Transspinal direct current stimulation alters neuronal excitability but not homosynaptic inhibition in human spinal cord injury: a pilot clinical trial.

Transspinal stimulation with direct current or at intensities and frequencies that produces 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 changes in neurophysiological biomarkers after multiple sessions of transspinal direct current stimulation (tsDCS) in people with and without SCI. Nine SCI and 10 healthy control subjects received daily cathodal tsDCS over the Thoracic 10 vertebra while supine with an average stimulation intensity of 2.28 ± 0.02 mA. SCI and healthy control 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 did not alter the strength of homosynaptic depression in any subject group but reversed postactivation depression to facilitation in AIS D subjects. tsDCS resulted in depression of reflex excitability in both subject groups. No significant changes in clinically assessed hyperreflexia were observed in SCI subjects. The results suggest decreased reflex hyperexcitability without recovery of spinal inhibitory control in the injured human spinal cord after multiple sessions of tsDCS. More systematic investigations on reorganization of spinal and cortical interneuronal networks are needed to delineate the tsDCS-induced neuroplasticity in people with SCI to develop effective treatments.