Testing the vestibular-ocular reflexes: abnormalities of the otolith contribution in patients with neuro-otological disease

Interaction between otolith organ and semicircular canal vestibulo-ocular reflexes during eccentric rotation in humans

Controversy remains about the linearity of the interaction between horizontal semicircular canal and otolith organ vestibulo-ocular reflexes (VORs) in the generation of horizontal eye movements during head movements including both rotational and translational components. We used three eccentric rotation techniques to investigate this interaction in human subjects: (1) the tangential interaural acceleration was varied using three head positions (on-axis, 25 and 40 cm ahead of the rotational axis), while angular head velocity remained unchanged; (2) the magnitude of the angular head velocity was varied with head eccentricity to keep the tangential interaural acceleration unchanged; (3) the subject's head was oriented either upright or 90 degrees forward from upright (nose-down). Experiments were performed in complete darkness with the subjects remembering a close earth-fixed target (20 cm distant) while being rotated at 1.2 and 1.8 Hz. Our data showed that the translational component of the VOR evoked during eccentric yaw rotation increased proportionally with an increase in head eccentricity, i.e. with tangential acceleration. We also found that the translational component of the VOR was equal for motion stimuli producing identical interaural tangential accelerations even when angular velocities differed. In addition, we found that the translational component of the VOR evoked during head upright eccentric rotation was equal to the translational VOR evoked during nose-down rotation for a given stimulus and head eccentricity. We conclude that these three findings are in agreement with what would be expected from a linear interaction (i.e. algebraic summation) between otolith organ and horizontal canal VORs for the generation of horizontal compensatory eye movements during head motion.

Otolith function: basis for modern testing

The major challenge in developing a robust test of otolith function, particularly with regard to linear vestibulo-ocular reflex (LVOR) and perceptual measures, is to find a way in which graded lesions are reflected in graded response properties and abnormalities. The ability of the vestibulo-ocular reflex (VOR) to compensate and adapt to dysfunction and pathology presents formidable challenges for registering localizing clinical findings, whether in the angular vestibulo-ocular reflex (AVOR), the LVOR, or both. Based on a variety of considerations, various forms of eccentric rotation seem to provide the most convenient, and potentially the most useful, means to generate motion profiles from which otolith function can be directly assessed. Both translational and tilt responses can be recorded depending on the stimulus profile. The near-centric version is particularly enticing because of the ability to study one labyrinth at a time, much like calorics. In that case and in others in which the tilt-LVOR is prominent, measures of the perceived visual vertical are useful and by all accounts similar to ocular torsion. The latter does hold the important advantage of being an objective measure, requiring no intervention on the part of the patient. The translational-LVOR can be derived from eccentric rotation responses with the head displaced forward as well as backward, while viewing near targets in hopes of generating a large addition or subtraction (even inversion) of an otherwise AVOR-driven reflex. These considerations provide an impetus to pursue improved methods of quantifying otolith function in a clinical population. The sobering caveat is that the diagnosis of total unilateral vestibular loss presents little challenge either clinically or by classic testing (e.g., calorics), and yet most of our efforts in developing quantifiable measures of dysfunction over the years have yielded results that are modest and hardly compelling.

Unilateral vestibular neuritis with otolithic signs and off-vertical axis rotation

Off-vertical axis rotation (OVAR) at constant velocity is a dynamic otolith stimulus that induces horizontal and vertical eye movement responses. To determine the value of this examination as a test for unilateral otolithic hypofunction, we compared the OVAR responses of patients suffering from acute vestibular neuritis (VN) without any sign of otolith affection, with those of patients suffering from acute VN with otolithic signs. The horizontal eye movement bias component shows directional preponderance (DP) significantly higher in patients with otolithic signs than in patients not presenting them. However, as bias DP also reflects the imbalance between right and left horizontal canals activity, this greater bias DP could be explained by the more severe canals impairment-evaluated by caloric test-found in patients with otolithic signs. No significant difference can be shown on horizontal modulation. The DP of vertical modulation is significantly higher in patients presenting otolithic signs than in patients not presenting them: in the case of otolithic signs, the responses are smaller during rotations toward the affected side. Therefore, this variable could be used as an indication of unilateral otolithic hypofunction.

The human vertical vestibulo-ocular reflex during combined linear and angular acceleration with near-target fixation

The purpose of this study was to examine the effect of fixation target distance on the human vestibuloocular reflex (VOR) during eccentric rotation in pitch. Such rotation induces both angular and linear acceleration. Eight normal subjects viewed earth-fixed targets that were either remote or near to the eyes during whole-body rotation about an earth-horizontal axis that was either oculocentric or 15 cm posterior (eccentric) to the eyes. Eye and head movements were recorded using magnetic search coils. Using a servomotor-driven chair, passive whole-body rotations were delivered as trains of single-frequency sinusoids at frequencies from 0.8 to 2.0 Hz and as pseudorandom impulses of acceleration. In the light, the visually enhanced VOR (VVOR) was recorded while subjects were asked to fixate targets at one of several distances. In darkness, subjects were asked to remember targets that had been viewed immediately prior to the rotation. In order to eliminate slip of the retinal image of a near target when the axis of rotation of the head is posterior to the eyes, the ideal gain (compensatory eye velocity divided by head velocity) of the VVOR and VOR must exceed 1.0. Both the VOR and VVOR were found to have significantly enhanced gains during sinusoidal and pseudorandom impulses of rotation (P < 0.05). Enhancement of VVOR gain was greatest at low frequencies of head rotation and decreased with increasing frequency. However, enhanced VOR gain only slightly exceeded 1.0, and VVOR gain enhancement was significantly lower than the expected ideal values for the stimulus conditions employed (P < 0.05). During oculocentric rotations with near targets, both the VOR and VVOR tended to exhibit small phase leads that increased with rotational frequency. In contrast, during eccentric rotations with near targets, there were small phase lags that increased with frequency. Visual tracking contributes during ocular compensatory responses to sustained head rotation, although the latency of visual tracking reflexes exceeds 100 ms. In order to study initial vestibular responses prior to modification by visual tracking, we presented impulses of head acceleration in pseudorandom sequence of initial positions and directions, and evaluated the ocular response in the epoch from 25 to 80 ms after movement onset. As with sinusoidal rotations, pseudorandom eccentric head rotation in the presence of a near, earth-fixed target was associated with enhancement of VVOR and VOR gains in the interval from 25 to 80 ms from movement onset. Despite the inability of visual tracking to contribute to these responses, VVOR gain significantly exceeded VOR gain for pseudorandom accelerations. This gain enhancement indicates that target distance and linear motion of the head are considered by the human ocular motor system in adjustment of performance of the early VOR, prior to a contribution by visual following reflexes. Vergence was appropriate to target distance during all VVOR rotations, but varied during VOR rotations with remembered targets. For the 3-m target distance, vergence during the VOR was stable over each entire trial but slightly exceeded the ideal value. For the 0.1-m near target, instantaneous vergence during the VOR typically declined gradually in a manner not corresponding to the time course of instantaneous VOR gain change; mean vergence over entire trials ranged from 60 to 90% of ideal, corresponding to target distances for which ideal gain would be much higher than actually observed. These findings suggest a dissociation between vergence and VOR gain during eccentric rotation with near targets in the frequency range from 0.8 to 2.0 Hz.

Unilateral peripheral semicircular canal lesion and off-vertical axis rotation

Off vertical axis rotation (OVAR) is a stimulus that can be used to assess the otolith-ocular reflex. However, experimental data suggest that isolated unilateral lesion of the lateral semicircular canal (SCC) nerve could modify responses to OVAR. Thus, to determine what nystagmus variables are not affected by SCC dysfunction and might be used as indices of otolithic disease, responses to OVAR were compared in 39 healthy controls and in 19 patients suffering from acute unilateral vestibular neuritis (VN), without any sign of otolith dysfunction. Horizontal and vertical slow phase velocities (SPV) were measured during earth vertical axis rotation (EVAR), and during OVAR at a tilt angle of 9 degrees and rotation velocity of 60 degrees/s. During OVAR, horizontal SPV consists of a sinusoidal modulation superimposed on a sustained bias opposite to the rotation. Vertical SPV consists of a sinusoidal modulation without bias. In patients, the bias shows directional preponderance (DP) toward the healthy side, strongly correlated to EVAR nystagmus DP. It would therefore simply reflect an imbalance, produced by the unilateral peripheral vestibular lesion, between right and left vestibular nuclei activity. On the other hand, vertical and horizontal modulations are not significantly different in patients and controls. Since the cause and the site of VN are not known, we cannot be sure that patients had pure SCC deafferentation. However, as all of them had SCC paresis it is concluded that OVAR modulations are not affected by a strong dysfunction of the pathways issued from the SCCs.

Off-axis rotation as a test of otolith function

This experiment was performed to determine whether linear acceleration causes a change in the vestibulo-linear reflex (VOR) that could be exploited as a research and/or clinical test of otolith function. The effect of otolith stimulation on the VOR was studied in the horizontal and vertical planes in 10 cats. The VOR was induced by pulses of angular acceleration (velocity ramps). During some of the test runs, a centripetal linear acceleration stimulus was also applied to stimulate the otolith organs. Using the stimuli combination as tested in this project, the authors found that otolith stimulation affected the vertical VOR but not the horizontal VOR. The vertical VOR was asymmetric. While the methods used in this study are useful and the equipment requirements make these techniques generally impractical for clinical testing.

Investigations of disorders of balance

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Otolith-ocular testing in human subjects

Assessment of the otolith-ocular reflex of human subjects involves linear acceleration and/or changes in the orientation of the head with respect to gravity. Several such stimuli are currently under investigation regarding their applicability to the evaluation of patients with dizziness and balance disorders. Discussed in this paper are off-vertical axis rotation, eccentric rotation, pitch and roll rotation, and linear acceleration. For each of these stimuli, basic principles, normative human data, and patient data are described. Although none of these methods are currently established for clinical use, each of them, especially off-vertical axis rotation and linear acceleration, have the potential for developing into a clinically useful method for assessing otolith function in man.

Testing otolith function

Otolithic signals contribute to; (1) perception of orientation and linear motion, (2) generate compensatory eye movements in response to linear acceleration of the head and (3) participate in the co-ordination of movement and balance. Tests of these functions shown to be useful in identifying clinical disorders have been reviewed: (1) Evaluation of orientation to gravity, as estimated by adjustment of the visual vertical, indicates deranged otolith function at a peripheral or central level and the sensitivity of this test can be enhanced by performing estimates during centrifugation on a motorised turntable. Estimation of thresholds of self motion on a parallel swing identifies global reduction or unilateral loss of peripheral function, with central disorders awaiting study. (2) Otolith ocular reflexes to linear head translation can be used to demonstrate overall integrity of peripheral function and reveal central abnormalities. Counter-rolling responses to head roll-tilt and measurements of cyclodeviation of the eyes demonstrate functional asymmetries, with some lateralising value, particularly in central lesions. Global function and asymmetries may also be evaluated by 'head eccentric' rotational testing, which adds a tangential linear acceleration to the angular stimulus. The linear acceleration enhances the canal response by adding an otolith component. (3) Latency and amplitude of surface electro-myography (EMG) responses in the limbs to sudden falls, which can be recorded with the subject suspended on a hinged bed, indicate gross peripheral abnormality of function and can lateralize disorders of CNS motor pathways. It is concluded that some tests of otolith function can be of use in indicating global loss of peripheral otolith function, others are capable of lateralizing a marked loss of function and all have the potential to give information about central disorders. They all have to be interpreted within the clinical context and, unfortunately, none have yet been shown to be sensitive to partial, particularly unilateral, dysfunction.