[An unusual cause for unilateral eye pain with double vision].

A 48-year-old man was diagnosed with IgG4-related disease (IgG4-RD) during an evaluation for exophthalmos and unexplained pain on the left side of his face. The diagnosis was made by ruling out all other possible conditions and was based on histopathological findings. After initiating immunosuppressive therapy with corticosteroids (GC), most of the symptoms resolved. As the GC-dose was gradually reduced, a relapse occurred, leading to two courses of Rituximab. Reflecting the success of the therapy, long-term follow-up revealed a marked radiological reduction in the retrobulbar mass and a significant clinical improvement in pain.