Resumen en Inglés:

Abstract Immunoglobulin G4-related disease (IgG4-RD) is a systemic fibroinflammatory condition that can mimic malignancy, infection, or other autoimmune disorders, causing significant diagnostic challenges. Neurological involvement is uncommon, most frequently presenting as hypertrophic pachymeningitis or hypophysitis, but it may also manifest as inflammatory ophthalmic or cerebral pseudotumors. We herein describe a complex case of neurological IgG4-RD with a 10-year disease course, initially presenting with focal epilepsy and later evolving into a tumefactive brain lesion. This case highlights key clinical lessons: neurological IgG4-RD may remain subclinical, with only functional symptoms preceding structural abnormalities; the histopathological demonstration of storiform fibrosis and IgG4-positive plasma cell infiltration remain the diagnostic cornerstone; and systemic assessment is essential due to frequent multiorgan involvement. Finally, we discuss strategies to interpret brain-biopsy findings in inflammatory brain disorders and emphasize the importance of maintaining a high index of suspicion for IgG4-RD in atypical presentations.