Transcranial direct current stimulation in individuals with spinal cord injury: Assessment of autonomic nervous system activity

Background: We hypothesized in this study that transcranial direct current stimulation (tDCS) of primary motor cortex could exert top-down modulation over subcortical systems associated with autonomic control and thus be useful to revert some of the dysfunctional changes found in the autonomic nervous system (ANS) of subjects with spinal cord injuries (SCI). Objective: To explore the acute effect of tDCS on ANS indexed by Heart Rate Variability (HRV) in individuals with SCI and analyze whether this effect depends on the gender, degree, level and time of injury. Methods: In this randomized, placebo-controlled, crossover, double-blinded study, 18 adults with SCI (32.9 +/- 7.9 years old) were included; the intervention consisted of a single 12-minute session of active tDCS (anodal, 2 mA) and a control session of sham tDCS applied over Cz (bihemispheric motor cortex). HRV was calculated using spectral analysis. Low-frequency (LF), high-frequency (HF), and LF/HF ratio variables were evaluated before, during, and post tDCS. Results: A two-way repeated measures ANOVA showed that after active (anodal) stimulation, LF/HF ratio was significantly increased (P = 0.013). Therewas a trend for an interaction between time and stimulation for both LF andHF(P = 0.052). Paired exploratory t-tests reported effects on the difference of time [post-pre] between stimulation conditions for LF (P = 0.052), HF (P = 0.052) and LF/HF (P = 0.003). Conclusion: Anodal tDCS of the motor cortex modulated ANS activity in individuals with SCI independent of gender, type and time of lesion. These changes were in the direction of normalization of ANS parameters, thus confirming our initial hypothesis that an enhancement of cortical excitability by tDCS could at least partially restore some of the dysfunctional activity in the ANS system of subjects with SCI.