The Vestibulo-ocular Reflex and Head Impulse Testing

Abstract

The major role of the vestibulo-ocular reflex (VOR) is to stabilize the eyes for clear vision during movement in daily life. Periodic head movements during normal life activities move the head at fundamental frequencies of 2–4 Hz, with harmonics extending above these frequencies. The other ocular control systems are relatively insensitive above 2 Hz and therefore the VOR functions as the primary control system for visual stabilization during locomotion. Traditional testing using caloric stimulation of the lateral semicircular canal is limited to a very low-frequency response, and only one canal of the labyrinth is tested. Traditional rotational chair systems test only the lateral semicircular canal and at relatively low-frequency levels. At a constant rotational velocity, the semicircular canals are not stimulated, only with acceleration or deceleration. This chapter is to cover active and passive head movements to test the VOR from 2 to 6 Hz, using the autorotational test and the video head impulse test (vHIT) that simulates more authentic daily VOR challenges. Eye movements resulting from low-frequency rotational testing are composed of slow and fast phases of nystagmus. The slow phase is directed by VOR, and the fast phase is a result of compensation by the paramedian pontine reticular formation which causes the saccadic eye movements. Linear analysis of the VOR cumulative eye position results in relatively accurate gain and phase because of the apparent linearity of the system. Phase and gain change abnormalities are the basis of the testing paradigm.