Influence of stimulus frequency-amplitude relation of spinal cord stimulation on reflex responses in individuals with spinal cord injury

Abstract

Spinal cord injury (SCI) is a damage to the spinal cord that disrupts the connectivity between brain and the spinal neural networks caudal to the lesion and the peripheral nervous system, resulting in loss of partial or complete motor control below the injury level. It is associated with spasticity, chronic pain, among other problems. Spinal Cord Stimulation (SCS) has shown to be a promising approach to improve the motor function and manage complications such as spasticity and pain. Specifically, SCS has shown to be able to ameliorate the spasticity when high frequencies are applied. It has been suggested that, by depolarizing the posterior roots, epidural (eSCS) and transcutaneous (tSCS) are able to access the surviving neural centers below the SCI, a neuromodulation process is conducted in synergy with the residual influence from the brain and other supraspinal centers . However, since each SCI case alters the nervous system in a unique way, it has been observed that using the fixed parameters for modulation of spasticity might fail. This thesis presents the effects of stimulation intensity and frequency on the behavior of the motor output and how the interaction of these parameters affects the level of motor suppression, which ultimately leads to spasticity control. Sustain tSCS was applied at the T11—T12 vertebral level of three subjects with complete SCI to evoked sustain responses in the lower limbs. The neuromuscular activity was monitored via EMG from quadriceps (Q), hamstrings (H), tibialis anterior (TA) and triceps surae (TS) of both legs. In order to assess the suppressing effect of different stimulation frequencies and intensities, the neuromuscular responses were quantified. An additional validation was carried on with eSCS data obtained from an early study. Threshold intensities eliciting the posterior root reflexes were inspected with low frequency (2 Hz) stimulation and afterwards the suppression frequencies were estimated by increasing the stimulation frequency gradually at different supra-threshold stimulation strengths. The results have showed variety among not only the patients but also the muscles. In almost all cases with a few exceptions, it has been shown that at constant stimulation strength, responses evoked to the stimulation are suppressed with increasing frequency. However, when the stimulation intensity is increased the activity that has been previously suppressed at certain frequency came back. All our results show a dependency between stimulation frequency and intensity in terms of suppressing the motor output –controlling the spasticity. Therefore, it is concluded that in order to increase the efficiency of SCS-based anti-spastic treatments, an assessment of the frequency-intensity interaction has to be done to properly define the stimulation parameters, helping this way to increase the success rate of the technique.