Abstract
We examined the eye movement response patterns of a group of patients with bilateral vestibular loss (BVL) during suppression head impulse testing. Some showed a new saccadic strategy that may have potential for explaining how patients use saccades to recover from vestibular loss. Eight patients with severe BVL [vestibulo-ocular reflex (VOR) gains less than 0.35 and absent otolithic function] were tested. All patients were given the Dizziness Handicap Inventory and questioned about oscillopsia during abrupt head movements. Two paradigms of video head impulse testing of the horizontal VOR were used: (1) the classical head impulse paradigm [called head impulse test (HIMPs)]-fixating an earth-fixed target during the head impulse and (2) the new complementary test paradigm-fixating a head-fixed target during the head impulse (called SHIMPs). The VOR gain of HIMPs was quantified by two algorithms. During SHIMPs testing, some BVL patients consistently generated an inappropriate covert compensatory saccade during the head impulse that required a corresponding large anti-compensatory saccade at the end of the head impulse in order to obey the instructions to maintain gaze on the head-fixed target. By contrast, other BVL patients did not generate this inappropriate covert saccade and did not exhibit a corresponding anti-compensatory saccade. The latencies of the covert saccade in SHIMPs and HIMPs were similar. The pattern of covert saccades during SHIMPs appears to be related to the reduction of oscillopsia during abrupt head movements. BVL patients who did not report oscillopsia showed this unusual saccadic pattern, whereas BVL patients who reported oscillopsia did not show this pattern. This inappropriate covert SHIMPs saccade may be an objective indicator of how some patients with vestibular loss have learned to trigger covert saccades during head movements in everyday life.
Highlights
Bilateral vestibular loss (BVL) patients are often severely handicapped during head movements in their daily life
In suppression head impulse paradigm (SHIMP), after each rapid head impulse, the healthy subject generated a large anticompensatory saccade that was in the same direction as the head turn, in order to return their gaze to the head-fixed target due to healthy horizontal vestibulo-ocular reflex (VOR) (HVOR) (Figure 2B)
In the SHIMPs protocol the apparently automatic covert saccade is inappropriate—in the usual head impulse test (HIMP) paradigm such a covert saccade would be appropriate because it would be acting to compensate for the head turn to keep the eyes on the target, but in SHIMPs the covert saccade is opposite to the instructed task, because it takes the eyes off the target
Summary
Bilateral vestibular loss (BVL) patients are often severely handicapped during head movements in their daily life. Patients often complain of oscillopsia when turning the head rapidly in the horizontal plane, surprisingly some do not complain of oscillopsia during such rapid head movements. In HIMPs, subjects are instructed to maintain gaze on an earth-fixed target during brief, abrupt, unpredictable, horizontal head turns to the left or right. The compensatory horizontal slow phase eye velocity matches head velocity, so the gain of the horizontal VOR (HVOR) is around 1.0 [7] and so overt or covert compensatory saccades are only small or are absent [7]. BVL patients show significantly lower HVOR gain for both horizontal directions and always generate large compensatory covert and/ or overt saccades to regain the earth-fixed fixation target [8]. We use the standard terminology [9]: a “compensatory” saccade is one which is opposite to the direction of head turn whereas an “anti-compensatory” saccade is one which is in the same direction as the direction of head turn
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