Isolated Orbital Involvement of IgG4-Related Disease Mimicking Malignancy

Immunoglobulin G4-related disease (IgG4-RD) is a rare, immune-mediated systemic condition that can affect the orbit. In this report, we describe a 76-year-old female patient who experienced a 4-month history of pain in the right orbit and vision loss. Imaging studies revealed a mass occupying the right orbital apex. Although the patient’s serum IgG4 level was within normal limits, which complicated the diagnostic process, histopathological evaluation revealed significant tissue infiltration by IgG4-positive plasma cells, confirming the diagnosis of IgG4-RD. Following treatment with azathioprine and methylprednisolone, the lesion regressed; however, the vision loss became permanent. Isolated orbital involvement in IgG4-RD is rare and can mimic neoplasms. Early diagnosis and combination therapy may help prevent disease progression, although complete functional recovery is not always possible.