A 38-year-old Indian woman with a 3-year history of Meniere disease (left ear) presents with her third severe vertigo attack this month. Previous attacks lasted 4–6 hours with associated nausea and vomiting. Audiometry shows progressive low-to-mid frequency sensorineural hearing loss on the left, with incomplete recovery after the last two episodes. She has been on a low-sodium diet and hydrochlorothiazide 25 mg daily for 18 months with diminishing benefit. She is now unable to work due to unpredictable attacks. What is the most appropriate next step in management?