Immediate Effects of Transcutaneous Spinal Stimulation on Stretch-Induced Spasticity in Persons with Spinal Cord Injury

<b>Background/Objectives</b>: Transcutaneous spinal stimulation (TSS) is a noninvasive stimulation approach for spasticity reduction in people with spinal cord injury (SCI). We enrolled 17 individuals with SCI who experience lower extremity hyperreflexia for this randomized crossover study to compare single-session effects of 3 TSS conditions: single-site continuous (SS-CONT), single-site burst (SS-BURST), and dual-site continuous (DS-CONT). <b>Methods</b>: Each TSS condition was delivered for 30 min with participants in supine via a cathode over the thoracic spine (T11-T12) and an anode over the abdomen. A second cathode was placed over the lumbar spine (L1/2 or L2/3) for DS-CONT. SS-CONT and DS-CONT stimulation was delivered as continuous 50 Hz stimulation with a 1 ms pulse width. SS-BURST stimulation was delivered as 4 bursts/second of 50 Hz stimulation with a 1 ms pulse width. Pendulum test first swing excursion (FSE) and ankle clonus drop test first drop excursion (FDE) were measured at baseline and immediately post-intervention to assess quadriceps and soleus spasticity, respectively. FSE and FDE of the first trial (FSE_T1 and FDE_T1) and the average of 3 trials (FSE_avg and FDE_avg) were included in analyses. Subgroup analyses were performed based on baseline level of spasticity (high vs. low). <b>Results</b>: Between-condition analyses showed no significant differences; however, SS-CONT (FSE_T1<i>d</i> = 0.30, FSE_avg<i>d</i> = 0.27) and DS-CONT (FSE_T1<i>d</i> = 0.33, FSE_avg<i>d</i> = 0.12) stimulation demonstrated the largest effect sizes for FSE measures, and SS-CONT (FDE_T1<i>d</i> = 0.32, FDE_avg<i>d</i> = 0.31) stimulation demonstrated the largest effect size for FDE measures. Significant fair correlations between baseline FSE measures and change in FSE were identified when all conditions were combined. A significant fair correlation between baseline FDE_T1 and change in FDE_T1 was identified when data were collapsed across conditions. In subgroup analyses, only participants with high baseline quadriceps spasticity showed a significant decrease in quadriceps spasticity with DS-CONT (∆FSE_T1 = 14.8 ± 13.0°), SS-BURST (∆FSE_T1 = 4.1 ± 4.5°), and with all conditions combined (∆FSE_T1 = 11.3 ± 16.5°, ∆FSE_avg = 7.2 ± 13.1°). For participants with low baseline soleus spasticity, DS-CONT stimulation significantly increased soleus spasticity (∆FDE_T1 = -12.2 ± 9.3°, ∆FDE_avg = -8.5 ± 8.4°). <b>Conclusions</b>: When data were collapsed across conditions, TSS did not result in a significant reduction in quadriceps or soleus spasticity. Continuous stimulation at both single- and dual-sites was associated with the largest effect on quadriceps spasticity when all participants were combined. Lastly, TSS reduced spasticity in a severity-dependent manner.