IgG4-related orbital disease masquerading as thyroid eye disease, vice versa, or both?

A 40 year-old male presented after one year of unilateral, progressive, steroid-responsive, orbital inflammatory disease causing proptosis, extraocular muscle (EOM) restriction, and compressive optic neuropathy. The development of anti-thyroidal antibodies prompted the diagnosis of thyroid eye disease (TED); however, the prolonged active phase, remarkable reversibility of ophthalmic features with high-dose corticosteroids, unilaterally of disease, uncharacteristic EOM involvement (including both obliques), and the absence of autoimmune thyroid disease provoked consideration of alternative diagnoses. Inferior oblique biopsy stained positive for IgG4 with histologic features atypical of TED. The patient received rituximab for presumed IgG4-related orbital disease (IgG4-ROD) with subsequent reversal of compressive optic neuropathy, near complete resolution of EOM restriction, and improved proptosis, the latter two of which are not routinely anticipated in advanced TED. The possible role for B-cell depletion in both TED and IgG4-ROD suggests a degree of overlap in the underlying immune-related pathophysiology that is yet to be defined.