How to do and why perform the skull vibration-induced nystagmus test.

The skull vibration-induced nystagmus test (SVINT) is a global vestibular test stimulating otoliths and semicircular canals at a frequency of 100Hz, not modified by vestibular compensation, which may reveal vibration-induced nystagmus (VIN). Bone-conducted vibration applied to the mastoid processes and the vertex instantaneously induces predominantly low-velocity (∼10°/s) horizontal nystagmus, with rapid phases beating away from the affected side in patients with unilateral vestibular loss (UVL). VIN starts and stops immediately with stimulation, is continuous, reproducible, beats in the same direction irrespective of which mastoid process is stimulated, with no or little habituation. The SVINT acts like a vestibular Weber test. In peripheral UVL, the SVINT is a good marker of vestibular asymmetry and demonstrates pathological nystagmus beating towards the healthy side in 90% of cases of vestibular neuritis, 71% of cases of Menière's diseases and 44 to 78% of vestibular schwannomas. In superior semicircular canal dehiscence, VIN usually beats towards the affected side due to facilitation of bone conduction related to the presence of a third window. Stimulation of the vertex is more effective than in UVL patients, with sensitivity extending to higher frequencies, up to 700Hz. Observation of vibration-induced nystagmus then reveals equally represented vertical, torsional, and horizontal components beating towards the affected ear, suggesting dominant, but not exclusive, stimulation of the dehiscent superior semicircular canal.