Abstract
Purpose: Previous studies in healthy individuals showed that high-frequency but not the low frequency non-invasive vagal nerve stimulation improves olfactory function in healthy individuals. The present study aimed to analyze the potential effects of non-invasive, high-frequency transcutaneous auricular vagal nerve stimulation (TA-VNS) on olfactory function in patients with olfactory dysfunction (OD) and healthy individuals.
Methods: Patients with post-COVID-19 associated OD (n = 10) and normosmic individuals (n = 30) performed olfactory testing (Sniffin` Sticks), trigeminal testing (lateralization test), attentional tests (d2 test), ratings of odor intensity and pleasantness before and after receiving a TA-VNS for 10 min. Patients with OD received only TA-VNS while normosmic subjects underwent, in addition to TA-VNS, placebo stimulation (control condition) with electrode placement of TA-VNS, and active-control transcutaneous stimulation at the forearm in a randomized order with a minimum 24-h interval between sessions.
Results: In patients with OD intensity ratings for fish and pleasantness ratings for eucalyptol increased and olfactory discrimination improved (p = 0.017) as opposed to olfactory threshold (p = 0.18). For both, patients with OD and healthy individuals results of the d2 attention test improved following the TA-VNS. In healthy individuals, there were no differences in olfactory discrimination nor threshold regarding the 3 stimulation modalities (TA-VNS, placebo (control condition), transcutaneous stimulation).
Conclusion: These results seem to underline the functional connection of the olfactory system with the afferent vagus network. However, a ceiling effect for olfactory discrimination did not allow to observe a possible improvement in normosmic individuals. Further studies in patients with OD are needed to characterize this potential therapeutic effect of TA-VNS.