Chronic Unilateral Proptosis in Primary Care: A Case of IgG4-Related Orbital Disease.

Immunoglobulin G4-related orbital disease (IgG4-ROD) is an uncommon fibroinflammatory condition that may closely resemble malignant orbital lesions, posing a significant diagnostic challenge. We report the case of a 50-year-old man who presented with a four-year history of progressive right-sided proptosis. Computed tomography and magnetic resonance imaging revealed enhancing extraconal orbital masses involving the lacrimal gland and extraocular muscles, with imaging characteristics including restricted diffusion that strongly suggested orbital lymphoma. Given the indeterminate radiological findings, the patient underwent anterior orbitotomy and incisional biopsy. Histopathological examination demonstrated dense lymphoplasmacytic infiltration and collagenous fibrosis with abundant IgG4-positive plasma cells and an elevated IgG4/IgG ratio, establishing the diagnosis of IgG4-ROD and excluding malignancy. The postoperative course was uneventful, and further evaluation was arranged to assess for systemic involvement. This case highlights the substantial clinical and radiological overlap between IgG4-ROD and orbital lymphoma and underscores the limitations of imaging in achieving a definitive diagnosis. Recognition of characteristic patterns of orbital involvement, particularly when the lacrimal gland and extraocular muscles are affected, may raise suspicion for IgG4-ROD; however, histopathological confirmation remains essential. Early and accurate diagnosis is critical to avoid misclassification as a malignant process, prevent inappropriate treatment, and ensure appropriate management and surveillance. This report emphasizes the importance of maintaining a broad differential diagnosis when evaluating chronic orbital masses and reinforces the pivotal role of tissue diagnosis in guiding clinical decision-making.