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.
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
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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