Recovery times of stance and gait balance control after an acute unilateral peripheral vestibular deficit.

BACKGROUND: Acute unilateral peripheral vestibular deficit (aUPVD) patients have balance deficits that can improve after several weeks. Determining differences in vestibulo-spinal reflex (VSR) influences on balance control and vestibular ocular reflex (VOR) responses with peripheral recovery and central compensation would provide insights into CNS plasticity mechanisms. Also, clinically, knowing when balance control is approximately normal again should contribute to decisions about working ability after aUPVD. Usually VORs are employed for this purpose, despite a lack of knowledge about correlations with balance control. Given this background, we examined whether balance and VOR measures improve similarly and are correlated. Further whether balance improvements are different for stance and gait.

METHODS: 26 patients were examined at onset of aUPVD, and 3, 6 and 13 weeks later. To measure balance control and thereby assess the contribution of VSR influences during stance and gait, body-worn gyroscopes mounted at lumbar 1-3 recorded the angular velocity of the lower trunk in the roll (lateral) and pitch (anterior-posterior) directions. These signals were integrated to yield angle deviations. To measure VOR function, rotating chair (ROT) tests were performed with triangular velocity profiles with accelerations of 20°/s(2) and 5°/s(2), and caloric tests with bithermal (44 and 30°C) water irrigation of the external auditory meatus. Changes in average balance and VOR measures at the 4 examination time points were modelled with exponential decays. Improvements were assumed to plateau when model values were to within 10% of steady state.

RESULTS: Balance improvement rates were task and direction dependent, ranging from 3-9 weeks post aUPVD, similar to the range of ROT VOR improvement rates. Stance balance control improved similarly in the pitch and roll directions. Both reached steady state at 7.5 weeks. However, changes in visual and proprioceptive influences on stance sway velocities continued to decrease in favour of vestibular influences for over 10 weeks with the visual influence being correlated with ROT deficit side responses (R= 0.475). Spontaneous nystagmus and stance roll velocity were weakly correlated (R= 0.24). Pitch control during gait tests improved faster than roll. Gait speed was slower and only recovered normal velocity at 6-9 weeks. Pitch velocity when walking eyes closed was correlated (R= 0.38) with ROT asymmetry. Other balance and VOR measures were more weakly correlated (R< 0.2) even if these had similar improvement rates.

CONCLUSIONS: These results indicate that balance control for stance improves equally fast in the pitch and roll directions. For gait, pitch control improves faster than roll. On average, stance and gait tests show normal balance control at 6-9 weeks post aUPVD onset. As few balance measures are correlated with those of VOR function and then with low (R< 0.5) coefficients, we suggest that VOR tests should not be used to assess improvements in balance control after aUPVD. The lack of strong correlations between balance and VOR measures included in this study during peripheral recovery and central compensation of aUPVD supports the hypothesis that recovery of balance function after an aUPVD involves different CNS pathways and neural plasticity mechanisms.