In the present study, limitations in daily life after uVN was measured by DHI (18). In the present study, average of DHI (38.52) at the 1st test corresponded to the results of previous studies for acute uVN, ranging 37 ~ 45 (19–21). DHI improved significantly between the 1st and the 2nd test. Although DHI improved between 2nd and 3rd tests, difference was not statistically significant. The authors think that statistical insignificance between 2nd and 3rd test resulted from the large inter-personal variations. Considering DHI scores less than 5 in healthy controls (21) and improvement to near normal value after 6 months (20), These findings suggest that perception of limitation in daily life after uVN persists after 3 months of onset. The results of vestibular score and composite score from CDP showed significant improvements between the 1st and 2nd tests. Compared to the normal reference values (22), vestibular score and composite scores at the 2nd test in the present study improved to the values within one SD of normal reference values. These findings suggest early return of vestibular function or limitation of CDP which does not reflect medio-lateral stability in the present study.
Characteristics of gait with poor balance (23) or perceptive dizziness (24) 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 to healthy controls; slow walking speed, reduced cadence, and short step length (10, 11). The present study focused on the longitudinal change of gait function after uVN which had relatively fast recovery of vestibular function but prolonged limitation in daily living activity or social participation. In the present study, walking speeds, stride length and cadence did not show significant differences between uVN and control at the 1st, 2nd and 3rd tests, respectively. These findings suggest that overall walking function represented by walking speed is within normal after acute stage of uVN. However, speed and stride length showed significant improvement between the 1st and the 3rd test. These findings suggest continuous improvement of overall walking function during recovery stage (2 weeks to 3 months). Participants with uVN showed wider step-width at the 1st test than the controls. Step-width improved significantly between the 1st and 2nd test, thereby showing no significant difference to the controls at the 2nd test. Step width is related with foot placement strategy in balance control by widening base of support. Above results suggest that widening of step width is a temporary compensation strategy occurring during early recovery stage. Participants with uVN did not show significant difference of stance phase proportion, compared to the control. Although stance phase proportion in the affected side significantly improved between the 1st and 2nd test, it was within normal range, even at the 1st test. Increased stance phase proportion is also related with balance control by reducing swing phase which corresponds to single support phase of opposite side and is dynamically unstable. These findings also suggest that increased stance phase proportion is a temporary compensation strategy during recovery stage. From the gait metrics in the present study, widening of step width and increasing of stance phase proportion may be compensation strategies observed in the early recovery stage of uVN. Even after these compensation strategies subsides, improvement of overall walking function represented by walking speed continues during recovery stage. Although spatio-temporal metrics are reliable, easy to uptake and most frequently studied, they have limitations in providing direct evidence of biomechanical or motor control evidence of balance control.
For vestibular disorders, most previous studies using CDP reported the CoM-CoP relationship in sagittal plane without that in frontal plane. However, previous authors postulated that CoM-CoP relationship in frontal plane might be more relevant to gait stability than that in sagittal plane (25). Medio-lateral 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 hypo-function or cerebellar ataxia (26, 27). However, position of CoM close to the CoP was related with fall and excessive lateral momentum of CoM was identified in balance-impaired elderly (28). We think that increased distance between CoM and CoP during stance phase may result from the compensation related with widening of base of support.
In the present study, IA_min in non-affected side at the 1st test was significantly larger than the control. This finding may results from postural sway to the affected side and increased step width in uVN. Although there was trend to decrease IA_min in non-affected side, the time effect on this metric was not significant. The authors think that this statistical insignificance results from large variances. Therefore, future study should be conducted to confirm the result of the present study. There were no significant differences of IA_min between control and the affected side of uVN at the 1st, 2nd and 3rd tests. These results suggest maintenance of relationship between CoM and CoP during recovery stage of uVN while the step width increases at the 1st test. This supports that the CoM-CoP relationship in frontal plane is dominant constraint for maintenance of gait. In addition to the foot placement strategy which is represented by step width, ankle strategy may also influence on the CoM-CoP relationship after onset of uVN. Therefore, future studies should be conducted to reveal the influence of ankle strategy on uVN.
In the present study, uVN showed significantly larger IA_var in the affected side at the 1st study, compared to controls. In addition, IA_var decreased significantly at the 3rd study in the affected side, compared to the 1st study. This finding indicates continuous improvement of variability during at least 2 months after uVN onset. Human motor performance is generated inherently ‘noisy’ nervous system which results from stochastic events at the level of ion channels, synapses, neurons and neural networks (29). After uVN, noise in nervous system increases, thereby increasing uncertainty and variability. It is widely believed that motor control is optimized for current performance, and that variability that interferes with this goal should be minimized (30). In other view, variability in motor performance is a means of exploring motor spaces which reinforce motor learning (31). We think that increased variability after acute stage of uVN suggests existence of actively ongoing adaptation process in vestibular system. Therefore, this period is clinically significant for long term progress and 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 limitations. Gait metrics were not evaluated during acute stage of uVN due to safety issues, thereby making time differences between 3D gait analysis and other tests. Duration of follow up was too short to investigate complete recovery of participants. Sample size was small for the t-test to compare uVN and control results. Although education of vestibular ocular rehabilitation exercise was provided at acute stage of uVN, quantitative monitoring for this exercise program was not implemented. Future study with longer follow up, larger sample size and control of rehabilitation program should be conducted to verity the results from the present study.