Autonomic laterality in caloric vestibular stimulation

Abstract

Background: Caloric stimulation of the vestibular system is associated with autonomic response. The lateralization in the nervous system activities also involves the autonomic nervous system.

Aim: To compare the effect of the right and left ear caloric test on the cardiac sympathovagal tone in healthy persons.

Methods: This self-control study was conducted on 12 healthy male volunteers. The minimal ice water caloric test was applied for vestibular stimulation. This was done by irrigating 1 milliliter of 4 ± 2 °C ice water into the external ear canal in 1 s. In each experiment, only one ear was stimulated. For each ear, the pessimum position was considered as sham control and the optimum position was set as caloric vestibular stimulation of horizontal semicircular channel. The order of right or left caloric vestibular stimulation and the sequence of optimum or pessimum head position in each set were random. The recovery time between each calorie test was 5 min. The short-term heart rate variability (HRV) was used for cardiac sympathovagal tone metrics. All variables were compared using the analysis of variance.

Results: After caloric vestibular stimulation, the short-term time-domain and frequency-domain HRV indices as well as, the systolic and the diastolic arterial blood pressure, the respiratory rate and the respiratory amplitude, had no significant changes. These negative results were similar in the right and the left sides. Nystagmus duration of left caloric vestibular stimulations in the optimum and the pessimum positions had significant differences (e.g., 72.14 ± 39.06 vs 45.35 ± 35.65, P < 0.01). Nystagmus duration of right caloric vestibular stimulations in the optimum and the pessimum positions had also significant differences (e.g., 86.42 ± 67.20 vs 50.71 ± 29.73, P < 0.01). The time of the start of the nystagmus following caloric vestibular stimulation had no differences in both sides and both positions.

Conclusion: Minimal ice water caloric stimulation of the right and left vestibular system did not affect the cardiac sympathovagal balance according to HRV indices.

Keywords: Autonomic; Caloric stimulation; Heart rate variability; Laterality; Vestibular system.

Figure 1

Schematic drawing of horizontal semi-circular canal axis and its alignment with gravity plane in supine and upright orientations. A: Arrow shows the angle between horizontal canal and vertical line in supine position; B: Arrow shows the vertical position of horizontal canal following the 30˚ forward tilt in supine position e.g., the optimum position in supine state; C: Arrow shows the horizental position of horizontal canal following the 60˚ backward tilt in supine position e.g., the pessimum position in supine state; D: Arrow shows the angle between horizontal canal and horizental line in upright position; E: Arrow shows the horizental position of horizontal canal following the 30˚ forward tilt in upright position e.g., the pessimum position in upright state; F: Arrow shows the vertical position of horizontal canal following the 60˚ backward tilt in upright position e.g., the optimum position in upright state. In optimum position the horizontal canal axis is vertical and in the gravity plane (B and F). In pessimum position this axis is horizontal and eliminates the effect of gravity on endolymph convection (C and E).

Figure 2

The study design and timeline. Each horizontal arrow is equal to 5 min.

Figure 3

The mean ± SD of respiratory rate before vestibular stimulation, after it and following the last recovery stage (n = 12).

Figure 4

The mean ± SD of respiratory amplitude before vestibular stimulation, after it and following the last recovery stage (n = 12).

Figure 5

The mean ± SD of nystagmus duration in seconds after caloric vestibular stimulation of each ear in the optimum (+30º) and in the pessimum (+60º) conditions (n = 12).