INTRODUCTION

Since 1978, the World Health Organization (WHO) recommends the insertion of complementary and alternative medicine (CAM) or integrative and complementary practices (ICP) in public health systems (e.g., Unified Health System, SUS). In Brazil, ICPs increased following the approval by the Ministry of Health (MS) in 2006 to include non-pharmacological and more natural therapies, such as Tai Chi, Qigong (Lian Gong), Yoga, Mat Pilates, workplace exercises, therapeutic exercises, manipulative resources, acupuncture, and meditation¹. In 2017 there was an increase in ICPs used, and auriculotherapy (AT) was included. Since AT can be applied at several levels of health care, such as basic, specialized, and hospital, the proposal is to prevent health problems, to promote recovery, health, and the non-abandonment of the conventional medicine treatment^2-5. Given the effectiveness and low cost of AT, the Federal University of Santa Catarina, in partnership with the MS, promoted throughout Brazil the “Training in Auriculotherapy for Primary Care Health Professionals,” enabling and promoting the integration of this resource to ICP⁶.

AT, ear acupuncture and ear acupressure are synonymous with centuries-old therapy via stimulation of the pinna to relieve pathological conditions in the body. AT has two main lines of reasoning that explain its principles, the French school (Paul Nogier) and the Chinese school (Traditional Chinese Medicine - TCM)⁷. In 1957, in France, AT was driven by the cartography proposed by Paul Nogier, scheming an inverted fetus in the ear as a somatotopic map representing reflex parts of body stimulation. Theorizing that symptoms and diseases are projected in specific regions in the ear, as it is one of the few anatomical structures formed by endoderm, mesoderm, and ectoderm (three embryonic layers), which may hypothetically represent all parts of the body⁸. This triggered the study of its neurophysiological basis of action and its recognition by WHO in 1987, which identifies it as an acupuncture microsystem capable of intervening in the body as a whole. The standardization of an international nomenclature took place in 1990, with updates of such information occurring to the present day, merging the two principles (French and Chinese)^9-11. In addition to Paul Nogier’s view, for more than 2000 years, AT has been dated in Asian culture and explained by the regulation of “vital energy” (Qi), which circulates through the meridians and collateral channels. When there is an imbalance of a person’s Qi, he/she becomes vulnerable to disease, and AT would be able to harmonize such flow by minimizing symptoms (TCM concept)^7,9,10. AT treatment may be isolated or in combination with another intervention (e.g., kinesiotherapy), temporary, aiming at discharge, or referral to exercise groups¹.

Pain, in its various forms of manifestation, is a common cause of medical consultations and other health professionals (e.g., physiotherapists), and chronic pain is prevalent in the Brazilian population equal to or greater than worldwide (10.1-55.5%), representing one of the biggest challenges for public health^12,13. It impacts the functionality, quality of life, productivity, generates economic damage (personal, family, business and government), as well as the indiscriminate use of drugs and thus their possible adverse effects. Therefore, complementary therapies are part of the list of pain control options, as they have lower risk, lower cost, and are less invasive than the usual approach^10,11,14.

The concepts described serve as a starting point for AT studies. However, by focusing on the care of painful conditions, the analysis of its relationship with pain modulation should be understood. Neurophysiologically, stimuli at the nerve endings of the pinna are transmitted via the spinal and cranial nerves (peripheral nervous system, PNS) to the central nervous system (CNS), releasing neurotransmitters that regulate the control mechanisms of endogenous pain. When activated, the descending neural pathway releases endogenous opioids (endorphins) in the posterior horn of spinal cord (PHSC), making it difficult for the CNS to spread and perceive the pain stimulus (pain descending inhibitory pathways, extra segmental or supraspinatus mechanism). Another mechanism of nociceptive modulation is the so-called Gate Control Theory (segmental or spinal mechanism), which transmits non-painful stimuli via myelinated afferent fibers (Aβ), as opposed to harmful stimuli from poorly myelinated (Aδ) or unmyelinated (C) fibers balancing painful sensation in PHSC^15,16. Both routes are the most attributed to AT, aiming to justify their effects in pain conditions. Inflammation control would be linked to points in the anatomical region of the cavum conchae (e.g., lung 1 and 2), which stimulates the vagus nerve by releasing acetylcholine, a neurotransmitter that inhibits the release of tumor necrosis factor-alpha (TNF-α, proinflammatory cytokine) by macrophages, thereby minimizing inflammation (cholinergic reflex)^17-19.

AT points can be stimulated in a variety of ways such as seeds (mustard or rapeseed), acupuncture needles (facial or systemic), magnetic pellets, semi-permanent needles, electrophototherapy (laser or transcutaneous electrical nerve stimulation (TENS) and the fingers themselves. However, the seeds are low cost and have the advantage that patients self-stimulate the points and thus have been the most used^7,10. The seeds should be stimulated three to four times a day, for a minute or until the site becomes sensitive, with weekly changes upon reassessment of the case. However, there are reports of staying with the same application for up to one month, and the total treatment time ranges from 2-10 weeks^7,10,14. A systematic review with meta-analysis concluded that AT might be effective in relieving acute and chronic pain, reducing its intensity within the first 48 hours of treatment initiation, and being a safe resource¹¹. Another study describes pain remission time ranging immediately up to 6 months¹⁰. Although there are systematic reviews^7,8,10,11,14 addressing AT and pain, they did not propose a unified database of possible site choice, their possible combinations, or describe the location of such sites, thus becoming the goal of this study. In addition to mentioning the main sites indicated for painful cases, the aim was to identify their locations, facilitating the clinical practice.

CONTENTS

In order to achieve the proposed objective, a systematic review of reviews that addressed AT and pain was chosen (Overview). PEDro, Pubmed, Scielo and LILACS databases were accessed in February 2019. The keywords and Boolean indexes were used as follows: Auriculotherapy AND pain; ear acupuncture AND pain, ear acupressure AND pain; auricular therapy AND pain; auricular medicine AND pain. These words should be present in the title or abstract for the articles to be selected. If in doubt, the studies were fully verified. Filters were applied seeking only systematic reviews that were in humans, without restriction on the date of publication. Two evaluators made the selection and assessment by the Assessment of Multiple Systematic Reviews (AMSTAR)⁸ tool, discussing the score in case of divergence. Although revisions on the theme are not an unpublished subject, the report of the points or which were the most used are not always described, making clinical practice and methodological reproduction difficult^8,11. Therefore, books were also consulted to provide the best description of the use of points, as well as their location. Table 1 shows the results of the selection in the databases, and table 2 shows the studies with the application of the AMSTAR tool in order of the highest score.

Figure 1 shows the subdivision of the pinna necessary to understand and facilitate the interpretation of the description of the sites in table 3. It is noteworthy that there are normal anatomical variations from person to person, so at first, the identification of the structures, as well as the search for specific sites, require practical training⁹.

Figure 1 Anatomical division of the pinnaSF = scaphoid fossa; TF = triangular fossa; UC = upper conchae or cymba; LC = lower conchae or cavum; IN = intertragical notch. 

Table 3 shows the main points for the relief of painful conditions according to the unification of several studies, and figure 2 shows where they are anatomically^{6,10,14,22-25}.

Figure 2 Location of the main analgesic pointsMR = muscle relaxation; L1 = lung 1; H = heart; L2 = lung 2. 

This does not mean that all should be applied in a single session but rather selected according to the combinations already described and added to the painful area to be treated (Figure 3).

Figure 3 Somatotopic map of the reflex region affected and to be chosen to treat 

For example, ShenMen, Kidney, Sympathetic (auriculocybernetics) + lumbar (affected region - AR). However, a systematic review demonstrated the possibility of choosing 15 different points only for the treatment of low back pain, with ShenMen and subcortex being the most frequent⁷. This demonstrates considerable variability among the studies analyzed, as it is suggested to select about 4-6 treatment points²⁶. Therefore, it is impracticable to propose associations for each painful case in this format; a book would be needed to address this objective since more than 200 AT points were identified⁸. In any case, some previously used combinations serve as the basis, such as the described auriculocybernetics itself or the following combinations: ShenMen, thalamus, lung + affected region, somatotopic correspondence of reflexively compromised body area (AR)¹⁴; ShenMen + AR (10); thalamus, analgesic + AR)²³; ShenMen, subcortex + AR⁷.

There have been many reported conditions where AT can help to relieve the pain. Some of these indications are as follows: 1) cancer-associated pain, knee arthroscopy, femur fracture, hip arthroplasty¹⁴; in dysmenorrhea, postoperative pain, hip fracture, low back pain, bone marrow aspiration, acute and chronic pain¹⁰; 2) spinal pain, lumbar sciatic pain, cramps, stiff neck, fibromyalgia, rheumatic pain, phantom pain, amputation stump pain, herpes zoster, pain after fractures in general, trigeminal neuralgia, toothache, headache, migraine and tension headache²³; 3) chronic low back pain, muscle spasm, whiplash injuries, traumatic pain, inflammation after joint sprains, osteoarthrosis, pelvic and abdominal pain, shoulder impingement syndrome, adhesive capsulitis, bursitis, lateral epicondylitis , carpal tunnel syndrome and joint pain²².

Usichenko, Lehmann and Ernst²¹ did not state which AT points were applied in the clinical trials analyzed and their results in statistical terms, which hinders the interpretation and reproducibility of new research. This study was conducted over a decade ago, and others with the same methodological design came later, being more careful. Murakami, Fox and Dijkers¹¹ highlighted the fact that AT has results as good as their comparative groups, has temporary adverse effects and is less degrading than medications (pain in the site, which can make it difficult to sleep, skin irritation, slight bleeding, dizziness, and nausea) and the application is quick and accessible. However, they expected a more significant reduction in pain. Asher et al.¹⁴ came to a more positive conclusion than previous authors, stating superior pain minimization results when AT was compared with a control group or placebo compared to the same comparison made with systemic acupuncture. Contradicting another systematic review, which shows that AT was not superior to the placebo group and that its effects begin to diminish three months after the end of its application, they nevertheless described it as promising and capable of reducing pain²⁰. Jiang et al.¹⁸ found a positive and lasting effect of AT on pain and that adverse effects are insufficient for patients to abandon treatment. In order to resolve these doubts, studies such as Moura et al.²⁷, with 110 participants, treatment group, placebo, and control, should be encouraged, as they allow reliable conclusions to be based.

The justification for the effects of AT seems to be linked not only to the penetration of the needle into the pinna but to the choice of ideal sites¹⁴. This is an extensive discussion, but four possible explanations are elucidated: (1) AT acts by a mechanism other than systemic acupuncture; (2) action similar to acupuncture, which would activate meridians, regularization of organ function, Qi and Blood, with consequent normalization of painful pathways (TCM); (3) hypersensitive reflex neuronal pathways that connect the auricular microsystem to the corresponding somatotopic region in the brain, which through the spinal cord reaches the corresponding painful area; (4) AT does not depend on specific points, but rather on the stimulated region^7,10,14,20. The fourth explanation comes from the stimuli in the cavum conchae region, innervated by the vagus nerve, to be able to induce parasympathetic stimulation. Therefore, analgesia would be caused by the application site and not by the selection of points. That is, AT can function via the central pain control mechanism. However, if the analgesia provided is by specific points or stimulated region, it remains under discussion¹⁴. What is known is that auricular stimulation is a scientifically validated method, even by functional non-invasive magnetic resonance imaging of brain neuromodulation¹⁷. The possibility of acting by central descending pain inhibitory mechanism was reinforced by the fact that the effects of AT are blocked by the use of the opioid antagonist naloxone. AT would still be able to increase pain tolerance. Therefore, there is variability in the explanation that indicates the action of AT in pain, but it shows the interest of the scientific community in this microsystem, making it the most studied⁷.

Yeh et al.¹⁰ also reported significant pain relief with AT compared to the control or placebo group, noting that the quality of the studies was moderate to high. One of the difficulties with AT effectiveness reported by these authors would be the limited number of placebo group studies, only 32% of studies involving AT and pain. The patient expectation about the treatment, the relationship with the therapist, and the placebo effect itself may affect the results obtained with AT, but they are present in any other treatment modality. The ShenMen point and the reflex points corresponding to the affected region are the most commonly used in practice, according to the authors mentioned, information that corroborates the findings in table 2.

In short, the need for blind, randomized, sample-based clinical trials to define the number of participants should be encouraged when investigating the effectiveness of any treatment method, as well as the review of reviews. The possibility of using this study design, review of reviews, on a specific theme (AT and pain), demonstrates the interest of the scientific community in the topic. So far, the conclusions are cautious, but the systematic reviews that have been included have shown high methodological quality (8-10/11 - AMSTAR) and agree on the following aspects: AT is an adjunct technique to be used to manage pain; to reduce the use of analgesic drugs, minimizing tolerance and adverse effects. It is a treatment with low risk, low cost, and easy administration^{7,10,11,14,20,21}.

CONCLUSION

AT has favorable results regarding its effects on pain (although its mechanisms of action continue to be studied), showing to be promising as adjunctive therapy to conventional treatment. The ShenMen point, reflex points corresponding to the site in the affected body, and cavum conchae stimulation (e.g., lung point), seems to be the most favorable combination for better pain relief results.