Using Unidirectional Rotations to Improve Vestibular System Asymmetry in Patients with Vestibular Dysfunction

Abstract

The vestibular system provides information about head movement and mediates reflexes that contribute to balance control and gaze stabilization during daily activities. Vestibular sensors are located in the inner ear on both sides of the head and project to the vestibular nuclei in the brainstem. Vestibular dysfunction is often due to an asymmetry between input from the two sides. This results in asymmetrical neural inputs from the two ears, which can produce an illusion of rotation, manifested as vertigo. The vestibular system has an impressive capacity for compensation, which serves to rebalance how asymmetrical information from the sensory end organs on both sides is processed at the central level. To promote compensation, various rehabilitation programs are used in the clinic; however, they primarily use exercises that improve multisensory integration. Recently, visual-vestibular training has also been used to improve the vestibulo-ocular reflex (VOR) in animals with compensated unilateral lesions. Here, a new method is introduced for rebalancing the vestibular activity on both sides in human subjects. This method consists of five unidirectional rotations in the dark (peak velocity of 320°/s) toward the weaker side. The efficacy of this method was shown in a sequential, double-blinded clinical trial in 16 patients with VOR asymmetry (measured by the directional preponderance in response to sinusoidal rotations). In most cases, VOR asymmetry decreased after a single session, reached normal values within the first two sessions in one week, and the effects lasted up to 6 weeks. The rebalancing effect is due to both an increase in VOR response from the weaker side and a decrease in response from the stronger side. The findings suggest that unidirectional rotation can be used as a supervised rehabilitation method to reduce VOR asymmetry in patients with longstanding vestibular dysfunction.

Figure 1:. Unidirectional rotation decreases asymmetry between the two sides.

(A) Schematic showing the hypothesis behind the unidirectional rotation. Stimulation of the side with lower response (LR) and inhibition of the side with the higher response (HR, red arrows) will result in a change in commissural inputs as well as direct afferent inputs. This results in an increase in the response of LR neurons and decrease in the asymmetry between the two sides (black arrows). (B) Experimental design and rotational paradigms.

Figure 2:. Short-term and long-term effect of the unidirectional rotation.

(A) In the first session and 70 min after unidirectional rotation, peak eye velocity (°/s) showed a 14% increase in response to rotations toward the side with lower response (LR) and 16% decrease for rotations toward the side with higher response (HR, n = 16). Although these changes were not statistically significant (for LR: 25.0 ± 2.2 vs. 26.75 ± 5.3 °/s, paired Student’s t-test, p = 0.23; for HR: 35.0 ± 3.6 vs. 26.0 ± 4.4 °/s, paired Student’s t-test, p = 0.15), they resulted in a decrease in overall asymmetry. Error bars represent SEM. (B) Corresponding DP values decreased significantly (paired Student’s t-test, p = 0.0006) and reached normal values. Error bars represent SEM. (C) The effect of the unidirectional rotation stayed for a longer time period and was cumulative. Presession values were measured before rehabilitation in a session and post-session values were measured 70 min after rehabilitation in that session. Negative DP values indicate reversal of the direction of asymmetry compared to the beginning of the study. Sessions are comparable to the Figure 1B schematic. Error bars represent SEM. This figure has been modified from Sadeghi et al..