A 24-year-old man complained of a right temporal headache for four weeks. The patient denied any trauma or previous anticoagulation use. He also reported tender right facial swelling. His physical exam was unrevealing except for right cranial nerve (CN) VI palsy, right parotid enlargement, and cervical adenopathy. Laboratory findings were significant for mild leukopenia at 3300 cells/uL. The computed tomography (CT) scan obtained showed a chronic left subdural effusion with a 4 mm midline shift and confirmed right parotid enlargement and cervical lymphadenopathy. Surgical burr hole evacuation was done and the fluid was sent for wound culture analysis. The infectious diseases service recommended initiating antibiotics, which were later stopped due to cerebrospinal fluid (CSF) cultures with no growth of any organisms. His CN VI palsy resolved during admission. The patient was discharged with follow-up for biopsy. The patient was lost to follow-up. The patient presented to the emergency department (ED) three months later, with a left-sided frontal headache. A repeat CT scan showed a new, right-sided fluid collection outside the brain parenchyma. Burr hole evacuation was done again and purulent fluid was drained. Antibiotics were held this time, but anti-tuberculous therapy was initiated empirically. The otolaryngology service was consulted and a lymph node biopsy was performed. The pathology showed histiocytic necrotizing lymphadenitis. A dural biopsy was done as well and was consistent with histiocytic necrotizing lymphadenitis involving the dura. Cultures from the subdural fluid did not grow any organism. The patient remained neurologically intact. He improved after surgery was done to drain the fluid and was managed by analgesics. The cultures from the extra-parenchymal fluid collection remained negative for pathogens and tuberculous mycobacteria. The patient was discharged with rheumatology clinic follow-up. He saw the rheumatologist six weeks after the discharge. During his clinic visit, the patient reported no recurrence of headaches, fevers, rash, or joint pain. Our patient had a rare presentation of Kikuchi-Fujimoto disease, in which he had a subdural fluid collection resulting in neurological complications that required surgical intervention.

Nonvasculitic autoimmune meningoencephalitis (NAIM) is a rare condition describing a syndrome of steroid-responsive encephalopathy in patients with similar clinical and pathologic features. It can be associated with autoimmune diseases, such as Sjogren's syndrome and autoimmune thyroiditis. Brain biopsies usually show inflammatory cells without evidence of vasculitis. In this article, we present a patient who developed NAIM after receiving rituximab, a B-cell-depleting therapy for rheumatoid arthritis. The brain biopsy showed a lack of B lymphocytes in the brain tissue, and the patient responded well to intravenous immunoglobulins. We further discuss the role of B lymphocytes and specific regulatory B lymphocytes in suppressing autoimmunity in the brain and propose that the depletion of regulatory B cells may contribute to the pathogenesis of NAIM. This case illustrates a potential side effect of rituximab and demonstrates the importance of regulatory B cells in maintaining the immune response.