Sensory re-weighting for balance control and the effects of ankle foot orthoses and stance width: A comparison of people with diabetic peripheral neuropathy and healthy participants
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Background: Diabetic peripheral neuropathy (DPN) is diagnosed clinically as a loss of sensation in the feet and affects over 2 million people in the UK. One of the functional effects of DPN is a decrease in standing stability giving rise to a risk of falls. In an attempt to stabilise in the mediolateral direction, people with DPN frequently walk with a wider base of support and stand with a larger stance width. This is often seen in the elderly and is not always beneficial for stability contributing to falls risk. Standing balance requires the integration of sensory information from somatosensory, vestibular and visual systems. Alterations in distal sensory input may result in a re-weighting of the effectiveness of remaining sensations in mediating a stabilising postural response; termed sensory re-weighting. Alterations in posture such as adopting a wider stance width and wearing Ankle Foot Orthoses (AFOs) may also affect sensory input as well as altering the mechanics of the ankle and hip joints. The impact of distal sensory loss on the sensory control of balance in people with DPN compared to the healthy population is unknown. Moreover, it is not known whether standing balance or the sensory control of balance is affected by the adoption of an increased stance width and wearing (AFOs) that restrict mediolateral ankle motion. A better understanding of the mechanisms underlying balance dysfunction in diabetic peripheral neuropathy and how it might be manipulated could inform the development of future interventions to improve balance. Aim: To explore the effects of ankle foot orthoses and stance width on standing balance and the sensory control of mediolateral balance in people with DPN and healthy controls. Objectives: To assess how mediolateral postural stability and the sensory control of balance is affected by (a) AFO use and alterations in stance width in healthy participants (study 1) (b) acute distal sensory loss in healthy participants (study 2) (c) chronic sensory loss in people with DPN and how this in turn is modulated by AFO use and alterations in stance width (study 3). Methods: Postural stability and the response to selective muscle vibration that stimulates muscle spindle afferents was measured by 3D motion analysis. Study 1 investigated the effects of stance width and AFOs on postural sway and the response to selective hip proprioception stimulation induced by vibration of the hip abductors in healthy participants. Study 2 investigated the effect of an acute reduction of somatosensory information induced by cooling in healthy participants on the response to ankle evertor and hip abductor vibration. This provided a model of the acute effects of sensory loss. Study 3 compared healthy people with people with chronic DPN. It investigated the impact on stance stability and whether there was a change in the postural response (gain) to ankle evertor and hip abductor vibration. It further explored the effect of altering the stance width and wearing an AFO on stability and the postural response to hip abductor vibration. Results: Study 1: In healthy controls postural sway was significantly reduced when wearing an ankle foot orthoses and when standing at wider stance widths. Whilst this was also seen during balance perturbation, trunk motion increased at larger stance widths. This could be the result of the AFO restricting ankle motion and affecting the interpretation of the hip vibratory input by the postural control system. Study 2: Experimental reduction in distal sensation by cooling resulted in a reduction in postural responses to ankle evertor muscle vibration. Conversely postural responses at the level of the hip, to proximal (hip) muscle vibration, significantly increased. Study 3: Baseline sway velocity was higher in people with DPN compared to healthy controls. Postural strategies were modified in the DPN group, with increased motion at more proximal segments of the shoulder and head. In both groups, AFO and stance width significantly reduced baseline sway velocity, and the size of postural responses (translations) to hip abductor muscle vibration. Conclusion: Alterations in stance width and the use of AFOs can affect postural sway and the response to selective proprioceptive stimulation. Whilst acute reductions in distal sensory loss are associated with sensory re-weighting of distal and proximal proprioceptive information this is not seen in people with chronic DPN, possibly resulting from long term adaptive changes in the multi-sensory control of balance. Novel differences were found in postural strategies between healthy and DPN groups. The increase in head and trunk motion in people with DPN may have a negative impact on visual acuity and therefore a risk factor for falls. In people with diabetic peripheral neuropathy AFOs and increased stance width led to a reduction in postural response size and postural sway. The effect of AFO on sway velocity was more pronounced in those with DPN at smaller stance widths. Clinically this suggests that an AFO could be used in those with diabetic peripheral neuropathy to slow down the velocity of sway and increase stability.