[A case of IgG4-related pia mater inflammation].

A 74-year-old man presented with slowly progressive ataxia and impaired consciousness. Brain MRI demonstrated hyperintensity on diffusion-weighted imaging (DWI) in the bilateral frontal subdural spaces and cerebral sulci, mild FLAIR hyperintensity in the subarachnoid space, and gadolinium enhancement along the pia mater of the left frontal cortex, suggesting meningitis. Cerebrospinal fluid analysis revealed pleocytosis with elevated protein but normal glucose, and aseptic meningitis was suspected based on the clinical course and laboratory findings. Because his serum IgG4 level was elevated, IgG4-related disease was considered as an underlying etiology. Brain biopsy showed inflammatory cell infiltration in the dura mater, arachnoid mater, and cortical surface, while clusters of CD138-positive plasma cells were observed only in the pia mater adjacent to the brain parenchyma. Immunostaining revealed that more than 40% of IgG-positive plasma cells were IgG4-positive, with approximately 100 IgG4-positive cells per high-power field, without κ/λ light-chain restriction. Although storiform fibrosis and obliterative phlebitis were absent, the findings were compatible with IgG4-related disease in combination with the clinical presentation. The patient was treated with intravenous methylprednisolone followed by oral prednisolone, resulting in rapid improvement of consciousness and reduction of leptomeningeal enhancement on MRI. This case represents a rare form of IgG4-related leptomeningitis in which the pia mater is the primary site of involvement. The findings underscore the importance of considering IgG4-related disease in the differential diagnosis of aseptic meningitis, particularly when imaging suggests pia mater inflammation and serum IgG4 levels are elevated.