Superior Canal Dehiscence Syndrome: Relating Clinical Findings With Vestibular Neural Responses From a Guinea Pig Model.

Hypothesis: In superior canal dehiscence (SCD), fluid displacement of the endolymph activates type I vestibular hair cells in the crista of the affected canal and thus irregular superior canal (SC) neurons in Scarpa's ganglion, which provides the neurophysiological basis for the clinical presentation of SCD.

Background: Patients with SCD display sound- and vibration-induced vertigo/nystagmus and increased amplitudes of vestibular evoked myogenic potentials.

Methods: Extracellular recordings from n = 25 primary vestibular neurons of 16 female guinea pigs were analyzed.

Response of the inner ear to lipopolysaccharide introduced directly into scala media.

In an attempt to develop an animal model of immune mediated Meniere's disease, we have injected lipopolysaccharide (LPS) directly into scala media of guinea pigs and monitored functional and morphological changes over a period of 6 weeks. Depending on the concentration of LPS, changes ranged from moderate-to-severe hearing loss and endolymphatic hydrops with minimal cellular infiltrate or fibrosis, to dense cellular infiltration that filled the scalae. Interestingly, higher concentrations of LPS not only induced severe cellular infiltration, hydrops, and hearing loss, but also a substantial enlargement of the endolymphatic duct and sac.

Suppression of the vestibular short-latency evoked potential by electrical stimulation of the central vestibular system.

In an attempt to view the effects of the efferent vestibular system (EVS) on peripheral dynamic vestibular function, we have monitored the Vestibular short-latency Evoked Potential (VsEP) evoked by pulses of bone conducted vibration during electrical stimulation of the EVS neurons near the floor of the fourth ventricle in the brainstem of anesthetized guinea pigs. Given the reported effects of EVS on primary afferent activity, we hypothesized that EVS stimulation would cause a slight reduction in the VsEP amplitude. Our results show a substantial (>50%) suppression of the VsEP, occurring immediately after a single EVS current pulse.

The response of guinea pig primary utricular and saccular irregular neurons to bone-conducted vibration (BCV) and air-conducted sound (ACS).

Unlabelled: This study sought to characterize the response of mammalian primary otolithic neurons to sound and vibration by measuring the resting discharge rates, thresholds for increases in firing rate and supra-threshold sensitivity functions of guinea pig single primary utricular and saccular afferents. Neurons with irregular resting discharge were activated in response to bone conducted vibration (BCV) and air conducted sound (ACS) for frequencies between 100 Hz and 3000 Hz. The location of neurons was verified by labelling with neurobiotin.

Neural basis of new clinical vestibular tests: otolithic neural responses to sound and vibration.

Extracellular single neuron recording and labelling studies of primary vestibular afferents in Scarpa's ganglion have shown that guinea-pig otolithic afferents with irregular resting discharge are preferentially activated by 500 Hz bone-conducted vibration (BCV) and many also by 500 Hz air-conducted sound (ACS) at low threshold and high sensitivity. Very few afferent neurons from any semicircular canal are activated by these stimuli and then only at high intensity. Tracing the origin of the activated neurons shows that these sensitive otolithic afferents originate mainly from a specialized region, the striola, of both the utricular and saccular maculae.