Visual Input Is the Main Trigger and Parametric Determinant for Catch-Up Saccades During Video Head Impulse Test in Bilateral Vestibular Loss

Abstract

Patients with vestibular deficit use slow eye movements or catch-up saccades (CUS) to compensate for impaired vestibulo-ocular reflex (VOR). The purpose of CUS is to bring the eyes back to the visual target. Covert CUS occur during high-velocity head rotation and overt CUS are generated after head rotation has stopped. Dynamic visual acuity is improved with an increased rate and gain of CUS. Nevertheless, the trigger and the parametric determinants of CUS are still under debate. To clarify the underlying mechanism, especially the visual contribution, we analyzed the number, amplitude and latencies of the CUS in relation with the extent of VOR deficiency. The head and eye movements were recorded in 17 patients with bilateral vestibular loss (BVL) and in 33 subjects with normal VOR gain using the Video Head Impulse Test (vHIT) in two conditions: with visible target and in darkness with an imaginary target. Our study shows that in darkness without visible target the number of CUS is significantly reduced and the relationship between the amplitude of CUS and gaze position error is lost. Results showed that there is a correlation between the number of CUS and the drop in VOR gain. CUS occurring during the head movement and when the head remained still were not always sufficiently accurate. Up to four consecutive CUS could be required to bring eyes back to the visible target. A positive correlation was found between the amplitude of overt saccades with visible target and the gaze position error, namely the remaining eye movement to reach the target. These results suggest that the visual inputs are the main trigger and parametric determinant of the CUS or at least the presence of a visual target is necessary in most cases for a CUS to occur.

Head impulse compensatory saccades: Visual dependence is most evident in bilateral vestibular loss

“…As reported by numerous previous investigators [5,7,22,23,25,[57][58][59][60][61], we found that as the VOR loss increased compensatory saccade characteristics generally followed a similar pattern: the amplitude, frequency, and clustering increased while the latency became earlier (Fig 8). The rate of these changes to the reduction in VOR gain depended on both the visual condition and saccade number.…”

“…In contrast, after the bilateral loss of the compensatory velocity reflex in BVL, we found that subjects relied heavily in visual input-possibly either a position or velocity signal [54]-to generate the required position change, especially after the head stopped moving (Fig 1). Previous studies of compensatory saccades and posture after BVL also found an increased dependence on vision for position corrections [7,22,23,55]. Surprisingly, we found that in the horizontal plane a single bilaterally deafferented (BVD) subject could still regularly generate early saccades without visual fixation (Fig 1), supporting a proprioceptive trigger [26], although these saccades were much smaller in amplitude and scattered in latency.…”

“…Saccades were more affected in the vertical planes (Fig 1). Like BVD, saccades after BVL show a much greater dependence on visual fixation (Fig 3) [23]. The early latency, large amplitude, very frequent saccades generated by complete BVD with vision and complete UVD (UVD ipsi ) without vision demonstrates that: for early saccades, residual vestibular input is just as adequate as visual input, and that early saccade triggering has sensory flexibility.…”

“…Vestibular nuclei asymmetry, from visual and vestibular inputs [19], also triggers 'quick-phase' saccades in precisely the opposite direction of ongoing smooth VOR drift [20,21], while head-impulse compensatory saccades are required in the same direction as the smooth VOR [4,5,22]. In complete darkness, to imaginary targets compensatory saccades were found to be reduced in frequency and amplitude after sub-total bilateral vestibular loss (BVL) [23]. While saccades were abolished in subjects with complete BVL when tested in the dark [24], this effect of vision was not complete for short-latency saccades after one subject with sub-total BVL, implying any residual vestibular input may also contribute [25].…”

Quantitative Analysis of Saccade Gain in Video Head Impulse Testing

“…9 However, another study showed that there was no correlation between covert saccades and VOR gain. 27 In the patient group, our data showed that CS gain was negatively correlated with VOR gain and the correlation was strong, which suggested that the amplitude of combined CSs (overt and covert CSs) might be programmed based on residual VOR gain. In case of vestibular deficits, covert saccades have been proved to account for most eye displacement-compensating head movements and improved dynamic visual acuity, although they were not corrected with gaze position errors.…”