External modification of lumbosacral spinal networks in individuals with spinal cord injury.

Abstract

Spinal cord injury impairs the neural connectivity between the brain and the spinal network as well as the peripheral nervous system. Electrical stimulation below the injury zone can be used as a tool to evoke spinal reflexes and activate spinal networks by depolarization of the posterior roots of the lumbosacral spinal cord. Transcutaneous spinal cord stimulation (tSCS) is a non-invasive alternative to stimulation with epidurally implanted electrodes. The thesis presents preliminary results on the modification of the excitability of lumbosacral spinal networks. Therefore, tSCS modifies the motor output in clinically complete spinal cord injured (SCI) subjects. Electrical stimulation was applied at the T11-T12 vertebral level eliciting posterior root reflexes (PRRs) in the lower limbs using surface electrodes. EMG signals were derived bilaterally from rectus femoris (RF), hamstring (H), tibialis anterior (TA) and triceps surae (TS). The reflex responses showed the excitability level of the lumbosacral spinal networks. In every subject, the reflex nature was tested with double pulses having an inter-pulse interval of 35 ms. Three different intervention protocols were assessed in the preliminary study. First, sustained stimulation below the motoric threshold intensity at rates of 20 pps and 30 pps was used as conditioning at the same vertebral level. The results of this test showed mainly suppressive effects on PRR responses in most muscle groups, whereas only in the thigh muscles a response facilitation was observed. For the second assessment, repetitive stimulation above the motoric threshold at lower stimulation rates (1 to 10 pps) caused no modification of the response amplitudes, in contrast to higher rates (20 and 30 pps) where the amplitudes were significantly suppressed in every subject. In the third intervention, the study addressed modification of spasticity during a passive hip and knee flexion. It was shown that spastic motor output of the SCI individual was completely suppressed with sustained stimulation at 80 pps. In all three parts of the preliminary study, spinal cord stimulation showed a positive effect on modification capability of the spinal motor output. In further studies, a larger subject group will be addressed to confirm these findings.