Directional abnormalities of vestibular and optokinetic responses in cerebellar disease

Classification of short and long term mild traumatic brain injury using computerized eye tracking

Accurate, and objective diagnosis of brain injury remains challenging. This study evaluated useability and reliability of computerized eye-tracker assessments (CEAs) designed to assess oculomotor function, visual attention/processing, and selective attention in recent mild traumatic brain injury (mTBI), persistent post-concussion syndrome (PPCS), and controls. Tests included egocentric localisation, fixation-stability, smooth-pursuit, saccades, Stroop, and the vestibulo-ocular reflex (VOR). Thirty-five healthy adults performed the CEA battery twice to assess useability and test-retest reliability. In separate experiments, CEA data from 55 healthy, 20 mTBI, and 40 PPCS adults were used to train a machine learning model to categorize participants into control, mTBI, or PPCS classes. Intraclass correlation coefficients demonstrated moderate (ICC > .50) to excellent (ICC > .98) reliability (p < .05) and satisfactory CEA compliance. Machine learning modelling categorizing participants into groups of control, mTBI, and PPCS performed reasonably (balanced accuracy control: 0.83, mTBI: 0.66, and PPCS: 0.76, AUC-ROC: 0.82). Key outcomes were the VOR (gaze stability), fixation (vertical error), and pursuit (total error, vertical gain, and number of saccades). The CEA battery was reliable and able to differentiate healthy, mTBI, and PPCS patients reasonably well. While promising, the diagnostic model accuracy should be improved with a larger training dataset before use in clinical environments.

Vestibulo-spatial navigation: pathways and sense of direction

Aims of the present article are: 1) assessing vestibular contribution to spatial navigation, 2) exploring how age, global positioning systems (GPS) use, and vestibular navigation contribute to subjective sense of direction (SOD), 3) evaluating vestibular navigation in patients with lesions of the vestibular-cerebellum (patients with downbeat nystagmus, DBN) that could inform on the signals carried by vestibulo-cerebellar-cortical pathways. We applied two navigation tasks on a rotating chair in the dark: return-to-start (RTS), where subjects drive the chair back to the origin after discrete angular displacement stimuli (path reversal), and complete-the-circle (CTC) where subjects drive the chair on, all the way round to origin (path completion). We examined 24 normal controls (20-83 yr), five patients with DBN (62-77 yr) and, as proof of principle, two patients with early dementia (84 and 76 yr). We found a relationship between SOD, assessed by Santa Barbara Sense of Direction Scale, and subject's age (positive), GPS use (negative), and CTC-vestibular-navigation-task (positive). Age-related decline in vestibular navigation was observed with the RTS task but not with the complex CTC task. Vestibular navigation was normal in patients with vestibulo-cerebellar dysfunction but abnormal, particularly CTC, in the demented patients. We conclude that vestibular navigation skills contribute to the build-up of our SOD. Unexpectedly, perceived SOD in the elderly is not inferior, possibly explained by increased GPS use by the young. Preserved vestibular navigation in cerebellar patients suggests that ascending vestibular-cerebellar projections carry velocity (not position) signals. The abnormalities in the cognitively impaired patients suggest that their vestibulo-spatial navigation is disrupted.NEW & NOTEWORTHY Our subjective sense-of-direction is influenced by how good we are at spatial navigation using vestibular cues. Global positioning systems (GPS) may inhibit sense of direction. Increased use of GPS by the young may explain why the elderly's sense of direction is not worse than the young's. Patients with vestibulo-cerebellar dysfunction (downbeat nystagmus syndrome) display normal vestibular navigation, suggesting that ascending vestibulo-cerebellar-cortical pathways carry velocity rather than position signals. Pilot data indicate that dementia disrupts vestibular navigation.

Neurophysiology of the optokinetic system

This chapter provides a review of early studies into the neural substrate for optokinetic-vestibular responses. Properties and connections of retinal and brainstem neurons contributing to optokinetic responses in the afoveate rabbit are summarized. Electrophysiological and lesion studies provide support for confluence of optokinetic and vestibular signals in the vestibular nucleus to provide the brain's estimate of self-rotation. Evidence for optokinetic-vestibular symbiosis in humans comes from the observation that individuals who have lost vestibular function show no optokinetic after-nystagmus in darkness, following full-field stimulus motion. An anatomical scheme for brainstem elaboration of optokinetic responses is proposed and cerebellar contributions are reviewed.

Head-impulse tests aid in differentiation of multiple system atrophy from Parkinson's disease

The integrity of the vestibulo-ocular reflex (VOR) remains to be delineated in patients with parkinsonism. We aimed to define the findings of the VOR using head-impulse tests (HITs) and their differential diagnostic value in patients with Parkinson's disease (PD) and multiple system atrophy (MSA). From December 2019 to January 2021, 30 patients with PD and 23 patients with MSA (17 with cerebellar-type MSA and 6 with parkinsonian-type MSA) had a video-oculographic recording of HITs at two university hospitals in South Korea. Reversed (p = 0.034) and perverted (p = 0.015) catch-up saccades were more frequently observed in MSA than in PD during HITs. The gain difference between the ACs and the PCs were larger in MSA than in PD (p = 0.031), and positively correlated with the disease duration in patients with MSA (Spearman's coefficient = 0.512, p = 0.012). Multivariate logistic regression analysis showed that reversed (p = 0.044) and perverted (p = 0.039) catch-up saccades were more frequently associated with MSA than with PD during HITs. In conclusion, HITs aid in differentiation of MSA from PD, and may serve as a surrogate marker for the clinical decline.

Retinal Image Slip Must Pass the Threshold for Human Vestibulo-Ocular Reflex Adaptation

We sought to determine whether repeated vestibulo-ocular reflex (VOR) adaptation training to increase the VOR gain (eye/head velocity) had a lasting effect in normal subjects and whether there was a retinal image slip tolerance threshold for VOR adaptation. We used the unilateral incremental VOR adaptation technique and horizontal active (self-generated, predictable) head impulses as the vestibular stimulus. Both active and passive (imposed, unpredictable) head impulse VOR gains were measured before and after unilateral incremental VOR adaptation training. The adapting side was pseudo-randomized for left or right. We tested ten normal subjects over one block (10 sessions over 12 days) of VOR adaptation training and testing, immediately followed by a second block (5 sessions over 19 days) of testing only without training. Our findings show robust short-term VOR adaptation of ~ 10 % immediately after each 15-min training session, but that the daily pre-adaptation gain was most different on days 1 and 2, and for subsequent training days before saturating to ~ 5 % greater than the pre-adaptation gain on day 1. This increase was partially retained for 19 days after regular training stopped. The data suggest that stable vision in normal subjects is maintained when there is < 5 % deviation in VOR gain from the original baseline, which corresponds to < 9°/s retinal image slip. Below this threshold, there is poor adaptive drive to return the gain to its original baseline value.

Clinical implications of head-shaking nystagmus in central and peripheral vestibular disorders: is perverted head-shaking nystagmus specific for central vestibular pathology?

BACKGROUND AND PURPOSE

The patterns of head-shaking nystagmus (HSN) aid in differentiation between central and peripheral vestibular disorders, and perverted HSN (pHSN) has been considered a central sign. The aim was to determine the characteristics of HSN in a large number of patients with either peripheral or central vestibular disorders in a dizziness clinic of a university hospital.

METHODS

The medical records of 7544 dizzy patients were reviewed during a year and 822 patients with a clinical diagnosis of vestibular disorders were recruited. The findings of spontaneous nystagmus (SN) and HSN in these patients were compared with those of healthy controls (n = 48).

RESULTS

A total of 217 of the 822 patients (26.4%) were classified as having a central vestibular disorder, whilst 397 (48.3%) had a peripheral vestibular disorder. In the peripheral vestibular disorder group, SN was observed in 14.1% and HSN in 40.8%, amongst whom 24.1% were the pHSN form. In the central group, SN was observed in 17.5% and HSN in 24.0% of whom 57.7% was pHSN. HSN was more frequently observed in the peripheral vestibular disorder group than in the central group (40.8% vs. 24.0%, P < 0.01). However, the proportion of pHSN was significantly increased in the central group compared to the peripheral vestibular patient group (57.7% vs. 24.1%, P < 0.01).

CONCLUSIONS

Since pHSN is not specific for central vestibular disorders, other clinical features should be considered in pursuing a central lesion in patients with pHSN.

Eye Movement Disorders and the Cerebellum

The cerebellum works as a network hub for optimizing eye movements through its mutual connections with the brainstem and beyond. Here, we review three key areas in the cerebellum that are related to the control of eye movements: (1) the flocculus/paraflocculus (tonsil) complex, primarily for high-frequency, transient vestibular responses, and also for smooth pursuit maintenance and steady gaze holding; (2) the nodulus/ventral uvula, primarily for low-frequency, sustained vestibular responses; and (3) the dorsal vermis/posterior fastigial nucleus, primarily for the accuracy of saccades. Although there is no absolute compartmentalization of function within the three major ocular motor areas in the cerebellum, the structural-functional approach provides a framework for assessing ocular motor performance in patients with disease that involves the cerebellum or the brainstem.

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.

Classification of vestibular signs and examination techniques: Nystagmus and nystagmus-like movements

This paper presents a classification and definitions for types of nystagmus and other oscillatory eye movements relevant to evaluation of patients with vestibular and neurological disorders, formulated by the Classification Committee of the Bárány Society, to facilitate identification and communication for research and clinical care. Terminology surrounding the numerous attributes and influencing factors necessary to characterize nystagmus are outlined and defined. The classification first organizes the complex nomenclature of nystagmus around phenomenology, while also considering knowledge of anatomy, pathophysiology, and etiology. Nystagmus is distinguished from various other nystagmus-like movements including saccadic intrusions and oscillations.

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Past and Present of Eye Movement Abnormalities in Ataxia-Telangiectasia

Ataxia-telangiectasia is the second most common autosomal recessive hereditary ataxia, with an estimated incidence of 1 in 100,000 births. Besides ataxia and ocular telangiectasias, eye movement abnormalities have long been associated with this disorder and is frequently present in almost all patients. A handful of studies have described the phenomenology of ocular motor deficits in ataxia-telangiectasia. Contemporary literature linked their physiology to cerebellar dysfunction and secondary abnormalities at the level of brainstem. These studies, while providing a proof of concept of ocular motor physiology in disease, i.e., ataxia-telangiectasia, also advanced our understanding of how the cerebellum works. Here, we will summarize the clinical abnormalities seen with ataxia-telangiectasia in each subtype of eye movements and subsequently describe the underlying pathophysiology. Finally, we will review how these deficits are linked to abnormal cerebellar function and how it allows better understanding of the cerebellar physiology.

Approach to the Examination and Classification of Nystagmus

BACKGROUND AND PURPOSE

Physical therapists caring for patients with neurologic or vestibular disorders must routinely examine and characterize nystagmus and other oscillatory eye movements. Often, the diagnosis hinges on proper interpretation of the nystagmus pattern. This requires understanding the terminology surrounding the numerous attributes and influencing factors of nystagmus, a systematic approach to the examination, and a classification structure that guides practitioners to the specific nystagmus type and subsequent evaluation and management.

SUMMARY OF KEY POINTS

Nystagmus is an involuntary, rapid, rhythmic, oscillatory eye movement with at least 1 slow phase. Jerk nystagmus has a slow phase and a fast phase. Pendular nystagmus has only slow phases. Nystagmus is distinguished from other types of oscillatory eye movements, such as saccadic intrusions or oscillations. Characterizing nystagmus requires clearly describing its trajectory. This includes choosing a reference frame to describe the axes or planes and direction of eye movements. Several attributes are used to describe nystagmus: binocularity, conjugacy, velocity, waveform, frequency, amplitude, intensity, temporal profile, and age at first appearance. Several factors may influence nystagmus, including gaze position, visual fixation, vergence, and a variety of provocative maneuvers. Classification of nystagmus may be organized by physiologic or pathologic nystagmus versus other nystagmus-like movements. Pathologic nystagmus may be spontaneous, gaze-evoked, or triggered by provocative maneuvers. The combination of attributes allows differentiation between the many peripheral and central forms.

RECOMMENDATIONS FOR CLINICAL PRACTICE

Therapists should carefully examine and characterize the trajectory and other attributes and influencing factors of nystagmus to accurately classify it and arrive at the correct diagnosis.

Recent advances in head impulse test findings in central vestibular disorders

The head impulse test (HIT) is used to evaluate the vestibulo-ocular reflex (VOR) during a high-velocity head rotation. Corrective catch-up saccades that occur during or after the HITs usually indicate peripheral vestibular hypofunction, whereas in acute vestibular syndrome, normal clinical (bedside) HITs should prompt a search for a central lesion. However, recent quantitative studies that evaluated HITs using magnetic search coils or video-based techniques have demonstrated that specific patterns of HIT abnormalities are associated with central vestibular disorders. While normal clinical HITs are typical of central lesions, discrepancies have been observed between clinical and quantitative HITs. The horizontal head impulse VOR gains can be significantly reduced unilaterally or bilaterally (positive HITs) in lesions involving the vestibular nucleus, nucleus prepositus hypoglossi, or flocculus. In diffuse cerebellar lesions, the VOR gain during horizontal head impulses may increase (hyperactive) with corrective saccades directed the opposite way. The presence of cross-coupled vertical corrective saccades during horizontal HITs is also suggestive of diffuse cerebellar lesions. Lesions involving the vestibular nucleus, medial longitudinal fasciculus, and cerebellum may show decreased or increased gains of the VOR during vertical HITs. Defining the differences in patterns observed during abnormal HITs may help practitioners localize the responsible lesions in both central and peripheral vestibulopathy.

Approach to dizziness in the emergency department

Acute dizziness/vertigo is among the most common causes for visiting the emergency department. The traditional approach to dizziness starts with categorizing dizziness into four types: vertigo, presyncope, disequilibrium, and nonspecific dizziness. However, a recently proposed approach begins with classifying dizziness/vertigo as acute prolonged spontaneous dizziness/vertigo, recurrent spontaneous dizziness/vertigo, recurrent positional vertigo, or chronic persistent dizziness and imbalance. Vestibular neuritis and stroke are key disorders causing acute prolonged spontaneous dizziness/vertigo, but the diagnosis of isolated vascular vertigo has increased by virtue of developments in clinical neurotology and neuroimaging. However, a well-organized bedside examination appears more sensitive than brain imaging in diagnosing strokes presenting with acute dizziness/vertigo. A detailed history is vital to diagnose recurrent spontaneous dizziness/vertigo since confirmatory diagnostic tests are usually unavailable. Isolated positional vertigo is usually caused by benign paroxysmal positional vertigo, which can be treated at the bedside. In recent years, marked progress has occurred in the evaluation/management of acute dizziness/vertigo. However, even with developments in imaging technology, the diagnosis of acute dizziness/vertigo largely relies on bedside examination.

Vertical components of head-shaking nystagmus in vestibular neuritis, Meniere's disease and migrainous vertigo

OBJECTIVES

To describe vertical and horizontal components of head-shaking nystagmus (HSN) in various vestibular disorders.

DESIGN

Retrospective case review.

SETTING

Tertiary care academic referral centre.

PARTICIPANTS

Head-shaking nystagmus was assessed in 66 vestibular neuritis (VN) patients at acute (<7 days) and follow-up (2 months), and 65 Meniere's disease (MD) and 76 migrainous vertigo (MV) in interictal period.

MAIN OUTCOME MEASURES

Head-shaking nystagmus was categorised as pure horizontal, pure vertical or mixed. Horizontal HSN was classified as monophasic or biphasic and paretic or recovery. Vertical HSN was classified as upbeat or downbeat.

RESULTS

Abnormal HSN (pathologic monophasic, biphasic or delayed-peak HSN) showed different positive rates depending on the vestibular disorders and compensation (94% in acute VN; 89% in FU VN; 78% in MD; 50% in MV). Paretic HSN with the nystagmus towards the lesioned side was the most common type in VN and MD; however, recovery HSN with the nystagmus towards the intact side could be rarely observed especially in patients with MD or compensated VN. Vertical nystagmus could be combined with horizontal HSN, and upbeat HSN was observed in most (83%) of the patients with acute VN, but downbeat HSN was common in follow-up VN (83%), MD (97%) and MV (85%). Weak perverted HSN, which is assumed to be a central nystagmus, was rarely observed in MD and MV (6-9%), but not in VN.

CONCLUSIONS

Head-shaking nystagmus (HSN) in horizontal plane is a valuable tool in the assessment of vestibular imbalance. Common observation of upbeat HSN in acute VN and downbeat HSN in follow-up VN, MD and MV suggests that vertical components are possibly related to the involvement of vestibular apparatus and compensation. Weak perverted HSN and delayed-peak HSN were rarely observed in MD and MV, and never observed in VN, suggesting that it is possibly related to either asymmetrically impaired vertical canals or misorientation of the velocity-storage system.

Quantitative evaluation of human cerebellum-dependent motor learning through prism adaptation of hand-reaching movement

The cerebellum plays important roles in motor coordination and learning. However, motor learning has not been quantitatively evaluated clinically. It thus remains unclear how motor learning is influenced by cerebellar diseases or aging, and is related with incoordination. Here, we present a new application for testing human cerebellum-dependent motor learning using prism adaptation. In our paradigm, the participant wearing prism-equipped goggles touches their index finger to the target presented on a touchscreen in every trial. The whole test consisted of three consecutive sessions: (1) 50 trials with normal vision (BASELINE), (2) 100 trials wearing the prism that shifts the visual field 25° rightward (PRISM), and (3) 50 trials without the prism (REMOVAL). In healthy subjects, the prism-induced finger-touch error, i.e., the distance between touch and target positions, was decreased gradually by motor learning through repetition of trials. We found that such motor learning could be quantified using the “adaptability index (AI)”, which was calculated by multiplying each probability of [acquisition in the last 10 trials of PRISM], [retention in the initial five trials of REMOVAL], and [extinction in the last 10 trials of REMOVAL]. The AI of cerebellar patients less than 70 years old (mean, 0.227; n = 62) was lower than that of age-matched healthy subjects (0.867, n = 21; p < 0.0001). While AI did not correlate with the magnitude of dysmetria in ataxic patients, it declined in parallel with disease progression, suggesting a close correlation between the impaired cerebellar motor leaning and the dysmetria. Furthermore, AI decreased with aging in the healthy subjects over 70 years old compared with that in the healthy subjects less than 70 years old. We suggest that our paradigm of prism adaptation may allow us to quantitatively assess cerebellar motor learning in both normal and diseased conditions.

Rebound upbeat nystagmus after lateral gaze in episodic ataxia type 2

Rebound nystagmus is a transient nystagmus that occurs on resuming the straight-ahead position after prolonged eccentric gaze. Even though rebound nystagmus is commonly associated with gaze-evoked nystagmus (GEN), development of rebound nystagmus in a different plane of gaze has not been described. We report a patient with episodic ataxia type 2 who showed transient upbeat nystagmus on resuming the straight-ahead position after sustained lateral gaze that had induced GEN and downbeat nystagmus. The rebound upbeat nystagmus may be ascribed to a shifting null in the vertical plane as a result of an adaptation to the downbeat nystagmus that developed during lateral gaze.

Bedside evaluation of dizzy patients

In recent decades there has been marked progress in the imaging and laboratory evaluation of dizzy patients. However, detailed history taking and comprehensive bedside neurotological evaluation remain crucial for a diagnosis of dizziness. Bedside neurotological evaluation should include examinations for ocular alignment, spontaneous and gaze-evoked nystagmus, the vestibulo-ocular reflex, saccades, smooth pursuit, and balance. In patients with acute spontaneous vertigo, negative head impulse test, direction-changing nystagmus, and skew deviation mostly indicate central vestibular disorders. In contrast, patients with unilateral peripheral deafferentation invariably have a positive head impulse test and mixed horizontal-torsional nystagmus beating away from the lesion side. Since suppression by visual fixation is the rule in peripheral nystagmus and is frequent even in central nystagmus, removal of visual fixation using Frenzel glasses is required for the proper evaluation of central as well as peripheral nystagmus. Head-shaking, cranial vibration, hyperventilation, pressure to the external auditory canal, and loud sounds may disclose underlying vestibular dysfunction by inducing nystagmus or modulating the spontaneous nystagmus. In patients with positional vertigo, the diagnosis can be made by determining patterns of the nystagmus induced during various positional maneuvers that include straight head hanging, the Dix-Hallpike maneuver, supine head roll, and head turning and bending while sitting. Abnormal smooth pursuit and saccades, and severe imbalance also indicate central pathologies. Physicians should be familiar with bedside neurotological examinations and be aware of the clinical implications of the findings when evaluating dizzy patients.

Isolated vestibular nuclear infarction: report of two cases and review of the literature

Cerebral infarction presenting with isolated vertigo remains a diagnostic challenge. To define the clinical characteristics of unilateral infarctions restricted to the vestibular nuclei, two patients with isolated unilateral vestibular nuclear infarction had bedside and laboratory evaluation of the ocular motor and vestibular function, including video-oculography, bithermal caloric irrigation, the head impulse test (HIT) using magnetic scleral coils, and cervical and ocular vestibular-evoked myogenic potentials (VEMPs). We also reviewed the literature on isolated vertigo from lesions restricted to the vestibular nuclei, and analyzed the clinical features of seven additional patients. Both patients showed spontaneous torsional-horizontal nystagmus that beat away from the lesion side, and direction-changing gaze-evoked nystagmus. Recording of HIT using a magnetic search coil system documented decreased gains of the vestibular-ocular reflex for the horizontal and posterior semicircular canals on both sides, but more for the ipsilesional canals. Bithermal caloric tests showed ipsilesional canal paresis in both patients. Cervical and ocular VEMPs showed decreased or absent responses during stimulation of the ipsilesional ear. Initial MRIs including diffusion-weighted images were normal or equivocal, but follow-up imaging disclosed a circumscribed acute infarction in the area of the vestibular nuclei. Infarctions restricted to the vestibular nuclei may present with isolated vertigo with features of both peripheral and central vestibulopathies. Central signs should be sought even in patients with spontaneous horizontal-torsional nystagmus and positive HIT. In patients with combined peripheral and central vestibulopathy, a vestibular nuclear lesion should be considered especially when hearing is preserved.

Isolated floccular infarction: impaired vestibular responses to horizontal head impulse

Isolated floccular infarction is extremely rare, and impairments of the vestibulo-ocular reflex (VOR) have not been explored in humans with isolated floccular lesions. The purpose of this study was to examine and report selective impairment of VOR in response to high acceleration using head impulse in a patient with isolated floccular infarction. The patient underwent bedside and laboratory evaluation of vestibular function, which included video-oculography, ocular torsion and the subjective visual vertical, cervical and ocular vestibular-evoked myogenic potentials, bithermal caloric irrigation, rotatory chair test, and the head impulse test (HIT) using search coils. A 70-year-old woman with a unilateral floccular infarction presented with an acute vestibular syndrome with spontaneous nystagmus beating to the lesion side, impaired ipsilesional pursuit, contraversive ocular torsion and tilt of the subjective visual vertical. With rotatory chair testing at low frequencies, horizontal VOR gains were increased. However, VOR gains were decreased with the higher-frequency, higher-speed HIT. While HIT is often normal in patients with central vestibular disorders, decreased HIT responses do not exclude an isolated cerebellar lesion as a cause of the acute vestibular syndrome.

The bedside examination of the vestibulo-ocular reflex (VOR): an update

Diagnosing dizzy patients remains a daunting challenge to the clinician in spite of modern imaging and increasingly sophisticated electrophysiological testing. Here we review the major bedside tests of the vestibulo-ocular reflex and how, when combined with a proper examination of the other eye movement systems, one can arrive at an accurate vestibular diagnosis.

Patterns of spontaneous and head-shaking nystagmus in cerebellar infarction: imaging correlations

Horizontal head-shaking may induce nystagmus in peripheral as well as central vestibular lesions. While the patterns and mechanism of head-shaking nystagmus are well established in peripheral vestibulopathy, they require further exploration in central vestibular disorders. To define the characteristics and mechanism of head-shaking nystagmus in central vestibulopathies, we investigated spontaneous nystagmus and head-shaking nystagmus in 72 patients with isolated cerebellar infarction. Spontaneous nystagmus was observed in 28 (39%) patients, and was mostly ipsilesional when observed in unilateral infarction (15/18, 83%). Head-shaking nystagmus developed in 37 (51%) patients, and the horizontal component of head-shaking nystagmus was uniformly ipsilesional when induced in patients with unilateral infarction. Perverted head-shaking nystagmus occurred in 23 (23/37, 62%) patients and was mostly downbeat (22/23, 96%). Lesion subtraction analyses revealed that damage to the uvula, nodulus and inferior tonsil was mostly responsible for generation of head-shaking nystagmus in patients with unilateral posterior inferior cerebellar artery infarction. Ipsilesional head-shaking nystagmus in patients with unilateral cerebellar infarction may be explained by unilateral disruption of uvulonodular inhibition over the velocity storage. Perverted (downbeat) head-shaking nystagmus may be ascribed to impaired control over the spatial orientation of the angular vestibulo-ocular reflex due to uvulonodular lesions or a build-up of vertical vestibular asymmetry favouring upward bias due to lesions involving the inferior tonsil.

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.

Characterizing high-velocity angular vestibulo-ocular reflex function in service members post-blast exposure

Blasts (explosions) are the most common mechanism of injury in modern warfare. Traumatic brain injury (TBI) and dizziness are common sequelae associated with blasts, and many service members (SMs) report symptoms worsen with activity. The purpose of this study was to measure angular vestibulo-ocular reflex gain (aVOR) of blast-exposed SMs with TBI during head impulse testing. We also assessed their symptoms during exertion. Twenty-four SMs recovering from TBI were prospectively assigned to one of two groups based on the presence or absence of dizziness. Wireless monocular scleral search coil and rate sensor were used to characterize active and passive yaw and pitch head and eye rotations. Visual analog scale (VAS) was used to monitor symptoms during fast walking/running. For active yaw head impulses, aVOR gains were significantly lower in the symptomatic group (0.79 ± 0.15) versus asymptomatic (0.87 ± 0.18), but not for passive head rotation. For pitch head rotation, the symptomatic group had both active (0.915 ± 0.24) and passive (0.878 ± 0.22) aVOR gains lower than the asymptomatic group (active 1.03 ± 0.27, passive 0.97 ± 0.23). Some SMs had elevated aVOR gain. VAS scores for all symptoms were highest during exertion. Our data suggest symptomatic SMs with TBI as a result of blast have varied aVOR gain during high-velocity head impulses and provide compelling evidence of pathology affecting the vestibular system. Potential loci of injury in this population include the following: disruption of pathways relaying vestibular efference signals, differential destruction of type I vestibular hair cells, or selective damage to irregular afferent pathways-any of which may explain the common discrepancy between reports of vestibular-like symptoms and laboratory testing results. Significantly reduced pitch aVOR in symptomatic SMs and peak symptom severity during exertional testing support earlier findings in the chronic blast-exposed active duty SMs.

Isolated nodular infarction

BACKGROUND AND PURPOSE

Isolated nodular infarction has rarely been described in human. The purpose of this study is to report clinical and laboratory findings of isolated nodular infarction.

METHODS

Eight patients with isolated nodular infarction were recruited from 6 hospitals in Korea. All patients underwent a complete and standardized neurotological evaluation including ocular torsion, bithermal caloric tests, and rotatory chair test in addition to MRI and MR angiography.

RESULTS

All patients presented with isolated vertigo and moderate to severe imbalance. The most common manifestation was unilateral nystagmus and falling in the opposite direction, which mimicked peripheral vestibulopathy. Six patients had unilateral lesion, and 2 showed bilateral lesions. The direction of the spontaneous nystagmus was all ipsilesional in the unilateral lesion. However, head impulse and bithermal caloric tests were normal. Other findings include periodic alternating nystagmus, perverted head shaking nystagmus, paroxysmal positional nystagmus, and impaired tilt suppression of the postrotatory nystagmus. Hypoplasia of the ipsilesional vertebral artery was the only abnormal finding on MR angiography in 3 patients. The prognosis was excellent.

CONCLUSIONS

Isolated nodular infarction mostly presents with isolated vertigo mimicking acute peripheral vestibulopathy. However, severe imbalance and a negative head impulse test are important clinical discriminants between nodular infarcts and peripheral vestibular dysfunction. The findings of isolated nodular infarctions are consistent with impaired gravito-inertial processing of the vestibular signals and disrupted nodular inhibition on the vestibular secondary neurons and the velocity storage mechanism.

Kinematics of the Rotational Vestibuloocular Reflex: Role of the Cerebellum

We studied the effect of cerebellar lesions on the 3-D control of the rotational vestibuloocular reflex (RVOR) to abrupt yaw-axis head rotation. Using search coils, three-dimensional (3-D) eye movements were recorded from nine patients with cerebellar disease and seven normal subjects during brief chair rotations (200°/s2 to 40°/s) and manual head impulses. We determined the amount of eye-position dependent torsion during yaw-axis rotation by calculating the torsional-horizontal eye-velocity axis for each of three vertical eye positions (0°, ±15°) and performing a linear regression to determine the relationship of the 3-D velocity axis to vertical eye position. The slope of this regression is the tilt angle slope. Overall, cerebellar patients showed a clear increase in the tilt angle slope for both chair rotations and head impulses. For chair rotations, the effect was not seen at the onset of head rotation when both patients and normal subjects had nearly head-fixed responses (no eye-position-dependent torsion). Over time, however, both groups showed an increasing tilt-angle slope but to a much greater degree in cerebellar patients. Two important conclusions emerge from these findings: the axis of eye rotation at the onset of head rotation is set to a value close to head-fixed (i.e., optimal for gaze stabilization during head rotation), independent of the cerebellum and once the head rotation is in progress, the cerebellum plays a crucial role in keeping the axis of eye rotation about halfway between head-fixed and that required for Listing's Law to be obeyed.

Asymmetry of the Pitch Vestibulo-Ocular Reflex in Patients with Cerebellar Disease

Responses to pitch head impulses were measured in 15 patients with cerebellar degeneration and downbeat nystagmus and in 5 control subjects, using three-axis search coils. For each subject, response gains were calculated as (1) the ratio of instantaneous vertical eye velocity to pitch head velocity (at 30 ms and at 70 ms into the response), and (2) the ratio of peak eye velocity to peak head velocity. Gains varied more widely among patients. When calculated at 70 ms and using peak values, patients had higher gains for downward pitch, and normal subjects had symmetric gains. At 30 ms, gains were more symmetric in patients, but at that point control subjects actually had lower gains for downward pitch. Thus, the pitch gain ratio (ratio of downward gain to upward gain) was consistently greater in patients than in normal subjects. Because downward impulses were generally faster, eye velocities during downward and upward pitch for equivalent head speeds were also compared. Again, patients had higher gains for downward pitch. These results are consistent with the authors' hypothesis that cerebellar disease results in a higher sensitivity of anterior than posterior semicircular canal pathways, perhaps through loss of inhibition from the flocculus/paraflocculus complex on anterior canal secondary neurons in the vestibular nuclei. The pitch gain asymmetry was larger than, and did not correlate with, the velocity of spontaneous upward drift. This supports the notion that other mechanisms are likely to contribute to downbeat nystagmus in these patients.

Eye movements of the murine P/Q calcium channel mutant tottering, and the impact of aging

Mice carrying mutations of the gene encoding the ion pore of the P/Q calcium channel (Cacna1a) are an instance in which cerebellar dysfunction may be attributable to altered electrophysiology and thus provide an opportunity to study how neuronal intrinsic properties dictate signal processing in the ocular motor system. P/Q channel mutations can engender multiple effects at the single neuron, circuit, and behavioral levels; correlating physiological and behavioral abnormalities in multiple allelic strains will ultimately facilitate determining which alterations of physiology are responsible for specific behavioral aberrations. We used videooculography to quantify ocular motor behavior in tottering mutants aged 3 mo to 2 yr and compared their performance to data previously obtained in the allelic mutant rocker and C57BL/6 controls. Tottering mutants shared numerous abnormalities with rocker, including upward deviation of the eyes at rest, increased vestibuloocular reflex (VOR) phase lead at low stimulus frequencies, reduced VOR gain at high stimulus frequencies, reduced gain of the horizontal and vertical optokinetic reflex, reduced time constants of the neural integrator, and reduced plasticity of the VOR as assessed in a cross-axis training paradigm. Unlike rocker, young tottering mutants exhibited normal peak velocities of nystagmus fast phases, arguing against a role for neuromuscular transmission defects in the attenuation of compensatory eye movements. Tottering also differed by exhibiting directional asymmetries of the gains of optokinetic reflexes. The data suggest at least four pathophysiological mechanisms (two congenital and two acquired) are required to explain the ocular motor deficits in the two Cacna1a mutant strains.

Cerebellar Disease Alters the Axis of the High-Acceleration Vestibuloocular Reflex

L. W. Schultheis and D. A. Robinson showed that the axis of the rotational vestibuloocular reflex (RVOR) cannot be altered by visual-vestibular mismatch (“cross-axis adaptation”) when the vestibulocerebellum is lesioned. This suggests that the cerebellum may calibrate the axis of eye velocity of the RVOR under natural conditions. Thus we asked whether patients with cerebellar disease have alterations in the RVOR axis and, if so, what might be the mechanism. We used three-axis scleral coils to record head and eye movements during yaw, pitch, and roll head impulses in 18 patients with cerebellar disease and in a comparison group of eight subjects without neurologic disease. We found distinct shifts of the eye-velocity axis in patients. The characteristic finding was a disconjugate upward eye velocity during yaw. Measured at 70 ms after the onset of head rotation, the median upward gaze velocity was 15% of yaw head velocity for patients and <1% for normal subjects ( P < 0.001). Upward eye velocity was greater in the contralateral (abducting) eye during yaw and in the ipsilateral eye during roll. Patients had a higher gain (eye speed/head speed) for downward than for upward pitch (median ratio of downward to upward gain: 1.3). In patients, upward gaze velocities during both yaw and roll correlated with the difference in anterior (AC) and posterior canal excitations, scaled by the respective pitch gains. Our findings support the hypothesis that upward eye velocity during yaw results from AC excitation, which must normally be suppressed by the intact cerebellum.

Vertical conjugate eye deviation in postresuscitation coma

Vertical eye deviation in hypoxic coma is considered to be rare. In contrast, we found that in a consecutive series of 50 postresuscitation comatose patients, 28 (56.0%) developed tonic upward or downward eye deviation. We suggest that both the upward and the downward deviations resulted from diffuse cerebrocerebellar damage sparing the brainstem. Upward deviation is an early sign, whereas downward deviation appears later and generally implies a transition to the vegetative state.

Evidence for interacting cortical control of vestibular function and spatial representation in man

The objective of this research was to determine the possible relation between deficits in spatial representation capability and vestibular function following cortical lesions. We thus investigated vestibulo-ocular behaviour in a group of 14 patients with unilateral cortical damage involving the occipito-temporo-parietal junction. Patients were divided in three sub-groups: (1) Group R+: five patients with right sided cortical lesions associated with a left hemi-neglect, (2) Group R-: four patients with right sided cortical lesions with no hemi-neglect and (3) Group L: five patients with left-sided cortical lesions. The patient groups were compared to a group of eight healthy age-matched subjects. The vestibulo-ocular reflex (VOR) was tested in complete darkness by rotating the subject around the vertical axis by sinusoidal rotation at different frequencies, and by steps of acceleration or deceleration. The nystagmus slow phase velocity was measured and plotted as a function of the head velocity and the VOR parameters including gain, bias, time constant and phase were calculated. The cortical lesions induced a significant VOR asymmetry in terms of: a directional preponderance of the VOR gain to the contralesion side, only during sinusoidal rotation, and, in contrast, a VOR bias and a directional preponderance of the VOR time constant and of the nystagmus frequency to the side of the cortical lesion. These latter VOR deficits were the most significant in the R+ group, i.e. in right cortical lesions with hemi-neglect syndrome. These results demonstrate in man, the existence of a cortical influence on vestibular function related to the mechanisms of spatial representation.

The cerebellar contribution to eye movements based upon lesions: binocular three-axis control and the translational vestibulo-ocular reflex

The study of human cerebellar patients and monkeys with experimental cerebellar lesions has taught us much about the role of the cerebellum in normal ocular motor control. Here we emphasize recent findings that point to a role for the cerebellum in (1) the control of the three-dimensional axis about which the eye rotates in response to visual and vestibular stimuli, and (2) the generation of the translational VOR. Findings in cerebellar patients include abnormalities of eye torsion during attempted fixation that suggest a cerebellar role in the control of torsion so that Listing's law is obeyed. Abnormal torsion during vertical pursuit suggests that central processing of information for smooth pursuit may be based upon a phylogenetically old, semicircular canal coordinate scheme. Inappropriate and disconjugate vertical and torsional eye movements ("cross-coupling") occur during brief, high-acceleration rotations of the head. This suggests a role for the cerebellum in the binocular control of the rotation axis of the VOR. Finally, abnormalities of the modulation of the translational VOR with near viewing in cerebellar patients, but with sparing of the very initial 25-30 msec of response, suggests an important role for the cerebellum in the translational VOR.

Calcium channelopathy mutants and their role in ocular motor research

Thanks to technical advances in eye movement recording, the mouse is destined to become increasingly important in ocular motor research. An advantage of this species is the wide range of existing mutant strains and techniques to generate new mutations affecting specific cell types. Mutations of ion channels may be used to modulate the intrinsic properties of neurons, and this approach may generate insight into the degree to which neuronal computations depend upon those intrinsic properties as opposed to the properties of circuits of neurons. Dendritic calcium currents carried by P-type voltage-activated calcium channels have been widely postulated to perform important computational functions in cerebellar Purkinje cells. Mutations of this channel lead to human diseases, and several ataxic strains of mice are now known to harbor mutations of this calcium channel. Murine P-channel mutants such as rocker are ataxic, but have normal or near-normal numbers of cerebellar Purkinje cells and thus offer the opportunity to study the effects of biophysical perturbations as opposed to outright cell destruction or inactivation. Initial studies of rocker mice reveal an array of ocular motor abnormalities, including static hyperdeviation of the eyes and an attenuation of vestibulo-ocular reflex gains at high stimulus frequencies. The pattern of gain and phase abnormalities is entirely different in lurcher, an ataxic mutant in which Purkinje cells degenerate. The ocular motor abnormalities of rocker progress with animal age, underscoring the importance of careful attention to animal age when performing ocular motor studies in this short-lived species.