Skull Vibration Induced afternystagmus: A new clinical indicator of superior canal dehiscence.

Vibrations applied to the cranium induce, in 55%-92% of patients with CT-verified superior canal dehiscence (SCD), a perstimulatory nystagmus most often ipsilaterally beating when the vertex location is stimulated at 100Hz. The skull vibration-induced nystagmus (SVIN) test is a bone-conducted (BC) Tullio phenomenon in patients with a 3rd mobile window syndrome (TMWS). Here, we describe the technical cues and procedures to obtain an afternystagmus regularly observed in 55% of a series of 43 patients with SCD chosen as examples and stimulated at 100Hz and higher frequencies by bone-conducted vibrations (BCV). Only 6% of 32 patients with total unilateral vestibular loss (TUVL) show this afternystagmus with a very small slow phase velocity. The SVIN afternystagmus is characterized by a persistent nystagmus with a slow decay after stimulation withdrawal and is associated with dizziness. This distinctive sign in SCD is usually not observed in TUVL patients but only in some other TMWS. Its production likely relies on a cupula deflection secondary to endolymph flow and stimulation of regular discharging neurons by the BCV in SCD but not TUVL patients. It is a simple, specific, robust sign of SCD and TMWS, and its sensitivity is comparable to air-conducted Tullio phenomenon and other compressional tests.