STABILITY OF LOCOMOTION AND BIPEDAL STANCE IN UNILATERAL VESTIBULOPATHY

Auteurs: 
McCrum, Christopher
Affiliatie: 
NUTRIM School of Nutrition and Translational Research in Metabolism, MUMC+, Human Movement Science, Maastricht, The Netherlands
Supervisie: 
Dr. Meijer, Kenneth

This master’s thesis project consisted of three studies analysing gait and bipedal stance in patients with unilateral peripheral vestibular disorder (UPVD) and healthy adults. UPVD disrupts vestibular sensory input, affecting upper body motor control and postural stability [1-3]. However, the effects of UPVD on gait stability and the effects on the plasticity of gait stability in UPVD are also relatively unknown. This knowledge has important implications for balance and gait training in patients with vestibulopathy and may help explain the increased falls risk seen in UPVD. Additionally, it is unclear how stance stability relates to stability in dynamic settings, during which falls more frequently occur, despite stance posturography being commonly used to assess balance control in clinical settings. However, by systematically disturbing the sensory systems during stance, the contribution of each system to stability control can be estimated. This project aimed to determine sensory contributions to standing balance in UPVD, how stance stability relates to dynamic gait stability, and most importantly, how dynamic gait stability and adaptation are affected by UPVD.

Methode: 
STUDY 1: The first study [4] aimed to determine the degree of dynamic gait stability control and the potential for gait stability adaptation in healthy participants and in UPVD patients by analysing recovery responses and adaptation to repeated trip perturbations during walking. Seventeen middle-aged adults with UPVD and 17 age-, sex-, height- and body weight-matched healthy control subjects walked on a treadmill at 1.4m/s and experienced eight unexpected trip perturbations during the right leg swing phase. The perturbation consisted of a single unilateral resistance of 2.1kg, applied using an electronically driven magnet system via a Teflon cable and ankle strap. For each perturbation, the margin of stability (MoS; state of the centre of mass in relation to the base of support) was determined at touchdown of the perturbed leg and the following six recovery steps [5]. STUDY 2: Having found deficiencies in gait stability and in gait stability adaptation in vestibulopathy patients, this study aimed to determine if stance posturography, as commonly used in clinics to assess balance in these patients, showed any relationships with gait stability following a single trip. 13 UPVD patients and 13 healthy matched controls were unexpectedly tripped during the right leg swing phase while walking on a treadmill. The margin of stability (MoS) was calculated at touchdown (TD) of the perturbed step and the first six recovery steps to analyse recovery and dynamic stability control [5]. Bipedal stance on a force plate was used to assess stability over 30s with eyes open and closed. Postural stability was evaluated by means of the total excursion distance of the centre of pressure (COPPath), the peak-to-peak distance in the anterior-posterior direction (COPPeak) and the distances between the most anterior and posterior points of the COPPath and the anatomical boundaries of the base of support (COPAmin and COPPmin). Pearson correlations between the tasks were conducted. STUDY 3: The aim of this third study was to examine and compare the limits of stance stability and the contribution of the sensory systems to upright stance in UPVD patients and healthy subjects. The same participants from study 1 participated in study 3. Centre of pressure (COP) trajectories were assessed using force plate during different postural tasks: forwards and backwards leaning, and 30s upright standing with and without Achilles tendon vibration, each with eyes open and closed. Stance stability was evaluated using COPPath, COPAmin and COPPmin. The corrected COPAmin and COPPmin were additionally assessed by accounting for the base of support boundaries using the data from the leaning tasks. A two-way repeated measures ANOVA was used to identify potential differences in the analysed parameters.
Resultaat: 
STUDY 1: The first trip reduced the MoS of the tripped leg in both groups equally (p<0.05), although while controls needed five steps to return to baseline stability level, the patients did not recover to normal walking stability in six steps. By the final (8th) trip, controls needed two steps and patients needed three steps to recover stability, both thereby improving. MoS for the tripped leg increased (p<0.05) after repeated trips for controls, indicating feedback-driven locomotor adaptation, but the patients’ MoS for the tripped leg did not improve. This suggests that while the recovery behaviour improved in general in the UPVD patients, the initial reactive response of the perturbed limb did not improve. STUDY 2: Following the trip, healthy subjects returned to MoS baseline in five steps but patients did not recover within the six steps. Patients showed higher total COPPath and COPPeak (EO and EC) and a more posterior COP in relation to the base of support boundary (COPPmin). Significant correlations were found only between COPPmin (EO and EC) and MoS of the second to fourth steps post-perturbation (0.4 ≤ r ≤ 0.57; p < 0.05). STUDY 3: UPVD patients showed a tendency for smaller limits of stability during leaning. Significant subject group and task condition effects were found (p<0.05) for COPPath, (higher values for patients compared to controls). UPVD patients showed significantly (p<0.05) lower COPPmin values compared to the control group for all conditions. These differences were more pronounced when the corrected COPPmin was used. There were no effects for the COPAmin or the corrected COPAmin. Visual disturbance alone lead to a distinct postural backward sway in both groups, becoming more pronounced in combination with Achilles tendon vibration.
Conclusie: 
UPVD patients have a diminished ability to control and recover dynamic gait stability after an unexpected trip and lower static postural stability control compared to healthy matched controls, particularly in the posterior direction. However, UPVD patients remain capable of some gait adaptation following repeated trip perturbations, suggesting that gait training may be of benefit to these patients. It appears that trip recovery and static postural control rely on different control mechanisms and that posturography may be of limited use in predicting dynamic stability during perturbed walking. As expected, UPVD patients showed marked decreases in postural stability with visual and proprioceptive sensory disturbance. Finally, the individual limits of stability should be considered in future, as differences from healthy subjects may lead to erroneous comparisons when assuming an equivalent base of support.
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