Sympathetic skin responses in adult humans during sequential swallowing

Autonomic changes, especially those of sympathetic skin responses (SSR), during sequential water swallowing (SWS) have not been systematically investigated. This study aims to electrophysiologically examine these autonomic changes (SSR and heart rate) that occur during 50 ml sequential water swallowing from a cup.

Fifty-eight normal healthy adults were included in the study. Their submental muscle activity, respiratory activity, heart rate changes, and sympathetic skin responses were recorded during 50 ml water swallowing. In addition, we requested subjects to imagine drinking water as they did just before. The same recordings were performed during this imagination period.

SSR appeared at the beginning and at the end of SWS in 52% of subjects. A first sympathetic skin response was evoked at the onset of SWS, and a second one appeared 8.6±1.7 seconds after the first one and at the end of swallowing. Similar double SSRs were also obtained during imagination in most investigated subjects (33 out of 35 of selected subjects in a total group of 58 subjects). Swallowing tachycardia was observed during the SWS-associated apnea period, but not during the imagination period. Heart rate significantly increased during the SWS-associated apnea period.

The first SSR that appeared at the onset of swallowing is likely related to arousal. The appearance of a second response is a novel finding, which is probably related to the activity of subtil corticosubcortical networks. While discrete/single swallows can be used to evoke SSRs, SWS is unlikely to be clinically useful in its current form. In contrast, swallowing tachycardia could be a useful tool to examine dysphagic patients.