IgG4-related systemic disease as a cause of "idiopathic" orbital inflammation, including orbital myositis, and trigeminal nerve involvement.

IgG4-related systemic disease (IgG4-RD) is an inflammatory condition of unknown etiology that has been identified as the cause of tumefactive lesions in a number of tissues and organs. The role of the IgG4 remains to be clarified fully, but the histopathologic diagnosis hinges upon the finding of IgG4-bearing plasma cells in addition to characteristic morphologic features, with or without elevated seum IgG4. We present a 56-year-old man with orbital pseudotumor in whom, after 30 years of intractable disease, biopsy showed IgG4-RD involving the lacrimal gland, extraocular muscles, intraconal fat, and trigeminal nerve. Six months after initiating treatment with rituximab, his disease remained dormant, with improvement in his proptosis and normalization of serum IgG4 levels. We review the differential of idiopathic orbital inflammatory disease, including IgG4-RD, and emphasize the need for biopsy for accurate diagnosis and to guide appropriate treatment.