A 4-year-old child is diagnosed with pulmonary tuberculosis and started on standard 4-drug ATT (HRZE). After 2 weeks of treatment, the child develops sudden onset of severe headache, neck stiffness, and altered sensorium. CSF analysis shows lymphocytic pleocytosis (180 cells/μL, predominantly lymphocytes), elevated protein (120 mg/dL), and low glucose (28 mg/dL). What is the most appropriate immediate next step?