Literature Review of Research on Chronic Pain and Yoga in Military Populations

Background: Although yoga is increasingly being provided to active duty soldiers and veterans, studies with military populations are limited and effects on chronic pain are largely unknown. We reviewed the existing body of literature and provide recommendations for future research. Methods: We conducted a literature review of electronic databases (PubMed, PsychINFO, Web of Science, Science Citation Index Expanded, Social Sciences Citation Index, Conference Proceedings Citation Index—Science, and Conference Proceedings Citation Index—Social Science & Humanities). The studies were reviewed for characteristics such as mean age of participants, sample size, yoga type, and study design. Only peer-reviewed studies were included in the review. Results: The search yielded only six studies that examined pain as an outcome of yoga for military populations. With one exception, studies were with veteran populations. Only one study was conducted with Operation Enduring Freedom (OEF) or Operation Iraqi Freedom (OIF) veterans. One study was a randomized controlled trial (RCT). Four of the five studies remaining used pre/post design, while the last study used a post-only design. Conclusions: Studies on the use of yoga to treat chronic pain in military populations are in their infancy. Methodological weaknesses include small sample sizes, a lack of studies with key groups (active duty, OEF/IEF veterans), and use of single group uncontrolled designs (pre/post; post only) for all but one study. Future research is needed to address these methodological limitations and build on this small body of literature.


Introduction
Rates of chronic pain in the military are alarmingly high, ranging from 25% to 82% among active duty and veteran populations [1][2][3][4][5]. Data on veterans from the nationally representative National Health Interview Survey reveal high rates: 65.5% reported being in pain. Particularly concerning is that 9.1% of veterans reported severe pain [6]. The prevalence of severe pain was striking for younger veterans who served in Operation Enduring Freedom and Operation Iraqi Freedom (OEF/OIF) in Afghanistan and Iraq. Active duty service members also experience high rates of chronic pain; one study reported a prevalence of 44% after combat deployment [5]. that 36% of active duty personnel reported using at least one CIH modality in the past 12 months, yet only 6.8% reported using yoga or some other form of movement therapy during that same span [32]. Even as yoga classes have become more common at VHA facilities targeting chronic pain management, only a small number of veterans use these services [29]. This stagnation in the use of yoga suggests a need to revise not only the content of the military's pain management model, but also the way in which that model is promoted to veterans and active duty soldiers.
Given the military's promotion of yoga to treat chronic pain, research is needed to test its effectiveness among active duty service members and veterans. To date, a review has not been completed of the effectiveness of yoga on pain in military populations. Such a review is needed to inform future research.

Methods
We conducted a literature review of the following electronic databases through April 2017: PubMed, PsychINFO, Web of Science, Science Citation Index Expanded, Social Sciences Citation Index, Conference Proceedings Citation Index-Science, and Conference Proceedings Citation Index-Social Science & Humanities. The search included a range of studies, from single group observations to randomized controlled trials. We used the following search terms: yoga, military, veteran, active duty, and chronic pain. Inclusion criteria included instruction of yoga, use of quantitative measures that assessed the effect of yoga on pain, and publication in a peer-reviewed journal. Studies that included only qualitative data were excluded. In addition, we focused our review on yoga studies that included movement, and excluded studies that focused only on breathing exercises, including mindful breathing. We recognize that other types of yoga, such as Sudarshan yoga, consist of controlled breathing techniques [33]. We also excluded yoga nidra, or iRest [34], from this review. Yoga nidra consists of simple breathing exercises and noticing, without attempting to change, different parts of the body; and choosing a sankalpa-a phrase or short sentence of a goal or intention to manifest in one's life [35]. More than 50 veterans' hospitals and military installations across the United States are implementing iRest [36], although there are limited studies of iRest in military populations.

Results
Tables 1 and 2 delineate the variables that were extracted from each of the studies: demographics (participant age, sex, race/ethnicity, marital status, and employment status), sample size, yoga (content, duration, and frequency of sessions), and methods and results (study design, dependent variables, and general findings). This search yielded six studies that fit our criteria. Although our primary focus was on military populations with chronic pain, we broadened our search to include studies that were conducted in military populations who did not necessarily have chronic pain but that did measure pain outcomes.  Note. PTSD, posttraumatic stress disorder.
The tables summarize the studies; we delineate the results, strengths, and weaknesses of each study in this section. Three of the publications are by Groessl and colleagues [37,38,42]. Two of the papers are from the same study, with a greater number of enrollees over time. The 2008 paper [37], which used a single-group, pre/post design, reported on the effect of Anusara yoga on pain, depression, fatigue, and quality of life (mental and physical) among veterans experiencing chronic low back pain. Significant decreases were seen in pain as well as in depression and fatigue. The study is notable in its inclusion of measures of depression, fatigue, and quality of life -domains that are related to chronic pain. Groessl's second study [38] builds on the first with an expanded sample of subsequent enrollees. The study examined gender differences in the effects of the Anusara yoga program. As compared to men, women reported significantly greater decreases in pain, depression, and fatigue, as well as greater improvements in mental health-related quality of life. A strength of the study is the larger sample size than Groessl's earlier study. However, this larger sample size is tempered by the study's focus on gender differences, with only 13 women participating. The third study was an RCT conducted with veterans with chronic low back pain [42]. Significant differences were seen between the yoga and the control groups for reduced pain at 6-month follow-up, as well as for reduced pain intensity at 12 weeks (immediately after completion of the yoga sessions) and at 6-month follow-up. This is the only study that used the gold standard, an RCT. The study is also notable in its focus on back pain. Multiple studies have found that back pain is one of the most common pain conditions [43,44]. To illustrate, data from the population-based National Health Interview Survey reveal that half of veterans reported back pain [6].
Groll and colleagues [39] carried out a study of yoga with active and retired members of the Canadian armed forces. Participants with PTSD reported significant pre/post differences, with decreased depression and anger and increased mental health-related quality of life after participating in the program. The PTSD group also exhibited a trend toward decreased pain. By contrast, findings for participants without PTSD were nonsignificant for all outcomes, including pain. A strength of the study is that it examined findings among study participants who had PTSD as well as those who did not. As has been stated, chronic pain is highly comorbid with PTSD [4,[7][8][9][10]. Another strength is assessment of multiple outcomes that are comorbid with chronic pain, although the primary focus of the paper was on PTSD. A weakness is the small sample number of those who did not have PTSD (n = 14), which may explain the lack of findings for these participants. This study was conducted with Canadian participants, whereas the other studies reported here were U.S. samples.
King, Beehler, and Wade [45] studied a yoga intervention for older veterans who were cancer survivors. The study used a pre/post design. There were no significant pre/post changes for any outcomes: pain, anxiety, insomnia, fatigue, or PTSD. A strength of the study is its wide array of outcomes addressing comorbid conditions (anxiety, PTSD) and quality of life (insomnia, fatigue). Another strength is adaptation of yoga to the age of the participants (mean age 66 years) and their medical conditions. Specifically, the physical poses were completed in a chair, the pace of the intervention was slowed, and facilitators spoke loudly. A weakness of the study is its small sample size (N = 15), which likely precluded finding significant effects.
Schulz-Heik's study [41] compared yoga across two conditions: in person and via telehealth. Participants were not randomized to condition. Participants reported on whether yoga led to improved mental health and physical functioning. Only post-data were collected. There were no significant differences between the two conditions on any of the outcomes (back pain; headaches; upset stomach; constipation or diarrhea; sleep difficulties; low energy level; irritability; angry outbursts; difficulty concentrating; depression; anxiety; exaggerated startle response; and repeated, disturbing memories). More than 80% of participants reported decreased pain, depression, and anxiety and increased energy level. One weakness of the study is the wide variability in the number of sessions (range from 1 to more than 21 sessions). An additional weakness is that data were collected only after the yoga intervention. The study is innovative in its examination of yoga via telehealth. Given the long distances that some veterans must travel to a Veterans Affairs health care facility, this modality has the potential to increase access to yoga.

Discussion
Chronic pain is a serious problem in active duty and veteran populations and presents treatment challenges. A well-established body of literature underscores that rates of chronic pain among active duty and veteran populations are exceedingly high [5,6]. Chronic pain also affects quality of life, emotional functioning, and coping strategies. In addition, chronic pain negatively affects relationships and physical and social functioning [46,47]. Treating chronic pain is compounded by its high co-occurrence with PTSD [8]. Individuals with both chronic pain and PTSD have more frequent health problems, greater pain-related disability, higher ratings of pain, and increased functional impairment [48]. Further compounding the treatment of chronic pain is the use of opioids. Despite their widespread use, opioids have limited effectiveness and can lead to abuse or dependence [19].
The military is promoting the use of CIH strategies, including yoga, to address chronic pain. DoD's Pain Management Task Force recommended the use of CIH modalities to treat chronic pain [27]. Research on CIH strategies is being conducted, but very little is known about the effectiveness of these approaches for chronic pain in military populations. The NIH's National Center for Complementary and Integrative Health (NCCIH), the DoD, and the Department of Veterans Affairs have developed a partnership [49] that includes funding from both the NCCIH and the DoD for research on mind-body interventions, including yoga, to treat chronic pain. This partnership has the potential to move the field forward in determining the effectiveness of mind-body interventions with active duty soldiers and veterans in treating chronic pain. Research is particularly needed with younger OEF/IEF veterans who served in Afghanistan and Iraq, for whom the prevalence of severe pain is particularly high.
Research on the effectiveness of yoga with active duty and veteran populations to treat chronic pain is in its infancy. Our review identified only six studies, only one of which was an RCT. The remaining studies were pre/post (four studies) or post-only (one study) designs. Despite this limited literature base, these studies contain several strengths. In addition to pain, the studies assessed a wide array of outcomes. These measures consider conditions that co-occur with pain, such as insomnia [4,50] and PTSD [9]. Studies also included both sexes. One study looked at yoga delivered via telehealth, a modality that provides greater access than in-person administration.
The current body of literature also has several limitations. Sample sizes were small, and all but one of the studies were conducted with veterans; the other was conducted with a combination of veterans and active duty soldiers. Two studies compared two yoga conditions (yoga in person or by telehealth; men and women). However, the small sample sizes in these studies likely precluded finding significant differences. In addition, in one study, participants could attend as many or as few classes as desired. With one exception, studies used uncontrolled designs.

Implications for Research and Practice
The studies in this review inform directions for future research. Particularly needed are RCTs with larger sample sizes to have sufficient statistical power to detect change. It is striking that we found no studies that included only active duty service members. Active duty service members likely experience and receive treatment for more recent chronic pain. As such, their response to yoga may differ from veterans whose chronic pain is not as recent. In addition, only one of the studies included OEF/IEF veterans. More than 2 million troops were deployed to Iraq and Afghanistan [51]; the estimated number of active duty service members is over 1 million, with an additional 801,000 in reserve components [52]. In addition, as noted, chronic pain is highly comorbid with PTSD [4,12]. It is possible that a more complex, trauma-informed yoga intervention is needed to reduce both chronic pain and PTSD, given that these conditions mutually maintain each other. Specifically, chronic pain may aggravate PTSD symptoms in terms of increased perceived pain, and disability. Similarly, the physiological and affective components of PTSD may amplify chronic pain symptoms [53].
We further recommend including measures of chronic pain in studies testing yoga with other conditions (e.g., PTSD) to increase the body of research on the effects of yoga.
A major practice consideration is integration of yoga in military settings. A recent report on CIH services in the military health system indicated that most military treatment facilities (MTFs) offered at least one type of CIH service [54]. Nonetheless, as noted previously, only 6.8% of active duty personnel reported using yoga in the last year [32]. Barriers to CIH service provision included a lack of available providers, support by the facility, provider awareness and interest, and space. We recommend that attention be given to the integration of yoga into MTFs, including increasing awareness among providers and incorporating yoga into chronic pain treatment protocols [55].

Conclusions
In conclusion, our literature review yielded only a small number of studies that tested the effects of yoga on pain in active duty or veteran populations. Sample sizes were small, and with one exception, studies used nonrandomized pre/post or post-only designs. Studies that have greater methodological rigor, such as RCTs, and that include a wide array of outcomes related to comorbid conditions are needed. Despite these limitations, this small body of literature sets the stage for future research with increased methodological rigor. Taken as a whole, results indicate that yoga shows promise for active duty and veteran personnel who experience pain. Consideration can be given to working with MTFs to integrate yoga into routine recommended care for chronic pain patients seeking treatment.