A 42-year-old woman presents with a 6-month history of episodic vertigo lasting 4–6 hours, accompanied by fluctuating sensorineural hearing loss, tinnitus, and aural fullness in the left ear. Audiometry confirms low-frequency sensorineural hearing loss. MRI of the internal acoustic meatus is normal. She has failed conservative management with salt restriction and diuretics for 3 months. What is the most appropriate next step in management?