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Due to the popularity of the use of peppermint for digestive discomfort, its role on pain mediation has been studied more vastly. In fact, clinical trials have demonstrated decreased subjective pain in those with irritable bowel syndrome (IBS) using this oil.80,81 One mechanism suggested is related to the mediation of TRPM8, a transient receptor potential (TRP) cation channel, activated by menthol, a main constituent of found in peppermint essential oil. TRPM8 activation induces smooth muscle contractions inversely relational to temperature and also involves the initiation of Rho-kinase, leading to smooth muscle contraction. There is also some controversial evidence on menthol modulating intracellular calcium stores.
In one review article L-menthol was reported to potentially be involved in modulating arachidonic acid metabolism into LTB4 (leukotriene B4) and PGE2 (Prostaglandin E2), markers of the inflammatory pathways of lipoxygenase and cyclooxygenase, respectively.80
Combinations of Essential Oils
In a randomized trial that included 60 participants with a history of neck pain and a Neck Disability Index (NDI) score of 10%, the use of a 3% concentration of an essential oil cream was evaluated for efficacy in pain relief. The dose used was 2 g of the cream daily applied for four weeks. The cream consisted of four essential oils: marjoram, black pepper, lavender, and peppermint. An unscented cream was given to the control group. Measurements for effective outcomes post intervention, as compared with baseline, included a visual analogue scale (VAS), NDI, pressure pain threshold (PPT, which used a pressure meter in the midpoint between the seventh cervical vertebra and the acromion connection), and neck-joint range (via Motion Analysis System [MAS] at the occiput, C2, C7, and shoulder level).
An independent-sample t-test indicated that the groups significantly differed with regard to age only (p = 0.04). Within each group, there were nonsignificant differences in the number of pain episodes per week and intensity, duration, associated symptoms, location, and cause of pain. The study found that the VAS scores improved in both participant groups after the intervention; however, the NDI and PPT values were only improved with the experimental group. The authors stated that the VAS scores may be more of an indicator of subjective pain, whereas the NDI and PPT may be a better overall evaluation of chronic neck pain. Furthermore, neck range of motion in the experimental group with aromatherapy significantly increased following the intervention.82
Although there were some limitations in the trial, such as the short duration of the study and translation to those with more severe neck pain, the study demonstrated a significant difference in pain modulation with essential oils.
An experiment using a quasi-experimental design with a nonequivalent control group, pre- and posttest consisted of 40 subjects who were examined in regard to the role of aromatherapy in pain, depression, and life satisfaction. An essential oils blend of lavender, marjoram, eucalyptus, rosemary, and peppermint mixed with a carrier oil to a 1.5% dilution was used. The authors reported that aromatherapy significantly decreased scores for pain and depression for the experimental group compared with the control group, though there was no difference in life satisfaction reported.83 The article was in Korean, so unbiased conclusions could not be fully examined. Limitations may have included a small sample, the dilution technique of the oil, and that subjective physiological measures weren't examined.
In another nonequivalent Korean trial with 58 terminal cancer patients, the researchers assessed for changes in anxiety, depression, and pain when using hand massage with or without essential oils. The carrier oil used for both conditions was a sweet almond oil. The almond oil was used alone in the control group. In the experimental group, almond oil was mixed in an equal ratio of bergamot, lavender, and frankincense diluted to 1.5%. The hand massage was done for 5 minutes for 7 days. The authors found that the experimental group had a more significant difference in changes in pain and depression scores than the controls. Although the trial design has limitations, including generalizability, the subjective measurements of decrease in pain as compared with controls lend support for the pain-modulating effects of essential oils.84
In a controlled, randomized, double-blind clinical trial, the use of essential oil massage was evaluated for moderating pain in 48 subjects diagnosed with primary dysmenorrhea and who had a 10-point numeric rating scales that were more than 5. The control group received a synthetic fragrance in an unscented jojoba cream, whereas the experimental group consisted of a 2:1:1 ratio blend of lavender (Lavandula officinalis), clary sage, and marjoram (Origanum majorana) diluted in unscented jojoba cream at 3% concentration.
The cream was applied daily by the participants' massaging it on the lower abdomen starting from the end of their last menses to the beginning of the next. The dose was two 1 g spoonfuls daily. The verbal rating scale (VRS) and the numeric rating scale (NRS) were analyzed on the first to third days of the first menstrual cycle and post intervention on the same days of the second cycle.
The NRS, VRS, and duration of pain were significantly decreased in the essential oils group, whereas the control group did not experience a decrease in duration of pain after one menses cycle. The main analgesic components of the essential oils were determined to be linalyl acetate, linalool, eucalyptol, and beta-caryophyllene.85
Limitations include generalizing to other populations, the effects of massage alone on pain, and the potential estrogenic effects present in the synthetic cream (endocrine disruptors).85-88
Popular Essential Oils Categorized by Action
In a systematic review of essential oils used in aromatherapy, the authors categorized essential oil use in the following manner: cosmetic, massage, medical (treating diseases), olfactory (impacting mood and relaxation), and psychoaromatherapy (the psychological impact from aromas' effect on memories and emotions).
Within this context of respect for the multimodal actions of essential oils, 10 common essential oils were reviewed. The following were reported by the authors to modulate pain as follows89:
1. Clary sage: reduces cortisol, assists with menstrual periods, and eases tension and muscle cramps89,90
2. Eucalyptus: regulation of the nervous system relating to neuralgia, headache, and debility, treatment for joint and muscle pains (rheumatoid arthritis), and for muscle and joint pains and aches89,91
3. Geranium: sedative properties, nerve tonic, and may help the patient in coping with the pain in cancer89
4. Lavender: for the treatment of abrasions, burns, stress, headaches, muscle pain, and primary dysmenorrhea
5. Lemon: may help with labor pain, nausea, vomiting, and ulcers89,92
6. Peppermint: relief of pain spasms, arthritic problems, digestive pain and IBS, and inflammation
7. Roman chamomile: assist with modulation of "inflammation, muscle spasms, menstrual disorders, insomnia, ulcers, wounds, gastrointestinal disorders, rheumatic pain, and hemorrhoid."89,93
8. Rosemary: soothes menstrual cramps, contains the anti-inflammatory constituent 1-8 cineole
9. Tea tree: anti-inflammatory, and antimicrobial support in painful infections89,94
10. Ylang ylang: powerful sedative proper-ties, and useful for trauma and shock89,95
In regards to direct pain management, the authors listed the following oils:
Eucalyptus smithii (gully gum)
Lavandula angustifolia (lavender)
Matricaria recutita (German chamomile)
Leptospermum scoparium (manuka)
Origanum majorana (sweet marjoram)
Pinus mugo var. pumilio (dwarf pine)
Rosmarinus officinalis ct. camphor (rosemary)
Zingiber officinale (ginger)89
Bioindividuality in Odor Processing Experiences and Clinical Implications with Essential Oils
Due to the complexity of aromas consisting of potentially hundreds of molecules, humans tend to weave them into a conceptual whole. Specifically, the brain encodes odor stimuli and synthesizes them into a "unified perceptual experience."96 This unique experience of smells has implications for selecting aromas for different individuals for therapeutic use.
Specifically, it has been found that perceptions and environmental conditions associated with certain scents can vary from person to person. This means that the same smell could evoke differing emotional responses from one person to the next.33,34 Furthermore, one's internal physiology can also impact odor perception and response. This means that one's current state at the time of stimulus presentation may determine if it is deemed pleasant or unpleasant.97,98
For example, in one study, the scent of orange syrup was considered pleasant in a subject in the fasting state; however, after ingestion of a glucose load, the olfactory stimulus was deemed "unpleasant."97 Furthermore, personality can also bias how emotional information is processed, and people who are emotionally liable may be more reactive to unpleasant sensations.99
This means that perception does not occur in isolation based solely on the stimulus of one odorant. Rather, olfaction is intertwined into a diverse context of physiological conditions and also psychological states.97,98 All these unique factors suggest that it is important to determine emotional liability, personality, and the internal state of your patient when choosing an aromatic oil to modulate pain and other physiological effects.
Putting it more crudely, essential oils have the molecular mechanisms to modulate pain response. Do not cancel this desired response out with an olfactory stimulus that is unpleasant and could subsequently ignite the amygdala emotional responses involved in pain perception.
Gender and Pain
Another factor to be aware in response to odorant stimuli is gender. It has also been implicated in how one processes pain during states of emotional stress. Specifically, females tend to have more sensitivity to pain, respond differently to pain medication, and those with high anxiety states tend to have a lower threshold for pain stimuli.100,101
Therefore, using subjective and autobiographical memories modulated by volatile oils could be very helpful in individualized pain perception, but especially for females who are anxious.
In Part 1 of this article series, the complex interactions between odorants modulating emotional and physiological responses was reviewed. Specifically, the critical role of the amygdala's response to odorant stimuli, and the interwoven neurological overlap to its pain processing functions, were discussed. How this knowledge served in treating chronic pain was also established.
In this article, the biochemical, physiological, and psychological effects of essential oils were explored. Support for the role of physiological effects from the secondary metabolites modifying biochemical pathways was explained in order for the clinician to understand that the effect of aromatherapy is inclusive. Specifically, it supports but goes well beyond the effects of olfaction alone. The term pyschoaromatherapy may more precisely portray the impact of essential oils on various perceptual states, including mood and pain perception.
Finally, the importance of environmental association patterns, gender, and internal states was also discussed as modifying responses to odors. This means that those who implement essential oils must be aware of any potential biases against the smell they or their patients may have when initiating therapy. I have found it is best to match positive associations of an essential oil's aroma with its biochemical and stimulating or relaxing effect on the nervous system.
In conclusion, the use of essential oils is an ancient practice that is proving to be a powerful medicine. They may appear a simple tool, but these secondary metabolites are complex in medicinal actions which assist in modulating pain response. From a societal perspective, their more popularized use could help alleviate the chronic pain epidemic in a holistic and integrative manner, one whiff at a time.
Sarah LoBisco, ND, is a graduate of the University of Bridgeport's College of Naturopathic Medicine (UBCNM). She is licensed in Vermont as a naturopathic doctor and holds a bachelor of psychology degree from State University of New York at Geneseo. Dr. LoBisco is a speaker on integrative health, has several publications, and has earned her certification in functional medicine. Dr. LoBisco currently incorporates her training as a naturopathic doctor and functional medicine practitioner through writing, researching, private practice, and her independent contracting work for companies regarding supplements, nutraceuticals, essential oils, and medical foods. Dr. LoBisco also enjoys continuing to educate and empower her readers through her blogs and social media. Her recent blog can be found at www.dr-lobisco.com.
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