Vestibulo-ocular reflex

By Marcello Cherchi, MD PhD

For patients

The vestibulo-ocular reflex (VOR) is a function that keeps your eyes steady on a target even when your head moves. Many diseases can interfere with the proper functioning of the VOR, and some of these diseases can make a person feel disequilibrium. Your doctor may perform one or more bedside tests to screen for a problem with the VOR, but more definitive assessment may require testing with special equipment.

For clinicians

Practical summary

The vestibulo-ocular reflex (VOR) maintains an image stable on the retina despite rotational or translational movements of the head.

Physiology and neuroanatomy

The function of the vestibulo-ocular reflex (VOR) is to stabilize an image on the retina despite rotational and/or translational movements of the head (Rinaudo et al. 2019). The VOR consists of a the rotational VOR (which detects angular acceleration) and the translational VOR (which detects linear acceleration). The rotational VOR (rVOR) can be assessed to some degree with bedside testing, and in greater detail with instrumented otovestibular testing. The translational VOR (tVOR) is more difficult to assess, and usually requires instrumentation in a special research laboratory.

The Figure below, from Zee (Zee 2006), illustrates rotational and translational movements.

In tabular format:

	Rotation around axis (angular movement)	Translation along axis (linear movement)
	When acceleration around this axis occurs, the ocular motor responses are driven by the canal-ocular reflex	When acceleration along this axis occurs, the ocular motor responses are driven by the otolith-ocular reflex
Inter-aural axis (“left and right”)	Pitch	Heave
Rostro-caudal axis (“up and down”)	Yaw	Bob
Naso-occipital axis (“fore and aft”)	Roll	Surge

The rotational vestibulo-ocular reflex (rVOR)

Ideally the rotational VOR (rVOR) should rotate the eyes within the orbits by an amount that is equal in magnitude, but opposite in direction, to that of the head, thereby offsetting the head movement and maintaining the point of regard stable on the fovea (in other words, the gain of eye rotation to head rotation is 1.0). The rVOR responds to pitch (rotation around the inter-aural axis), yaw (rotation around the rostro-caudal axis) and roll (rotation around the naso-occipital axis).

The neuroanatomy of the rVOR is well understood (Bronstein, Patel, Arshad 2015). During rotational acceleration of the head towards one side there is excitation of the ipsilateral lateral semicircular canal, and inhibition of the contralateral lateral semicircular canal. Afferent vestibular excitation from a given lateral semicircular canal passes to the vestibular (Scarpa’s) ganglion, through the superior division of the vestibular nerve and into the ipsilateral medial vestibular nucleus (MVN) situated in the mid dorsal portion of the lower pons and upper medulla. From the MVN there are some projections through the ascending tract of Dieters to the ipsilateral oculomotor nucleus which stimulates contraction of the medial rectus of the ipsilateral eye; there are other projections to the contralateral abducens nucleus which in turn stimulates contraction of the lateral rectus of the contralateral eye. In addition, from the MVN, there are projections to the ipsilateral abducens nucleus; from the abducens nucleus there are projections that inhibit the ipsilateral lateral rectus, and other projections to the contralateral oculomotor nucleus from which projections inhibit the contralateral medial rectus. Afferent vestibular inhibition from a given lateral semicircular canal will have effects opposite that described above.

The VOR can be assessed for any pair of coplanar semicircular canals; the relevant neuroanatomy is different for each canal (Bronstein, Patel, Arshad 2015). The Figure below, from Leigh and Zee (Leigh and Zee 2015), illustrates the neuroanatomy of all rotational vestibular ocular reflexes.

The VOR can adapt by altering its gain (Watanabe, Hattori, Koizuka 2003). The most common circumstance in which such adaptation occurs is when a patient dons refractive lenses (spectacles or contact lenses) to correct for hyperopia (farsightedness) or myopia (nearsightedness) (Cannon et al. 1985). Such adaptive changes in the rVOR gain occur relatively quickly and can be durable even after the refraction is discontinued (Mahfuz et al. 2020). This phenomenon applies mostly to spectacles, because contact lenses move with the eyeball and thus do not induce adaptive changes to the rVOR (Eggers and Zee 2003).

Because of this, if a patient habitually uses lenses (whether spectacles or contact lenses) for visual correction, then they should also wear them during testing (bedside or instrumented) of the rotational vestibulo-ocular reflex, otherwise one may obtain false positive results.

What tests can assess the rotational vestibulo-ocular reflex?

On bedside testing, the middle frequency range of the vestibular tuning spectrum can be evaluated with the ophthalmoscope test and with dynamic visual acuity testing, while the high frequency range can be tested with head impulse testing.

On instrumented otovestibular testing, the low frequency range of the vestibular tuning spectrum can be evaluated with caloric testing, the middle frequency range can be evaluated with rotatory chair testing, and the high frequency can be evaluated with video head impulse testing.

The translational vestibulo-ocular reflex (tVOR)

The translational vestibulo-ocular reflex (tVOR), also called the linear VOR or otolith-ocular reflex, maintains an image stable on the retina despite translational movements of the head — mostly heave (translation along the inter-aural axis) and bob (translation along the rostro-caudal axis), less surge (translation along the naso-occipital axis).

The translational VOR (tVOR) responds to translational movements, specifically linear acceleration, along the inter-aural (heave) axis (Crane et al. 2003; King et al. 2019; Ramat, Straumann, Zee 2005; Ramat and Zee 2002, 2003; Tian, Crane, Demer 2003; Tian et al. 2002; Walker, Shelhamer, Zee 2004), naso-occipital (surge) axis (Seidman, Paige, Tomko 1999; Semrau, Wei, Angelaki 2006; Tian, Mokuno, Demer 2006), rostro-caudal (bob) axis (Bush and Miles 1996; Cheng and Walker 2016; Kutz et al. 2020; Liao et al. 2008; Liao et al. 2009) or oblique axes (Tomko and Paige 1992). The tVOR is mediated by the otolith system, which feeds into the neural integrator (Hegemann et al. 2000) and can be modulated by viewing distance (Busettini, Miles, Schwarz 1991; Busettini et al. 1994; Cheng and Walker 2016; King et al. 2019; Paige et al. 1998; Schwarz and Miles 1991; Wei and Angelaki 2004) and other visual influences (Dits, King, van der Steen 2013; Gianna, Gresty, Bronstein 2000; Hashiba et al. 1995; Liao et al. 2011; Paige et al. 1998), and by somatosensory inputs (Kutz et al. 2020). The tVOR can be adversely affected by various pathologies such as unilateral vestibular weakness (Crane et al. 2005; Lempert et al. 1998; Lempert, Gresty, Bronstein 1999; Tian, Ishiyama, Demer 2007), bilateral vestibular weakness (Lempert, Gresty, Bronstein 1999), cerebellar disorders (Anastasopoulos et al. 1998; Baloh, Yue, Demer 1995; Crane, Tian, Demer 2000) and skew deviation (Schlenker et al. 2009). Many natural head movements comprise a mix of rotational and translational movements, thereby activating rVOR and tVOR simultaneously (Anastasopoulos et al. 1996; Angelaki, Green, Dickman 2001; Branoner and Straka 2018; Crane and Demer 1997, 1999; Crane, Tian, Demer 2000; Koizuka et al. 2000; Laurens, Straumann, Hess 2010; McCrea and Chen-Huang 1999; Meng et al. 2005; Paige et al. 1998; Ramaioli et al. 2019; Schneider et al. 2014).

What tests can assess the translational vestibulo-ocular reflex?

The tVOR is more difficult to study than the rVOR. Accurate study requires specialized equipment largely restricted to research laboratories, such as human centrifuges, Moog platforms, parallel swings, and linear accelerating sleds.

Although the ocular motor responses of the tVOR are harder to study, it is still possible to assess the otolith system using cervical vestibular evoked myogenic potentials and ocular vestibular evoked myogenic potentials.

Diseases that affect the vestibulo-ocular reflex

Any lesion affecting any of the neuroanatomical pathways described earlier can cause vestibular weakness and thereby influence the vestibulo-ocular reflex — and here we use the term “lesion” loosely, since not only focal lesions, but also systemic issues (such as pharmacologic vestibular suppression) can cause such affectation.

It is less common for the vestibulo-ocular reflex to be abnormally elevated (i.e., the gain is >1).  Examples include some cerebellar ataxias (Zee et al. 1976) and nitrous oxide toxicity (Hamilton et al. 1986).

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