A 38-year-old woman undergoing spinal anesthesia for cesarean section develops sudden onset of severe headache, neck stiffness, and fever (38.5°C) on postoperative day 2. CSF analysis shows pleocytosis (WBC 450/μL, predominantly lymphocytes), elevated protein (120 mg/dL), and normal glucose. Gram stain and bacterial culture are negative. What is the most appropriate next step in management?