In the present study, perception of limitations in the activities of daily life after uVN were measured using the DHI 17. The average DHI (38.52) in the 1st test corresponded to the results of previous studies for acute uVN (range: 37-45) 18-20. DHI improved significantly between the 1st and the 2nd tests. Although the DHI improved between the 2nd and 3rd tests, the difference was not statistically significant. We speculate that statistical insignificance between the 2nd and 3rd tests resulted from the large inter-personal variations. Considering DHI scores less than 5 in the control group 20, these findings suggest that the perception of limitation in the activities of daily life after uVN persists after 3 months of onset and support a previous study result that DHI improved to near normal values after 6 months 19. The results of the vestibular score and composite score from the CDP showed significant improvements between the 1st and 2nd tests. Compared to the normal reference values 21, the vestibular score and composite scores in the 2nd test in this study improved to the values within one SD of the normal reference values.
Gait characteristics with poor balance 22 or perceptive dizziness 23 are slow walking speed, short stride/step length, increased percentage of double support and stance phases, increased step width, and increased spatio-temporal variability. Previous investigations for vestibular disorders demonstrated gait characteristics different from healthy controls: slow walking speed, reduced cadence, and short step length 9,10. This study focused on the longitudinal change of gait function after uVN, which had a relatively fast recovery of vestibular function but a prolonged limitation in the activities of daily living or social participation. In this study, walking speed, stride length, and cadence did not show significant differences between participants with uVN and control in the 1st, 2nd, and 3rd tests, respectively. These findings suggest that the overall walking function represented by walking speed is within the normal range after the acute stage of uVN. However, speed and stride length showed significant improvement between the 1st and the 3rd test. These findings suggest a continuous improvement of overall walking function during the recovery stage (two weeks to three months). Participants with uVN showed wider step width in the 1st test than the controls. Step width improved significantly between the 1st and 2nd tests, thereby showing no significant difference from the controls in the 2nd test. Step width is related to the foot placement strategy in balance control by widening the base of support (BoS). The above results suggest that widening the step width is a temporary compensation strategy occurring during the early recovery stage. Participants with uVN did not show a significant difference in stance phase proportion compared to the control. Although stance phase proportion in the affected side significantly improved between the 1st and 2nd tests, it was within the normal range, even in the 1st test. Increased stance phase proportion is also related to balance control by reducing the swing phase which corresponds to the single support phase of the opposite side and is dynamically unstable. These findings also suggest that an increased stance phase proportion is a temporary compensation strategy during the recovery stage. From the gait metrics in the present study, widening of step width and increasing stance phase proportion may be compensation strategies observed in the early recovery stage of uVN. Even after these compensation strategies subsided, improvement of the overall walking function, represented by walking speed, continued during the recovery stage. Although spatio-temporal metrics are reliable, easy to take up, and most frequently studied, they have limitations in providing direct evidence of biomechanical or motor control and evidence of balance control.
For vestibular disorders, most previous studies using CDP reported the CoM-CoP relationship only in the sagittal plane. However, the CoM-CoP relationship in the frontal plane might be more relevant to gait stability than in the sagittal plane 24. Mediolateral stability during gait is maintained when CoM and extrapolated CoM are controlled within the BoS. In the previous studies, the maximum horizontal separation distance between CoM and CoP during stance was reported to sensitively quantify gait instability in patients with bilateral vestibular hypofunction or cerebellar ataxia 25,26. The position of CoM close to the CoP was related to falls, and excessive lateral momentum of CoM was identified in the balance-impaired elderly 27. We assume that the increased distance between CoM and CoP during the stance phase may result from a compensation related to the widening of the BoS.
In this study, IA_min in the non-affected side in the 1st test was significantly larger than that in the control. This finding may result from relative postural sway on the affected side and increased step width in uVN. Although there was a trend to decrease IA_min in the non-affected side, the time effect on this metric was not significant. We contemplate that this statistical insignificance results from large inter-personal variances. Therefore, future studies should be conducted to verify the results of the present study. There were no significant differences in IA_min between the control and the affected side of uVN in the 1st, 2nd, and 3rd tests. These results suggest maintenance of the relationship between CoM and CoP during the recovery stage of uVN, while the step width increases in the 1st test. This supports that the CoM-CoP relationship in the frontal plane is a dominant constraint for maintaining gait. In addition to the foot placement strategy, which is represented by step width, the ankle strategy may also influence the CoM-CoP relationship by medio-lateral shifting of CoP. Therefore, future studies should be conducted to reveal the influence of ankle strategy on uVN.
In this study, uVN showed significantly larger IA_var in the affected side in the 1st test compared to controls. Additionally, IA_var decreased significantly in the 3rd test on the affected side compared to that in the 1st test. We speculate that IA_var reflect the mediolateral balance control more directly than step width and proportion of stance phase which may be compensation strategies. Therefore, this finding indicates a continuous improvement of mediolateral balance control at least two months after uVN onset. Human motor performance is generated by an inherently ‘noisy’ nervous system, which results from stochastic events at the level of ion channels, synapses, neurons, and neural networks 28. After uVN, noise in the nervous system increases, thereby increasing uncertainty and variability in vestibular nervous system. It is widely believed that motor control is optimized for current performance; the variability that interferes with this goal should be minimized 29. However, variability in motor performance is a mean of exploring motor spaces that reinforce motor learning 30. We think that the increased variability after the acute stage of uVN suggests the existence of an actively ongoing adaptation process in the vestibular system. Therefore, this period is clinically significant for long-term progress. Furthermore, more active rehabilitation should be provided because vestibular rehabilitation interventions interact with the recovery mechanism during the critical plastic time window of internal reorganization processes 2.
This study has some limitations. First, gait metrics were not evaluated during the acute stage of uVN due to safety issues, making time differences between 3D gait analysis and other tests. Second, the duration for follow-up was too short to investigate the complete recovery of participants. Third, the sample size was too small for the t-test to compare the results of uVN and control. Finally, although vestibular ocular rehabilitation exercise was provided at the acute stage of uVN, quantitative monitoring for this exercise program was not implemented. Future studies with longer follow-up periods, larger sample sizes, and control of rehabilitation programs should be conducted to verify the results of this study.