Vestibular evoked myogenic potentials in practice: Methods, pitfalls and clinical applications.

Vestibular evoked myogenic potentials (VEMPs) are a useful and increasingly popular component of the neuro-otology test battery. These otolith-dependent reflexes are produced by stimulating the ears with air-conducted sound or skull vibration and recorded from surface electrodes placed over the neck (cervical VEMPs) and eye muscles (ocular VEMPs). VEMP abnormalities have been reported in various diseases of the ear and vestibular system, and VEMPs have a clear role in the diagnosis of superior semicircular canal dehiscence. However there is significant variability in the methods used to stimulate the otoliths and record the reflexes. This review discusses VEMP methodology and provides a detailed theoretical background for the techniques that are typically used. The review also outlines the common pitfalls in VEMP recording and the clinical applications of VEMPs.

Differential effect of dopa and subthalamic stimulation on vestibular activity in Parkinson's disease.

Postural disturbances in advanced Parkinson's disease are less responsive to therapy than other cardinal motor signs. The vestibulocollic reflex represents one brain-stem neuronal circuit involved in postural adjustments. The objective of this study was to investigate the vestibulocollic reflex in parkinsonian patients and the effects of subthalamic stimulation and dopa by recording vestibular-evoked myogenic potentials. After overnight withdrawal of medication, 20 patients with Parkinson's disease with (6 men, 4 women; mean age, 64.4 ± 2.2 years) or without (8 men, 2 women; mean age, 62.7 ± 3.9 years) implanted subthalamic electrodes in different treatment conditions were compared with 10 age-matched controls (5 men, 5 women; mean age, 59.6 ± 2.4 years). Vestibular-evoked myogenic potentials were recorded by electromyographic surface electrodes applied to both sternocleidomastoid muscles (band-pass filter, 8-1600 Hz; sampling rate, 5 kHz) and averaged in response to bilateral auditory tone bursts (120 dB SPL; sine waves, 7 ms; 1000 Hz) applied through earphones. Adjusted vestibular-evoked myogenic potential amplitudes were significantly smaller in parkinsonian patients than in controls, in particular in patients without surgery. Administration of dopa, but not subthalamic stimulation, significantly increased amplitudes. Onset latencies were similar for all groups and treatment conditions. Decreased vestibular-evoked myogenic potential amplitudes in parkinsonian patients suggest reduced vestibular nuclei excitability within the brain stem, which is modulated by dopa but not by subthalamic stimulation. This suggests different pathways for the action of both treatment modalities in Parkinson's disease and may explain clinical differences in terms of postural disturbances. © 2012 Movement Disorder Society.

Ocular vestibular evoked myogenic potentials produced by impulsive lateral acceleration in unilateral vestibular dysfunction.

OBJECTIVE: To deduce the connectivity underlying ocular vestibular evoked myogenic potentials (OVEMPs) recorded from two sites and produced by lateral transmastoid stimulation in patients with unilateral vestibular dysfunction. METHODS: OVEMPs were recorded using lateral impulsive stimuli delivered by a hand-held minishaker placed at the mastoid. Twelve patients were tested using the typical OVEMP recording montage placed inferior to the eyes. In a subset of 6 patients, recordings were also made using a lateral electrode montage. The majority of patients were tested following surgery for inner ear disease. Patient responses were compared to those in normal subjects under similar recording conditions. RESULTS: For the inferior montage, regardless of which mastoid was stimulated, deficits were observed only from the eye opposite the affected ear. In contrast, OVEMPs recorded using the lateral electrode montage showed changes on both sides. CONCLUSIONS: OVEMPs produced using lateral transmastoid stimulation and recorded from beneath the eyes are generated by a crossed vestibulo-ocular pathway while the projections underlying the lateral responses are likely to be bilateral. SIGNIFICANCE: The vestibular-ocular connectivity underlying the OVEMPs recorded from inferior and lateral recording sites differs. For clinical use, the inferior recording site is the simplest to interpret.

Hypokalemic weakness in hyperaldosteronism: activity-dependent conduction block.

The authors describe a 48-year-old man who presented with acute weakness. Serum K+ was 1.7 mmol/L, and investigations established hyperaldosteronism. Nerve excitability studies during hypokalemia demonstrated that axons were of high threshold with a fanning out of threshold electrotonus, consistent with hyperpolarization. Activity-dependent conduction block was induced by voluntary contraction. Excitability abnormalities resolved with K+ replacement. Activity-dependent conduction block induced by normal activity may contribute to weakness and paralysis developing with hypokalemia.

Characteristics and clinical applications of vestibular-evoked myogenic potentials.

A recent technique of assessing vestibular function, the vestibular-evoked myogenic potential (VEMP), is an otolith-mediated, short-latency reflex recorded from averaged sternocleidomastoid electromyography in response to intense auditory clicks delivered via headphones. Since their first description 10 years ago, VEMPs are now being used by investigators worldwide, and characteristic changes observed with aging and in a variety of peripheral and central vestibulopathies have been described. Additional methods of evoking VEMPs, which use air- and bone-conducted short-tone bursts, forehead taps, and short-duration transmastoid direct current (DC) stimulation, have been described, and these complement the original technique. Click-evoked VEMPs are attenuated or absent in a proportion of patients with vestibular neuritis, herpes zoster oticus, late Meniere disease, and vestibular schwannomas; their amplitudes are increased and thresholds are pathologically lowered in superior semicircular canal dehiscence presenting with the Tullio phenomenon. VEMPs evoked by clicks and DC are useful when monitoring the efficacy of intratympanic gentamicin therapy used for chemical vestibular ablation. Prolonged p13 and n23 peak latencies and decreased amplitudes have been observed in association with central vestibulopathy. VEMPs evoked by clicks are a robust, reproducible screening test of otolith function. DC stimulation enables differentiation of labyrinthine from retrolabyrinthine lesions; bone-conducted stimuli permit VEMP recording despite conductive hearing loss and deliver a relatively larger vestibular stimulus for a given level of auditory perception.