Vestibulo-Ocular Reflexes

Vestibulo-Ocular Reflexes

The vestibulo-ocular reflexes (VOR) are a physiological mechanism which generates eye rotations that compensate for head movements, especially during locomotion, so stabilizing the retinal image on the fovea. VORs depend upon the integrity of the connections between the semicircular canals of the vestibular system (afferent limb of reflex arc) and oculomotor nuclei in the brainstem (efferent limb). Loss of vestibular function, as in acute bilateral vestibular failure, causes gaze instability due to loss of VORs, causing the symptom of oscillopsia (q.v.) when the head moves. As well as vestibular input, compensatory eye rotations may also be generated in response to visual information (pursuit-optokinetic eye movements) and neck proprioceptive information; anticipatory eye movements may also help stabilize the retinal image.

VORs are also useful in assessing whether ophthalmoplegia results from a supranuclear or infranuclear disorder, since in the former the restriction of eye movement may be overcome, at least in the early stages, by the intact VOR (e.g., the supranuclear gaze palsy in the vertical plane in progressive supranuclear palsy).
VORs are difficult to assess in conscious patients because of concurrent pursuit-optokinetic eye movements, and because rotation of the head through large angles in conscious patients leads to interruption of VORs by vestibular nystagmus in the opposite direction (optokinetic nystagmus). The head impulse test (q.v.) may be used to test VORs in conscious patients, for example those with vertigo in whom vestibular failure is suspected. VOR may also be assessed using a slow (0.5-1.0 Hz) doll’s head maneuver while directly observing the eyes ("catch up" saccades may be seen in the absence of VOR), measuring visual acuity (dynamic visual acuity, or illegible E test; dropping two to three lines on visual acuity with head movement vs. normal if VOR impaired), and ophthalmoscopy (optic disc moves with head if VOR abnormal).
In unconscious patients, slow phase of the VORs may be tested by rotating the head and looking for contraversive conjugate eye movements (oculocephalic responses, doll’s head eye movements) or by caloric testing. VORs are lost in brainstem death.
Another important element of VOR assessment is suppression or cancellation of VOR by the pursuit system during combined head and eye tracking. VOR suppression may be tested by asking the patient to fixate on their thumbs with arms held outstretched while rotating at the trunk or sitting in a swivel chair. VOR suppression can also be assessed during caloric testing: when the nystagmus ceases with fixation, removal of the fixation point (e.g., with Frenzel’s glasses) will lead to recurrence of nystagmus in normals but not in those with reduced or absent VOR suppression. VOR suppression is impaired (presence of nystagmus even with slow head movements) in cerebellar and brainstem disease.

References

Bronstein AM. Vestibular reflexes and positional manoeuvres. Journalof Neurology, Neurosurgery and Psychiatry 2003; 74: 289-293
Leigh RJ, Brandt T. A reevaluation of the vestibulo-ocular reflex: new ideas of its purpose, properties, neural substrate, and disorders. Neurology 1993; 43: 1288-1295

Cross References

Caloric testing; Coma; Doll’s eye maneuver, Doll’s head maneuver; Hallpike maneuver, Hallpike test; Head impulse test; Ocular tilt reaction; Oculocephalic response; Oscillopsia; Supranuclear gaze palsy; Vertigo