## 4 Discussion  
The use of TENS in the management of xerostomia has been investigated in different etiological contexts, encompassing radiotherapy-induced xerostomia to cases linked to systemic, pharmacological, or hormonal conditions [10; 11; 12]. The mechanism of action of TENS on the parotid glands is not yet clear, although it may stimulate the auriculotemporal nerve. In our study, we found that the application of TENS to both sides of the face on the parotid glands, in 15 min sessions once a week for three consecutive weeks, resulted in a significant reduction in the perception of mouth dryness (VAS-X and XI), and a significant increase in the production of salivary fluid. In addition, the quality of life tended to increase (OHIP-14), although this increase was not statistically significant. [PERSON] et al. [23] assessed electrostimulation devices in a clinical assay, showing a statistically significant increase in the production of saliva in the group of patients with Sjogren syndrome. These results suggest that TENS can stimulate functional residual glands with a profile of reduced secondary effects, as compared with pharmacological therapies. Nevertheless, the study did not explore the subjective impact of the symptoms or the quality of life. In another non-randomized clinical study in post-menopausal women with xerostomia associated to hormonal changes, conducted by [PERSON] et al. (2016) [16], the application of TENS found that 90% of the participants showed a significant increase in salivary flow, independently of the initial severity of mouth dryness. The study results suggest that the treatment may have a dose-dependent effect, as the VAS-X scores progressively decreased after each session. This underlines the need for a more in-depth assessment. After analyzing the protocols, a lack of consensus in the literature was found on the optimum duration and frequency of the TENS sessions for the treatment of xerostomia. In [PERSON] et al. [24; 25], the total duration of the treatment was 12 weeks, with two sessions per week, and each session lasting 20 min, while in [PERSON] et al. [26] and [PERSON] et al. [27], the authors did not specify the total duration of the treatment, only the duration of each session (5 min and 10 min, respectively). On the other hand, in [PERSON] et al. [28], the treatment lasted 4 weeks, with two 20 min sessions per week. The importance of adjusting the TENS parameters to optimize its benefits and minimize the possible adverse effects are underlined in diverse studies. [PERSON] et al. (2009) [29] performed a clinical study with 100 patients with xerostomia due to diverse causes, finding that in 85%, an increase in saliva production was observed after the application of TENS. Nevertheless, 4% indicated a decrease in salivary flow, which was attributed perhaps to the use of inadequate intensity parameters or frequency of stimulation, as the salivary reflex may be altered when the brain perceives a stimulus to be painful. Similarly, [PERSON] et al. (2022) [30] assessed hemodialysis patients affected by xerostomia, dividing the patients into two groups with different configurations of TENS. The results showed that group 1 (G1), with a configuration of 50 Hz and 250 us, obtained better results than group 2 (G2), configured with 2 Hz and 50 us. Based on these findings, a dose of 50 Hz and 250 us was used in the treatments in the present study. The criteria to define the intensity of the current in all of the studies was defined according to the comfort of the subjects. Only [PERSON] et al. [26] reported the intensity tolerated by the patient during the intervention, which was 38.8 \(\pm\) 7.5 mA. Our results show that the TENS therapy provides an average relief of 6 to 7 h after each treatment session, with a notable decrease in the subjective scores. This efficacy, together with the absence of secondary systemic effects, establishes the TENS therapy as a promising option as compared with conventional treatments, such as mechanical methods, sialogogue drugs, and saliva substitutes.