Retro-labyrinthine Lesion Site Detected by Galvanic Vestibular Stimulation Elicited Vestibular-evoked Myogenic Potentials in Patients with Auditory Neuropathy.

OBJECTIVE: Auditory neuropathy (AN) is a unique pattern of hearing loss with preservation of hair cell function. The condition is characterized by the presence of otoacoustic emissions (OAE) or cochlear microphonic (CM) responses with severe abnormalities of the auditory brainstem response (ABR). The vestibular branches of the VIII cranial nerve and the structures innervated by it can also be affected. However, the precise lesion sites in the vestibular system are not well characterized in patients with AN. METHODS: The air-conducted sound (ACS) vestibular-evoked myogenic potentials (VEMPs) and galvanic vestibular stimuli (GVS)-VEMPs were examined in 14 patients with AN. RESULTS: On examination of VEMPs (n=14, 28 ears), the absent rates of ACS-cervical VEMP (cVEMP), ACS-ocular VEMP (oVEMP), GVS-cVEMP, GVS-oVEMP and caloric test were 92.9% (26/28), 85.7% (24/28), 67.9% (19/28), 53.6% (15/28), and 61.5% (8/13), respectively. Impaired functions of the saccule, inferior vestibular nerve, utricle, superior vestibular nerve, and horizontal semicircular canal were found in 25.0% (7/28), 67.9% (19/28), 32.1% (9/28), 53.6% (15/28) and 61.5% (8/13) patients, respectively. On comparing the elicited VEMPs parameters of AN patients with those of normal controls, both ACS-VEMPs and GVS-VEMPs showed abnormal results in AN patients (such as, lower presence rates, elevated thresholds, prolonged latencies, and decreased amplitudes). CONCLUSION: The study suggested that patients with AN often have concomitant vestibular disorders. Retro-labyrinthine lesions were more frequently observed in this study. GVS-VEMPs combined with ACS-VEMPs may help identify the lesion sites and facilitate detection of areas of vestibular dysfunction in these patients.

Comparing activated brain regions between noisy and conventional galvanic vestibular stimulation using functional magnetic resonance imaging.

OBJECTIVE: Galvanic vestibular stimulation (GVS) enhances vestibular sensory inputs in vestibular afferents. However, it is unclear whether noisy and conventional GVS activate different regions of the brain. The purpose of this study was to investigate the differences in activated brain regions between those two interventions using functional MRI (fMRI). METHODS: Twenty-four healthy volunteers who met the inclusion/exclusion criteria were randomly assigned to the noisy GVS or conventional GVS groups. Brain activity was measured during stimulation and compared with that during resting fMRI. This study used a blocked design comprising four task-rest blocks, each consisting of a 30-s period of vestibular stimulation followed by a 30-s period of rest. We evaluated the differences in contrast images between the noisy and conventional GVS groups. RESULTS: The noisy GVS group showed significantly increased activation in the vestibular system-related brain regions, including the insula and central operculum. The conventional GVS group showed significant activity in multisensory areas, including the supramarginal gyrus, central operculum and opercular part of the inferior frontal gyrus. Thus, the noisy GVS group showed significantly increased activity in the insula, putamen and central operculum compared with the conventional GVS group. CONCLUSIONS: Noisy GVS could increase brain activity in the insular peripheral region compared to conventional GVS. Our results extend the literature about the importance of the stochastic resonance of noise addition for the vestibular system.