Immunoglobulin G4-related disease manifesting as peripheral neuropathy: A rare clinical symptom due to rare autoimmune disease

Nervous system involvement in immunoglobulin G4-related disease (IgG4-RD) has been rarely reported. We describe an unusual case of IgG4-RD manifested as paresthesia in the right lower extremity. A 51-year-old male presented with paresthesia in the right S1–S3 regions. A neurological examination revealed peripheral neuropathy. Blood examination results were normal, barring slightly elevated IgG levels. Initial magnetic resonance imaging of the swollen right S1 and S2 nerve roots revealed lymphoma, schwannoma, and sarcoidosis. However, following the biopsy, the pathological findings were not typical of these diseases. Abdominal computed tomography revealed perirenal lesions, and IgG4-RD was suspected. The patient had a serum IgG4 level of 724 mg/dL. Additional pathological evaluations of the swollen S1 nerve revealed findings that corresponded to the diagnostic criteria for IgG4-RD. Oral steroid therapy was initiated, which improved paresthesia, and the swollen S1 nerve root gradually shrank. This report highlights a rare case of IgG4-RD involving nerve roots that neurosurgeons should consider.