A 28-year-old man from Mumbai is admitted on day 7 of dengue fever. He has recovered from fever, platelet count is now 120,000/μL (rising), haematocrit is 38% (down from 42% on admission), and urine output is 1.2 mL/kg/hr. He is haemodynamically stable. However, he now complains of severe headache and neck stiffness. CSF shows 120 cells/μL (90% lymphocytes), protein 80 mg/dL, glucose 55 mg/dL (serum glucose 110 mg/dL), and Gram stain is negative. What is the most appropriate next step?