Comparative evaluation of salivary flow rate in patients with oral submucous fibrosis before and after transcutaneous electrical nerve stimulation

Abstract

Background

Oral submucous fibrosis (OSMF) is a premalignant condition characterized by progressive fibrosis of the oral mucosa. It may extend to involve minor salivary glands, resulting in reduced salivary secretion and xerostomia. Pharmacological sialogogues which are traditionally used to manage hyposalivation are often associated with adverse effects, limiting their long-term use. Hence, safer non-pharmacological alternatives are warranted.

Aim

To evaluate and compare salivary flow rate (SFR) in patients with OSMF before and after Transcutaneous electric nerve stimulation (TENS) therapy.

Methods

A prospective interventional study, approved by the Institutional Ethics Committee (IEC) included clinically diagnosed 60 OSMF patients. Xerostomia was evaluated subjectively using the xerostomia inventory. Unstimulated SFR was measured using the Modified Schirmer Test (MST) at baseline. TENS therapy was administered using a standard protocol and post intervention SFR values were recorded at first visit and follow-up appointments. Pre and post SFR were statistically analysed to determine the effect of TENS therapy and correlated with OSMF grading.

Results

A statistically significant improvement in SFR was seen after TENS therapy across all grades of OSMF. However, the extent of improvement varied with disease severity, with Grade I showing maximum increase, followed by Grade II and Grade III, while Grade IV showed least improvement in SFR. Progressive improvement in SFR was noted with repeated stimulation.

Conclusion

TENS seems to be a safe, non-invasive and effective modality for enhancing salivary flow in patients with OSMF and could be considered a valuable adjunctive modality for managing OSMF associated hyposalivation.

Trial registration

Clinical Trials of Registry in India (CTRI/2024/06/068807), registered on June 12 2024.

Background

Oral submucous fibrosis (OSMF) is now considered a significant public health issue, especially in South and Southeastern Asian countries, in view of its close association with the habitual areca nut consumption. The etiology of OSMF is complex; but among several proposed causative factors, the most important one that remains consistently recognized is the habitual areca nut consumption. Areca nut contains biologically active compounds like arecoline, arecaidine and guvacoline which have the ability to trigger fibroblast proliferation, enhance collagen production and then prevent collagen degradation [1–3].

The adverse effects of areca nut chewing are not limited to oral mucosa but may extend to adjacent structures including major salivary glands. But the major salivary glands are not directly involved in the fibrotic process. However, their functional output may be secondarily reduced due to altered neural stimulation and reduced masticatory activity [4, 5].

Saliva is an essential oral fluid that is crucial for the preservation of oral health, although its importance is frequently underestimated. It performs multiple protective and functional roles in the oral environment, involving lubricating the oral tissues, shielding oral mucosa, pellicle formation, facilitation of digestion and having therapeutic properties. Saliva is composed of several enzymes, immunoglobulins, glycoproteins and antimicrobial peptides with antiviral, antibacterial, and antifungal characteristics [6]. When present in normal quantity and biochemical composition, saliva contributes significantly to the host defense system by safeguarding the oral cavity against various diseases. Any change in salivary flow negatively impacts saliva pH and buffering capacity, thereby increasing susceptibility to enamel demineralization. Hyposalivation or impaired taste buds are the causes of taste loss [7]. Furthermore, decreased salivary secretion predisposes individuals to several oral complications such as halitosis and dental caries, which commonly affects smooth surfaces and areas like the lower incisor region which are usually less prone to caries [8].

Hyposalivation in OSMF is primarily due to progressive fibrosis involving minor salivary glands resulting in ductal obstruction, acinar atrophy, reduced vascular supply and distorted neural innervation. These pathological alterations culminate in reduced salivary secretion and clinically manifest as xerostomia. This consequent impairment in oral lubrication adversely affects mastication, speech, deglutition, nutritional intake and overall quality of life (QoL) of affected patients. Therefore, effectively managing hyposalivation is an imperative therapeutic objective in the comprehensive treatment of individuals with OSMF [7, 9].

Traditionally, pharmacological sialogogues including sorbitol, xylitol and pilocarpine are used to increase salivation in hyposalivatory patients. But sialogogues are having adverse effects such as increased sweating, diarrhoea and cardiovascular problems among others which can limit their long-term use. Accordingly, other safer, non-pharmacological alternatives including Transcutaneous electric nerve stimulation (TENS) has been reported in the literature to treat hyposalivation in conditions such as postmenopausal, diabetes, postradiotherapy etc [10–12].

TENS is an easy, cost-effective, non-invasive and non-pharmacological technique. Traditionally limited to managing the pain, but the effect of TENS on salivary flow has not been thoroughly considered in the clinical studies. Components of the salivary reflex arc, including afferent sensory pathways, central salivatory nuclei and efferent parasympathetic fibers supplying the salivary glands are activated in an attempt to elucidate its underlying mechanism. Electrical stimulation of nerves, such as the auriculotemporal nerve, has been shown to increase parasympathetic activity, enhancing salivary secretion. In theory, the application of electric impulses to any one of these three components of the salivary reflex pathway may enhance salivary output and help to mitigate the complications associated with prolonged hyposalivation [10, 11].

Although a number of previous studies reported improvement in salivary flow in conditions like post-menopausal, radiotherapy patients with use of TENS evidence correlating the improvement in salivary flow in OSMF is sparse [10, 12]. This study was undertaken to assess and compare salivary flow rate (SFR) in OSMF patients before and after TENS therapy and to correlate improvement in SFR with OSMF gradings.

Materials and methods

This prospective hospital-based interventional study was carried out in Oral Medicine and Radiology department over a period of two years. A comprehensive study protocol was published earlier [13]. Ethical approval was taken from the “Institutional Ethics Committee” of Datta Meghe Institute of Higher Education and Research, Sawangi (Meghe), Wardha, (IEC- DMIHER(DU)/IEC/2024/224). All the participants were enrolled in the study after taking informed consent in written form.

Patients aged 18–60 years with a clinical diagnosis of OSMF, irrespective of gender and presence or absence of subjective xerostomia, were involved in the study. A detailed habit history was obtained from all participants, and only individuals reporting areca nut chewing habits for more than 6 months were included in the study. Patients above 60 years of age, those undergoing treatment for OSMF, individuals with salivary gland disorders, systemic diseases or psychological stress affecting salivation and those receiving medications known to alter salivary flow were the exclusion criteria. Also, the patients with a history of radiotherapy, psychological illness other oral mucosal lesions, denture wear, smoking, alcohol consumption or prolonged deleterious oral habits were excluded.

Sample size calculation

• n₁ = Sample size of Group 1

• n₂ = Sample size of Group 2

• σ₁ = Standard deviation of Group 1

• σ₂ = Standard deviation of Group 2

• Δ = Difference in group means

• κ = Ratio = n₂/n₁

• Z₁₋α/₂ = Two-sided Z value (e.g., Z = 1.96 for 95% confidence interval)

• Z₁₋β = Z value corresponding to study power

Mean salivary flow in stimulated saliva continuous mode = 1.55

Mean salivary flow in stimulated saliva in burst mode = 1.21

σ1 = SD of salivary flow in stimulated saliva continuous mode = 0.31

σ2 = SD of salivary flow in stimulated saliva in burst mode = 0.31

For detecting mean difference of 0.34 i.e. ∆ =1.55–1.21 = 0.34

Power of the Test: 80%.

Level of significance: 5%( 95% confidence interval) [14].

Grouping

According to the functional classification system established by Khanna JN and Andrade NN in 1995, the study participants were categorized into four groups, with 15 patients each in Grade I, Grade II, Grade III and Grade IV OSMF [15].

Standardization of salivary flow assessment

To reduce bias in results due to diurnal fluctuations, all assessments were performed between 09.00 and 11.00 am and participating individuals were not allowed to eat or drink two hours prior to the appointed time. Each participant was assessed for hyposalivation by subjective and objective method [16].

Subjective assessment of xerostomia

Subjective xerostomia was evaluated using Xerostomia inventory (XI), an 11-item questionnaire intended to evaluate the intensity of dry mouth symptoms. Responses were counted on a 5-point Likert scale, with higher cumulative scores indicative of greater severity of xerostomia [17]. The XI was used as a screening tool for subjective xerostomia.

Objective assessment of SFR

Unstimulated SFR was measured by Modified Schirmer Test (MST) [16, 17]. Salivary flow was assessed using the MST and the results were expressed as wetting length in millimeters (mm) throughout the study. Baseline values represented unstimulated salivary flow, whereas post-treatment measurements represented stimulated salivary flow immediately following TENS application. The patient was instructed to swallow all saliva in their mouth before the procedure, to stop swallowing throughout it and to rest for three to five minutes. In order to avoid the test strip from coming into contact with the tongue during the process, the patient was instructed to place their tongue on their hard palate. The MST strip was placed in a vertical position using a cotton plier and rounded end of the strip was positioned at the floor of the mouth (Fig. 1). Saliva moved up the strip when its round end comes into touch with wetness, and wetness of strip was measured at three minutes (Fig. 3A). Baseline unstimulated salivary flow was recorded prior to the intervention and recorded in a master sheet.

Fig. 1.

Placing the Modified Schirmer strip on the floor of mouth while tongue resting on palate

Fig. 3.

A and B Comparison of measurement of MST wetting before (12 mm) andafter TENS therapy (17 mm) in OSMF patient

TENS intervention

After baseline salivary assessment, TENS therapy was administered to evaluate its effect on salivary flow. A 4-channel digital TENS unit (LCS-184DX, Life Care Systems, India) was used in continuous mode with a square-wave output at a frequency of 40 Hz for 15 min per session. Pulse width information was not reported in the manufacturer’s technical documentation and therefore could not be specified. The intensity was adjusted according to patient tolerance (0–99 mA output range). Surface electrode pads (approximately 6.5 cm diameter) were placed bilaterally over the parotid gland region at standardized anatomical locations anterior to the ears, secured with an elastic belt, and positioned consistently for all participants throughout the study (Fig. 2).

Fig. 2.

Patient positioning and placement of the electrode pads during TENS therapy

Post intervention salivary assessment

After TENS therapy- Again MST was done to measure stimulated SFR in OSMF patients following the same standardized protocol (Fig. 3B). This procedure was repeated over four consecutive appointments to assess cumulative or sustained changes in SFR.

Statistical analysis

All the data were collected into tabular format and statistically analysed using descriptive and inferential statistics. Changes in salivary flow across repeated appointments were evaluated using repeated measures ANOVA, and paired t-tests were used for pre- and post-treatment comparisons. The analysis was performed using SPSS 27.0 software version and a p value < 0.05 was considered statistically significant.

Results

The patients in this study were between the ages of 18 and 58 years, with a mean age of 37.66 ± 10.22. The age group of 38–47 years contributed to the largest percentage of patients (33.33%), followed by age group of 28–37 years old (31.67%), 18–27 years age group (18.33%) and least number of patients were observed in the 48–58 years age group (16.67%). Out of 60 patients, 46 (76.67%) were males and 14 (23.33%) were females, showing a clear male predominance among the patients diagnosed with OSMF.

Patients were equally distributed across the grades of OSMF in four groups. Amongst all, fifty-seven patients (95%) reported the presence of xerostomia, while 3 patients (5%) did not suffer from xerostomia.

A statistically significant increase in salivary flow was observed at each visit following treatment when compared with pre-treatment values as shown in Table 1. The fourth appointment showed the maximum improvement, with mean salivary flow increasing from 25.28 ± 2.86 mm to 27.83 ± 3.18 mm with mean difference of 2.55 ± 1.19 mm (p = 0.0001). Overall, the results indicate a progressive and statistically significant improvement in salivary flow following TENS therapy across consecutive appointments, as confirmed by repeated measures ANOVA (Table 1, Fig. 4).

Table 1.

Grade wise comparison of mean difference in MST wetting (mm) before and after TENS therapy across four appointments in OSMF patients

OSMF Grade	1st Appointment (Mean ± SD)	2nd Appointment (Mean ± SD)	3rd Appointment (Mean ± SD)	4th Appointment (Mean ± SD)	p-value
Grade I	2.66 ± 1.11	3.10 ± 0.92	3.60 ± 0.88	4.00 ± 0.75	0.0001*
Grade II	1.53 ± 0.83	1.98 ± 0.85	2.30 ± 0.89	2.60 ± 0.91	0.0001*
Grade III	1.40 ± 0.63	1.72 ± 0.65	1.98 ± 0.66	2.20 ± 0.67	0.0001*
Grade IV	1.06 ± 0.79	1.18 ± 0.70	1.30 ± 0.65	1.40 ± 0.63	0.0001*

*Repeated measures ANOVA showed significant differences across appointments for both pre-treatment (F = 4540.28, p < 0.001) and post-treatment (F = 4445.44, p < 0.001) salivary flow measurements

Fig. 4.

Comparison of MST wetting (mm) at pre-treatment 1st appointment (baseline) with post treatment 4th appointment in all patients (60 patients)

However, on correlating the effect of TENS with OSMF grading, Grade I OSMF patients demonstrated the greatest response with mean difference in SFR increasing progressively from 2.66 ± 1.11 mm at the first appointment to 4.00 ± 0.75 mm at the fourth appointment. Grade II patients showed a consistent improvement, with mean difference rising from 1.53 ± 0.83 mm to 2.60 ± 0.91 mm over four sessions. Although patients with Grade III OSMF exhibited a statistically significant increase in salivary flow at all appointments, the magnitude of improvement was comparatively lower ranging from 1.40 ± 0.63 mm to 2.20 ± 0.67 mm. Grade IV OSMF patients also showed a significant stimulatory response to TENS therapy. Nonetheless, the extent of improvement was limited with mean difference increasing from 1.06 ± 0.79 mm at the first appointment to 1.40 ± 0.63 mm at the fourth appointment. Overall, the results demonstrate a grade dependent response with greater improvement observed in early stages of OSMF and progressively reduced response in advanced stages. (Table 1).

Inter-appointment comparison of post treatment SFR revealed a progressive and statistically significant increase in SFR across all four appointments. A significant difference was observed between the first and second appointments (mean difference = 0.88 ± 0.49 mm; p = 0.0001). The comparison between the first and third appointments demonstrated a higher increase (mean difference = 1.61 ± 0.76 mm; p = 0.0001), while the first and fourth appointments showed the maximum improvement in SFR (mean difference = 2.35 ± 1.14 mm; p = 0.0001). Similarly, statistically significant difference was observed between the second and third appointments, second and fourth appointments and third and fourth appointments (p = 0.0001), indicating a consistent increase in SFR with repeated TENS therapy.

Amongst xerostomia patients, almost similar observations are noted as there was a statistically significant increase in SFR values at all four follow-up sessions following treatment compared with values before treatment. Mean SFR showed gradual improvement from 1.04 ± 0.13 mm at baseline to 1.16 ± 0.15 mm at the fourth visit (p = 0.0001), confirming the efficacy of TENS in improving salivary flow in xerostomia patients (Table 2).

Table 2.

Comparison of MST measurements (mm) before and after TENS therapy among participants reporting xerostomia (57 patients)

Appointment	Pre Test	Post Test	Mean Difference	t-value
First Appointment	23.63 ± 2.78	25.26 ± 3.10	1.04 ± 0.13	11.77,p = 0.0001,S
Second Appointment	24.01 ± 2.82	26.15 ± 3.05	0.81 ± 0.10	19.91,p = 0.0001,S
Third Appointment	24.63 ± 2.77	26.91 ± 3.03	0.94 ± 0.12	18.31,p = 0.0001,S
Fourth Appointment	25.10 ± 2.82	27.63 ± 3.13	1.16 ± 0.15	16.35,p = 0.0001,S

Discussion

The present study meticulously evaluated alterations in salivary flow and the therapeutic effect of a well-known physiotherapeutic intervention TENS to increase salivation in patients with OSMF. With respect to age distribution, demographic characteristics of this study population revealed a preponderance of middle-aged individuals, with a mean age of 37.66 ± 10.22 years. This finding is comparable to the previous studies reported by Dyasnoor et al. and Passi et al. They observed that OSMF predominantly affects individuals in third to fifth decades of life. This age group is most frequently affected by OSMF due to prolonged use of areca nut-containing products [1, 18]. On the contrary, the study by Panda et al. found higher prevalence of OSMF in a younger group, i.e. 15–25 age range [19].

In this study, 57 patients (95%) reported xerostomia. Here the findings are comparable to the previous studies by Gupta et al., Khader et al. and Zope AB et al. The reasons for reduced salivation in OSMF are well documented in the literature [3, 8, 20].

As demonstrated by Pindborg and Sirsat, the fibrosis of the submucosa may extend around the salivary glands ducts, leading to ductal narrowing and obstruction and thereby interfering with normal salivary outflow, contributing to hyposalivation in OSMF patients [21]. Tilakaratne et al. highlighted the role of cytokine mediated fibrosis and ischemic changes in salivary gland and thus xerostomia in OSMF patients [2].

Irrespective of the pathophysiological mechanism underlying the salivary hypofunction, the results of the current study clearly showed that TENS was effective in increasing salivary secretion. These findings are supported by Ingle et al. who confirmed a strong correlation between increasing stages of fibrosis in OSMF and progressive minor salivary gland involvement, reduced mouth opening and increased oral dryness thereby explaining the reduced baseline salivary flow observed in advanced stages and comparatively limited response to stimulation [5].

The action of TENS in improving SFR is mediated through the auriculotemporal nerve which carries secretomotor fibres to the parotid gland. TENS activates afferent sensory pathways that transmit impulses to salivatory nuclei located in the medulla oblongata. Thus, activating parasympathetic reflex and resulting in improvement of salivary secretion. Low-frequency stimulation, such as 40 Hz used in the current study, preferentially stimulates the parasympathetic fibers, leading to the secretion of thin, watery, serous saliva, which is highly useful in patients with xerostomia. This neurophysiological mechanism has been uniformly stated in studies by Konidena et al., Paim et al., and Salimi et al., who all found that TENS therapy significantly improved SFR [22–24].

Ramadoss et al. reported significant improvement in salivary flow among geriatric patients with xerostomia following low-level transcutaneous electro-neurostimulation. Although the study population differed from the present OSMF cohort, their findings support the role of neuromodulatory electrical stimulation as a non-invasive method for enhancing salivary gland function [25].

In the present study, irrespective of the grades of OSMF, all the patients with xerostomia showed significant rise in salivary flow following TENS treatment. Similar improvements in salivary flow following electrical stimulation have also been reported by Yang et al. in patients with xerostomia undergoing hemodialysis [26].

Farooqui et al. reported a significant increase in salivary flow following electrical stimulation in OSMF patients. Similar findings were observed in the present study, where repeated TENS sessions resulted in progressive improvement in salivary flow measurements [27].

A statistically significant rise in salivary flow was found to occur after TENS during each of the four treatment sessions, thus establishing the presence of both immediate and cumulative stimulation. Comparison between appointments for post-treatment values also supported an incremental improvement in SFR. The mean difference between the first and second post-treatment appointments was 0.88 ± 0.49 mm, while the difference between the first and fourth post-treatment appointments was 2.35 ± 1.14 mm. These results indicate that the impact of TENS is more than just a short-term effect but rather a cumulative effect on the salivary glands.

Similar cumulative changes have been reported by Tabrez et al. who observed a 38.5% increase in salivary flow following repeated TENS sessions in diabetic patients. These findings further support the sustained neuromodulatory effect of electrical stimulation [28].

Deshpande et al. also demonstrated improved salivary flow following neuromodulatory stimulation in OSMF patients. Despite differences in stimulation techniques, both the present study and that of Deshpande et al. support the role of non-invasive stimulation in enhancing salivary gland function [29].

Long-term benefits of electrostimulation have also been reported by Strietzel et al. and Alajbeg et al., suggesting the potential for sustained improvement in salivary function [9, 10]. Further studies with longer follow-up are required to evaluate the long-term efficacy of TENS in OSMF patients.

Although there was a positive and statistically significant effect of TENS therapy on SFR in all grades of OSMF, improvement in SFR was relatively lower in the advanced stages of OSMF. This is consistent with the outcomes of Ingle et al., which showed that the advanced stages of OSMF are associated with higher salivary gland fibrosis and ductal involvement, thus reducing the functional reserve of the salivary gland tissue [5].

Overall, the findings of the present study, when considered in the context of the existing literature, support the effectiveness of TENS as a non-invasive adjunctive modality for improving salivary flow. While it may not be possible to completely normalize salivary function in all patients, particularly in more advanced pathological states, even small increments in salivary secretion can be of immense benefit. It is well known that the improved salivary flow enhances oral lubrication, facilitate mastication and swallowing, reduce xerostomia-related discomfort, and contribute to improved oral health and quality of life in patients with OSMF. Therefore, the observed statistically significant increase in salivary flow also carries potential clinical relevance.

Limitations

The study was limited by the absence of a control group, relatively small sample size, and short follow-up period. Salivary flow was assessed using the MST rather than conventional volumetric sialometry. In addition, factors such as duration and frequency of areca nut use, habit cessation status, and duration of OSMF were not evaluated and may have influenced the findings. Further randomized controlled studies with larger sample sizes and longer follow-up are warranted.

Conclusion

The present study has shown a statistically significant and progressive increase in SFR with TENS in patients with OSMF. The progressive increase in SFR after each treatment session suggests an immediate and cumulative stimulatory effect, despite the presence of fibrotic changes. Although the SFR did not return to normal in all patients, the degree of improvement was significant and suggests that neuromodulatory stimulation can counteract the salivary hypofunction associated with oral submucous fibrosis to some extent. Based on the current study, TENS can be considered a promising modality warranting further controlled investigation for increasing salivary secretion in patients with OSMF.

Abbreviations

OSMF

Oral Submucous Fibrosis

SFR

Salivary Flow Rate

TENS

Transcutaneous Electrical Nerve Stimulation

XI

Xerostomia Inventory

MST

Modified Schirmer Test

IEC

Institutional Ethics Committee

SPSS

Statistical Package for the Social Sciences

SD

Standard Deviation

QoL

Quality of Life

Hz

Hertz

Authors' contributions

TT: Conceptualization, Literature search, Manuscript writing, review, and editing, statistical analysis. SD: Supervision, Manuscript writing, review, and editing. RS: Manuscript writing, review, and editing.

Funding

Open access funding provided by Datta Meghe Institute of Higher Education and Research. No funding was received.

Data availability

All data related to this manuscript is available with the author on request.

Declarations

Ethics approval and consent to participate

Ethical approval was taken from the “Institutional Ethics Committee” of Datta Meghe Institute of Higher Education and Research, Sawangi (Meghe), Wardha, (IEC- DMIHER(DU)/IEC/2024/224). The study was conducted in accordance with the ethical standards of the institutional and national research committee and with Declaration of Helsinki and its later amendments or comparable ethical standards. Written informed consent to participate was obtained from all of the participants prior to their inclusion in the study.

Consent for publication

Written informed consent was obtained from all participants for publishing clinical data and any identifying images included in the study.

Competing interests

The authors declare no competing interests.