A 40 years old woman presents with a 1-year history of recurrent episodes of vertigo. The vertigo spells are described as a sensation of the room spinning that lasts from 20 minutes to a few hours and may be associated with nausea and vomiting. The spells are incapacitating and are accompanied by dizziness, vertigo and disequilibrium, which may last for days. No loss of consciousness is reported. The patient also reports aural fullness, tinnitus, and hearing loss in the right ear that is more pronounced around the time of her vertigo spells. Physical exam reveals a horizontal nystagmus, and she is unable to maintain her position during Romberg testing or tandem stepping test. She turns toward the right side and she is unable to walk tandem. Her cerebellar function tests are normal. What is the most likely diagnosis?