Proptosis Revealing a Neurological Form of IgG4-Related Disease: A Case Report and Literature Review.

IgG4-related disease (IgG4-RD) is a chronic, immune-mediated fibro-inflammatory condition characterized by tissue infiltration of IgG4-positive plasma cells, with a continually expanding spectrum of clinical manifestations. Neurological involvement is uncommon, and imaging findings are often nonspecific, frequently delaying diagnosis. The condition generally carries a favorable prognosis due to its responsiveness to corticosteroid therapy. We report the case of a 30-year-old man presenting with a nine-month history of progressive, painless left-sided proptosis and eyelid swelling, without focal neurological deficits. Imaging revealed an orbital lesion extending posteriorly into the intracranial compartment. Orbital decompression via an eyebrow incision was performed, and histopathological examination confirmed IgG4-RD. The patient was subsequently treated with systemic corticosteroids, with a favorable clinical outcome. This case highlights the diagnostic challenge posed by IgG4-related orbital disease (IgG4-ROD) with neurological involvement, given the limited specificity of imaging in distinguishing it from other orbital and skull-base pathologies. It underscores the importance of maintaining a broad differential diagnosis for proptosis and the essential role of histopathological confirmation in guiding timely, appropriate management.