Transspinal direct current stimulation for multiple sessions alters neuronal excitability but not homosynaptic inhibition in people with and without Spinal Cord Injury

Transspinal stimulation with direct current or at intensities and frequencies that produce intermittent depolarization of motoneurons can be an adjunct treatment strategy for spasticity and recovery of movement in persons with spinal cord injury (SCI). The main objective of this study was to assess neuroplasticity after multiple sessions of transspinal direct current stimulation (tsDCS) in people with and without SCI. Nine SCI and 10 noninjured subjects received daily cathodal tsDCS over Thoracic 10 while supine with an average stimulation intensity of 2.28 ± 0.02 mA. SCI and noninjured subjects received an average of 15 and 10 stimulation sessions, respectively. Before and 1–2 days post intervention, we assessed changes in soleus H-reflex recruitment input-output curves, homosynaptic depression and postactivation depression. tsDCS for approximately 1 hour did not alter the strength of homosynaptic depression in both subject groups, but reversed postactivation depression to facilitation in AIS C-D subjects. tsDCS resulted in depression of reflex excitability in both subject groups, but without significant changes in clinically assessed hyperreflexia. The results indicate decreased reflex hyperexcitability without recovery of spinal inhibitory control in the injured human spinal cord after tsDCS. More systematic investigations are needed to delineate the tsDCS-induced neuroplasticity of spinal neuronal networks in people with SCI and thus be able to develop effective treatments.