Horizontal Eye Position Affects Measured Vertical VOR Gain on the Video Head Impulse Test.

Leigh A Mcgarvie,Marta Martinez-Lopez,Ian S Curthoys,Ann M Burgess,Hamish G Macdougall

doi:10.3389/fneur.2015.00058

Abstract

Background/hypothesis: With the video head impulse test (vHIT), the vertical VOR gain is defined as (vertical eye velocity/vertical head velocity), but compensatory eye movements to vertical canal stimulation usually have a torsional component. To minimize the contribution of torsion to the eye movement measurement, the horizontal gaze direction should be directed 40° from straight ahead so it is in the plane of the stimulated canal plane pair. Hypothesis: as gaze is systematically moved horizontally away from canal plane alignment, the measured vertical VOR gain should decrease.Study design: Ten healthy subjects, with vHIT measuring vertical eye movement to head impulses in the plane of the left anterior-right posterior (LARP) canal plane, with gaze at one of five horizontal gaze positions [40°(aligned with the LARP plane), 20°, 0°, −20°, −40°].Methods: Every head impulse was in the LARP plane. The compensatory eye movement was measured by the vHIT prototype system. The one operator delivered every impulse.Results: The canal stimulus remained identical across trials, but the measured vertical VOR gain decreased as horizontal gaze angle was shifted away from alignment with the LARP canal plane.Conclusion: In measuring vertical VOR gain with vHIT the horizontal gaze angle should be aligned with the canal plane under test.

Highlights

The new video head impulse test provides objective, accurate measures of the semicircular canal response to its physiological stimulus – the angular acceleration during head rotation [1,2,3,4]
Results:The canal stimulus remained identical across trials, but the measured vertical vestibulo-ocular reflex (VOR) gain decreased as horizontal gaze angle was shifted away from alignment with the left anterior-right posterior (LARP) canal plane
In measuring vertical VOR gain with video head impulse test (vHIT) the horizontal gaze angle should be aligned with the canal plane under test

Summary

Introduction

The new video head impulse test (vHIT) provides objective, accurate measures of the semicircular canal response to its physiological stimulus – the angular acceleration during head rotation [1,2,3,4]. The subject is instructed to maintain fixation on an earth-fixed target at a distance of about a meter while the operator delivers small, brief, passive, head turns in the plane of the semicircular canals being tested. These turns (head impulses) are unpredictable in size, direction, velocity, and timing, and are measured exactly. The compensatory eye movement response is recorded by a small, light, fast, head-mounted camera to measure the VOR. This is a major step forward in clinical evaluation of peripheral vestibular function.

Methods

Results

Discussion

Conclusion