Tilt suppression of vestibulo-ocular reflex in patients with cerebellar lesions

Amplification of vibration induced nystagmus in patients with peripheral vestibular loss by head tilt

Introduction

In patients with unilateral loss of vestibular function (UVL) vibration of the skull leads to a response of the vestibulo-ocular reflex (VOR) called vibration-induced nystagmus (VIN), with slow phases usually directed toward the paretic ear. This response is thought to result from the difference between the neural discharge in semicircular canal afferents from the healthy and the affected labyrinth. The brain interprets this difference as a sustained imbalance in angular (rotational) vestibular tone, which in natural circumstances would only occur when the head was rotating at a constant acceleration.

Methods

To study this effect, we used a contemporary model of the neural network that combines sensory information about head rotation, translation, and tilt relative to gravity to estimate head orientation and motion. Based on the model we hypothesize that in patients with UVL, the brain may estimate not only a "virtual" rotation from the induced canal imbalance but also a subsequent "virtual" translation from the incorrect computation of the orientation of the head relative to gravity. If this is the case, the pattern of vibration-induced nystagmus will depend on the orientation of the head relative to gravity during the stimulation. This model predicts that this "virtual" translation will alter the baseline VIN elicited with the head upright; augmenting it when the affected ear is down and diminishing it when the affected ear is up.

Results

Confirming this hypothesis, we recorded VIN in 3 patients with UVL (due to vestibular neuritis) in upright, right ear-down, and left ear-down positions and each showed the expected pattern.

Discussion

From a practical, clinical view, our results and modeling suggest that positional VIN might reveal a hidden imbalance in angular vestibular tone in patients with UVL, when patients have equivocal signs of a vestibular imbalance, such as a minute amount of spontaneous or vibration-induced nystagmus with the head upright. This research provides insights into the underlying mechanisms of vestibular processing, the analysis of nystagmus in patients with UVL, and guides the design of a new bedside diagnostic test to assess vestibular function in patients with dizziness and imbalance.

Modeling the effect of gravity on periodic alternating nystagmus

Periodic alternating nystagmus (PAN) is a rare oscillatory ocular motor disorder. The effects of gravity on the dynamic behavior of PAN can be studied by monitoring the nystagmus while changing head orientation. Previous studies of patients with PAN reached different conclusions about the effect of changing the orientation of the head relative to gravity on the ongoing PAN, either no effect or a damping of the nystagmus within several minutes. What neuronal circuits could account for the difference in the effects of gravity among PAN patients? We modeled how the brain resolves the tilt-translation ambiguity in normal individuals and added an unstable, oscillatory vestibular system generating PAN. PAN was suppressed in our patient in ear-down positions, in a similar pattern to that of a previously reported patient. This effect was simulated by reducing the gain of the projection of the "rotation feedback" loop to the velocity-storage integrator to approximately 5% of its normal value. With normal "rotation feedback" PAN is expected to dissipate quickly as soon as the head is rotated away from upright position. Moreover, by disconnecting the rotation feedback completely (gain = zero) the model simulated PAN that was reported to be unaffected by gravity. Thus, understanding the effect of this single parameter, the gain of the rotation feedback, can explain the observed variability among our own and previous studies.

The otolith vermis: A systems neuroscience theory of the Nodulus and Uvula

The Nodulus and Uvula (NU) (lobules X and IX of the cerebellar vermis) form a prominent center of vestibular information processing. Over decades, fundamental and clinical research on the NU has uncovered many aspects of its function. Those include the resolution of a sensory ambiguity inherent to inertial sensors in the inner ear, the otolith organs; the use of gravity signals to sense head rotations; and the differential processing of self-generated and externally imposed head motion. Here, I review these works in the context of a theoretical framework of information processing called the internal model hypothesis. I propose that the NU implements a forward internal model to predict the activation of the otoliths, and outputs sensory predictions errors to correct internal estimates of self-motion or to drive learning. I show that a Kalman filter based on this framework accounts for various functions of the NU, neurophysiological findings, as well as the clinical consequences of NU lesions. This highlights the role of the NU in processing information from the otoliths and supports its denomination as the "otolith" vermis.

Is There an “Acquired Idiopathic Head-Shaking Nystagmus”?—A Discussion of Mechanisms and Clinical Implications Based on a Case Report

Background

Head-shaking nystagmus (HSN) occurs in both peripheral and central vestibular disorders. In most cases, HSN can be attributed to an asymmetric peripheral vestibular input or a structural lesion mostly in the cerebellum affecting the central velocity storage mechanism. An isolated HSN is very rare.

Case Presentation

We report on a young female patient with the clinical picture of recurrent episodes of vertigo, which were induced by fast head movements and were accompanied by a severe right-beating HSN with a long time constant of 60 s. There was no other clinical and instrument-based evidence of peripheral vestibular dysfunction (including video head impulse test, caloric test, vestibular-evoked myogenic potentials) or indication of a structural lesion in the nodulus, uvula or pontomedullary brainstem on fine-slice magnetic resonance imaging. She had no previous history of migraine, hearing deficits, or other focal neurological symptoms. Diagnostic criteria for vestibular paroxysmia, vestibular migraine, benign peripheral paroxysmal vertigo, or any other known vestibular disorders were not fulfilled. Chart review in the database of the German Center for Vertigo and Balance Disorders indicated eight additional patients with a similar clinical phenotype between 2018 and 2022.

Conclusion

We propose a clinical entity called acquired idiopathic head shaking nystagmus (aiHSN) as a rare cause of episodic vertigo induced by fast head movements. Nystagmus characteristics suggest a subtle functional pathology of the central velocity storage mechanism in the nodulus and uvula, which is exacerbated during symptomatic episodes.

The behavior of the optokinetic system

Optokinetic responses in several species are compared, describing differences in afoveate and foveate animals, and the effects of visual testing conditions, including directions of stimulus motion. Smooth pursuit contributes to responses to full-field visual motion in foveate species; in the latter, measurement of optokinetic after-nystagmus in darkness allows investigation of the optokinetic system. The concept of optokinetic-vestibular symbiosis and velocity storage are discussed, pertinent electrophysiological studies (such as vestibular nucleus neurons that respond to both optokinetic and vestibular stimuli) are reviewed and a model is developed. The different purposes and properties of optokinetic responses (to maintain clear vision during self-rotation) and smooth pursuit (to visually track a moving target) are clarified.

Central Ocular Motor Disorders: Clinical and Topographic Anatomical Diagnosis, Syndromes and Underlying Diseases

The key to the diagnosis of ocular motor disorders is a systematic clinical examination of the different types of eye movements, including eye position, spontaneous nystagmus, range of eye movements, smooth pursuit, saccades, gaze-holding function, vergence, optokinetic nystagmus, as well as testing of the function of the vestibulo-ocular reflex (VOR) and visual fixation suppression of the VOR. This is like a window which allows you to look into the brain stem and cerebellum even if imaging is normal. Relevant anatomical structures are the midbrain, pons, medulla, cerebellum and rarely the cortex. There is a simple clinical rule: vertical and torsional eye movements are generated in the midbrain, horizontal eye movements in the pons. For example, isolated dysfunction of vertical eye movements is due to a midbrain lesion affecting the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), with impaired vertical saccades only or vertical gaze-evoked nystagmus due to dysfunction of the Interstitial nucleus of Cajal (INC). Lesions of the lateral medulla oblongata (Wallenberg syndrome) lead to typical findings: ocular tilt reaction, central fixation nystagmus and dysmetric saccades. The cerebellum is relevant for almost all types of eye movements; typical pathological findings are saccadic smooth pursuit, gaze-evoked nystagmus or dysmetric saccades. The time course of the development of symptoms and signs is important for the diagnosis of underlying diseases: acute: most likely stroke; subacute: inflammatory diseases, metabolic diseases like thiamine deficiencies; chronic progressive: inherited diseases like Niemann-Pick type C with typically initially vertical and then horizontal saccade palsy or degenerative diseases like progressive supranuclear palsy. Treatment depends on the underlying disease. In this article, we deal with central ocular motor disorders. In a second article, we focus on clinically relevant types of nystagmus such as downbeat, upbeat, fixation pendular, gaze-evoked, infantile or periodic alternating nystagmus. Therefore, these types of nystagmus will not be described here in detail.

Isolated cerebellar nodulus infarction: Two case reports and literature review

BACKGROUND

Isolated cerebellar nodulus infarction (ICNI) is rare and has great clinical similarity with acute peripheral vestibulopathy (APV), from which it is difficult to distinguish. We report two cases of ICNI followed by a literature review to identify the discriminant clinical elements that differentiate ICNI from APV.

METHODS

We describe in detail our 2 cases. Besides, a literature search in Medline via PubMed and Scopus was performed up to May 17, 2020. Clinical characteristics, mainly of well-described cases, were extracted and analyzed.

RESULTS

Our search yielded 43 total publications, among which 13 were selected, including 23 patients. Spontaneous or positional rotatory vertigo with unidirectional spontaneous horizontal nystagmus, associated with the postural imbalance and unilateral lateropulsion or fall on Romberg's test, was the most common clinical picture. According to our literature review, the discriminant clinical elements which differentiate ICNI from APV were direction-changing gaze-evoked nystagmus, bilateral lateropulsion or fall on Romberg's test, and normal horizontal head impulse test. Our two patients reported a positional fleeting abnormal visual perception of spatial orientation of objects. We proposed this symptom as a discriminant clinical element.

CONCLUSION

The ICNI constitutes a difficult differential diagnosis of APV. Through our two patients reported here, we proposed a supplementary discriminant symptom helpful for the clinical diagnosis.

Alexander's Law During High-Speed, Yaw-Axis Rotation: Adaptation or Saturation?

Objective: Alexander's law (AL) states the intensity of nystagmus increases when gaze is toward the direction of the quick phase. What might be its cause? A gaze-holding neural integrator (NI) that becomes imperfect as the result of an adaptive process, or saturation in the discharge of neurons in the vestibular nuclei? Methods: We induced nystagmus in normal subjects using a rapid chair acceleration around the yaw (vertical) axis to a constant velocity of 200°/second [s] and then, 90 s later, a sudden stop to induce post-rotatory nystagmus (PRN). Subjects alternated gaze every 2 s between flashing LEDs (right/left or up/down). We calculated the change in slow-phase velocity (ΔSPV) between right and left gaze when the lateral semicircular canals (SCC) were primarily stimulated (head upright) or, with the head tilted to the side, stimulating the vertical and lateral SCC together. Results: During PRN AL occurred for horizontal eye movements with the head upright and for both horizontal and vertical components of eye movements with the head tilted. AL was apparent within just a few seconds of the chair stopping when peak SPV of PRN was reached. When slow-phase velocity of PRN faded into the range of 6-18°/s AL could no longer be demonstrated. Conclusions: Our results support the idea that AL is produced by asymmetrical responses within the vestibular nuclei impairing the NI, and not by an adaptive response that develops over time. AL was related to the predicted plane of eye rotations in the orbit based on the pattern of SCC activation.

[Ocular movement and nystagmus: basics and clinical diagnosis]

BACKGROUND

Eye movements are one of the most complex motor functions of the central nervous system (CNS). Eye movement disorders including nystagmus occur in diseases of the CNS and the vestibular system. A systematic clinical examination often allows a topodiagnostic classification of the lesion.

OBJECTIVE

The basics of eye movements, the role of the cerebellum, the clinical examination of the oculomotor system, and the most important forms of nystagmus and their diagnostic implications are described.

MATERIALS AND METHODS

A literature analysis assessing eye movements, cerebellar control of eye movements, clinical examination of eye movement, and nystagmus was performed.

RESULTS

In disorders of eye movement, diseases of the oculomotor cranial nerves and the central nervous structures are to be distinguished from diseases of the orbit, eye muscles, and motor end plates. The former result in cranial nerve-related paralysis of extraocular muscles, disturbances of saccadic and smooth pursuit eye movements, vertical or horizontal gaze palsy, internuclear ophthalmoplegia, or impaired gaze holding. Nystagmus in combination with other disturbances of ocular movement is highly related to a lesion within the CNS. Intense nystagmus with a rotatory component that decreases during fixation usually has a peripheral vestibular cause.

CONCLUSION

Clinical examination of eye movements and nystagmus enables the diagnosis of typical eye movement disorders with a strong relation to distinct lesions of the CNS or the peripheral vestibular pathway.

Quantitative oculomotor and nonmotor assessments in late-onset GM2 gangliosidosis

OBJECTIVE

A cross-sectional study was performed to evaluate whether quantitative oculomotor measures correlate with disease severity in late-onset GM2 gangliosidosis (LOGG) and assess cognition and sleep as potential early nonmotor features.

METHODS

Ten patients with LOGG underwent quantitative oculomotor recordings, including measurements of the angular vestibulo-ocular reflex (VOR), with results compared to age- and sex-matched controls. Disease severity was assessed by ataxia rating scales. Cognitive/neuropsychiatric features were assessed by the cerebellar cognitive affective syndrome (CCAS) scale, Cerebellar Neuropsychiatric Rating Scale, and sleep quality evaluated using subjective sleep scales.

RESULTS

Oculomotor abnormalities were found in all participants, including 3/10 with clinically normal eye movements. Abnormalities involved impaired saccadic accuracy (5/10), abnormal vertical (8/10) and horizontal (4/10) pursuit, reduced optokinetic nystagmus (OKN) responses (7/10), low VOR gain (10/10), and impaired VOR cancellation (2/10). Compared to controls, the LOGG group showed significant differences in saccade, VOR, OKN, and visually enhanced VOR gains. Severity of saccadic dysmetria, OKN, and VOR fixation-suppression impairments correlated with ataxia scales (p < 0.05). Nine out of ten patients with LOGG had evidence of the CCAS (5/10 definite, 2/10 probable, 2/10 possible). Excessive daytime sleepiness was present in 4/10 and 8/10 had poor subjective sleep quality.

CONCLUSIONS

Cerebellar oculomotor abnormalities were present in all patients with LOGG, including those with normal clinical oculomotor examinations. Saccade accuracy (dorsal cerebellar vermis localization), fixation suppression, and OKN gain (cerebellar flocculus/paraflocculus localization) correlated with disease severity, suggesting that quantitative oculomotor measurements could be used to track disease progression. We found evidence of the CCAS, suggesting that cerebellar dysfunction may explain the cognitive disorder in LOGG. Sleep impairments were prevalent and require further study.

The Superiority of the Otolith System

BACKGROUND

The peripheral vestibular end organ is considered to consist of semi-circular canals (SCC) for detection of angular accelerations and the otoliths for detection of linear accelerations. However, otoliths being phylogenetically the oldest part of the vestibular sensory organs are involved in detection of all motions.

SUMMARY

This study elaborates on this property of the otolith organ, as this concept can be of importance for the currently designed vestibular implant devices. Key Message: The analysis of the evolution of the inner ear and examination of clinical examples shows the robustness of the otolith system and inhibition capacity of the SCC. The otolith system must be considered superior to the SCC system as illustrated by evolution, clinical evidence, and physical principles.

Perverted Downward Corrective Saccades During Horizontal Head Impulses in Chiari Malformation

The mechanism of perverted vertical responses during horizontal head impulse tests (HITs) requires further elucidation. A 47-year-old woman with a Chiari malformation showed alternating skew deviation, downbeat nystagmus with an increasing slow phase velocity, impaired smooth pursuit, and upward ocular deviation during horizontal HITs and corrective downward saccades in the presence of normal bithermal caloric tests and intact tilt suppressions of the post-rotatory nystagmus. These findings suggest dysfunction of the inferior cerebellum including the tonsil, nodulus, and uvula. We propose that disruption of signals from the medial part of the vestibulocerebellum, which normally inhibits the lateral and anterior canal pathways, may elicit an upward misdirection of the eye velocity during rapid horizontal head rotation. Otherwise, the Chiari malformation may have directly affected the brainstem structures involved in the direction matrix of the vestibulo-ocular reflex.

Brain Stem and Audio-Vestibular Regulation

The central pathologies present with perverted auditory perception and compromised postural control. Considering the existing controversy this study involves assessments of 100 cases of post fossa tumefactions in which a detailed clinical and neuro-otological (pure tone audiometry, electronystagmography, brainstem evoked response audiometry) profile is compared with their imaging patterns. The CP angle schwannomas (N = 26) presented with abnormal speech tests (N = 18), abnormal auditory adaptation (N = 7) and ABR with pathologically increased latency of wave V (N = 32), poor formation of wave I (N = 31) along with abnormal inter-wave interval (N = 32). In lesions (N = 32) compressing deeper nuclei, vermis and axial parts of brain stem, a gross truncal ataxia, incoordination, nystagmus, speech defects, subtotal deafness and bilateral ABR abnormalities were observed. The abnormal optomotor activities were seen as saccadic (N = 44) and deformed slow pursuit eye movements (N = 20). Inability to sustain holding function resulted in gaze nystagmus (N = 71), and poor timing manifested as fixation overshoots (N = 42). The midline cerebellar and upper brain stem lesions revealed bilateral OKN abnormalities whereas paramedian pathology showed only ipsilateral distortion. Caloric tests revealed culmination frequency as the most sensitive parameter for assessment of the hypo-reflexia in diffuse cerebellopathies while slow phase velocity in cases of posterior fossa lesion. The caloric hypo-activity appears to be of a better localizing value than the directional preponderance. The slow pursuit tracking revealed Type III curve perhaps due to defective regulation of slow movements in partially intact cerebellum (N = 15), while gross cerebellar dysfunctioning resulted into Type IV curve (N = 5).

Visual and Vestibular Induced Eye Movements in Verbal Children and Adults with Autism

This study assessed the functionality of vestibular, pursuit, and saccade circuitry in autism across a wide age range. Subjects were 79 individuals with autism (AUT) and 62 controls (CON) aged 5 to 52 years with IQ scores > 70. For vestibular testing, earth-vertical axis rotation was performed in darkness and in a lighted visual surround with a fixation target. Ocular motor testing included assessment of horizontal saccades and horizontal smooth pursuit. No between-group differences were found in vestibular reflexes or in mean saccade velocity or accuracy. Saccade latency was increased in the AUT group with significant age-related effects in the 8-18 year old subgroups. There was a trend toward decreased pursuit gain without age effects. Normal vestibular-induced eye movements and normal saccade accuracy and velocity provide the most substantial evidence to date of the functional integrity of brainstem and cerebellar pathways in autism, suggesting that the histopathological abnormalities described in these structures may not be associated with intrinsic dysfunction but rather reflect developmental alterations related to forebrain cortical systems formation. Increased saccade latency with age effects adds to the extensive existing evidence of altered function and maturation of cortical systems in autism.

The cerebellar nodulus: perceptual and ocular processing of graviceptive input

Current concepts postulate a decisive role of the cerebellar nodulus in the processing of otolith input. We hypothesized that nodular lesions abolish otolith-perceptual integration, predicting alignment of perceived direction of earth vertical with the z-axis of the head and not with gravity. In an 80-year-old patient with acute heminodular infarction, the subjective visual vertical deviated contralesionally by -21.1° when the patient was upright. After subtracting this offset, perceived vertical closely matched the patient's head orientation when the patient was roll-tilted. Otolith-ocular reflexes remained normal. This is the first report on abolished earth verticality perception in heminodular stroke and underlines the importance of the nodulus in spatial orientation.

The neuroanatomical correlates of training-related perceptuo-reflex uncoupling in dancers

Sensory input evokes low-order reflexes and higher-order perceptual responses. Vestibular stimulation elicits vestibular-ocular reflex (VOR) and self-motion perception (e.g., vertigo) whose response durations are normally equal. Adaptation to repeated whole-body rotations, for example, ballet training, is known to reduce vestibular responses. We investigated the neuroanatomical correlates of vestibular perceptuo-reflex adaptation in ballet dancers and controls. Dancers' vestibular-reflex and perceptual responses to whole-body yaw-plane step rotations were: (1) Briefer and (2) uncorrelated (controls' reflex and perception were correlated). Voxel-based morphometry showed a selective gray matter (GM) reduction in dancers' vestibular cerebellum correlating with ballet experience. Dancers' vestibular cerebellar GM density reduction was related to shorter perceptual responses (i.e. positively correlated) but longer VOR duration (negatively correlated). Contrastingly, controls' vestibular cerebellar GM density negatively correlated with perception and VOR. Diffusion-tensor imaging showed that cerebral cortex white matter (WM) microstructure correlated with vestibular perception but only in controls. In summary, dancers display vestibular perceptuo-reflex dissociation with the neuronatomical correlate localized to the vestibular cerebellum. Controls' robust vestibular perception correlated with a cortical WM network conspicuously absent in dancers. Since primary vestibular afferents synapse in the vestibular cerebellum, we speculate that a cerebellar gating of perceptual signals to cortical regions mediates the training-related attenuation of vestibular perception and perceptuo-reflex uncoupling.

Slowed saccades and increased square wave jerks in essential tremor

BACKGROUND

Eye movements in essential tremor (ET) are poorly described and may present useful information on the underlying pathophysiology of the disorder.

METHODS

Sixty patients with ET, including 15 de novo untreated patients, and 60 age-matched controls constitute the study population. A video-based eye tracker was used to assess binocular eye position. Oculomotor function was assessed while subjects followed random horizontally and vertically step-displaced targets.

RESULTS

For all reflexive saccades, latencies were increased in ET subjects by a mean of 16.3% (p<0.01). Saccades showed reduced peak velocities with a lengthy, wavering velocity plateau, followed by slowed decelerations. For larger 30°+ saccades, peak velocities were decreased by a mean of 25.2% (p<0.01) and durations increased by 31.8% (p<0.01). The frequency of square wave jerks (SWJs) in patients was more than triple that of controls (p<0.0001). Despite frequent interruptions by SWJs, fixations were otherwise stable and indistinguishable from controls (root mean square [RMS] velocity, p = 0.324). The abnormal eye movement parameters were independent of disease duration, tremor severity, and medication therapy.

DISCUSSION

In contrast to normally swift onset and efficient acceleration/deceleration movements, saccades in ET are characterized by abnormally prolonged latencies and slowed velocity profiles. Although ET subjects maintain highly stable fixations, they are interrupted by increased numbers of SWJs. This study reveals novel oculomotor deficits in ET, which are distinct from the eye movement dysfunction of other movement disorders, supporting a role for eye tracking to assist in the differential diagnoses of not only atypical, but also more common movement disorders.

Effects of 4-aminopyridine on nystagmus and vestibulo-ocular reflex in ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a progressive neurodegenerative disorder with prominent eye movement deficits localizing to the cerebellum. We sought to determine if 4-aminopyridine (4-AP), which putatively enhances the precision of Purkinje neurons, could improve the disorders of eye movements and vestibular function in A-T. The influence of 4-AP on disorders of eye movements and vestibular function was studied in four A-T patients. The effects on the cerebellar control of vestibulo-ocular reflex (VOR) was quantitatively assessed by the decay time constant of per- and post-rotational nystagmus during constant velocity en bloc rotations. The length of the VOR time constant determines the fidelity of the vestibular velocity storage, a neural mechanism that increases the bandwidth of VOR under cerebellar control. The VOR time constant was not increased in A-T patients. The latter is explained by the extent of cerebellar lesion as previously described in A-T and other cerebellar disorders. Nevertheless, 4-AP shortened the VOR time constant during horizontal rotations. Severe disinhibition of velocity storage in subjects with putatively profound cerebellar degeneration manifest periodic alternating nystagmus (PAN). Among two A-T subjects who manifested PAN, 4-AP reduced the peak slow phase velocity of the more severely affected individual and abrogated the PAN in the other. Two A-T subjects manifested horizontal and vertical spontaneous nystagmus (SN) in primary gaze, 4-AP reduced its slow phase velocity. We conclude that in subjects with A-T 4-AP has a prominent effect on the ocular motor and vestibular deficits that are ascribed to the loss of cerebellar Purkinje neurons.

Is vestibular self-motion perception controlled by the velocity storage? Insights from patients with chronic degeneration of the vestibulo-cerebellum

Background

The rotational vestibulo-ocular reflex (rVOR) generates compensatory eye movements in response to rotational head accelerations. The velocity-storage mechanism (VSM), which is controlled by the vestibulo-cerebellar nodulus and uvula, determines the rVOR time constant. In healthy subjects, it has been suggested that self-motion perception in response to earth-vertical axis rotations depends on the VSM in a similar way as reflexive eye movements. We aimed at further investigating this hypothesis and speculated that if the rVOR and rotational self-motion perception share a common VSM, alteration in the latter, such as those occurring after a loss of the regulatory control by vestibulo-cerebellar structures, would result in similar reflexive and perceptual response changes. We therefore set out to explore both responses in patients with vestibulo-cerebellar degeneration.

Methodology/Principal Findings

Reflexive eye movements and perceived rotational velocity were simultaneously recorded in 14 patients with chronic vestibulo-cerebellar degeneration (28–81yrs) and 12 age-matched healthy subjects (30–72yrs) after the sudden deceleration (90°/s2) from constant-velocity (90°/s) rotations about the earth-vertical yaw and pitch axes. rVOR and perceived rotational velocity data were analyzed using a two-exponential model with a direct pathway, representing semicircular canal activity, and an indirect pathway, implementing the VSM. We found that VSM time constants of rVOR and perceived rotational velocity co-varied in cerebellar patients and in healthy controls (Pearson correlation coefficient for yaw 0.95; for pitch 0.93, p<0.01). When constraining model parameters to use the same VSM time constant for rVOR and perceived rotational velocity, moreover, no significant deterioration of the quality of fit was found for both populations (variance-accounted-for >0.8).

Conclusions/Significance

Our results confirm that self-motion perception in response to rotational velocity-steps may be controlled by the same velocity storage network that controls reflexive eye movements and that no additional, e.g. cortical, mechanisms are required to explain perceptual dynamics.

Double impairment: clinical identification of 33 cases of cerebellar ataxia with bilateral vestibulopathy

OBJECTIVE

Following recently described small series of patients with the syndrome of cerebellar ataxia with bilateral vestibulopathy (CABV), the authors undertook a careful clinical and laboratory assessment of patients who presented to their unit with characteristics of this syndrome.

STUDY DESIGN

Case note review.

SETTING

Tertiary, university-based, multidisciplinary neurootology clinic.

SUBJECTS

Thirty-three patients whose characteristics fit this syndrome.

METHODS

Patients presenting to the Multidisciplinary Neurotology Clinic with characteristics of CABV were entered into a bespoke database. This was analyzed to identify the clinical findings and results of vestibular investigations for this group.

RESULTS

Patients presented at a mean age of 54 years (SD, 17.6) with symptoms having been present for a median of 3 years (interquartile ratio, 2.0-9.5). Caloric testing greatly underestimated the disorder, being subnormal in only 18% of patients; the head-thrust test was abnormal and dynamic visual acuity testing was abnormal 88% and 91% of the time, respectively. Of the patients, 76% demonstrated gaze-evoked nystagmus. Impaired smooth pursuit (97% of patients showed low gain with saccadic corrections) and impaired cancellation of the vestibulo-ocular reflex (in 97% of patients) were found. Impaired saccular otolithic function was abnormal in 33%, adding to patient imbalance.

CONCLUSION

The unique double-pathway balance impairment in CABV patients causes a high prevalence of subnormal function of both central and peripheral vestibular function. This is an easily missed clinical entity that is often associated with normal caloric investigations. As many patients with this syndrome are poor candidates for vestibular rehabilitation therapy, resources are better devoted to the early implementation of assistance with their safe ambulation and activities of daily living.

Cerebellum and ocular motor control

An intact cerebellum is a prerequisite for optimal ocular motor performance. The cerebellum fine-tunes each of the subtypes of eye movements so they work together to bring and maintain images of objects of interest on the fovea. Here we review the major aspects of the contribution of the cerebellum to ocular motor control. The approach will be based on structural–functional correlation, combining the effects of lesions and the results from physiologic studies, with the emphasis on the cerebellar regions known to be most closely related to ocular motor function: (1) the flocculus/paraflocculus for high-frequency (brief) vestibular responses, sustained pursuit eye movements, and gaze holding, (2) the nodulus/ventral uvula for low-frequency (sustained) vestibular responses, and (3) the dorsal oculomotor vermis and its target in the posterior portion of the fastigial nucleus (the fastigial oculomotor region) for saccades and pursuit initiation.

Ataxia telangiectasia: a “disease model” to understand the cerebellar control of vestibular reflexes

Experimental animal models have suggested that the modulation of the amplitude and direction of vestibular reflexes are important functions of the vestibulocerebellum and contribute to the control of gaze and balance. These critical vestibular functions have been infrequently quantified in human cerebellar disease. In 13 subjects with ataxia telangiectasia (A-T), a disease associated with profound cerebellar cortical degeneration, we found abnormalities of several key vestibular reflexes. The vestibuloocular reflex (VOR) was measured by eye movement responses to changes in head rotation. The vestibulocollic reflex (VCR) was assessed with cervical vestibular-evoked myogenic potentials (cVEMPs), in which auditory clicks led to electromyographic activity of the sternocleidomastoid muscle. The VOR gain (eye velocity/head velocity) was increased in all subjects with A-T. An increase of the VCR, paralleling that of the VOR, was indirectly suggested by an increase in cVEMP amplitude. In A-T subjects, alignment of the axis of eye rotation was not with that of head rotation. Subjects with A-T thus manifested VOR cross-coupling, abnormal eye movements directed along axes orthogonal to that of head rotation. Degeneration of the Purkinje neurons in the vestibulocerebellum probably underlie these deficits. This study offers insights into how the vestibulocerebellum functions in healthy humans. It may also be of value to the design of treatment trials as a surrogate biomarker of cerebellar function that does not require controlling for motivation or occult changes in motor strategy on the part of experimental subjects.

The anatomy and physiology of the ocular motor system

Accurate diagnosis of abnormal eye movements depends upon knowledge of the purpose, properties, and neural substrate of distinct functional classes of eye movement. Here, we summarize current concepts of the anatomy of eye movement control. Our approach is bottom-up, starting with the extraocular muscles and their innervation by the cranial nerves. Second, we summarize the neural circuits in the pons underlying horizontal gaze control, and the midbrain connections that coordinate vertical and torsional movements. Third, the role of the cerebellum in governing and optimizing eye movements is presented. Fourth, each area of cerebral cortex contributing to eye movements is discussed. Last, descending projections from cerebral cortex, including basal ganglionic circuits that govern different components of gaze, and the superior colliculus, are summarized. At each stage of this review, the anatomical scheme is used to predict the effects of lesions on the control of eye movements, providing clinical-anatomical correlation.

The cerebellar nodulus/uvula integrates otolith signals for the translational vestibulo-ocular reflex

Background

The otolith-driven translational vestibulo-ocular reflex (tVOR) generates compensatory eye movements to linear head accelerations. Studies in humans indicate that the cerebellum plays a critical role in the neural control of the tVOR, but little is known about mechanisms of this control or the functions of specific cerebellar structures. Here, we chose to investigate the contribution of the nodulus and uvula, which have been shown by prior studies to be involved in the processing of otolith signals in other contexts.

Methodology/Principal Findings

We recorded eye movements in two rhesus monkeys during steps of linear motion along the interaural axis before and after surgical lesions of the cerebellar uvula and nodulus. The lesions strikingly reduced eye velocity during constant-velocity motion but had only a small effect on the response to initial head acceleration. We fit eye velocity to a linear combination of head acceleration and velocity and to a dynamic mathematical model of the tVOR that incorporated a specific integrator of head acceleration. Based on parameter optimization, the lesion decreased the gain of the pathway containing this new integrator by 62%. The component of eye velocity that depended directly on head acceleration changed little (gain decrease of 13%). In a final set of simulations, we compared our data to the predictions of previous models of the tVOR, none of which could account for our experimental findings.

Conclusions/ Significance

Our results provide new and important information regarding the neural control of the tVOR. Specifically, they point to a key role for the cerebellar nodulus and uvula in the mathematical integration of afferent linear head acceleration signals. This function is likely to be critical not only for the tVOR but also for the otolith-mediated reflexes that control posture and balance.

Motion sickness induced by otolith stimulation is correlated with otolith-induced eye movements

This article addresses the relationships between motion sickness (MS) and three-dimensional (3D) ocular responses during otolith stimulation. A group of 19 healthy subjects was tested for motion sickness during a 16 min otolith stimulation induced by off-vertical axis rotation (OVAR) (constant velocity 60 degrees /s, frequency 0.16 Hz). For each subject, the MS induced during the session was quantified, and based on this quantification, the subjects were divided into two groups of less susceptible (MS-), and more susceptible (MS+) subjects. The angular eye velocity induced by the otolith stimulation was analyzed in order to identify a possible correlation between susceptibility to MS and 3D eye velocity. The main results show that: (1) MS significantly correlates in a multiple regression with several components of the horizontal vestibular eye movements i.e. positively with the velocity modulation (P<0.01) and bias (P<0.05) of the otolith ocular reflex and negatively with the time constant of the vestibulo-ocular reflex (P<0.01) and (2) the length of the resultant 3D eye velocity vector is significantly larger in the MS+ as compared with the MS- group. Based on these results we suggest that the CNS, including the velocity storage mechanism, reconstructs an eye velocity vector modulated by head position whose length might predict MS occurrence during OVAR.

Preserved otolith function in patients with cerebellar atrophy and bilateral vestibulopathy

Cerebellar degeneration affects vestibular function. For instance, with lesions of the cerebellar flocculus, the ability to adaptively modify the VOR gain is markedly reduced, and cerebellar patients may even demonstrate severe vestibular deficits. We report five patients (m=3, f=2) with cerebellar disease, in whom search-coil head impulse testing revealed reduced gains of the angular VOR, while sacculus-mediated myogenic potentials were normal. Preserved static ocular counterroll in roll-tilt positions and prominent gravity-dependent modulation of downbeat nystagmus (DBN) along the pitch plane demonstrated the integrity of otolith (OL) function in these patients as well. Probably, at least in some cerebellar patients with marked floccular atrophy, the dissociation between impaired semicircular canal (SCC) function and preserved OL function may be explained by a predilection of the atrophic process for the flocculus and brainstem neurons involved in angular VOR gain control, while structures mediating OL function remain widely spared by the cerebellar degeneration. The exact pathomechanism leading to the vestibular impairment remains unclear: both a primary multi-system-type atrophy involving cerebellar and brainstem vestibular structures as well as a mechanism of secondary retrograde degeneration of floccular brainstem target neurons mediating SCC function seem plausible.

Labyrinthine lesions and motion sickness susceptibility

The angular vestibulo-ocular reflex (aVOR) has a fast pathway, which mediates compensatory eye movements, and a slow (velocity storage) pathway, which determines its low frequency characteristics and orients eye velocity toward gravity. We have proposed that motion sickness is generated through velocity storage, when its orientation vector, which lies close to the gravitational vertical, is misaligned with eye velocity during head motion. The duration of the misalignment, determined by the dominant time constant of velocity storage, causes the buildup of motion sickness. To test this hypothesis, we studied bilateral labyrinthine-defective subjects with short vestibular time constants but normal aVOR gains for their motion sickness susceptibility. Time constants and gains were taken from rotational responses. Motion sickness was generated by rolling the head while rotating, and susceptibility was assessed by the number of head movements made before reaching intolerable levels of nausea. More head movements signified lower motion sickness susceptibility. Labyrinthine-defective subjects made more head movements on their first exposure to roll while rotating than normals (39.8 +/- 7.2 vs 13.7 +/- 5.5; P < 0.0001). Normals were tested eight times, which habituated their time constants and reduced their motion sickness susceptibility. Combining data from all subjects, there was a strong inverse relationship between time constants and number of head movements (r = 0.94), but none between motion sickness susceptibility and aVOR gains. This provides further evidence that motion sickness is generated through velocity storage, not the direct pathway, and suggests that motion sickness susceptibility can be reduced by reducing the aVOR time constant.

Head-shaking nystagmus depends on gravity

In acute unilateral peripheral vestibular deficit, horizontal spontaneous nystagmus (SN) increases when patients lie on their affected ear. This phenomenon indicates an ipsilesional reduction of otolith function that normally suppresses asymmetric semicircular canal signals. We asked whether head-shaking nystagmus (HSN) in patients with chronic unilateral vestibular deficit following vestibular neuritis is influenced by gravity in the same way as SN in acute patients. Using a three-dimensional (3-D) turntable, patients (N = 7) were placed in different whole-body positions along the roll plane and oscillated (1 Hz, +/-10 degrees ) about their head-fixed vertical axis. Eye movements were recorded with 3-D magnetic search coils. HSN was modulated by gravity: When patients lay on their affected ear, slow-phase eye velocity significantly increased upon head shaking and consisted of a horizontal drift toward the affected ear (average: 1.2 degrees /s +/-0.5 SD), which was added to the gravity-independent and directionally nonspecific SN. In conclusion, HSN in patients with chronic unilateral peripheral vestibular deficit is best elicited when they are lying on their affected ear. This suggests a gravity-dependent mechanism similar to the one observed for SN in acute patients, i.e., an asymmetric suppression of vestibular nystagmus by the unilaterally impaired otolith organs.

Eye movement abnormalities in essential tremor may indicate cerebellar dysfunction

Experimental and clinical data indicate that the cerebellum is involved in the pathophysiology of advanced stages of essential tremor (ET). The aim of this study was to determine whether a dysfunction also affects cerebellar structures involved in eye movement control. Eye movements of 14 patients with ET and 11 age-matched control subjects were recorded using the scleral search-coil technique. Vestibular function was assessed by electro-oculography. Eight ET patients had clinical evidence of intention tremor (ET(IT)); six had a predominantly postural tremor (ET(PT)) without intention tremor. ET patients showed two major deficits that may indicate cerebellar dysfunction: (i) an impaired smooth pursuit initiation; and (ii) pathological suppression of the vestibulo-ocular reflex (VOR) time constant by head tilts ('otolith dumping'). In the step ramp smooth pursuit paradigm, the initial eye acceleration in the first 60 ms of pursuit generation was significantly reduced in ET patients, particularly in ET(IT) patients, by approximately 44% (mean 23.4 degrees/s(2)) compared with that of control subjects (mean 41.3 degrees/s(2)). Subsequent steady-state pursuit velocity and sinusoidal pursuit gain (e.g. 0.4 Hz: 0.90 versus 0.78) were also significantly decreased in ET patients, whereas pursuit latency was unaffected. The intention tremor score correlated with the pursuit deficit, e.g. ET(IT) patients were significantly more affected than ET(PT) patients. Gain and time constant (tau) of horizontal VOR were normal, but suppression of the VOR time constant by head tilt ('otolith dumping') was pathological in 41% of ET patients, particularly in ET(IT) patients. Saccades and gaze-holding function were not impaired. The deficit of pursuit initiation, its correlation with the intensity of intention tremor, and the pathological VOR dumping provide additional evidence of a cerebellar dysfunction in the advanced stage of ET, when intention tremor becomes part of the clinical symptoms, and point to a common pathomechanism. The oculomotor deficits may indicate an impairment of the caudal vermis in ET.

Evidence of normal cerebellar control of the vestibulo-ocular reflex (VOR) in children with high-functioning autism

The effect of "tilt-suppression" on post-rotatory vestibular nystagmus was investigated to assess the function of the caudal cerebellar vermis (lobules IX and X, or nodulus and uvula) in 13 school-age children with high-functioning autism (HFA) and 10 normal controls. Tilt-suppression of the vestibulo-ocular reflex (VOR) refers to the decreasing of the duration of post-rotatory vestibular nystagmus that occurs when the head is moved out of the plane in which it was located during the previous sustained constant-velocity rotation. The participant is rotated in a vestibular chair with the head upright and then the head is tilted forward just after the chair stops rotating. Such tilt-suppression is impaired with lesions of the cerebellar nodulus and portions of the uvula. Results show that children with HFA have normal post-rotatory nystasmus with the head upright and normal attenuation of post-rotatory nystagmus induced by head tilt. These behavioral findings suggest that lobules IX and X of the cerebellum are spared in high-functioning autism.

Otolith-ocular responses in familial episodic ataxia linked to chromosome 19p

We evaluated several members of a family with episodic ataxia type 2 linked to chromosome 19p by using a battery of vestibulo-ocular reflex (VOR) tests. Testing focused on the otolith-ocular reflex and semicircular canal-otolith interaction. Our aims were to improve understanding of the structures important for the VOR and to define further the range of vestibulo-ocular, in particular otolith-ocular, manifestations within a family with episodic ataxia. Ocular motor, semicircular canal-ocular, and semicircular canal-otolith interaction assessments suggested impairment of the vestibulo-cerebellum (ie, the flocculonodular lobe); the brainstem appeared to be relatively spared. Eye movements during constant velocity off-vertical axis rotation (OVAR), a pure otolith stimulus, indicated that the modulation component of the response was normal whereas the bias component was reduced or nearly absent. Based on these data, it appears that the cerebellum is not responsible for the generation of the modulation component of the response to OVAR. However, the bias component appears to depend on the caudal midline cerebellum.

International Cooperative Ataxia Rating Scale for pharmacological assessment of the cerebellar syndrome. The Ataxia Neuropharmacology Committee of the World Federation of Neurology

Despite the involvement of cerebellar ataxia in a large variety of conditions and its frequent association with other neurological symptoms, the quantification of the specific core of the cerebellar syndrome is possible and useful in Neurology. Recent studies have shown that cerebellar ataxia might be sensitive to various types of pharmacological agents, but the scales used for assessment were all different. With the long-term goal of double-blind controlled trials-multicentric and international-an ad hoc Committee of the World Federation of Neurology has worked to propose a one-hundred-point semi-quantitative International Cooperative Ataxia Rating Scale (ICARS). The scale proposed involves a compartimentalized quantification of postural and stance disorders, limb ataxia, dysarthria and oculomotor disorders, in order that a subscore concerning these symptoms may be separately studied. The weight of each symptomatologic compartment has been carefully designed. The members of the Committee agreed upon precise definitions of the tests, to minimize interobserver variations. The validation of this scale is in progress.

Neurotologic manifestations of Chiari 1 malformation

It is important for otolaryngologists to recognize Chiari malformations as part of the differential diagnosis of balance disorders because patients may initially be seen with symptoms referable to the vestibular system, including ataxia, nystagmus, or vertigo. The objective of this paper is to review the signs, symptoms, and vestibular test findings of a series of patients with Chiari 1 malformation. Six patients were identified by retrospective chart review with a diagnosis of Chiari malformation. Each patient had a complete otoneurologic examination and vestibular function testing. The results indicated that patients fell into two different vestibular test result profiles. First, patients with advanced symptoms demonstrated oculomotor dysfunction, central vestibular nystagmus, abnormal vestibular visual interaction, and abnormal tilt suppression of postrotatory nystagmus. On the other hand, a number of patients were identified with incidentally noted Chiari malformation on magnetic resonance imaging scan who had a vestibular test profile consistent with peripheral vestibulopathy without signs and symptoms of central nervous system dysfunction. Guidelines are provided to help determine the extent of the group of symptoms attributable to an incidentally discovered Chiari malformation.

Alternating skew on lateral gaze. Neuroanatomic pathway and relationship to superior oblique overaction

BACKGROUND

Previous studies of patients with heterogeneous, often diffuse neurologic disorders concluded that the neurologic substrate for alternating skew on lateral gaze may be localized at the level of the brain stem tegmentum or the cervico-medullary junction, or both. The localized nature of brain tumors offers an opportunity to further investigate the anatomic localization for this as well as other conditions.

METHODS

To test the hypothesis that cervico-medullary and cerebellar lesions are responsible for alternating skew on lateral gaze, the authors investigated a series of 50 children with brain tumors, 39 of whom showed neuro-ophthalmologic abnormalities on clinical testing. Seven children had alternating skew on lateral gaze.

RESULTS

All seven children with alternating skew on lateral gaze showed neoplastic involvement at the level of the cervico-medullary junction and/or the cerebellum on critical analysis of neuro-imaging studies.

CONCLUSION

The authors conclude that the neuroanatomic substrate for alternating skew on lateral gaze is localized at the level of the cervico-medullary junction and/or the cerebellum. Knowing that (1) alternating skew on lateral gaze closely mimics superior oblique overaction clinically, (2) superior oblique overaction is frequently found in patients with myelomeningocele, and (3) myelomeningocele is uniformly associated with Arnold-Chiari type II which includes cerebellar and cervico-medullary region abnormalities, the authors propose that alternating skew on lateral gaze and superior oblique overaction associated with myelomeningocele have similar neuroanatomic pathways.

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.

Static roll and the vestibulo-ocular reflex (VOR)

We measured the effect of static lateral tilt (roll) on the gain and time constant of the vestibulo-ocular reflex (VOR) in five normal subjects by recording both the horizontal and vertical components of eye velocity in space for rotation about an earth vertical axis with the head either upright or rolled to either side. The time constant of the VOR in the upright position was 19.6 +/- 3.2s (mean +/- standard deviation). The time constant of the horizontal component with respect to the head decreased to 15.7 +/- 4.0s for 30 degrees roll and to 12.7 +/- 2.7s for 60 degrees roll. The time constant of the vertical component with respect to the head was 11.0 +/- 1.4s for 30 degrees roll and 7.5 +/- 1.6s for 60 degrees roll. The gain of the horizontal VOR with respect to space did not vary significantly with roll angle but a small space-vertical component to the VOR appeared during all rotations when the head was rolled away from upright. This non-compensatory nystagmus built up to a maximum of 2-3 degrees/s at 17.0 +/- 4.7s after the onset of rotation and then decayed. These data suggest that static otolith input modulates the central storage of semicircular canal signals, and that head-horizontal and head-vertical components of the VOR can decay at different rates.

Vestibular responses in Wernicke's encephalopathy

Two patients with Wernicke's encephalopathy were evaluated with quantitative vestibulo-ocular reflex and ocular motor testing. Vestibulo-ocular reflex testing included caloric irrigation, earth vertical axis rotational sinusoids, and rotational impulses. Both patients demonstrated hypoactive vestibular responses to both caloric and rotational stimuli at the time of presentation. One patient had unbeating nystagmus that diminished with upgaze, downgaze, or convergence. Following treatment with thiamine, both patients' vestibular responses improved but remained abnormal, with a short vestibulo-ocular reflex time constant and increased low-frequency rotational phase lead. Impairment of the velocity storage element attributable to damage to the vestibular nucleus and nucleus prepositus hypoglossi may account for this permanent effect on the vestibulo-ocular reflex.