The use of exercise in the form of appropriate movements and sensory exposure (ie, vestibular rehabilitation) is currently considered the standard of care for individuals with peripheral vestibular disorders regardless of age and symptom duration.1–3 Customized vestibular rehabilitation provides greater benefit compared with a generic exercise program (eg, Cawthorne-Cooksey exercise) with significant improvements in subjective symptoms, dynamic visual acuity, gait, and postural stability.2,4–9 Approximately 50% to 80% of individuals completing a customized vestibular rehabilitation program show improvements in symptoms and postural stability.10–12 However, complete recovery is less common and occurs in approximately-of all cases.12,13 The reasons why some individuals with peripheral vestibular disorders do not fully recover are not entirely clear.
Individuals with a peripheral vestibular disorder may experience discomfort, postural instability, and symptoms of dizziness, light headedness, and/or disorientation in situations involving visual-vestibular conflict or intense visual motion stimulation (ie, supermarket aisles, crowds, watching moving scenes, and driving on highways). This phenomenon has been referred to as visual vertigo (VV),14 space and motion discomfort,15 visual vestibular mismatch,16 and motorists' disorientation syndrome.17 Studies have shown that visual motion stimuli (tilted or rotating visual surroundings) have a stronger influence on verticality perception and postural stability in individuals with VV than in individuals with vestibular dysfunction but without VV.14,18,19 Thus, individuals with VV are overly reliant on visual cues for both perception and postural responses (ie, visually dependent). This excessive reliance on visual cues is believed to be 1 of the factors underlying poor vestibular compensation, especially in situations causing visual-vestibular conflict in which there is a mismatch between visual and vestibular input regarding movement and orientation (ie, 1 sensory system is indicating movement and the other is not).14,18
One of the aims of vestibular rehabilitation is to desensitize the patients through progressive, structured exposure to symptom-provoking movements and situations. When the ability to select appropriate sensory input for postural stability is disrupted, exercises focus on asking the individual to maintain balance in situations in which the availability and accuracy of 1 or more sensory inputs are varied. For those individuals with visual dependency, the approach involves exercises where visual input is incorrect, conflicting, or absent, so that the individual learns to rely more on proprioceptive and available vestibular cues.20 Guerraz et al18 suggested that rehabilitation programs promoting desensitization and increased tolerance to visual stimuli through exposure to visual motion (ie, optokinetic stimulation) would be specifically beneficial for individuals with VV.
Earlier studies in individuals with a unilateral or bilateral peripheral vestibular disorder showed significant improvements in both postural stability and optokinetic nystagmus after 6 weeks of exposure to optokinetic stimulation.21,22 These studies, however, did not have a control group and did not assess either symptom severity or type. Furthermore, patients were in the subacute phase from which the majority of individuals will recover either spontaneously or with conventional therapy.23 Consequently, it was not possible to ascertain whether exposure to optokinetic stimulation provided any greater benefit compared with customized vestibular rehabilitation alone, and, if so, which subset of individuals would benefit most from this type of treatment. Essentially, the clinical efficacy of visual motion and visual-vestibular conflict stimulation remained unknown. These issues were addressed in a study that compared patients' responses with a customized regimen versus a customized regimen that also incorporated exposure to optokinetic stimulation via whole-body or visual environment rotators.24 The findings showed that customized vestibular rehabilitation incorporating optokinetic stimuli was more beneficial for improving dizziness, postural instability, and particularly VV symptoms in individuals with chronic peripheral vestibular symptoms.24 However, the 8-week, twice-weekly therapy sessions and the equipment used (ie, custom made, expensive, and space consuming) made it difficult to transfer this rehabilitation method from a research setting into everyday clinical practice. Furthermore, visual dependency measures were not obtained, and the mechanism of recovery could not be identified.
The purpose of this focused review is to provide an overview of recent work addressing (a) the use of high-tech versus low-tech optokinetic stimulation and the role of supervision and (b) the mechanism of recovery. Optokinetic stimulation is also discussed in relation to other new innovations in vestibular rehabilitation techniques and future work.
HIGH-TECH VERSUS LOW-TECH OPTOKINETIC STIMULATION AND THE ROLE OF SUPERVISION
High-tech visual motion stimulation in previous work has included approaches such as exposure to optokinetic disks with multicolored circles, an optokinetic drum with rotating chair and striped curtain, and moving rooms.20,22,24,25 Many clinicians do not have access to these types of high-tech equipment. Wrisley and Pavlou26 reported that similar, although less intense, stimulation could be provided by a “busy” screen saver (eg, mazes), a head-mounted display, or a DVD including visual stimulation recorded from the available clinical equipment. However, the individual effectiveness of any of these low-tech methods had not been investigated.
A recently completed study, Pavlou et al27 compared responses three groups to a customized vestibular exercise regimen in three groups of individuals with chronic peripheral vestibular disorder. Participants received either: (1) supervised exposure to optokinetic stimuli via a “high-tech” full-field visual environment rotator, (2) supervised exposure to optokinetic stimuli via a low-tech DVD, or (3) practiced customized vestibular exercises and the low-tech DVD at home without supervision.
The high-tech stimulus (Stimulopt; Framiral, Cannes, France) is commercially available (Fig. 1A and B), provides full-field visual motion in the y- and z-axes, and the direction and speed can be controlled. The DVD stimuli include individual 2-minute sequences of an optokinetic disk (Fig. 2A) or drum (Fig. 2B) moving in a clockwise, counterclockwise, vertical, or sinusoidal direction at varying speeds within a limited field of view. A full description of the DVD is included in Pavlou et al.24
Parameters such as the frequency, velocity, type of texture, stimulus area, and position of the stimulus within the visual field can influence the amplitude of visual evoked postural responses and perception of self-motion (ie, vection).25,29–32 However, preliminary data analysis from the aforementioned study27 indicates significant improvements for subjective VV, vestibular, and autonomic symptoms for all 3 groups with no significant differences among groups. Furthermore, improvements in postural stability and psychological status are specific to supervised intervention but not to a particular visual motion stimulus. These findings suggest that factors that were specifically different between the high- and low-tech stimuli including texture, area, and position within the visual field did not influence rehabilitation outcome. Further work is needed to identify the most beneficial parameters of optokinetic stimulation for rehabilitation outcome; however, currently, the visual motion DVD could be an economical and effective method of incorporating optokinetic stimulation into vestibular rehabilitation programs.
Previous studies investigating the effect of supervision on vestibular rehabilitation outcomes show mixed results,12,33 but it is generally advocated that some form of supervision is beneficial.4,5,24,34 The data from Pavlou et al27 further support this view as the findings indicate a direct relationship between supervision, motivation, and compliance. There was an unacceptable 55% drop-out rate for the nonsupervised group, compared with 10% for each of the 2 supervised groups. Patients in each of the supervised groups attended once-weekly therapy sessions for the duration of the study and practiced a customized home exercise program, including the visual motion DVD, on the days they did not attend the clinic. Furthermore, supervision is believed to improve psychological status by increasing patient confidence, providing reassurance, and emphasizing the positive effects of vestibular exercises.5,34 Black et al5 also suggested that frequent assessment of progress during supervised therapy allows the therapist to introduce more challenging balancing tasks in a timely fashion, and ensure that exercises are being performed correctly. It is hypothesized that these factors are meaningful, as significant improvement for psychological symptoms and postural stability was observed only with supervised therapy.27 It is clear that some form of supervision is necessary, but further studies are required to determine the most appropriate type and frequency of supervised sessions.
MECHANISM OF VESTIBULAR RECOVERY
Adaptation of specific vestibular parameters has been noted after exposure to optokinetic stimulation including changes in the gain of the vestibulo-ocular reflex in primates, healthy individuals, and individuals with a chronic peripheral vestibular disorder.4,21,35,36 Exposure to repetitive vestibular or optokinetic stimulation also reduces the duration of postrotational vestibular sensation in healthy individuals.37 However, although it is believed that improvements noted in VV symptoms in individuals with a peripheral vestibular disorder after exposure to optokinetic stimulation are due to a decreased reliance on vision for perceptual and postural responses, no studies to date have investigated visual dependency measures before and after rehabilitation.
A recently completed study aimed to further investigate this issue by measuring visual dependency before and after repeated exposure to optokinetic stimuli in healthy individuals.38 Participants were randomly allocated into either an intervention group that underwent graded exposure to visual motion stimuli for 5 consecutive days, or a control group that did not receive any intervention. Static and dynamic aspects of perceptual preferences for spatial orientation and postural sway measures with eyes open and closed, and in the presence of visual motion stimuli, were obtained at baseline and at the end of the intervention. Findings indicated significant improvements for both perceptual and postural responses only for the intervention group suggesting that short-term, graded, repeated exposure to visual-vestibular exercises induces plastic, adaptive changes that decrease the magnitude of visual dependency.38 Possible neural sites of action for the adaptive changes induced by visual-vestibular conflict and optokinetic stimulation are numerous because of the extensive convergence of these signals in the neuraxis. However, recent functional magnetic resonance imaging studies have provided some insight into activation patterns during small-field, horizontal, and vertical optokinetic stimulation.39–43 During small-field optokinetic stimulation, activation in cortical areas related to visual motion processing and control of eye movement are noted, along with deactivation of parietoinsular vestibular cortices.39,40,41 Neuronal substrates in the cerebellum and brainstem are also involved in the processing of horizontal and vertical optokinetic stimulation.42,43
Preliminary findings from Pavlou et al38 may have important implications for the treatment of visual dependence in individuals with and without vestibular dysfunction. It was initially believed that daily and intense exposure to optokinetic stimuli would be too nauseogenic and almost certainly intolerable for this client group.24 However, Corna et al44 compared intensive twice-daily (1 hour total) supervised exposure with either sinusoidal support surface translations or Cawthorne-Cooksey exercises for 5 consecutive days in a similar client group. Participants in this study were not only able to complete the training but also reported significant improvements in objective and subjective outcome measures. A study is currently under way at the Academic Department of Neuro-Otology, Imperial College London, United Kingdom, to investigate whether individuals with a peripheral vestibular disorder and VV symptoms are able to tolerate short-term intensive, graded exposure to vestibular exercises and optokinetic stimuli, and the impact of this exposure on rehabilitation outcomes in both the short and long term.
OPTOKINETIC STIMULATION AND OTHER INNOVATIVE VESTIBULAR REHABILITATION TECHNIQUES
Vestibular rehabilitation is a continuously emerging field with promising advances in treatment. In addition to optokinetic stimulation provided via high-tech simulator-based equipment and low-tech DVDs, various authors have also discussed the potential benefit of virtual reality in vestibular rehabilitation.45–48 They have recommend that virtual reality can be a useful therapeutic tool to improve postural stability and symptoms in situations that closely reflect conditions found in everyday environments (ie, supermarket aisles).46–48 It has also been suggested that wide field-of-view devices may be more beneficial in improving postural responses, particularly in individuals with VV.46 However, 2 studies have showed that using a limited field-of-view head-mounted device can also be beneficial in individuals with vestibular disorders, with improvements noted in vestibulo-ocular reflex gain and symptoms.49,50 Additional work is needed for comparing the efficacy of vestibular rehabilitation using virtual reality devices. Future studies should also compare different types of optokinetic stimulation (eg, simulator-based, DVD, virtual reality) to try to identify whether an optimal stimulus exists both in regards to treatment outcome and cost.
Other exciting advances include the use of balance prostheses, which provides information regarding head or body orientation through vibrotactile cues delivered to the trunk,51–53 head,54 or tongue.55,56 Studies have shown that these devices can improve postural stability and reduce the risk of falls in individuals with vestibular disorders while standing on a compliant surface and during computerized dynamic posturography, a standard clinical test of balance.51–56 Most work has included participants with bilateral vestibular hypofunction. The prosthesis works by translating information normally sensed by the vestibular system (ie, head movement and verticality) into a proprioceptive cue that is then integrated with remaining sensory information to improve standing balance.54,57,58 In most of the aforementioned studies training sessions are brief, no control group is included, and carryover of improvements to task performance without the tactile device is not assessed. Furthermore, no randomized, controlled trials have been conducted to investigate the clinical usefulness of balance prostheses within a vestibular rehabilitation program. It is clear that more studies are needed in this area. Future work should also investigate the potential benefit of combined techniques (ie, optokinetic stimulation and balance prostheses).
Customized vestibular rehabilitation incorporating optokinetic stimuli is more beneficial than the vestibular rehabilitation without optokinetic stimuli for improving dizziness, postural instability, and particularly VV symptoms in individuals with chronic peripheral vestibular symptoms. However, the high-tech equipment used and the frequency of treatment sessions make it difficult to transfer this rehabilitation method into everyday clinical practice The visual motion DVD may be an economical, clinic friendly, and effective method of incorporating optokinetic stimulation into vestibular rehabilitation programs. It is suggested, however, that some form of supervision is needed for greater compliance and improvements in postural stability and psychological state. It is hypothesized that improvements after exposure to optokinetic stimuli are due to plastic, adaptive changes in the magnitude of visual dependency at both perceptual and postural levels. It is therefore suggested that the treatment of visual dependency in individuals with and without vestibular dysfunction incorporates exposure to visual motion stimuli. However, future research is required to assess optimal treatment duration, stimulus, and long-term benefit.
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