IgG4-RD-Associated Mikulicz Syndrome Without Classic Systemic Involvement—A Case Report

Background: IgG4-related disease is a rare, chronic inflammatory disorder characterized by lymphoplasmacytic infiltration, ‘storiform’ fibrosis, and elevated IgG4 levels in affected tissues. This disease has a broad and heterogeneous clinical spectrum that includes four main phenotypes: pancreatic–hepatobiliary disease, retroperitoneal/aortic fibrosis, head and neck disease, and Mikulicz syndrome. Case Description: An 85-year-old male patient with a clinical presentation, which is unusual outside Asia, of IgG4-related disease phenotype Mikulicz syndrome, characterized by bilateral dacryoadenitis, orbital pseudotumor, and no evidence of significant systemic participation. Despite extensive involvement in the orbital and glandular region, the patient did not develop serious organ complications, a behavior rarely documented in the literature. Despite the serum IgG4 levels being normal (<135 mg/dL), the clinical and radiological picture suggested IgG4-RD, emphasizing the need for a biopsy for a definitive diagnosis. Histopathological examination revealed a dense lymphoplasmacytic infiltrate, storiform fibrosis, and more than 40% IgG4-positive cells, confirming the diagnosis. Results: Treatment with prednisone was initiated alongside azathioprine for long-term control. Calcium and vitamin D3 supplementation were added to prevent glucocorticoid-induced osteoporosis. Remarkable clinical improvement was observed within 24 h, with progressive orbital and glandular symptoms resolution. Over a year, the patient exhibited complete resolution of the orbital tumors, total recovery of vision, and no relapses. The only sequelae observed were dry eye. Conclusions: This case highlights the need to consider IgG4-RD with normal serum IgG4 levels, the importance of histopathology for diagnosis, and the efficacy of steroids as first-line treatment. A multidisciplinary approach is essential for timely treatment.