[A case of IgG4-related disease exclusively affecting pia matter and cerebral parenchyma].

A 61-year-old man presented with slowly progressive cognitive decline. Brain MRI revealed left frontal lobe lesions with gadolinium enhancement along pia mater. Brain biopsy was performed and histopathological findings was compatible with the diagnosis of IgG4-related disease (IgG4-RD). Serum IgG4 level was within a normal range, and no other systemic organs were suggested to be involved by clinical symptoms or laboratory findings. Intravenous methylprednisolone therapy followed by oral prednisone induction markedly improved the cognitive functions and MRI findings detected at the initial diagnosis. Our case highlights the importance of including IgG4-RD as one of the differential diagnosis when encountering the patients suffering from isolated cranial lesions even in the absence of normal serum IgG4 level.