With vestibular hypofunction, the balance part of the inner ear is not working properly. This can occur on one side (unilateral hypofunction), or on both sides of the head (bilateral hypofunction). Unilateral hypofunction can occur after damage from a variety of causes, including vestibular neuritis, vestibular schwannoma/treatments for vestibular schwannoma, and Meniere’s disease/treatments for Meniere’s disease. Bilateral hypofunction can occur after damage to the inner ear/balance nerve from a variety of causes, including exposure to aminoglycoside antibiotics (which are toxic to the inner ear), neurofibromatosis type II, CANVAS syndrome (cerebellar ataxia, neuropathy, and vestibular areflexia), autoimmune disease, congenital problems, trauma, superficial siderosis, and idiopathic causes. Both unilateral and bilateral vestibular hypofunction are diagnosed with VNG (showing absent/diminished inner ear responses to thermal changes on one or both sides), rotary chair testing (showing decreased gain, phase leads, and decreased time constant on the affected side), and head impulse testing (showing that the eyes slip off the target during a rapid head movement, and then quickly correct, known as a catch-up saccade). Hypofunction of the inner ear produces symptoms related to a loss of the normal balance reflexes- therefore patients can have oscillopsia (movement or bobbing of the visual world with head movement due to loss of the vestibulo-ocular reflex), dizziness, and postural instability. Both unilateral and bilateral vestibular hypofunction are treated with vestibular physical therapy. In unilateral hypofunction, treatment is aimed at promoting central compensation and relies on the other ear (the normal side) to perform most of the inner ear functions. In bilateral hypofunction, vestibular physical therapy can still be helpful, but therapy is often longer and more difficult, as the 
absence of all inner ear function means that you have to rely on other systems (such as the COR- cervical-ocular reflex) to sense head movements.