Modulation of autonomic function and heart rate variability by transcutaneous spinal cord stimulation and physical therapy in individuals with chronic spinal cord injury

Spinal cord injury causes motor, sensory, and autonomic impairments. Transcutaneous spinal cord stimulation (tSCS) has shown potential to improve autonomic regulation, particularly cardiovascular and bladder control. Heart rate variability (HRV) serves as a non-invasive marker of autonomic balance, reflecting modulation of cardiac sympathetic and parasympathetic activity. The purpose of this study was to evaluate the effect of combined tSCS and physical therapy (tSCS + PT) on HRV and autonomic function metrics on individuals with chronic spinal cord injury. Eight participants (2 with tetraplegia, 6 with paraplegia) received tSCS at 30 Hz targeting T11–L1 or C5–T1, depending on injury level. Each underwent two tSCS + PT sessions per week for 12 weeks, followed by a 4-week washout period. Autonomic assessments included the Autonomic Standard Evaluation Form (ASAF), Neurologic Bladder Symptoms Score (NBSS), blood pressure, heart rate, SpO2%, and core temperature. Electrocardiograms were recorded pre- and post-tSCS + PT at baseline, 12, and 16 weeks. Baseline effects of tSCS + PT revealed increased time-domain HRV parameters, suggesting enhanced cardiac parasympathetic activation (PSA). After 12 weeks, these parameters decreased, suggesting reduced cardiac PSA and enhanced sympathetic activation (SA). Similar results were found with frequency-domain parameters. Time-and frequency-domain parameters showed a tendency to revert to baseline levels after the 4-week therapy suspension, indicating that the effect of the therapy was temporary and induced by stimulation. Two commonly reported HRV parameters, SDNN and RMSSD, showed large effects (g = 1.64, CI 95% [0.30, 2.98], and g = 1.23, CI 95% [0.11, 2.42], respectively) at 12 weeks when comparing pre- vs. post-tSCS + PT. Moderate, subject-related differences across evaluations were found in ASAF and NBSS (p > 0.05). Physiological metrics across time points did not show statistically significant differences (p > 0.05). Physiological metrics across time points did not show statistically significant differences (p > 0.05). Combined tSCS + PT elicited improvements in autonomic clinical measures, without statistically significant changes in time-domain or frequency-domain HRV parameters. Nonetheless, trends and size effect in SDNN and RMSSD suggested that tSCS + PT induced midterm SA and a transient PSA immediately post-tSCS + PT. The observed changes in HRV following the withdrawal of tSCS support the interpretation that the autonomic modulation was directly induced by the applied therapy.