Effect of bone-conducted vibration of the midline forehead (Fz) in unilateral vestibular loss (uVL). Evidence for a new indicator of unilateral otolithic function

Recently, a new indicator of vestibular otolithic function has been reported: it is a series of negative-positive myogenic potentials recorded by surface electrodes on the skin beneath the eyes in response to bone-conducted vibration (BCV) delivered to the forehead at the hairline in the midline (Fz). The potential is called the ocular vestibular-evoked myogenic potential (oVEMP) and the first component of this (n10) is a small (approximately 8 microV), short latency (~ 10 ms), negative potential. In healthy subjects, who are looking up, the n10 responses to Fz bone-conducted vibration are symmetrical beneath the two eyes. In the present investigation, in 17 patients with unilateral surgical vestibular loss, marked asymmetries were observed between the n10 beneath the two eyes: n10 is small or absent beneath the eye on the side opposite the operated ear, confirming previous evidence that n10 is a crossed vestibulo-ocular response unlike p13 of bone-conducted vibration cervical VEMPs (cVEMPs) is a ipsilateral vestibular response and also it is absent in this type of subjects. These results, together with evidence from patients with superior vestibular neuritis allow us to conclude: the asymmetry of the n10 response to Fz bone-conducted vibration is an indicator of utricular macula/superior vestibular nerve dysfunction on the operated side in patients with unilateral vestibular loss.