Enhanced vestibulo-ocular reflex to electrical vestibular stimulation in Meniere's disease

Abstract

Meniere's disease is characterized by sporadic episodes of vertigo, nystagmus, fluctuating sensorineural hearing loss, tinnitus and aural pressure. Since Meniere's disease can affect different regions of the vestibular labyrinth, we investigated if electrical vestibular stimulation (EVS) which excites the entire vestibular labyrinth may be useful to reveal patchy endorgan pathology. We recorded three-dimensional electrically evoked vestibulo-ocular reflex (eVOR) to transient EVS using bilateral, bipolar 100-ms current steps at intensities of 0.9, 2.5, 5.0, 7.5 and 10.0 mA with dual-search coils in 12 unilateral Meniere's patients. Their results were compared to 17 normal subjects. Normal eVOR had tonic and phasic spatiotemporal properties best described by the torsional component, which was four times larger than horizontal and vertical components. At EVS onset and offset of 8.9 ms latency, there were phasic eVOR initiation (M = 1,267 °/s(2)) and cessation (M = -1,675 °/s(2)) acceleration pulses, whereas during the constant portion of the EVS, there was a maintained tonic eVOR (M = 9.1 °/s) at 10 mA. However in Meniere's disease, whilst latency of EVS onset and offset was normal at 9.0 ms, phasic eVOR initiation (M = 1,720 °/s(2)) and cessation (M = -2,523 °/s(2)) were enlarged at 10 mA. The initiation profile was a bimodal response, whilst the cessation profile frequently did not return to baseline. The tonic eVOR (M = 20.5 °/s) exhibited a ramped enhancement of about twice normal at 10 mA. Tonic eVOR enhancement was present for EVS >0.9 mA and disproportionately enhanced the torsional, vertical and horizontal components. These eVOR abnormalities may be a diagnostic indicator of Meniere's disease and may explain the vertigo attacks in the presence of declining mechanically evoked vestibular responses.

FIG. 1

The eVOR to EVS of intensities 0.9, 2.5, 5.0, 7.5 and 10.0 mA using a left cathode/right-anode configuration (lc/ra) from A a typical left Meniere’s disease patient (P1, see Table 1) compared to B an age-matched normal subject. In Meniere’s disease, ipsilesional torsional, vertical and horizontal eVOR were enhanced. Tonic eVOR in Meniere’s disease displayed a ramped enhancement which increased after phasic eVOR initiation till phasic eVOR cessation. Phasic eVOR initiation showed a bimodal response pattern, and phasic cessation did not return the tonic eVOR velocity past the baseline like normal eVOR (see arrows).

FIG. 2

Group means of the ipsilesional and contralesional eVOR from A Meniere’s disease patients (N = 12) compared to B normal subjects (N = 17) in response to EVS at 0.9, 2.5, 5.0, 7.5 and 10.0 mA in left-cathode/right-anode (lc/ra) or right-cathode/left-anode (rc/la) configurations. Meniere’s disease showed eVOR enhancements in torsional, vertical and horizontal components when compared to normal. In Meniere’s disease, ipsilesional tonic eVOR displayed a ramped enhancement which increased after phasic eVOR initiation till phasic eVOR cessation. Ipsilesionally, phasic eVOR initiation showed a bimodal response pattern, and phasic cessation did not return the tonic eVOR velocity past the baseline like normal eVOR (see arrows). Contralesional tonic and phasic eVORs were enhanced without the abnormal tonic and phasic eVOR characteristics.

FIG. 3

A Variations of ipsilesional tonic eVOR velocity in unilateral “definite” Meniere’s disease. All patients showed enhanced step and ramped torsional and horizontal components. At EVS offset, some patients returned to the baseline normally, whilst others were slower. Horizontal component in patient (P9) was inverted. These variations in response patterns probably reflect the patchy nature of Meniere’s pathology and the disease stage in each patient. B The ipsilesional tonic eVOR velocity in an early “possible” Meniere’s disease patient (P8) shows similar enhanced ramped torsional and horizontal components.

FIG. 4

Comparison of the group means ± 95 % confidence intervals of ipsilesional and contralesional A tonic eVOR and B phasic eVOR (initiation and cessation) in Meniere’s disease (N = 12) to normal subjects (N = 17). Comparisons of group mean magnitude of C tonic eVOR and D phasic eVOR in Meniere’s disease to normal subjects. Tonic eVOR was greater than normal for EVS intensities of 2.5, 5.0, 7.5 and 10.0 mA. Phasic eVOR was greater than normal only for EVS at 10.0 mA. Mean tonic eVOR magnitude at 10.0 mA EVS showed that total response was much larger than normal. Phasic eVOR magnitude at 10.0 mA EVS showed that a bimodal response with increased pulse duration in Meniere’s disease.