Suppression of saccadic intrusions in hereditary ataxia by memantine

[Update on central oculomotor disorders and nystagmus]

The diagnosis of ocular motor disorders and the different forms of a nystagmus is based on a systematic clinical examination of all types of eye movements: 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. Relevant anatomical structures are the midbrain, pons, medulla, cerebellum, and cortex. There is a simple clinical rule: vertical and torsional eye movements are generated in the midbrain, horizontal in the pons. The cerebellum is relevant for almost all types of eye movements; typical pathological findings are saccadic smooth pursuit, gaze-evoked nystagmus or dysmetric saccades.Nystagmus is defined as a rhythmic, most often involuntary eye movement. It normally consists of a slow (pathological) drift of the eyes and a fast central compensatory movement of the eyes back to the primary position (re-fixation saccade). There are three major categories: first, spontaneous nystagmus, i. e. nystagmus which occurs in the gaze straight ahead position as upbeat or downbeat nystagmus; second, nystagmus that becomes visible at eccentric gaze only and third, nystagmus which can be elicited by certain maneuvers, e. g. head-shaking, head positioning, air pressure or hyperventilation, most of which are of peripheral vestibular origin. The most frequent central types of spontaneous nystagmus are downbeat and upbeat, infantile, pure torsional, pendular fixation, periodic alternating, and seesaw nystagmus. Many types of central nystagmus allow a precise neuroanatomical localization: for instance, downbeat nystagmus, which is most often caused by a bilateral floccular lesion or dysfunction, or upbeat nystagmus, which is caused by a lesion in the mesencephalon or medulla oblongata. Examples of pharmacotherapy are the use of 4-aminopyridine for downbeat and upbeat nystagmus, memantine or gabapentin for fixation pendular nystagmus or baclofen for periodic alternating nystagmus.

Scale for Ocular motor Disorders in Ataxia (SODA)

Eye movements are fundamental diagnostic and progression markers of various neurological diseases, including those affecting the cerebellum. Despite the high prevalence of abnormal eye movements in patients with cerebellar disorders, the traditional rating scales do not focus on abnormal eye movements. We formed a consortium of neurologists focusing on cerebellar disorders. The consortium aimed to design and validate a novel Scale for Ocular motor Disorders in Ataxia (SODA). The primary purpose of the scale is to determine the extent of ocular motor deficits due to various phenomenologies. A higher score on the scale would suggest a broader range of eye movement deficits. The scale was designed such that it is easy to implement by non-specialized neurological care providers. The scale was not designed to measure each ocular motor dysfunction's severity objectively. Our validation studies revealed that the scale reliably measured the extent of saccade abnormalities and nystagmus. We found a lack of correlation between the total SODA score and the total International Cooperative Ataxia Rating Scale (ICARS), Scale for Assessment and Rating of Ataxia (SARA), or Brief Ataxia Rating Scale (BARS). One explanation is that conventionally reported scales are not dedicated to eye movement disorders; and when present, the measure of ocular motor function is only one subsection of the ataxia rating scales. It is also possible that the severity of ataxias does not correlate with eye movement abnormalities. Nevertheless, the SODA met the consortium's primary goal: to prepare a simple outcome measure that can identify ocular motor dysfunction in patients with cerebellar ataxia.

Reading in Children Who Survived Cerebellar Tumors: Evidence from Eye Movements

Cerebellar tumors often affect the eye movement centers located in vermis, negatively affecting cognitive development and learning abilities in children. Previous research has established that patients who survived cerebellar tumors tend to demonstrate various saccadic impairments (e.g., hypermetria) and poor gaze stability as compared to healthy controls. The aim of the current study was to evaluate the influence of oculomotor deficits in such patients on reading parameters. A total of 112 children (8–17 y.o.), 65 of whom survived cerebellar tumors, participated in the study. The study design included several oculomotor and reading tasks. Eye movements were recorded every 1/60 s monocularly with an Arrington eye tracker. We observed profound reading impairments in the patients as compared to healthy children, including longer reading time, greater numbers of fixations and regressive saccades, and longer fixation durations. We also found significant correlations between changes in basic oculomotor functions and reading parameters. The patients also demonstrated gaze fixation instability, large number of fixations, and long scanpath reflecting the return of the gaze to the already counted objects. Thus, oculomotor changes caused by cerebellar tumor and its treatment led to disturbances in such neurocognitive activity as reading. Our findings emphasize the necessity of considering these deficits in cerebellar tumor survivors when designing rehabilitation protocols.

Microsaccades in Applied Environments: Real-World Applications of Fixational Eye Movement Measurements

Across a wide variety of research environments, the recording of microsaccades and other fixational eye movements has provided insight and solutions into practical problems. Here we review the literature on fixational eye movements-especially microsaccades-in applied and ecologically-valid scenarios. Recent technical advances allow noninvasive fixational eye movement recordings in real-world contexts, while observers perform a variety of tasks. Thus, fixational eye movement measures have been obtained in a host of real-world scenarios, such as in connection with driver fatigue, vestibular sensory deprivation in astronauts, and elite athletic training, among others. Here we present the state of the art in the practical applications of fixational eye movement research, examine its potential future uses, and discuss the benefits of including microsaccade measures in existing eye movement detection technologies. Current evidence supports the inclusion of fixational eye movement measures in real-world contexts, as part of the development of new or improved oculomotor assessment tools. The real-world applications of fixational eye movement measurements will only grow larger and wider as affordable high-speed and high-spatial resolution eye trackers become increasingly prevalent.

Motion tracking of iris features to detect small eye movements

The inability of current video-based eye trackers to reliably detect very small eye movements has led to confusion about the prevalence or even the existence of monocular microsaccades (small, rapid eye movements that occur in only one eye at a time). As current methods often rely on precisely localizing the pupil and/or corneal reflection on successive frames, current microsaccade-detection algorithms often suffer from signal artifacts and a low signal-to-noise ratio. We describe a new video-based eye tracking methodology which can reliably detect small eye movements over 0.2 degrees (12 arcmins) with very high confidence. Our method tracks the motion of iris features to estimate velocity rather than position, yielding a better record of microsaccades. We provide a more robust, detailed record of miniature eye movements by relying on more stable, higher-order features (such as local features of iris texture) instead of lower-order features (such as pupil center and corneal reflection), which are sensitive to noise and drift.

Characteristic Eye Movements in Ataxia-Telangiectasia-Like Disorder: An Explanatory Hypothesis

Objective

To investigate cerebellar dysfunctions and quantitatively characterize specific oculomotor changes in ataxia-telangiectasia-like disorder (ATLD), a rare autosomal recessive disease caused by mutations in the MRE11 gene. Additionally, to further elucidate the pathophysiology of cerebellar damage in the ataxia-telangiectasia (AT) spectrum disorders.

Methods

Saccade dynamics, metrics, and visual fixation deficits were investigated in two Italian adult siblings with genetically confirmed ATLD. Visually guided saccades were compared with those of 40 healthy subjects. Steady fixation was tested in primary and eccentric positions. Quantitative characterization of saccade parameters, saccadic intrusions (SI), and nystagmus was performed.

Results

Patients showed abnormally hypermetric and fast horizontal saccades to the left and greater inaccuracy than healthy subjects in all saccadic eye movements. Eye movement abnormalities included slow eye movements that preceded the initial saccade. Horizontal and vertical spontaneous jerk nystagmus, gaze-evoked, and rebound nystagmus were evident. Fixation was interrupted by large square-wave jerk SI and macrosaccadic oscillations.

Conclusion

Slow eye movements accompanying saccades, SI, and cerebellar nystagmus are frequently seen in AT patients, additionally our ATLD patients showed the presence of fast and hypermetric saccades suggesting damage of granule cell-parallel fiber-Purkinje cell synapses of the cerebellar vermis. A dual pathogenetic mechanism involving neurodevelopmental and neurodegenerative changes is hypothesized to explain the peculiar phenotype of this disease.

Cerebellar fastigial nucleus: from anatomic construction to physiological functions

Fastigial nucleus (FN) is the phylogenetically oldest nucleus in the cerebellum, a classical subcortical motor coordinator. As one of the ultimate integration stations and outputs of the spinocerebellum, the FN holds a key position in the axial, proximal and ocular motor control by projecting to the medial descending systems and eye movement related nuclei. Furthermore, through topographic connections with extensive nonmotor systems, including visceral related nuclei in the brainstem, hypothalamus, as well as the limbic system, FN has also been implicated in regulation of various nonsomatic functions, such as feeding, cardiovascular and respiratory, defecation and micturition, immune, as well as emotional activities. In clinic, FN lesion or dysfunction results in motor deficits including spinocerebellar ataxias, and nonmotor symptoms. In this review, we summarize the cytoarchitecture, anatomic afferent and efferent connections, as well as the motor and nonmotor functions of the FN and the related diseases and disorders. We suggest that by bridging the motor and nonmotor systems, the cerebellar FN may help to integrate somatic motor and nonsomatic functions and consequently contribute to generate a coordinated response to internal and external environments.

The role of cerebellar circuitry alterations in the pathophysiology of autism spectrum disorders

The cerebellum has been repeatedly implicated in gene expression, rodent model and post-mortem studies of autism spectrum disorder (ASD). How cellular and molecular anomalies of the cerebellum relate to clinical manifestations of ASD remains unclear. Separate circuits of the cerebellum control different sensorimotor behaviors, such as maintaining balance, walking, making eye movements, reaching, and grasping. Each of these behaviors has been found to be impaired in ASD, suggesting that multiple distinct circuits of the cerebellum may be involved in the pathogenesis of patients' sensorimotor impairments. We will review evidence that the development of these circuits is disrupted in individuals with ASD and that their study may help elucidate the pathophysiology of sensorimotor deficits and core symptoms of the disorder. Preclinical studies of monogenetic conditions associated with ASD also have identified selective defects of the cerebellum and documented behavioral rescues when the cerebellum is targeted. Based on these findings, we propose that cerebellar circuits may prove to be promising targets for therapeutic development aimed at rescuing sensorimotor and other clinical symptoms of different forms of ASD.

Simultaneous recordings of human microsaccades and drifts with a contemporary video eye tracker and the search coil technique

Human eyes move continuously, even during visual fixation. These “fixational eye movements” (FEMs) include microsaccades, intersaccadic drift and oculomotor tremor. Research in human FEMs has grown considerably in the last decade, facilitated by the manufacture of noninvasive, high-resolution/speed video-oculography eye trackers. Due to the small magnitude of FEMs, obtaining reliable data can be challenging, however, and depends critically on the sensitivity and precision of the eye tracking system. Yet, no study has conducted an in-depth comparison of human FEM recordings obtained with the search coil (considered the gold standard for measuring microsaccades and drift) and with contemporary, state-of-the art video trackers. Here we measured human microsaccades and drift simultaneously with the search coil and a popular state-of-the-art video tracker. We found that 95% of microsaccades detected with the search coil were also detected with the video tracker, and 95% of microsaccades detected with video tracking were also detected with the search coil, indicating substantial agreement between the two systems. Peak/mean velocities and main sequence slopes of microsaccades detected with video tracking were significantly higher than those of the same microsaccades detected with the search coil, however. Ocular drift was significantly correlated between the two systems, but drift speeds were higher with video tracking than with the search coil. Overall, our combined results suggest that contemporary video tracking now approaches the search coil for measuring FEMs.

Diagnostic approach to abnormal spontaneous eye movements

PURPOSE OF REVIEW

Abnormal spontaneous eye movements, including nystagmus and saccadic intrusions, are often encountered in neurologic practice and can cause disabling visual symptoms, such as oscillopsia and blurred vision. This article reviews the spectrum of abnormal spontaneous eye movements and describes their characteristics, etiology, and management.

RECENT FINDINGS

The number of prospective, controlled, and masked clinical trials evaluating candidate treatments for abnormal spontaneous eye movements has increased significantly over the past decade. The findings of recent clinical trials are highlighted, and the dosing and potential side effects of proposed medical treatments are summarized.

SUMMARY

Abnormal spontaneous eye movements are often encountered in neurologic practice. Recent clinical trials provide an evidence base to guide treatment decisions for these disorders.

Ocular-motor profile and effects of memantine in a familial form of adult cerebellar ataxia with slow saccades and square wave saccadic intrusions

Fixation instability due to saccadic intrusions is a feature of autosomal recessive spinocerebellar ataxias, and includes square wave intrusions (SWI) and macrosaccadic oscillations (MSO). A recent report suggested that the non-competitive antagonist of NMDA receptors, memantine, could decrease MSO and improve fixation in patients with spinocerebellar ataxia with saccadic intrusions (SCASI). We similarly tested two sisters, respectively of 58 and 60 years, with an unrecognized form of recessive, adult-onset cerebellar ataxia, peripheral neuropathy and slow saccades, who showed prominent SWI and also complained with difficulty in reading. We tested horizontal visually guided saccades (10°–18°) and three minutes of steady fixation in each patient and in thirty healthy controls. Both patients showed a significant reduction of peak and mean velocity compared with control subjects. Large SWI interrupting steady fixation were prominent during steady fixation and especially following visually guided saccades. Eye movements were recorded before and during the treatment with memantine, 20 mg/daily for 6 months. The treatment with memantine reduced both the magnitude and frequency of SWI (the former significantly), but did not modified neurological conditions or saccade parameters. Thus, our report suggests that memantine may have some general suppressive effect on saccadic intrusions, including both SWI and MSO, thereby restoring the capacity of reading and visual attention in these and in other recessive forms of ataxia, including Friedreich’s, in which saccadic intrusions are prominent.

Saccades during attempted fixation in parkinsonian disorders and recessive ataxia: from microsaccades to square-wave jerks

During attempted visual fixation, saccades of a range of sizes occur. These “fixational saccades” include microsaccades, which are not apparent in regular clinical tests, and “saccadic intrusions”, predominantly horizontal saccades that interrupt accurate fixation. Square-wave jerks (SWJs), the most common type of saccadic intrusion, consist of an initial saccade away from the target followed, after a short delay, by a “return saccade” that brings the eye back onto target. SWJs are present in most human subjects, but are prominent by their increased frequency and size in certain parkinsonian disorders and in recessive, hereditary spinocerebellar ataxias. Here we asked whether fixational saccades showed distinctive features in various parkinsonian disorders and in recessive ataxia. Although some saccadic properties differed between patient groups, in all conditions larger saccades were more likely to form SWJs, and the intervals between the first and second saccade of SWJs were similar. These findings support the proposal of a common oculomotor mechanism that generates all fixational saccades, including microsaccades and SWJs. The same mechanism also explains how the return saccade in SWJs is triggered by the position error that occurs when the first saccadic component is large, both in the healthy brain and in neurological disease.

Consensus paper: roles of the cerebellum in motor control--the diversity of ideas on cerebellar involvement in movement

Considerable progress has been made in developing models of cerebellar function in sensorimotor control, as well as in identifying key problems that are the focus of current investigation. In this consensus paper, we discuss the literature on the role of the cerebellar circuitry in motor control, bringing together a range of different viewpoints. The following topics are covered: oculomotor control, classical conditioning (evidence in animals and in humans), cerebellar control of motor speech, control of grip forces, control of voluntary limb movements, timing, sensorimotor synchronization, control of corticomotor excitability, control of movement-related sensory data acquisition, cerebro-cerebellar interaction in visuokinesthetic perception of hand movement, functional neuroimaging studies, and magnetoencephalographic mapping of cortico-cerebellar dynamics. While the field has yet to reach a consensus on the precise role played by the cerebellum in movement control, the literature has witnessed the emergence of broad proposals that address cerebellar function at multiple levels of analysis. This paper highlights the diversity of current opinion, providing a framework for debate and discussion on the role of this quintessential vertebrate structure.

Triggering mechanisms in microsaccade and saccade generation: a novel proposal

Saccades are rapid eye movements that change the line of sight between successive points of fixation. Even as we attempt to fixate our gaze precisely, small rapid eye movements called microsaccades interrupt fixation one or two times each second. Although the neural pathway controlling saccade generation is well understood, the specific mechanism for triggering microsaccades is unknown. Here, we review the evidence suggesting that microsaccades and saccades are generated by the same neural pathway. We also discuss current models of how the saccadic system produces microsaccades. Finally, we propose a new mechanism for triggering both microsaccades and saccades, based on a circuit formed by omnipause and long-lead burst neurons and driven by activity in the superior colliculus. Our model differs from previous proposals in that it does not require superior colliculus activity to surpass a particular threshold to trigger microsaccades and saccades. Rather, we propose that the reciprocal inhibition between omnipause and long-lead burst neurons gates each microsaccadic or saccadic event, triggering the eye movement whenever the activity in the long-lead burst neurons overcomes the inhibition from the omnipause neurons.

Critical role of cerebellar fastigial nucleus in programming sequences of saccades

The cerebellum plays an important role in programming accurate saccades. Cerebellar lesions affecting the ocular motor region of the fastigial nucleus (FOR) cause saccadic hypermetria; however, if a second target is presented before a saccade can be initiated (double-step paradigm), saccade hypermetria may be decreased. We tested the hypothesis that the cerebellum, especially FOR, plays a pivotal role in programming sequences of saccades. We studied patients with saccadic hypermetria because of either genetic cerebellar ataxia or surgical lesions affecting FOR and confirmed that the gain of initial saccades made to double-step stimuli was reduced compared with the gain of saccades to single target jumps. Based on measurements of the intersaccadic interval, we found that the ability to perform parallel processing of saccades was reduced or absent in all of our patients with cerebellar disease. Our results support the crucial role of the cerebellum, especially FOR, in programming sequences of saccades.

Therapy for nystagmus

Pathological forms of nystagmus and their visual consequences can be treated using pharmacological, optical, and surgical approaches. Acquired periodic alternating nystagmus improves following treatment with baclofen, and downbeat nystagmus may improve following treatment with aminopyridines. Gabapentin and memantine are helpful in reducing acquired pendular nystagmus due to multiple sclerosis. Ocular oscillations in oculopalatal tremor may also improve following treatment with memantine or gabapentin. The infantile nystagmus syndrome (INS) may have only a minor impact on vision if "foveation periods" are well developed, but symptomatic patients may benefit from treatment with gabapentin, memantine, or base-out prisms to induce convergence. Several surgical therapies are also reported to improve INS, but selection of the optimal treatment depends on careful evaluation of visual acuity and nystagmus intensity in various gaze positions. Electro-optical devices are a promising and novel approach for treating the visual consequences of acquired forms of nystagmus.

Nystagmus and oscillopsia

The ocular motor system consists of several subsystems, including the vestibular ocular nystagmus saccade system, the pursuit system, the fixation and gaze-holding system and the vergence system. All these subsystems aid the stabilization of the images on the retina during eye and head movements and any kind of disturbance of one of the systems can cause instability of the eyes (e.g. nystagmus) or an inadequate eye movement causing a mismatch between head and eye movement (e.g. bilateral vestibular failure). In both situations, the subjects experience a movement of the world (oscillopsia) which is quite disturbing. New insights into the patho-physiology of some of the ocular motor disorders have helped to establish new treatment options, in particular in downbeat nystagmus, upbeat nystagmus, periodic alternating nystagmus, acquired pendular nystagmus and paroxysmal vestibular episodes/attacks. The discussed patho-physiology of these disorders and the current literature on treatment options are discussed and practical treatment recommendations are given in the paper.

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.

Distinctive features of saccadic intrusions and microsaccades in progressive supranuclear palsy

The eyes do not stay perfectly still during attempted fixation; fixational eye movements and saccadic intrusions (SIs) continuously change the position of gaze. The most common type of SI, square-wave jerks (SWJs), consists of saccade pairs that appear purely horizontal on clinical inspection: the first saccade moves the eye away from the fixation target, and after a short interval, the second saccade brings it back toward the target. SWJs are prevalent in certain neurological disorders, including progressive supranuclear palsy (PSP). Here, we developed an objective method to identify SWJs. We found that SWJs are more frequent, larger, and more markedly horizontal in PSP patients than in healthy human subjects. Furthermore, the loss of a vertical component in fixational saccades and SWJs was the eye movement feature that best distinguished PSP patients from controls. We moreover determined that, in PSP patients and controls, the larger the saccade the more likely it was part of a SWJ. Furthermore, saccades produced by PSP patients had equivalent properties whether they were part of a SWJ or not, suggesting that normal fixational saccades (microsaccades) are rare in PSP. We propose that fixational saccades and SIs are generated by the same neural circuit and that, both in PSP patients and in controls, SWJs result from a coupling mechanism that generates a second corrective saccade shortly after a large fixation saccade. Because of brainstem and/or cerebellum impairment, fixational saccades in PSP are abnormally large and thus more likely to trigger a corrective saccade, giving rise to SWJs.

Nystagmus and saccadic intrusions

We review current concepts of nystagmus and saccadic oscillations, applying a pathophysiological approach. We begin by discussing how nystagmus may arise when the mechanisms that normally hold gaze steady are impaired. We then describe the clinical and laboratory evaluation of patients with ocular oscillations. Next, we systematically review the features of nystagmus arising from peripheral and central vestibular disorders, nystagmus due to an abnormal gaze-holding mechanism (neural integrator), and nystagmus occurring when vision is compromised. We then discuss forms of nystagmus for which the pathogenesis is not well understood, including acquired pendular nystagmus and congenital forms of nystagmus. We then summarize the spectrum of saccadic disorders that disrupt steady gaze, from intrusions to flutter and opsoclonus. Finally, we review current treatment options for nystagmus and saccadic oscillations, including drugs, surgery, and optical methods. Examples of each type of nystagmus are provided in the form of figures.

Eye disorders in patients with multiple sclerosis: natural history and management

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system and leading cause of disability in young adults. Vision impairment is a common component of disability for this population of patients. Injury to the optic nerve, brainstem, and cerebellum leads to characteristic syndromes affecting both the afferent and efferent visual pathways. The objective of this review is to summarize the spectrum of eye disorders in patients with MS, their natural history, and current strategies for diagnosis and management. We emphasize the most common disorders including optic neuritis and internuclear ophthalmoparesis and include new techniques, such as optical coherence tomography, which promise to better our understanding of MS and its effects on the visual system.

Microsaccades: a neurophysiological analysis

Microsaccades are the largest and fastest of the fixational eye movements, which are involuntary eye movements produced during attempted visual fixation. In recent years, the interaction between microsaccades, perception and cognition has become one of the most rapidly growing areas of study in visual neuroscience. The neurophysiological consequences of microsaccades have been the focus of less attention, however, as have the oculomotor mechanisms that generate and control microsaccades. Here we review the latest neurophysiological findings concerning microsaccades and discuss their relationships to perception and cognition. We also point out the current gaps in our understanding of the neurobiology of microsaccades and identify the most promising lines of enquiry.