1. INTRODUCTION
Auricular Acupuncture (AA), also known as Auricular Therapy, is both a diagnostic method and a treatment used to relieve pain and alleviate addictions. The principles of AA are based on the Acupuncture of Traditional Chinese Medicine and upon Neuro-reflex therapy that were discovered in Europe [1]. An important basis in AA is the sensory correspondences between the areas of the pinna or tympanic membrane with other parts of the body, which are arranged in an inverted fetal pattern [2]. Auricular reflex points can be determined by measuring the skin resistor of points on the pinna or by observing physical changes in the auricular skin (such as color or protrusions while pressure is being applied [1]. AA is used for rapid relief of chronic pain and anxiety (effect felt within minutes of treatment). It can also be used to relieve the unpleasant symptoms of opioid withdrawal by reducing cravings for opioids [3].
The effects and biological mechanisms of AA on the human body have been increasingly observed in clinical and experimental studies, particularly the analgesic effects [1]. When the downstream neurotransmitter pathway is activated, endogenous opioids (beta – endorphins) which inhibit pain perception are released. Furthermore, according to the gate control theory (spinal segmentation mechanism), auricular acupuncture aids in the activation of pain-suppressive stimuli by myelinated afferent fibers (Aβ), as opposed to stimuli with damage from poorly myelinated (Aδ) or unmyelinated (C) fibers [4, 5, 6]. The trigeminal nerve, which is responsible for sensation in the face, has ramifications for the skin of the pinna [7]. The World Health Organization’s acupuncture map includes two points that are specifically listed as having functional effects on the maxillofacial region: the Tooth (LO1) and Jaw points (LO3) [8]. Referring to additional research worldwide, it can be seen that the Shen Men point (TF4), Sympathetic point (AH6a), and Adrenal gland point (TG2) on the pinna are commonly used in many diseases due to their effect on the autonomic nervous system [3, 9, 10, 11]. Clinical studies have shown that using AA on acupoints mentioned above has analgesic effects on facial pain relief [9, 10, 12]. However, previous studies mainly conducted interventions on patients with pain symptoms that can be explained based on the physiological basis of the disease related to the trigeminal nerve. The authors have not found any studies on healthy individuals to investigate whether acupuncture is truly related to the pain threshold in the facial area, which is governed by the trigeminal nerve. This research will provide a foundation for using acupuncture to treat facial pain caused by trigeminal nerve pathology. In addition, this is the first study in Vietnam to investigate the effect of AA on the facial pain threshold of healthy volunteers. Our finding suggests that AA can be used as a non-pharmacological adjunct to facial pain relief.
2. MATERIALS AND METHOD
This study was conducted in the Acupuncture Experimental Research Lab, Faculty of Traditional Medicine, University of Medicine and Pharmacy at Ho Chi Minh City from February 2021 to September 2022.
Study design: This was a pilot study with a crossover randomized controlled trial design. Crossover studies have two advantages over parallel studies and non-crossover longitudinal studies. First, the influence of confounding covariates is reduced because each crossover participant serves as their control. In a randomized non-crossover study, different treatment groups are often found to be unbalanced on several covariates. In a crossover randomized controlled trial design, such imbalances are unlikely (unless covariates were to change systematically during the study). Second, crossover designs are statistically more efficient and require fewer participants than non-crossover designs (including repeated measurement designs) [13]. Because pain threshold is subjective, each individual will have a different pain experience, making it difficult to conduct a parallel 2-arm study, and the crossover design helps eliminate this subjectivity.
Participants criteria included: Participants were between the ages of 18 and 30, with a BMI of 18 to 23 kg/m2, a heart rate of 60 to 99 beats per minute (bpm), a supine blood pressure of less than 140/90 mmHg, and no medical conditions. The participants had no prior exposure to auricular acupuncture. On the day of the AA, none of the participants reported a psychological stress issue (as evaluated by using Depression Anxiety Stress Scale-21 with stress point less than 15 points [14]).
Participant’s elimination criteria: Exclusion criteria included volunteers who were over the age limit, had acute diseases, used stimulants (coffee, alcohol, or cigarettes) within 24 hours of the study, exercised immediately before the study, or took heart rate or blood pressure medications within the previous month. Additionally, pregnant or menstruating females were excluded.
Criteria to stop research: Participants who expressed a desire to withdraw from the study or overreacted to parasympathetic stimulation symptoms at the stimulus site, such as dizziness, nausea, vomiting, and allergy, met the criterion. These incidents would be reported as adverse events.
The sample size n is calculated using the following formula:
Formula note: n is the sample size needed for each group in this study, and ES is effect size. ES value equivalent to Cohen’s D in the previous study is 0.51 [15]. The sample size required to detect an effect with power = 0.8 of standardized effect size (alpha = 0.05; beta = 0.20; C =7.85). Accordingly, the sample size for each group is 33, the total sample of the study is 66.
Participants were recruited, and the procedures were explained. Volunteers would sign an informed consent form prior to participating in the study.
In this study, after the informed consent forms were signed, participants were randomly assigned to two groups, each receiving a different set of treatments. Participants in group A received AA at TF4, AH6a, TG2, LO1, and LO3 points in the left auricle (phase 1), and one week later, sham acupuncture at the same points (phase 2). Participants in group B received AA at TF4, AH6a, TG2, LO1, and LO3 points in the right auricle (phase 1), and one week later, sham acupuncture at the same points (phase 2).
Device for measuring pain thresholds: The facial pain thresholds before and after AA or sham acupuncture were measured on both sides with the Multi-capacity Digital Force Gage FDIX of Wagner Inc (Newton Unit of Measurement).
Auricular acupuncture: We used the press needles (0.06 - 0.5mm diameter, 1.3-3.0mm length) for auricular acupuncture. For sham auricular acupuncture, we used pieces of tape that are similar in appearance to press needles. This sham acupuncture method was mentioned in Zhang’s study (2014) [16].
Participants were exposed to the environment for 10 minutes in order to stabilize their breathing rate, blood pressure, and heart rate, as well as to stop sweating [17].
The facial pain thresholds were recorded before and after performing AA or sham acupuncture. Table 1 describes the location of facial pain threshold survey points.
The acupuncturist has a medical doctor’s license and has been trained in acupuncture. Acupuncture was done at including Shenmen (TF4), Sympathetic point (AH6a), Adrenal gland points (TG2), Jaw (LO3), and Tooth (LO1). The location of acupoints was determined according to the World Federation of Acupuncture and Moxibustion Societies - WHO Regional Office for the Western Pacific in 2013 [8].
Both before and after acupuncture, auricles were cleaned with 70% alcohol, regardless of laterality.
In phase 1, the acupuncturist used 4 needles per patient (0.06 - 0.5mm diameter, 1.3-3.0mm length), with one needle for each acupoint. Every 5 minutes, the acupuncturist stimulated the acupoints by gently pressing down until a sensation of burning, soreness, numbness, distension, or heat was felt (this sensation is known as “de qi”) was felt for approximately 30 seconds. This was repeated 3 times while the volunteers rested. The needles were then removed.
In phase 2 (7 days later), the acupuncturist performed all the aforementioned routines except for using four pieces of tape instead of needles.
The auricular acupuncture process was described in Figure 2.
The facial pain thresholds on 9 points on both sides (see Table 1) before and after inserting needles was the primary outcome, quantitative variable, measured with the Multi-capacity Digital Force Gage FDIX of Wagner Inc (Newton Unit of Measurement).
3. RESULTS
There were no participants who dropped out or were excluded from the study. All 66 participants were recorded and their data were transferred to a blinded analyst. It took 40 minutes to finish one phase per participant and a total of 30 days for the whole study.
Table 2. General characteristics of participants included in each group at the start of the trial. There were no significant differences in sex, age between the experimental and control group (p > 0.05). The difference in basic characteristics of the experimental and control groups was not significant (t test, p > 0.05). Each participant’s heart rate, blood pressure, respiratory rate, SpO2, and BMI were within the normal range, which is necessary for safety participants.
The pain thresholds on the facial skin increased statistically significantly (p<0.05) after participants in group A received AA at TF4, AH6a, TG2, LO1, and LO3 points in the left auricle. In contrast, sham acupuncture did not significantly affect the facial pain thresholds. The results are described in Tables 3 and 4 below.
The pain thresholds on the facial skin increased statistically significantly (p<0.05) after participants in group B received AA at TF4, AH6a, TG2, LO1, and LO3 points in the right auricle. In contrast, sham acupuncture did not significantly affect the facial pain thresholds. The results are described in Tables 5 and 6 below.
4. DISCUSSION
The main finding of this study is that AA at TF4, AH6a, TG2, LO1, and LO3 of either the left or the right auricle significantly increased the pain threshold on both sides of the face (p<0.05).
This finding is consistent with the function of these acupoints as described in the World Federation of Acupuncture and Moxibustion Societies: AA in the Shenmen, Sympathetic point, Adrenal gland point, Jaw point, and Tooth point groups is effective in relieving pain and and anxiety [8].
Besides, the results of the study are consistent with the structure and main function of the trigeminal nerve, which is to provide sensation and inner movement to the face. Three branches on either side of the trigeminal nerve connect to various areas of the face. In the Meckel cave of the cranial cavity, these branches connect to the trigeminal ganglia. The ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves are the various branches [7]. There is limited research on the specific mechanism of action of auricular acupuncture on the trigeminal nerve, but some theories have been proposed. Auricular acupuncture involves the stimulation of specific points on the ear, which are believed to correspond to different areas of the body. One theory suggests that auricular acupuncture may modulate the activity of the trigeminal nerve by activating the vagus nerve. The vagus nerve has anti-inflammatory and analgesic properties, and it can modulate pain processing in the central nervous system [18]. Stimulation of the vagus nerve can also activate the release of endogenous opioids and other neuromodulators that can reduce pain perception and increase pain threshold [18]. Another theory proposes that auricular acupuncture may modulate the activity of the hypothalamic-pituitary-adrenal (HPA) axis, which is a pathway that regulates the body’s response to stress and inflammation. Acupuncture has been shown to affect the activity of the HPA axis, which can modulate pain processing in the central nervous system [19]. Overall, the mechanism of action of auricular acupuncture on the trigeminal nerve is likely similar to that of traditional acupuncture, involving the modulation of multiple pathways and systems in the central and peripheral nervous system [20, 21, 22]. However, further research is needed to fully understand the specific mechanisms underlying the effects of auricular acupuncture on the trigeminal nerve.
This study also has similarities with previous studies. According to Simmons and Oleson’s research, when applying AA, the threshold of dental pain increased statistically significantly (p<0.0001) when compared to the threshold of background tooth pain (dental pain threshold was determined using a hand-held dental pulp tester). When compared to the pre-intervention pain threshold, the average pain threshold increased by 18% (p<0.01), while the control group increased by only 0.85% [12]. In a group of patients with the temporomandibular joint disorder, the pain intensity (according to the VAS scale) decreased by 61% after 1 week and by 84% after 1 month of AA (p<0.01) [9]. Furthermore, in the same study, the group of participants who received AA improved significantly in lower jaw function and quality of life-related to mastication over time (p<0.01) [9]. Iunes’ study discovered pain relief in tender points in the mandibular posterior region (p=0.04) and the right side of the submandibular region (p=0.02) after AA, as well as pain relief with left lateral temporomandibular joint activity (p<0.01) [10]. The pain scores on the face pain scale (FPS-R) differed significantly between the two groups of dementia residents with acute pain in nursing homes. The AA group’s facial pain score was 1.84 ± 0.23, compared to 2.22 ± 0.81 in the sham AA group. Caregivers’ satisfaction and patient acceptance were significantly higher in the AA group than in the sham AA group [23].
New point
The above studies, however, have not yet comprehensively evaluated how AA affects the facial pain thresholds in healthy people. In addition, these studies only investigated the pain-relieving effects of AA in the left ear, without assessing the effect of AA in the right auricle. Meanwhile, our study looked at how the physiological pain threshold changed in healthy volunteers who received auricular acupuncture.
Conclusion
In conclusion, we examined changes in the facial pain thresholds after AA at TF4, AH6a, TG2, LO1, and LO3 of either the left or the right auricle in 66 healthy volunteers using a pilot study design. The results showed that AA at these points in either auricle increased the pain threshold of the facial skin statistically significantly (p<0.05). Our study is among the first to investigate the effects of AA on the facial pain threshold.
