Measuring optokinetic after-nystagmus: potential for detecting patients with signs of visual dependence following concussion

Biofluid, Imaging, Physiological, and Functional Biomarkers of Mild Traumatic Brain Injury and Subconcussive Head Impacts

Post-concussive symptoms are frequently reported by individuals who sustain mild traumatic brain injuries (mTBIs) and subconcussive head impacts, even when evidence of intracranial pathology is lacking. Current strategies used to evaluate head injuries, which primarily rely on self-report, have a limited ability to predict the incidence, severity, and duration of post-concussive symptoms that will develop in an individual patient. In addition, these self-report measures have little association with the underlying mechanisms of pathology that may contribute to persisting symptoms, impeding advancement in precision treatment for TBI. Emerging evidence suggests that biofluid, imaging, physiological, and functional biomarkers associated with mTBI and subconcussive head impacts may address these shortcomings by providing more objective measures of injury severity and underlying pathology. Interest in the use of biomarker data has rapidly accelerated, which is reflected by the recent efforts of organizations such as the National Institute of Neurological Disorders and Stroke and the National Academies of Sciences, Engineering, and Medicine to prioritize the collection of biomarker data during TBI characterization in acute-care settings. Thus, this review aims to describe recent progress in the identification and development of biomarkers of mTBI and subconcussive head impacts and to discuss important considerations for the implementation of these biomarkers in clinical practice.

Temporal dynamics of ocular torsion and vertical vergence during visual, vestibular, and visuovestibular rotations

Ocular torsion and vertical divergence reflect the brain's sensorimotor integration of motion through the vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) to roll rotations. Torsion and vergence however express different response patterns depending on several motion variables, but research on their temporal dynamics remains limited. This study investigated the onset times of ocular torsion (OT) and vertical vergence (VV) during visual, vestibular, and visuovestibular motion, as well as their relative decay rates following prolonged optokinetic stimulations. Temporal characteristics were retrieved from three separate investigations where the level of visual clutter and acceleration were controlled. Video eye-tracking was used to retrieve the eye-movement parameters from a total of 41 healthy participants across all trials. Ocular torsion consistently initiated earlier than vertical vergence, particularly evident under intensified visual information density, and higher clutter levels were associated with more balanced decay rates. Additionally, stimulation modality and accelerations affected the onsets of both eye movements, with visuovestibular motion triggering earlier responses compared to vestibular motion, and increased accelerations leading to earlier onsets for both movements. The present study showed that joint visuovestibular responses produced more rapid onsets, indicating a synergetic sensorimotor process. It also showed that visual content acted as a fusional force during the decay period, and imposed greater influence over the torsional onset compared to vergence. Acceleration, by contrast, did not affect the temporal relationship between the two eye movements. Altogether, these findings provide insights into the sensorimotor integration of the vestibulo-ocular and optokinetic reflex arcs.

A Proposed Mechanism for Visual Vertigo: Post-Concussion Patients Have Higher Gain From Visual Input Into Subcortical Gaze Stabilization

Purpose

Post-concussion syndrome (PCS) is commonly associated with dizziness and visual motion sensitivity. This case-control study set out to explore altered motion processing in PCS by measuring gaze stabilization as a reflection of the capacity of the brain to integrate motion, and it aimed to uncover mechanisms of injury where invasive subcortical recordings are not feasible.

Methods

A total of 554 eye movements were analyzed in 10 PCS patients and nine healthy controls across 171 trials. Optokinetic and vestibulo-ocular reflexes were recorded using a head-mounted eye tracker while participants were exposed to visual, vestibular, and visuo-vestibular motion stimulations in the roll plane. Torsional and vergence eye movements were analyzed in terms of slow-phase velocities, gain, nystagmus frequency, and sensory-specific contributions toward gaze stabilization.

Results

Participants expressed eye-movement responses consistent with expected gaze stabilization; slow phases were fastest for visuo-vestibular trials and slowest for visual stimulations (P < 0.001) and increased with stimulus acceleration (P < 0.001). Concussed patients demonstrated increased gain from visual input to gaze stabilization (P = 0.005), faster slow phases (P = 0.013), earlier nystagmus beats (P = 0.003), and higher relative visual influence over the gaze-stabilizing response (P = 0.001), presenting robust effect sizes despite the limited population size.

Conclusions

The enhanced neural responsiveness to visual motion in PCS, combined with semi-intact visuo-vestibular integration, presented a subcortical hierarchy for altered gaze stabilization. Drawing on comparable animal trials, findings suggest that concussed patients may suffer from diffuse injuries to inhibiting pathways for optokinetic information, likely early in the visuo-vestibular hierarchy of sensorimotor integration. These findings offer context for common but elusive symptoms, presenting a neurological explanation for motion sensitivity and visual vertigo in PCS.

Systematic review and meta-analysis of the diagnostic value of optokinetic after-nystagmus in vestibular disorders

Introduction

To date, no systematic review or meta-analysis has critically evaluated the relevance of using optokinetic after-nystagmus (OKAN) in diagnosis of vestibular disorders. To assess the role of OKAN in diagnosis of vestibular disorders, the OKAN time constant (TC) between patients with vestibular disorders and healthy participants will be compared.

Methods

Automated search strategies were carried out in the Embase, Medline PubMed, Web of Science, and Scopus databases from inception to December 2023. The following inclusion criteria were applied: (1) evaluation of OKAN in individuals with vestibular disorders, (2) clinical trials, and (3) inclusion of healthy individuals as the control group. Exclusion criteria were: (1) animal studies, (2) non-clinical trial study designs, (3) assessment of non-vestibular disorders, (4) no examination of OKAN TC, (5) only examination of healthy participants, (6) studies published in a language other than English, (7) no healthy participants as control group, (8) case reports, and (9) only abstract available. The random-effects model was used to pool the data. The Joanna Briggs Institute (JBI) Critical Appraisal Tools was used to assess the risk of bias. The quality assessment was performed with the aid of the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies, provided by NHLBI. The PRISMA guidelines were used as reporting guidelines. The main outcome of this study was the between-group mean difference (MDbetween) in OKAN TC and its 95% confidence interval between patients with vestibular disorders and healthy participants.

Results

Seven out of 244 screened articles were included that studied 289 participants. The overall mean difference (MD = -7.08) with a 95% CI of [-10.18; -3.97] was significant (p = 0.014). The heterogeneity was significant (p = 0.02). Quality assessment was generally good (76%). The risk of bias was low in five studies and moderate in two studies.

Conclusion

The results demonstrate that OKAN TC is significantly shorter in patients with vestibular disorders compared to healthy controls. This finding is important for future research, particularly with the emergence of novel clinical tools and diagnostic syndromes.

Systematic Review

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=442695.

Precision Concussion Management: Approaches to Quantifying Head Injury Severity and Recovery

Mitigating the substantial public health impact of concussion is a particularly difficult challenge. This is partly because concussion is a highly prevalent condition, and diagnosis is predominantly symptom-based. Much of contemporary concussion management relies on symptom interpretation and accurate reporting by patients. These types of reports may be influenced by a variety of factors for each individual, such as preexisting mental health conditions, headache disorders, and sleep conditions, among other factors. This can all be contributory to non-specific and potentially misleading clinical manifestations in the aftermath of a concussion. This review aimed to conduct an examination of the existing literature on emerging approaches for objectively evaluating potential concussion, as well as to highlight current gaps in understanding where further research is necessary. Objective assessments of visual and ocular motor concussion symptoms, specialized imaging techniques, and tissue-based concentrations of specific biomarkers have all shown promise for specifically characterizing diffuse brain injuries, and will be important to the future of concussion diagnosis and management. The consolidation of these approaches into a comprehensive examination progression will be the next horizon for increased precision in concussion diagnosis and treatment.

Clinical characteristics of visual motion hypersensitivity: a systematic review

This qualitative systematic review presents an overview of the state of the research relating to visual motion hypersensitivity (VMH) and offers a reference tool for future studies in the field. The study set out to identify and collate articles investigating risk groups with aberrant responses to visual motion as compared to healthy control groups, presenting evidence for risk factors associated with visual motion hypersensitivity. Data were synthesized into the state of the research and analyzed in the context of the clinical characteristics of each risk factor. Literature searches were performed on Medline Ovid, EMBASE, Web of Science, and Cinahl, identifying a total of 586 studies of which 54 were finally included. Original articles published between the dates of commencement for each database and 19th January 2021 were included. JBI critical appraisal tools were implemented for each corresponding article type. In total, the following number of studies was identified for each respective risk factor: age (n = 6), migraines (n = 8), concussions (n = 8), vestibular disorders (n = 13), psychiatric conditions (n = 5), and Parkinson's disease (n = 5). Several studies described VMH as the primary concern (n = 6), though these primarily included patients with vestibulopathies. There were considerable differences in the nomenclature employed to describe VMH, depending largely on the investigating group. An overview of investigated risk factors and their evaluation methods was presented in a Sankey diagram. Posturography was the most implemented methodology but due to diverse measurements meta-analyses were not possible. One may however note that while the easily implemented Vestibular Ocular Motor Screening (VOMS) was designed for concussed patients, it may prove useful for other risk groups.

Concussed patients with visually induced dizziness exhibit increased ocular torsion and vertical vergence during optokinetic gaze-stabilization

Visually Induced Dizziness (VID) is a common post-concussion sequalae that remains poorly understood and difficult to quantify. The present study aims to identify biomarkers for VID in the form of gaze-stabilizing eye movements. Nine patients with post-commotio VID and nine age-matched healthy controls were recruited by physiotherapists at a local neurorehabilitation centre. Torsional and vergence eye movements were recorded while participants viewed a series of optokinetic rotations where the central- and peripheral regions moved coherently, incoherently, or semi-randomly. Results showed that vergence and torsional velocities were increased in VID patients, reflecting increased oculomotor gain to visual motion, and that responses correlated with symptom severity. Coherent stimulation produced fastest torsional slow-phases across all participants; when faced with confliction directional information, eye movements tended to follow the direction of the central visual field, albeit at slower velocities than during coherent motion, meaning that while torsion was sensitive to visual content of the entire visual field it expressed directional preference to the central stimulation. In conclusion, post-commotio VID was associated with faster slow-phases during optokinetic gaze-stabilization, with both vergence and torsion being correlated to symptom intensity. As torsional tracking remains inaccessible using commercial eye-trackers, vertical vergence may prove particularly accessible for clinical utility.

Mild traumatic brain injury and sports-related concussion

Mild traumatic brain injury (mTBI) and concussion are equivalent terms for the sequela of injury to the head that disrupts brain functioning. Various forces may be causative from seemingly innocuous bumps to the head resulting from sports-related injuries to more severe blows to the head. However, the postconcussive motor, cognitive, emotional, and psychosocial sequelae can be just as devastating and long lasting, leading to loss of independent function and safe performance of activities. Taken together, they pose a significant challenge to recovery, requiring a multifaceted dynamic rehabilitative strategy. The current systems of health care pose challenges to suboptimal management of sports-related concussion (SRC) that goes beyond the acute injury, and into the school setting, failing to be identified by school staff, and inconsistencies in communicating medical information regarding school modifications, follow-up health services, or concussion-related educational services. Children who sustain SRC at different ages face different challenges. Young children face increased vulnerability due to SRC that coincides with periods of brain motor maturation and development.

Patterns of vestibular dysfunction in chronic traumatic brain injury

Background

Dizziness and imbalance are common following traumatic brain injury (TBI). While these symptoms are often attributed to vestibular dysfunction, the relative contribution of peripheral vs. central mechanisms is unclear. This study investigated the prevalence of semicircular canal and otolith abnormalities in a cohort of patients with chronic TBI and symptoms of dizziness or imbalance. The relationship between vestibular, oculomotor and posturography results was further explored.

Methods

Clinical records of patients attending the New Zealand Dizziness and Balance Centre from January 2015 to December 2019 were reviewed for consideration in the study. Inclusion required: an age of 18–80 years, a diagnosed TBI, and vestibular assessment using three-dimensional video head impulses (vHIT), cervical and ocular vestibular-evoked myogenic potentials (c and o VEMPs, respectively) and caloric testing. Severe TBI, pre-existing vestibular diagnoses, and incomplete test results were excluded. Rates of abnormalities were determined for each test and compared with results of oculomotor function testing and postural control, measured using the sensory organization test (SOT).

Results

Of 158 reviewed records, 99 patients aged 49 ± 15 years (59 female) fulfilled criteria for inclusion in the study. The median time between the head injury and the clinical assessment was 12 (IQR 6–21) months. Abnormalities involving one or more components of the vestibular labyrinth and/or nerve divisions were identified in 33 of 99 patients (33.3%). The horizontal semicircular canal was most frequently affected (18.2%), followed by the saccule (14.1%), utricle (8.1%), posterior (7.1%) and anterior (2.0%) semicircular canals. Vestibular test abnormalities were associated with skull-base fractures, superior canal dehiscence, and focal ear trauma. Oculomotor dysfunction and postural instability were recorded in 41.1 and 75.5% of patients, respectively. Postural instability correlated with abnormal oculomotor function (p = 0.008) but not peripheral vestibular hypofunction (p = 0.336).

Conclusions

Dizziness and/or imbalance in chronic TBI was associated with impaired postural stability for tasks requiring high levels of use of vestibular and visual input for balance. Vestibular hypofunction identified through vHIT, VEMP and caloric testing was recorded but was less common, except when the injury involved a fractured skull-base. There was no specific pattern of end-organ or nerve involvement which characterized this group of patients.

Alertness and Visual Attention Impact Different Aspects of the Optokinetic Reflex

Purpose

Assessing visual attention and alertness is of great importance in visual and cognitive neuroscience, providing objective measures valuable for both researchers and clinicians. This study investigates how the optokinetic response differs between levels of visual attention in healthy adults while controlling for alertness.

Methods

Twelve healthy subjects (8 men and 4 women; mean age = 33 ± 9.36) with intact gaze-stability, visual acuity, and binocularity were recruited. Subjects viewed a rotating visual scene provoking torsional optokinetic nystagmus (OKN) while wearing a video eye tracker in a seated head-fixed position. Tasks requiring focused, neutral, and divided visual attention were issued to each subject and the OKN was recorded. Pupil sizes were monitored as a proxy for alertness.

Results

Pupil dilation was increased for both focused and divided visual attention. The number of nystagmus beats was highest for the focused condition and lowest for the divided attentional task. OKN gain was increased during both focused and divided attention. The distribution of nystagmus beats over time showed that only focused attention produced a reliable adaptation of the OKN.

Conclusions

Results consequently indicate that OKN frequency is adaptive to a viewer's level of visual attention, whereas OKN gain is influenced by alertness levels. This pattern offers insight into the neural processes integrating visual input with reflexive motor responses. For example, it contextualizes why attention to visual stimuli can cause dizziness, as the OKN frequency reflects activity of the velocity storage mechanism. Additionally, the OKN could offer a possible venue for differentiating between visual attention and alertness during psychometric testing.

Effect of the Stimulus Duration on the Adaptation of the Optokinetic Afternystagmus

Observing a rotating visual pattern covering a large portion of the visual field induces optokinetic nystagmus (OKN). If the lights are suddenly switched off, optokinetic afternystagmus (OKAN) occurs. OKAN is hypothesized to originate in the velocity storage mechanism (VSM), a central processing network involved in multi-sensory integration. During a sustained visual rotation, the VSM builds up a velocity signal. After the lights are turned off, the VSM discharges slowly, with OKAN as the neurophysiological correlate. It has been reported that the initial afternystagmus in the direction of the preceding stimulus (OKAN-I) can be followed by a reversed one (OKAN-II), which increases with stimulus duration up to 15 min. In 11 healthy adults, we investigated OKAN following optokinetic stimulus lasting 30 s, 3-, 5-, and 10-min. Analysis of slow-phase cumulative eye position and velocity found OKAN-II in only 5/11 participants. Those participants presented it in over 70% of their trials with longer durations, but only in 10% of their 30 s trials. While this confirms that OKAN-II manifests predominantly after sustained stimuli, it suggests that its occurrence is subject-specific. We also did not observe further increases with stimulus duration. Conversely, OKAN-II onset occurred later as stimulus duration increased (p = 0.02), while OKAN-II occurrence and peak velocity did not differ between the three longest stimuli. Previous studies on OKAN-I, used negative saturation models to account for OKAN-II. As these approaches have no foundation in the OKAN-II literature, we evaluated if a simplified version of a rigorous model of OKAN adaptation could be used in humans. Slow-phase velocity following the trials with 3-, 5-, and 10-min stimuli was fitted with a sum of two decreasing exponential functions with opposite signs (one for OKAN-I and one for OKAN-II). The model assumes separate mechanisms for OKAN-I, representing VSM discharge, and OKAN-II, described as a slower adaptation phenomenon. Although the fit was qualitatively imperfect, this is not surprising given the limited reliability of OKAN in humans. The estimated adaptation time constant seems comparable to the one describing the reversal of the vestibulo-ocular reflex during sustained rotation, suggesting a possible shared adaptive mechanism.