See also: Physiology of the VOR
The vestibulo-ocular reflex (VOR) assures clear vision when the head moves, since it maintains foveation. The VOR therefore naturally maintains gaze when the head and body are moving, keeping the visual environment steady.
Typically, the eyes and the head make one single movement to a target. The head, having greater inertia than the eye, moves after the eye has made a saccade in the orbit. With small saccades, eye movement contributes most to the gaze shift, with head movement adding to both the speed and amplitude of the saccade, during which gaze stabilising reflexes are temporarily turned off. When the predetermined end-point of the gaze change is reached at the end of the saccade, the head continues to move, and VOR is turned on, and consequently the eyes move in the opposite direction, exactly counteracting the ongoing head movement and establishing fixation.
The VOR has three major planes of action: the rotational vestibulo-ocular reflex (rVOR) produces a slow-phase eye movement that compensates for horizontal (yaw), vertical (pitch) or torsion (roll) head rotations2. These three planes represent the three-dimensional space in which the vestibular and ocular motor systems responsible for spatial orientation, perception of self-movement, stabilization of gaze, and postural control operate.
The normal rVOR is perfectly compensatory in direction and speed during yaw and pitch head rotations. However, in roll rotation, the eye movements are also well aligned with the head in direction, but the gain (eye speed/head speed) is lower.
From: Arcoverde E, Duarte R, Barreto R, Magalhaes, J, Bastos C, Ing Ren T,Cavalcanti G. 2014. Enhanced real-time head pose estimation system for mobile device. Integrated Computer Aided Engineering. 21. 281-293. 10.3233/ICA-140462.
The VOR connects a set of extraocular eye muscles that are aligned with the respective planes of the horizontal, anterior, and posterior canals. The canals of both labyrinths form functional pairs in the horizontal and vertical working planes.
The canals are excited or inhibited in pairs: the horizontal right and left pair, and the vertical anterior canal of one side with the posterior canal of the opposite side.
The vertical planes of ‘pitch’ and ‘roll’ are a result of the wiring connecting the two vertical canals that are diagonal to the sagittal plane in the head. The canal pair functions determine rotatory acceleration and reacts to rotational movements of the head in the corresponding plane.
Without an intact VOR, the visual world would “slip” on the retina with every head movement, resulting in blurred vision. At usual frequences of everyday life, the gain of the VOR is 1.0, that is, eye velocity is precisely compensated by head velocity.
An exception, and a common example of a change in VOR gain, would be a patient wearing magnifying spectacles that magnify by 1.2. If the patient were to turn their head to the left, the image of the world would move at 120 degrees per second to the right (relative to the head) and not 100 degrees per second. In this situation, the eyes therefore abruptly are moving relatively too slowly and do not compensate accurately. To the patient, the world appears to shift to their right at 20 degrees per second, resulting in oscillopsia. Under such conditions, motor learning adjusts the gain of the VOR to produce more accurate eye motion, which is referred to as VOR adaptation, an indication of the underlying ability of the VOR to compensate to changing circumstances3. A further example of the VOR needing to be continuously adjusted is in the first years of life to order to compensate for significant changes in head circumference (~30% in the first year)4.
Perhaps the best known clinical case is a report by a physician, JC, in 1952, who suffered from complete vestibular loss following treatment with streptomycin (an ototoxic antibiotic) for leg infection. He described his symptoms following treatment, and commented that “by bracing my head between two of the metal bars at the head of the bed I found I could minimize the effect of the pulse beat that made the letters of the page jump and blur” and that when walking “in these corridors I had the peculiar sensation that I was inside a flexible tube, fixed at the end nearest me but swaying free at the far end”5.
Causes of dysfunction
- Bilateral vestibular loss
- Patients with bilateral medial longitudinal fasciculus injuries commonly complain of oscillopsia with head movements due to bilateral impairment of the vertical VOR. Information from the vertical (anterior + posterior) semicircular canals also travels via the MLF. This can be confirmed by impairment of the vertical dynamic visual acuity and with vertical head impulse testing (HIT) in the planes of anterior and posterior SCCs. Such patients would have a normal horizontal VOR (at least in the abducting eye), but would also have abnormal vertical smooth pursuit due to the the MLF lesions.
Examination
Normal subjects and patients with loss only of the VOR or only of smooth pursuit+optokinetic reflexes are still able to make smooth compensatory eye movements and maintain gaze.
Only patients with loss of both the VOR and smooth pursuit+optokinetic reflexes are unable to make fully compensatory smooth eye movements and therefore make a series of observable saccades in the compensatory direction to maintain gaze6 (Patient with CANVAS shown here)
The most common bedside test of the examination of the VOR is the head impulse test, which examines the VOR at a high frequency.
The VOR may also be evaluated at the bedside using visual enhancement of the VOR, and visual cancellation of the VOR: the patient is instructed to continuously fixate a feature of the examiner face (nose or bridge of nose), or an object across the room. The examiner then oscillates the patient’s head from side to side, at a frequency of about 0.5 to 1 Hz. In the absence of VOR, the patient’s eye movements will not be smooth but will be interrupted by “catch up” saccades towards the fixation target7.
(vv)VOR.mp4(tt)
From: U-M Kellogg Eye Center in Ann Arbor. Normal and Abnormal Eye Movements. Retrieved from: https://www.youtube.com/watch?v=rRDDKKqkdTg
(vv)VOR_Slow_and_Fast_.mp4(tt)
From: Gold D, Zee D. VOR (Slow and Fast). Video. [Neuro-Ophthalmology Virtual Education Library: NOVEL Web Site]. 2018. Available at: https://collections.lib.utah.edu/ark:/87278/s63b97tz
(vv)Video_e-1.mp4(tt)
From: Petersen JA, Wichmann WW, Weber KP. The pivotal sign of CANVAS. Neurology. 2013 Oct 29;81(18):1642-3. doi: 10.1212/WNL.0b013e3182a9f435. PMID: 24166963.