Abstract
Objective: This study sought to investigate whether the size of the target used in the horizontal vHIT has an effect on the saccade profile of healthy subjects, and to expand upon previous work linking age to the existence of small vHIT saccades.Methods: Forty eight participants were recruited between 18 and 77 years of age, with no history of vestibular, oculomotor or neurological conditions and a visual acuity of at least 0.3 LogMAR. Participants underwent four consecutive horizontal vHIT trials using the standard target size and three smaller targets. VOR gain and metrics for saccadic incidence, peak eye velocity and latency were then extracted from results.Results: Target size was a statistically significant influence on saccade metrics. As target size increased, saccadic incidence decreased while peak eye velocity and latency increased. However, a potential order effect was also discovered, and once this was corrected for the remaining effect of target size was small and is likely clinically insignificant. The effect of age was much stronger than target size; increasing age was strongly positively correlated with saccadic incidence and showed a medium size correlation with peak velocity, though not with saccadic latency.Conclusion: While this study suggests that target size may have a statistically significant impact on the vHIT saccade profile of normal subjects, age has a greater influence on the incidence and size of small vHIT saccades.
Highlights
The video head impulse test is a quantitative adaptation of the clinical head impulse test [1], and enables functional assessment of the high-frequency angular vestibulo-ocular reflex (VOR) in all three planes of head rotation [2, 3]
Mann-Whitney U-test (U = 231, p < 0.01). This is a known phenomenon in video head impulse test (vHIT) related to monocular recording; gains of the adducting eye always exceed those of the abducting eye [4, 20]
No alternative target sizes reduced the incidence of small saccades to zero, and this study suggests that is the target itself not the sole cause of small saccades, but that vHIT can be performed with a number of different sized targets without grossly affecting the saccade profile in a clinical context
Summary
The video head impulse test (vHIT) is a quantitative adaptation of the clinical head impulse test [1], and enables functional assessment of the high-frequency angular vestibulo-ocular reflex (VOR) in all three planes of head rotation [2, 3]. Jay Age Target Size vHIT calculates a measure of VOR gain (the ratio of eye to head velocity), which can be compared to stratified age-related normative data to diagnose vestibular dysfunction [4]. VHIT research focused predominantly on VOR gain and the presence or absence of catch-up saccades in known cases of vestibular loss. The existence of poorly defined small saccades in all vHIT results could lead to diagnostic uncertainty in the clinical environment. Whilst a vHIT result of low VOR gain combined with repeatable, high peak velocity catch-up saccades is unequivocal, there can be diagnostic uncertainty associated with the presence of saccades with normal VOR gain. There is a need for further investigation and understanding of vHIT saccade profiles in normal human physiology
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