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

Area of Science:

• Vestibular neurophysiology
• Oculomotor system research
• Diagnostic testing in audiology and neurology

Background:

• The video head impulse test (vHIT) measures vertical vestibulo-ocular reflex (VOR) gain.
• Compensatory eye movements in response to vertical canal stimulation often include a torsional component.
• Minimizing torsion's influence is key for accurate vertical VOR gain assessment.

Purpose of the Study:

• To investigate how horizontal gaze direction affects vertical VOR gain measurement using vHIT.
• To test the hypothesis that measured vertical VOR gain decreases as horizontal gaze deviates from the stimulated canal plane.

Main Methods:

• Ten healthy subjects underwent vHIT with head impulses in the left anterior-right posterior (LARP) canal plane.
• Vertical eye movements were recorded at five horizontal gaze positions: 40°, 20°, 0°, -20°, and -40° relative to the LARP plane.
• A prototype vHIT system was used to measure compensatory eye movements.

Main Results:

• Vertical VOR gain consistently decreased as the horizontal gaze angle shifted away from alignment with the LARP canal plane.
• The canal stimulus intensity remained constant across all tested gaze positions.

Conclusions:

• Optimal alignment of horizontal gaze with the tested canal plane is essential for accurate vertical VOR gain measurement using vHIT.
• Deviating gaze from the canal plane leads to an underestimation of vertical VOR gain.