After reading with great interest the meta-analysis by Hassan et al. (2023) published in Spinal Cord titled “Effect of transcutaneous direct current stimulation on spasticity in upper motor neuron conditions: a systematic review and meta-analysis” [1]. First, we would like to thank the authors for this interesting study, which explored the effects of transcutaneous spinal direct current stimulation (tsDCS) on spasticity. This study is commendable for its well-structured approach and contribution to the understanding of the potential applications of tsDCS in modulating spinal cord activity.

Our careful examination of the published work has revealed significant inconsistencies in the terminology employed, particularly in the conflation of distinct non-invasive spinal cord stimulation techniques included and mixed in the meta-analysis; each intervention produces markedly different neurophysiological effects and should not be confounded. One is tsDCS, which involves the application of galvanic or direct current at the spinal level through rubber pads, either on the skin with a conductive gel or within saline-soaked sponges, with an intensity usually ranging between 1.5 and 2.5 mA. The main proposed mechanism of action of tsDCS includes the induction of changes in the resting membrane potential of neurons, in a polarity-dependent way, and without inducing neuron depolarization. A recent systematic review published by our group has shown inconsistent effects of anodal tsDCS to decrease hypertonia, neuropathic pain intensity, and balance deficits in people with hereditary spastic paraplegia, multiple sclerosis, and primary orthostatic tremor, respectively [2].