A 28-year-old woman with drug-susceptible pulmonary TB completed 2 months of intensive phase (HRZE) and is now 1 month into the continuation phase (HR). She reports progressive tingling and numbness in both feet, with difficulty walking. Neurological examination confirms distal, symmetrical, lower-limb sensorimotor polyneuropathy. Serum vitamin B12 and folate levels are normal. What is the most appropriate next step in management?