Effects of Combined Transcranial Direct Current Stimulation and Oral Capsaicinoids on Swallowing Function: A Randomized Controlled Trial

BACKGROUND: The swallowing biomechanics may be modulated by cortical stimulation through transcranial direct current stimulation (tDCS) or sensory stimulation through oral capsaicinoids. However, the effects of combining these techniques remain relatively unexplored. OBJECTIVE: This pilot study investigated the effects of tDCS combined with oral capsaicinoids on swallowing function. METHODS: Twenty-one healthy individuals (mean age = 25.8 ± 9.9 years; 7 males, 14 females) received two experimental conditions: (1) anodal tDCS and oral capsaicinoids, and (2) sham tDCS and oral capsaicinoids. The experimental conditions were randomized in order and conducted at least 1 week apart. Anodal and sham tDCS were applied over the right pharyngeal motor cortex at 1 mA for 20 min and 30 s respectively. A 15-min break was given after tDCS, after which participants drank 150 mL of 10 μM capsaicinoid-containing solution. Swallowing function was assessed using the timed water swallow test (TWST) before, immediately after and 15 min after each experimental condition. RESULTS: The baseline volume per swallow, duration per swallow and swallowing capacity, as assessed by TWST, were comparable between the two conditions. Combined anodal tDCS and oral capsaicinoids significantly increased the volume per swallow compared with combined sham tDCS and oral capsaicinoids (p = 0.003). There were no significant effects of intervention on duration per swallow or swallowing capacity between the two conditions. CONCLUSIONS: We found that combined anodal tDCS and oral capsaicinoids increased the volume per swallow, suggesting potential changes in swallowing biomechanics. The findings imply that cortical stimulation may enhance the effects of peripheral sensory stimulation on the swallowing system.