A 28-year-old man with unilateral otosclerosis (left ear, air-bone gap 35 dB) undergoes successful left stapedectomy with ossicular reconstruction. Six weeks post-operatively, he reports sudden-onset severe vertigo, nausea, and nystagmus. Audiometry shows new sensorineural hearing loss (15 dB) in the operated ear. Otoscopy is normal. What is the most appropriate next step in management?