Reflex modulation and functional improvements following spinal cord stimulation for sensory restoration after lower-limb amputation

Abstract

BackgroundThe goal of this study was to characterize spinal reflexes and muscle activation in people with lower-limb amputation during use of a sensory neuroprosthesis. People with lower-limb amputations lack sensory inputs from their missing limb, which increases their risk of falling. People with lower-limb amputation exhibit co-contractions of antagonist muscles and reduced pelvic obliquity range-of-motion and pelvic drop. These motor functions are governed, in part, by sensory-mediated spinal reflexes; loss of somatosensation after amputation contributes to their dysfunction. Spinal cord stimulation (SCS) can restore sensation in the missing limb, but its effects on spinal reflex modulation and muscle activation have not been studied in people with lower-limb amputation.MethodsWe implanted percutaneous SCS electrodes over the lumbosacral enlargement in 3 people with trans-tibial amputation (2 diabetic neuropathy; 1 traumatic) for 28 or 84 days. SCS was used to restore sensation in the missing limb. We used electromyography (EMG) to record posterior root-muscle (PRM) reflexes and muscle activity of the residual limb. We characterized rate-dependent depression and recruitment properties of the PRM reflexes, measured changes in PRM amplitudes over time during quiet standing, and quantified changes in muscle activation and pelvic obliquity during walking with SCS over time.ResultsSCS evoked PRM reflexes in the residual limb muscles in all participants, which was confirmed by the presence of rate-dependent depression at stimulation frequencies ≥2Hz. Overall, there was broad activation of residual limb muscles with SCS. Sensations that were evoked exclusively in the residual limb were associated with a time-dependent increase in PRM reflex amplitude in proximal leg muscles; whereas sensations evoked the missing limb were associated with a progressive decrease in PRM reflex amplitude in all muscles except for medial gastrocnemius. During walking, co-contractions of knee antagonist muscles were reduced following multiple sessions of SCS-mediated sensory restoration. Additionally, the activation of the tensor fasciae latae muscle (hip abductor) increased during gait with SCS-mediated sensory restoration, which corresponded to an increase in pelvic obliquity range-of-motion and pelvic drop.ConclusionsRestoring sensation in the missing limb using SCS modulates spinal reflexes, reduces co-contractions of antagonist muscles, and improves pelvic obliquity.