Biopsy-Proven Cerebral Parenchymal Involvement in IgG4-Related Disease: A Case Report and a Literature Review

We present a case of biopsy-confirmed IgG4-related disease (IgG4-RD) presenting as a solitary brain lesion despite normal serum IgG4 levels, along with a literature review of 12 cases. A woman in her 70s presented with seizures and impaired consciousness. Contrast-enhanced brain magnetic resonance imaging (MRI) revealed an enhanced lesion in the frontal lobe white matter. Endoscopic brain biopsy demonstrated perivascular infiltration of plasma cells within the brain parenchyma, with 25 IgG4-positive plasma cells per high-power field and an IgG4/IgG-positive cell ratio of >40%. The patient's serum IgG4 level remained within the normal range. A diagnosis of IgG4-RD was established, and prednisolone therapy was initiated. Follow-up MRI showed marked improvement in the lesion. A literature review identified 12 IgG4-RD cases with brain parenchymal lesions. The literature review revealed that normal serum IgG4 levels are not uncommon in this manifestation and that limb motor dysfunction is the most frequent symptom. In most cases, glucocorticoids are the mainstay of treatment. In conclusion, IgG4-RD can present as an isolated brain parenchymal lesion without elevated serum IgG4. Brain biopsy is critical for accurate diagnosis to differentiate it from other conditions, such as malignant lymphoma, and to guide appropriate therapy.