Are Covert Saccade Functionally Relevant in Vestibular Hypofunction?

Abstract

The vestibulo-ocular reflex maintains gaze stabilization during angular or linear head accelerations, allowing adequate dynamic visual acuity. In case of bilateral vestibular hypofunction, patients use saccades to compensate for the reduced vestibulo-ocular reflex function, with covert saccades occurring even during the head displacement. In this study, we questioned whether covert saccades help maintain dynamic visual acuity, and evaluated which characteristic of these saccades are the most relevant to improve visual function. We prospectively included 18 patients with chronic bilateral vestibular hypofunction. Subjects underwent evaluation of dynamic visual acuity in the horizontal plane as well as video recording of their head and eye positions during horizontal head impulse tests in both directions (36 ears tested). Frequency, latency, consistency of covert saccade initiation, and gain of covert saccades as well as residual vestibulo-ocular reflex gain were calculated. We found no correlation between residual vestibulo-ocular reflex gain and dynamic visual acuity. Dynamic visual acuity performance was however positively correlated with the frequency and gain of covert saccades and negatively correlated with covert saccade latency. There was no correlation between consistency of covert saccade initiation and dynamic visual acuity. Even though gaze stabilization in space during covert saccades might be of very short duration, these refixation saccades seem to improve vision in patients with bilateral vestibular hypofunction during angular head impulses. These findings emphasize the need for specific rehabilitation technics that favor the triggering of covert saccades. The physiological origin of covert saccades is discussed.

Keywords: Bilateral vestibulopathy; Dynamic visual acuity; Eye movement record; Head impulse test; Oscillopsia; Vestibular areflexia.

Fig. 1

Schematic representation of change in head position and relative head velocity with time during measurement of the dynamic visual acuity. The optotype (letter “E” in this example) appears on the screen only when head rotation velocity is within a range of 200 to 300°/s for more than 50 ms (“Visual Stimuli”)

Fig. 2

Example of cursor placements on a covert saccade. Eye (^_____) and head (_{·········}) position and velocity traces for a head impulse test in a patient. The head position and velocity waveforms are inverted relative to eye position and velocity waveforms. The vertical cursors represent the starting position (PS) and ending positions (PE), the time of starting position (TS) and the time of ending position (TE) of the movement, and the maximum velocity of the movement (V)

Fig. 3

Boxplots of the gain of the vestibulo-ocular reflex (VOR), of covert saccades (CS), and of overt saccades (OS) during head impulse testing showing lower extreme, 1st quartile, median (cross), median (larger horizontal line), 3rd quartile, and upper extreme. Statistical differences (*) are considered significant with a p < 0.05

Fig. 4

Linear regression of CS rs_latency and DVA measures