Background::IgG4-related hypertrophic pachymeningitis is a relative newly recognized and rare manifestation of IgG4-related disease, an immune-mediated fibroinflammatory tumefactive disorder. Fewer than 80 patients have been reported in the literature, and it can mimic common neurosurgical conditions. We describe the clinical presentation of two patients that were initially considered to have a subdural collection, tuberculous meningitis, and a cervical spinal meningioma, but were eventually diagnosed with this disease.Case presentation::Two ethnic Chinese men, 86 and 62 years old, experienced a 4-week history of headache. Both patients had a history of autoimmune disease, namely glomerulonephritis and Grave's disease, respectively. Magnetic resonance brain imaging revealed diffuse dural thickening with the latter patient exhibiting homogeneous and intense gadolinium-contrast enhancement. Since the 86-year-old patient also had progressive bilateral visual loss, giant cell arteritis was suspected and a 2-week course of glucocorticoid therapy was prescribed, but his symptoms failed to improve. The 62-year-old patient also had accompanying low-grade fever and was treated empirically as having tuberculous meningitis although there were no confirmatory microbiological findings. This patient further developed right hemiparesis, and additional imaging revealed a C4/5 intradural-extramedullary contrast-enhancing lesion resembling a meningioma causing cord compression. Both patients underwent neurosurgical intervention with the former undergoing a dural biopsy and the latter having the cervical lesion resected. The final diagnosis was IgG4-related hypertrophic pachymeningitis with the hallmark histological features of lymphoplasmacytic infiltration of IgG4+ plasma cells, storiform fibrosis, and obliterative phlebitis. In addition, their serum IgG4 levels were elevated (i.e., > 135 mg/dL). Both patients received at least 6 months of glucocorticoid therapy while the latter also had azathioprine. Their symptoms improved significantly and recurrent lesions were not detected on follow-up imaging.Conclusions::A high index of suspicion for this condition is suggested when a male patient with a history of autoimmune disease and compatible radiological findings, experiences subacute headache that is disproportionate to the degree of dural involvement. Neurosurgeons should consider early meningeal biopsy to establish a definitive histological diagnosis in order for early effective immunosuppressive treatment to be initiated and to avoid unnecessary morbidity.“,”Background::IgG4-related hypertrophic pachymeningitis is a relative newly recognized and rare manifestation of IgG4-related disease, an immune-mediated fibroinflammatory tumefactive disorder. Fewer than 80 patients have been reported in the literature, and it can mimic common neurosurgical conditions. We describe the clinical presentation of two patients that were initially considered to have a subdural collection, tuberculous meningitis, and a cervical spinal meningioma, but were eventually diagnosed with this disease.Case presentation::Two ethnic Chinese men, 86 and 62 years old, experienced a 4-week history of headache. Both patients had a history of autoimmune disease, namely glomerulonephritis and Grave's disease, respectively. Magnetic resonance brain imaging revealed diffuse dural thickening with the latter patient exhibiting homogeneous and intense gadolinium-contrast enhancement. Since the 86-year-old patient also had progressive bilateral visual loss, giant cell arteritis was suspected and a 2-week course of glucocorticoid therapy was prescribed, but his symptoms failed to improve. The 62-year-old patient also had accompanying low-grade fever and was treated empirically as having tuberculous meningitis although there were no confirmatory microbiological findings. This patient further developed right hemiparesis, and additional imaging revealed a C4/5 intradural-extramedullary contrast-enhancing lesion resembling a meningioma causing cord compression. Both patients underwent neurosurgical intervention with the former undergoing a dural biopsy and the latter having the cervical lesion resected. The final diagnosis was IgG4-related hypertrophic pachymeningitis with the hallmark histological features of lymphoplasmacytic infiltration of IgG4+ plasma cells, storiform fibrosis, and obliterative phlebitis. In addition, their serum IgG4 levels were elevated (i.e., > 135 mg/dL). Both patients received at least 6 months of glucocorticoid therapy while the latter also had azathioprine. Their symptoms improved significantly and recurrent lesions were not detected on follow-up imaging.Conclusions::A high index of suspicion for this condition is suggested when a male patient with a history of autoimmune disease and compatible radiological findings, experiences subacute headache that is disproportionate to the degree of dural involvement. Neurosurgeons should consider early meningeal biopsy to establish a definitive histological diagnosis in order for early effective immunosuppressive treatment to be initiated and to avoid unnecessary morbidity.