Do Mindfulness-Based Interventions Reduce Pain Intensity? A Critical Review of the Literature

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1 bs_bs_banner Pain Medicine 2013; 14: Wiley Periodicals, Inc. REVIEW ARTICLE Do Mindfulness-Based Interventions Reduce Pain Intensity? A Critical Review of the Literature Keren Reiner, MA, MSc, Lee Tibi, BA, and Joshua D. Lipsitz, PhD Department of Psychology, Ben Gurion University of the Negev, Beer Sheva, Israel Reprint requests to: Joshua D. Lipsitz, PhD, Department of Psychology, Ben Gurion University of the Negev, POB 653, Beer Sheva 84105, Israel. Tel: (972) ; Fax: (972) ; joshual@bgu.ac.il. There are no conflicts of interest for any of the authors. Abstract Background. Mindfulness-based interventions (MBIs) emphasizing a nonjudgmental attitude toward present moment experience are widely used for chronic pain. Although changing or controlling pain is not an explicit aim of MBIs, recent experimental studies suggest that mindfulness practice may lead to changes in pain tolerance and pain intensity ratings. Objective. The objective of this review is to investigate the specific effect of MBIs on pain intensity. Methods. A literature search was conducted using the databases PUBMED and PsycINFO for relevant articles published from 1960 to December We additionally conducted a manual search of references from the retrieved articles. Only studies providing detailed results on change in pain intensity ratings were included. Results. Sixteen studies were included in this review (eight uncontrolled and eight controlled trials). In most studies (10 of 16), there was significantly decreased pain intensity in the MBI group. Findings were more consistently positive for samples limited to clinical pain (9 of 11). In addition, most controlled trials (6 of 8) reveal higher reductions in pain intensity for MBIs compared with control groups. Results from follow-up assessments reveal that reductions in pain intensity were generally well maintained. Conclusions. Findings suggest that MBIs decrease the intensity of pain for chronic pain. We discuss implications for understanding mechanisms of change in MBIs. Key Words. Mindfulness; Mindfulness-Based Interventions; Acceptance; Pain; Review Introduction Over the past three decades, mindfulness-based interventions (MBIs) have gained increasingly wide use for the treatment of chronic pain conditions [1]. Mindfulness emphasizes a detached observation, from one moment to the next, of a constantly changing field of objects [2], meaning that cognitions, perceptions, and emotions occupying awareness at the present moment are observed without judgment and without attempts to change or control them. A substantial body of research supports the benefit of MBIs for chronic pain. However, much of this research assesses global distress, functioning, and quality of life [1,3]. Recently, a few reviews and a meta-analysis have focused on the benefits of MBIs specifically for chronic pain [4,5]. However, these consider a range of outcomes in addition to pain intensity. To date, there has been no systematic review focusing specifically on the effect of MBIs on pain intensity. Core Features of Mindfulness Although MBIs are far from homogenous, they are characterized by three core features. The first is to observe the reality of the present moment by attending to objective qualities of present experience or situation existing in one s inner or outer world [6,7]. The second is to maintain one s attention to a single aspect of awareness and accept it as it is without acting, judging, or elaborating on its implications [6 8]. The third is to remain open to everything that is currently salient without attachment to any particular point of view or outcome [6 9]. These three features have also been described in terms of a twocomponent model, in which one component involves attention to the present moment, and the second component involves an attitude of acceptance and openness [8]. Technically, mindfulness is defined as a set of techniques designed to encourage deliberate, non-evaluative contact with events that are here and now [10]. As such, 230

2 Mindfulness-Based Interventions and Pain Intensity a range of approaches that encourage this mental stance may be considered mindfulness-based. MBIs for Pain There are currently several MBIs that have been shown to be beneficial for pain conditions. The best researched among these are mindfulness-based stress reduction (MBSR) [2] and acceptance and commitment therapy (ACT) [11]. In addition, researchers have studied other MBIs for chronic pain, including mindfulness meditation and emotion regulation therapy [12] and the Breathworks mindfulness-based pain management program [13]. Originally designed to improve self-management of chronic pain [2,14], MBSR was guided by the rationale that the practice of mindfulness would lead to a spontaneous uncoupling of the sensory component of pain from the cognitive and emotional components and, by doing so, reduce the amount of suffering caused by pain [2]. In MBSR, individuals learn the principles of mindfulness by practicing yoga and different meditations such as breathing meditation or body scan meditation. In addition, participants are encouraged to implement mindfulness into their daily living. A recent meta-analysis investigating the effects of MBSR for adults with chronic medical conditions (including chronic pain) concludes that evidence supports the efficacy of MBSR for reducing distress and disability in this population [3]. ACT [11] is also offered for chronic pain, as well as for a broad array of psychological problems. Acceptance methods combine mindfulness practice with work on personal values, behavioral commitment, and traditional behavior change strategies to help live a fuller life [15]. Core principles of ACT are introduced using metaphors, paradoxes, and experiential exercises [16]. As such, formal meditation is considered to be only one of multiple ways to acquire mindfulness skills. Several studies have shown that ACT is efficacious for chronic pain [15,17,18]. The Association Between Mindfulness and Pain In contrast to some other therapies, in which the explicit goal is to control and reduce intensity of pain, mindfulness offers an alternative perspective, suggesting that being mindful without necessarily changing the intensity of pain may have a therapeutic value of its own. From this perspective, MBIs target the psychological correlates of pain with the explicit goal of improving functioning and distress in general rather than reducing the intensity of pain [19,20]. Consistent with this approach, many studies of MBIs have emphasized broader outcomes such as quality of life, well-being [21 23], and psychological distress [1,3,24]. The degree to which MBIs may directly influence pain intensity has not been a primary focus of research. Recently, evidence from correlational and experimental studies suggests that mindfulness may affect pain intensity. For example, correlational studies find that higher levels of mindfulness are associated with lower pain intensity ratings in chronic pain populations [25 27]. Furthermore, a recent neuroimaging study by Grant et al. [28] showed that highly experienced Zen meditators had lower levels of pain sensitivity compared with controls, and that this was associated with thicker cortex in pain-related brain areas, indicating increased activity. Although causal direction cannot be inferred from these studies, findings are intriguing and challenge the notion that MBIs bypass pain intensity and exert their influence on broader mental stance only. Experimental studies of healthy participants further support the effects of MBIs on pain intensity. Zeidan et al. [29] found that 3 days of mindfulness meditation training lead to reduced sensitivity to pain, indicated by lower pain intensity ratings for predetermined electrical stimuli. Kingston et al. [30] showed that compared with guided imagery, participants receiving mindfulness training showed increased pain tolerance in the cold pressor procedure. Experimental studies also show that acceptance strategies lead to reductions in pain ratings and increases in pain tolerance [31 34]. Two recent reviews consider the effects of MBIs for chronic pain [4,5]. Findings from a review by Chiesa and Serretti [5] suggest that MBIs have a nonspecific effect on pain and depressive symptoms. Based on a meta-analysis, Veehof et al. [4] conclude that MBIs yield benefits for both pain and depression. However, despite describing the primary outcome as pain or pain intensity ([4], p. 534), both of these analyses included pain measures that combine pain intensity and other dimensions, such as pain interference (e.g., the Medical Outcomes Study Short Form-36 items [SF-36] bodily pain scale) [35] and pain-related affect (e.g., the McGill Pain Questionnaire) [36]. It is impossible to determine the degree to which improvement on these complex scales reflects change in pain intensity per se vs change in these other related dimensions. As such, it is not possible to draw conclusions regarding effects of MBIs on pain intensity. Another limitation of these two reviews is that they included studies of chronic pain only. As such, no consideration is given to the possible impact of MBIs on pain intensity in more diverse populations. Considering the experimental findings earlier in samples without clinical pain and the high prevalence of pain symptoms among undiagnosed individuals (e.g., [37]), it is important to consider whether MBIs may reduce pain intensity in other populations as well. The objective of this report is to systematically review evidence of the effect of MBIs on pain intensity ratings in clinical trials. To provide a broad context for this effect, we will examine all studies in which pain intensity was assessed before and after an MBI. We have organized the review into uncontrolled and controlled studies. Uncontrolled studies may provide evidence of pain reduction but cannot support a specific association with MBIs. Evidence 231

3 Reiner et al. from controlled trials would support a specific association. In addition to immediate outcome, follow-up results are included when available. Method Literature Search An electronic search was conducted in the databases PUBMED and PsycINFO from 1960 to December Databases were searched for English language articles using the following key words: mindfulness / mindfulness meditation / insight meditation / mindfulness based stress reduction / MBSR / acceptance based / acceptance and commitment therapy combined with Pain anywhere in the record. Reference lists from the obtained articles as well as reference list of recent mindfulness reviews were examined for additional articles. Two reviewers (K.R and L.T) independently assessed whether the studies met the inclusion criteria (see later) and interrater reliability was calculated to ensure consistency (kappa = 0.89). Disagreements were resolved by discussion with the third reviewer (J.L). Inclusion Criteria Studies were included if they utilized a standardized MBI teaching mindfulness techniques and examined its effect on pain intensity ratings from pretreatment to posttreatment. We considered interventions to be standardized if they included a minimum of six hourly sessions, and in the case of MBSR [2] or ACT [11], followed the basic curriculum of these treatments. Additional inclusions criteria were: 1) outcome measures assessed pain intensity using standardized scales, 2) data were available from at least 10 participants in the treatment group, and 3) participants were at least 18 years of age. Exclusion Criteria In order to limit our review to clinically relevant pain, we did not include experimental studies in which pain intensity was measured using laboratory-induced pain. Unpublished dissertations and conference presentations were excluded, as were studies addressing non-mindfulness forms of meditation, such as transcendental meditation. Pain Intensity Measures Consensus recommendations [38,39] advocate assessing pain intensity in clinical trials based on unidimensional scales. To this end, are asked to rate the intensity of their pain from e.g., no pain to worst pain possible on a visual or numerical scale with two end points representing these extremes. These measures are used in many, but not all trials investigating treatment effects on pain [40]. For this review, we included only studies for which unidimensional pain intensity ratings were available (either as a unidimensional scale of intensity or a single item or subscale of a more comprehensive scale). Table 1 summarizes all pain measures used to evaluate change in pain intensity in the reviewed studies. Quality Assessment Assessment of quality of the included studies was based on criteria recently presented by Moore et al. [41]: 1. Sequence generation: Was allocation to condition adequately randomized? 2. Allocation concealment: Was allocation undertaken independently and blind? 3. Blinding of assessors: Were assessors blind to the condition? 4. Incomplete outcome data: Were incomplete data adequately addressed? 5. Size: Was there sufficient statistical power to detect treatment effects? 6. Duration: Were there at least 3 weeks between pre and post assessments? Quality was considered high when at least five criteria were met, medium when 3 5 criteria were met, and low Table 1 Measure Pain intensity measures in the reviewed studies Description No. of Studies Unidimensional scales for pain intensity ratings Numerical rating A scale with 11, 21, or 101 point where the end points are the extremes of no scale (NRS) pain and worst possible pain. Visual analog A 10-cm line anchored by two verbal end points ( no pain and worst possible scale (VAS) pain ). Unidimensional items for pain intensity ratings Linear analog self-assessment scale (LASA)* Brief Pain Inventory (BPI) A multiscale instrument measuring 12 domains of well-being, among them is a single scale relating to pain severity. Pain intensity is rated on a scale of 0 ( as bad as it can be ) to 10 ( as good as it can be ). Two-scale instrument measuring pain intensity and the influence of pain on different aspects of daily life. Pain intensity items are rated on a numerical scale from 0 ( no pain ) to 10 ( worst pain imaginable ) * Higher scores represent less pain. 232

4 Mindfulness-Based Interventions and Pain Intensity when two or fewer criteria were met. Quality of studies was assessed by two independent raters. Disagreements were resolved through discussion. Analytic Strategy We report results in terms of significance (when available) and in terms of mean percentage change in pain intensity from pre to post intervention, calculated as: (group mean pre intervention group mean post intervention)/group mean pre intervention 100. Following Dworkin et al. [42], we consider changes between 10 20% as small, between 20 50% as moderate, and above 50% as substantial. For studies that provided sufficient data, effect sizes were calculated using Cohen s d. Following the recommendations of Rosenthal et al. [43], pre-post effect sizes were calculated for uncontrolled trials by subtracting the mean pretreatment pain score from the mean posttreatment pain score divided by the pooled standard deviations. For the controlled studies, we calculated controlled effect sizes. According to Cohen [44], effect sizes of 0.2, 0.5, and 0.8 are considered as small, medium, and large, respectively. All effect sizes were corrected for small sample bias as recommended by Hedges et al. [45]. Reasons for rejections Stage 1 (33) 1.Studies not published in English (1) 2.Studies were not published investigations, e.g. dissertations or chapters (32) Stage 2 (77) 1. Studies not including standardized MBI (11) 2. Studies not including pre-post pain intensity ratings (39) 3. No detailed results on pain intensity ratings (19) 4. Studies including experimental pain paradigm (8) Stage 3 (7) Stage 1 Total search results yielded 133 abstracts Stage 2 Selected abstracts 100 Stage 3 Selected abstracts 23 Results We retrieved a total of 133 studies in the computerized search. Only 16 trials met the inclusion criteria described earlier. Most of the excluded studies did not include prepost pain intensity ratings or did not provide detailed results on pain intensity ratings (58 studies). Specific reasons for exclusion are presented in Figure 1. Characteristics of Included Studies Sample size in the reviewed studies ranged from 14 to 228 participants, with a total of 1,404 participants. Eleven of the 16 studies (68.7%) included only chronic pain. Of the 11 studies including only chronic pain, six consisted of with heterogeneous pain diagnoses and five included homogenous samples with specific pain conditions (fibromyalgia three studies, rheumatoid arthritis one study, chronic back pain one study). The remaining five studies did not limit their sample to participants with pain conditions. Among these studies, two included normal undiagnosed participants, one study included mixed population with both physical and psychological diagnoses, one study included nursing home residents with mixed diagnosis, and one study included healthy pregnant women. The mean age of participants in the reviewed studies ranged from 30.4 to 85 years, with an overall mean of 48.1 years. Most studies had a majority of women ( %); two studies included women only. Three studies had roughly even numbers of men and women (ranges from % men). Based on assessment criteria, one study was rated as high quality and two studies were rated as medium quality. The remaining studies were all rated as low-quality studies (score of two or less). 1.Studies reporting data analysis regarding less than 10 treatment group participants (5) 2.Participants' minimum age was under 18 (2) We examine first uncontrolled studies, in which change in pain intensity was reported from pre to post intervention, and then controlled studies for which change in pain for the MBI group is compared with change in the control group. Uncontrolled Trials Eight of the 16 studies are uncontrolled trials (Table 2). Five of these studies employed MBSR [2] or an intervention based on MBSR with some revisions, and three studies used the ACT program [11]. Pre-Post Change in Pain Intensity Selected papers 16 Figure 1 Process of studies selection. MBI = mindfulness-based interventions. Three of the eight uncontrolled studies demonstrate a significant decrease in pain intensity ratings from baseline, with medium effect sizes. These studies investigated the effect of ACT on chronic pain [46 48] and report significant, although small reductions in pain ratings (mean percent change between 13 17%). A fourth study by 233

5 Reiner et al. Table 2 Uncontrolled trials grouped by participants diagnosis Results: Pain Intensity Reduction Study Type of Participants Mean Age (SD or Range) % Male Treatment Group N* Attrition (%) Pain Measures Pre-Post Differences Effect Size (d) (Mean % Change) FU Results Quality of Study Vowles et al. (2007) [48] Vowles & McCracken (2008) [46] Vowles & McCracken (2010) [47] Kaplan et al. (1993) [49] Flugel Colle et al. (2010) [21] McBee et al. (2004) [50] Chang et al. (2004) [51] Beddoe et al. (2009) [52] Chronic pain Chronic pain Chronic pain 44.2 (11.4) 37.7 Variant of ACT 47.3 (11.4) 35.8 Variant of ACT NRS P < (13%) Pain reduction maintained for 3 months (d = 0.48) NRS P < (17.1%) Pain reduction maintained for 3 months (d = 0.57) 46.1 (10.0) 35.8 ACT NRS P < (15.7% pre FU) Fibromyalgia Variant of MBSR Pain reduction maintained for 3 months VAS NA (8%) None Low Mixed diagnosis 46.7 (9.4) 12.5 MBSR LASA NS 0.42 (10.5%) None Low Nursing home residents 85 (68 92) 7.1 Variant of MBSR 14 NA VAS NS 0.12 (6.6%) None Low Not diagnosed 46.5 (12.1) 42.9 MBSR NRS NS (8% increase) Healthy pregnant women Variant of MBSR BPI NS 2nd trimester: 0.08 (6.25%) 3rd trimester: (25.5% increase) Medium Low Low None Low None Low * N = Number of participants after attrition. Percentages represent mean percentage change in pain, calculated as ([pre post]/pre 100). Insufficient data to calculate. Median age. ACT = acceptance and commitment therapy; BPI = Brief Pain Inventory; FU = follow-up; LASA = linear analog self-assessment; MBSR = mindfulness-based stress reduction; mixed diagnosis = mental and physical diagnosis; NA = not available; NS = nonsignificant; NRS = numerical rating scale; SD = standard deviation; VAS = visual analog scale. 234

6 Mindfulness-Based Interventions and Pain Intensity Kaplan et al. [49] investigated the effects of an MBSRbased intervention on fibromyalgia. The authors report that 65% of participants showed some improvement. However, mean percentage reduction in pain intensity was only 8% (no data are provided on the significance of decrease). Four of the eight uncontrolled studies failed to find a significant reduction in pain intensity following MBIs. A common feature of the samples in these studies is that they were not limited to participants with pain conditions. For example, Flugel Colle et al. [21] examined the impact of MBSR on participants with mixed diagnosis, of which 68.7% suffered a physical or psychological disorder, and the rest had no clinical diagnosis. The authors report that following treatment, changes in the intensity of pain were nonsignificant, although there was a trend toward improvement. Similarly, in a study investigating the effect of an MBI on nursing home residents with mixed diagnoses, pain reduction was nonsignificant [50]. Two of the studies which failed to find a significant change in pain intensity from baseline to post MBI included only healthy subjects. Chang et al. [51] did not find significant changes in pain intensity ratings following MBSR treatment for non-diagnosed participants. A second study [52] investigated the impact of mindfulness on pain intensity in a sample of healthy pregnant women who reported suffering from pain complaints pretreatment. Following an intervention of mindfulness-based yoga combined with the curriculum of MBSR, there was no significant reduction in pain intensity. Follow-Up of Uncontrolled Studies Three of the said studies included follow-up of treated participants. These studies were based on ACT and employed a 3-month follow-up period [46 48]. In these studies, follow-up assessments demonstrated a significant reduction in pain intensity from pretreatment to follow-up (between %), indicating sustained benefits of ACT in these trials. Summary of Uncontrolled Trials The results from the uncontrolled studies indicate significant reductions in pain intensity ratings following MBIs in studies investigating samples with chronic pain. In contrast, mixed and normal samples failed to show benefits for MBIs with respect to pain intensity. This may be due to a floor effect as mean levels of pain were not high in these groups prior to treatment. Results from follow-up investigations show that the immediate reductions in pain intensity of chronic pain following MBIs were generally sustained over a period of 3 months. While overall results suggest that MBIs do reduce pain intensity for participants with chronic pain and that these benefits are maintained in the short term, open trials such as these do not provide evidence of a specific association between mindfulness practice and pain reduction. It is possible that nonspecific aspects of treatment such as expectation of improvement or passages of time may be involved (e.g., [12]). It is important, therefore, to examine the impact of MBIs on pain reduction in controlled trials to determine whether there is a specific effect on pain intensity. Controlled Trials Eight of the qualified investigations are controlled designs (Table 3). The most commonly used MBI in these studies was MBSR or a modified intervention based on MBSR (five studies). Other studies included acceptance-based treatment for chronic pain [53], mindfulness meditation, and emotion regulation therapy [12], Breathworks mindfulnessbased pain management program [13], and mindfulnessbased chronic pain management course [54]. Three of the controlled studies compared an MBI with a non-active wait-list condition [53 55]. Three studies used an active control group such as treatment as usual [13,56], or social support, relaxation, and stretching exercises [57]. The remaining two studies were comparative trials, comparing MBIs with cognitive behavioral therapy (CBT; with an additional education-only control group) [12], or cognitive behavioral stress reduction [58]. It is noteworthy that allocation to an MBI or control group was randomized in only two of the controlled studies (Table 3). In the remaining studies, allocation was based on the order the participants attended the research or the location of participants. In one study, allocation was made based on the participant s choice of treatment [58]. Seven of the eight controlled studies reported a significant reduction in pain intensity following an MBI. The mean percent change in pain ratings in these studies varied from 8.1% to 49.4%. Controlled Comparison for Pain Intensity Reduction When comparing the effect of MBIs on pain intensity to control groups, six of the eight studies report significantly greater reductions in pain intensity for the MBI groups compared with controls (mean change in pain intensity for MBI groups ranged from 11.8% to 49.4%). Considering first comparison with inactive controls, all three studies found greater pain intensity reduction for the MBI group compared with control, with medium effect sizes. Goldenberg et al. [55] found that reduction in pain intensity was significantly greater for the MBI group compared with waiting list control group (14.4% reduction in mean pain intensity for the MBI group, data regarding change in pain intensity for controls are not available). Similarly, McCracken et al. [53] report significantly greater reduction in pain intensity after an ACT-based intervention compared with waiting phase (18.2% reduction in ACT group compared with 2.7% reduction in the waiting interval). Gardner-Nix et al. [54] also found that participants in a mindfulness-based course taught in person or through videoconference experienced significantly lower pain intensity levels at the end of the intervention compared with the wait-list control group. 235

7 Reiner et al. Table 3 Controlled trials grouped by research design Results: Pain Reduction Study Research Design Type of Participants Mean Age (SD or Range) % Male Treatment Group Control Group N* Rand Assn. Attrition (%) Pain Measures Effect Size (d) MBI > Control Mean % Change in MBI Group FU Results Quality of Study McCracken et al. (2005) [53] Goldenberg et al. (1994) [55] Grossman et al. (2007) [57] Gardner-Nix et al. (2008) [54] Cusens et al. (2009) [13] Esmer et al. (2010) [56] Zautra et al. (2008) [12] Smith et al. (2008) [58] Pre-pre-post (repeated measures) Chronic pain 44.4 (10.7) 35.8 ACT Waiting interval CT Fibromyalgia Variant of MBSR CT Fibromyalgia T: 54.4 (8.3) C: 48.8 (9.1) CT Chronic pain CT Chronic pain RCT Chronic back pain Comparative Rheumatoid arthritis Comparative Not diagnosed 52.0 (24 89) Wait-list T: 79 C: 42 0 MBSR Support, relaxation, stretching exercises 19.5 Variant of MBSR: T1: in-person teaching T2: videoconferencing Breathworks mindfulnessbased pain management program MBSR + traditional therapy Mindfulness meditation and emotion regulation therapy 108 No 23.9 NRS 0.64 (P < 0.01) 18.2% Pain reductions maintained for 3 months (d = 0.38) T: 39 C: 13 Wait-list T1: 99 T2: 57 C: 54 TAU T: 32 C: 18 TAU wait-list T: 15 C: 10 C1: CBT for pain C2: education only T: 47 C1: 51 C2: MBSR CBSR T: 36 C: 14 No T: 9.2 VAS (P = 0.05) 14.4% None Low No T: 10.2 C: 13.3 No T1: 49 T2: 30 C: 10 No T: 3 C: 10 Yes T: 21 C: 23.8 Yes T: 2 C1: 4 C2: 0 No T: 20 C: 26.3 VAS 0.44 (P < 0.05) 23.1% Pain reductions maintained for 3 years (d = 0.27) NRS Present: 0.45 Distant: 0.29 (P < 0.05) Present: 11.8% Distant: 8.1% Low Low None Low NRS 0.60 (NS) 11.2% None Low VAS 1.02 (P < 0.05) 29.7% Pain reductions maintained for 10 months (d = 0.10) Medium Daily NRS (NS) 14.5% None High VAS 0.4 (P < 0.01) 49.4% None Low * N = Number of participants after attrition. Percentages represent mean percentage change in pain, calculate as ([pre post]/pre 100). Insufficient data to calculate. Effect size was calculated based on comparison of mean pre-post differences between treatment and control groups and their pooled standard deviations (SDs), as these were the available data. ACT = acceptance and commitment therapy; C = control group; CBSR = cognitive-behavioral stress reduction; CBT = cognitive behavioral therapy; CT = controlled trial; FU = follow-up; MBI = mindfulness-based interventions; MBSR = mindfulness-based stress reduction; NRS = numerical rating scale; NS = nonsignificant; rand assn. = random assignment; RCT = randomized control trial; T = treatment group; TAU = treatment as usual; VAS = visual analog scale. 236

8 Mindfulness-Based Interventions and Pain Intensity In two of three studies comparing MBIs to an active control group, reduction in pain intensity was greater for the MBI group compared with control. First is a study by Esmer et al. [56] reporting significant and moderate reductions in pain intensity ratings following MBSR compared with nonsignificant reductions for traditional therapy control group, with a large effect size (d = 1.02). Second is a study by Grossman et al. [57] comparing MBI to a control group which utilized social support, relaxation, and stretching exercises. Results from this study indicate significantly greater pain reduction for the MBI group (23.1% reduction in mean pain intensity for the MBI group compared with an increase of 14.65% among controls) compared with the control intervention. On the other hand, one study comparing MBI to treatment as usual [13] failed to find significant differences in pain intensity ratings between MBI and control group receiving treatment as usual. Two studies compared MBIs with an active treatment based on CBT. As CBT has demonstrated efficacy for pain reduction [58], these comparative designs sought to show that the MBI achieved pain reduction equivalent to or greater than CBT. In one such study, Smith et al. [58] found substantial decreases in pain intensity ratings for the MBSR group (49.4% reduction for the MBSR group compared with 29.5% increase in the control group) and a moderate advantage in posttreatment mean effect size for MBSR vs CBT control group. However, in a second study comparing mindfulness meditation and emotion regulation therapy to CBT for pain and to an intervention based on education only, there were no differences, with significant pain reduction for the MBI (14.5% reduction) and both control groups (14.3% and 17.8% reductions in the CBT and education groups, respectively) [12]. Follow-Up of Controlled Studies Three of the earlier studies included follow-up of pain intensity ratings [53,56,57]. Results from these studies indicate that pain intensity reductions were generally maintained over 3-month to 3-year follow-up periods. Summary of Controlled Trials Of eight controlled studies, six found that pain intensity was more reduced in the MBI group compared with controls; one study found that an MBI produced similar pain reduction to CBT. Only one study failed to find a significant difference between MBI and a treatment as usual control group [13]. The results from the controlled studies thus provide support for a specific effect of MBIs on pain intensity. Results from follow-up assessments support that this effect is well maintained, as all three studies found that pain intensity was reduced at follow-up. Discussion Change in Pain Intensity Following MBIs The goal of this review was to examine systematically, the impact of MBIs on pain intensity. Results from most controlled and uncontrolled studies suggest that MBIs lead to reduced pain intensity, with medium effect sizes. This finding is consistent with the findings of a meta-analysis by Veehof et al. [4], reporting a medium effect size for pain improvement for chronic pain undergoing MBIs. However, magnitude of decrease in pain intensity varied greatly across studies (range of mean decrease: %). This variability raises the question of whether MBIs effect on pain intensity may be moderated by certain variables of the participants being treated. One obvious factor is whether participants in the trial had a pain condition. As would be expected, decrease in pain intensity was found more consistently in samples in which all participants had a chronic pain condition and was less consistent in mixed samples consisting e.g., of physical or mental diagnosis or no diagnosis. This finding is not surprising considering the fairly low baseline levels of pain in these studies. For example, Chang et al. [51] included normal participants without pain complaints in a trial of MBSR and reported that there were no significant changes in pain intensity ratings at the end of the intervention. The authors raise the possibility of a floor effect in explaining these results. It has also been suggested that the specific type of pain condition may moderate MBIs effect on pain intensity [60]. However, as very few studies included homogenous samples with specific pain conditions, it is difficult to draw conclusions in this regard. Nonetheless, it should be noted that in three of the reviewed studies, participants had a single diagnosis of fibromyalgia. Results from two of these studies show that reduction in pain intensity was greater for the MBI group compared with control [55,57]. Results from the third uncontrolled study show that pain intensity was reduced for the majority of the participants [49]. Thus, it is possible that MBIs may have unique benefits for this difficult to treat pain condition. Fibromyalgia has been hypothesized to involve central sensitization [61,62]. Thus, it is possible that MBIs, through one of the psychological mechanisms described later, may exert influence on descending pathways involved in pain modulation resulting in desensitization. Future research is needed to test this effect. Other personal characteristics such as age or gender may also moderate the effect of MBIs on pain. For example, one study including an elderly sample [50] failed to find significant pain reductions at the end of the intervention. It is possible that the advanced age make it difficult to comply with the intervention or that multiple, physical conditions in this age group were less likely to respond to the MBI. This review further suggests that MBIs may have a specific effect on pain intensity compared with no treatment, and to other active treatments. Six of eight controlled studies show significantly greater reductions in pain intensity for MBIs compared with control groups, and another study found similar efficacy for MBI and CBT intervention for chronic pain. This finding contrast with conclusions of a recent review on MBIs for chronic pain [5] indicating non- 237

9 Reiner et al. specific effects of MBIs on pain. The discrepancy may be explained by the inclusion of multidimensional pain measures which include evaluations of pain interference and affect in the previous review, rather than the more narrow aspect of pain intensity which was the focus of the present review. It is possible that a more comprehensive pain assessment may lead to greater variability in participant s reports thus mask significant effects on more narrow aspects of pain. Long-Term Effect of MBIs on Pain Intensity It has been suggested that participants in MBIs learn a life skill, and that the beneficial effects of this skill can gradually grow rather than diminish over time [63]. When examining follow-up assessments ranging from 3 months to 3 years, significant reductions in pain intensity were maintained in all studies, regardless of the assessment period, suggesting long-term benefits for chronic pain, regarding the intensity of their pain. An important limitation to consider is that not all of the treatment group participants completed follow-up assessments. It is possible that participants who did not experience pain improvement were inclined not to complete follow-up assessments, thus biasing these results. Furthermore, as none of the controlled studies included a follow-up assessment of pain intensity for the control group, we cannot make conclusions regarding specificity of long-term benefits of MBIs as compared with other treatments. More studies are needed with long-term follow-up of both MBIs and control groups so that comparisons could be made for maintenance of gains. Limitations Quality assessment indicates that there is a clear shortage of high-quality studies. Most prominent methodological problems involve handling of missing data and lack of randomization. A large majority of studies (13 of 16) include data only for treatment completers in their final outcome analyses. This strategy may lead to a bias of favoring treatment, as only the results of those who completed treatment are presented. Two of three studies that reported results for the intent to treat sample (i.e., for all who began treatment with last observation carried forward) found that reduction in pain intensity was not significant [13,21]. A third study found, based on intent to treat sample analyses, that reduction in pain intensity for the MBI was significant and comparable with that of CBT [12]. This being noted, it is somewhat encouraging that most reviewed studies had fairly low rates of attrition (<20% in 10 of 16 studies). Thus, it is not clear how dramatically outcome patterns would change with these additional analyses. Nevertheless, future clinical studies of MBIs should include intent to treat as well as completer analyses or utilize statistical methods (e.g., hierarchical linear modeling) to address the potential effects of drop out in these trials. Most (six of eight) of the controlled trials lacked randomization. In the non-randomized trials, allocation based on participants location or time of attending the study may have influenced between groups variance and this may partially account for the different results found between groups. In one study, a self-selection of treatment was used [58], which may also bias outcomes. Thus, there is an urgent need for high-quality studies which would overcome these limitations in the future and allow for a more critical evaluation of the impact of MBIs on pain intensity. Finally, only one study assessed the clinical significance [64] of changes in pain intensity. Clinical significance reflects the practical value or the clinical importance of the effect in the lives of participants [65], and this can be evaluated in several ways. However, most studies report only mean group differences, which may or may not be clinically meaningful. One study which did consider clinical significance in terms of number of participants achieving moderate improvement found that change was clinically significant [49]. Conclusions regarding benefits of MBIs could be more definitive had other studies reported e.g., how many participants achieved a moderate to substantial improvement in their pain ratings, or those who reached a predetermined threshold level of improvement (i.e., treatment responders ) [41]. The Association Between Mindfulness and Pain Intensity Pain is a multidimensional experience comprised of sensory, affective, and cognitive components [66]. Thus, change in pain intensity does not necessarily imply change in the global experience of pain [40]. Nor are we suggesting that pain intensity is the most important clinical outcome for pain-focused interventions. However, understanding the unique effect of MBIs on pain intensity has important implications for evaluating the robustness of the effects of this approach. There are also practical implications on expectations from MBIs as compared with medical or other treatments for pain. Furthermore, understanding the effect of mindfulness on pain intensity may help elucidate mechanism of change in MBIs, a topic about which there is very little agreement at present [67]. Researchers have proposed several possible mediating factors to explain mindfulness effects on pain, including acceptance and self-regulation [68,69]. It has been argued that by learning to accept pain, are able to reduce their avoidance and other attempts to control pain, and direct their efforts toward goals they can achieve [18]. This would eventually lead to improved quality of life without necessarily changing the severity of pain (Figure 2A). Given the findings of this review, which indicate that MBIs also affect pain intensity, one possibility is that reduced avoidance and increase in valuable activities will ultimately lead to reduction in pain intensity (Figure 2B). Thus, in contrast to other interventions (e.g., medication) in which change in pain intensity is thought to mediate effects of the treatment on functioning and distress, for MBIs changes in functioning and distress may mediate effects on pain intensity. 238

10 Mindfulness-Based Interventions and Pain Intensity Figure 2 Suggested mechanisms of change involved in mindfulness-based interventions (MBIs). Introducing possible roles of pain reduction: (A) traditional view: MBIs improve life quality by increasing acceptance without necessarily change pain perception; (B) suggested mechanism: MBIs increase acceptance which in turn led to behavioral change that results in improved quality of life and reduced pain. (C) MBIs influence pain perception directly which then mediate change in self-regulation and improved quality of life. A Mindfulness Intervention B Mindfulness Intervention C Mindfulness Intervention Increased Acceptance Less avoidance More valuable action Improved life quality Increased Acceptance Less avoidance More valuable action Improved life quality Reduced pain intensity Detaching cognitive/ emotional pain components Reduced pain intensity Improved self-regulation Improved life quality Alternatively, it is possible that reduction in pain intensity occurs much earlier in the process of change induced by MBIs. Following the original rationale of MBSR, attending mindfully to the experience of pain may detach the cognitive and emotional components of pain from the sensory component [2]. Thus, pain is perceived and experienced as less painful at the present moment. As human beings dedicate extensive resources to self-regulation (i.e., controlling and regulating their own cognitions, emotions, and behaviors) [70], a reduction in perceived pain intensity could reduce task load on the self-regulation system of chronic pain, which is often occupied by intense persistent pain [71]. This reduced load may in turn promote a more adaptable and flexible regulation of behavior, thoughts, and feelings, which would help comply with the MBI and also use it to further induce positive change in their lives (Figure 2C). It should be noted that the two mechanisms discussed earlier are not mutually exclusive and it is possible that both mechanisms contribute to decrease in pain intensity but operate along different pathways. Future research is needed to understand how and when change in pain intensity takes place during the process of MBIs. Future Directions Current findings may inform future development and refinements of MBIs. If being mindful can help one experience less pain in the present moment, then this approach would seem appropriate not only for chronic pain, but perhaps also for individuals with acute pain, such as pain accompanying medical procedures or labor. This question can be addressed by investigating mindfulness states and their influence on laboratoryinduced pain. Only a few studies have used this paradigm, with results indicating that relatively short mindfulness training may indeed influence pain tolerance and pain intensity to experimentally induced pain [29,30]. Other studies have investigated the influence of present moment mindfulness states on experimentally induced pain in highly trained mindful experts vs naïve participants [72,73]. The results from these studies indicate that being mindful while receiving a painful stimulus may have direct analgesic effects. However, this effect was seen only in very experienced mindfulness meditators. Clinical trials which utilize interventions such as MBSR or ACT provide initial evidence for the contribution of mindfulness states to reduced pain intensity. Experimental studies such as those discussed earlier provide examples of additional methodologies that may help elucidate the mindfulness pain relationship. Future research is needed to carefully evaluate mechanisms of change and to identify which specific aspects of MBIs (active ingredients) may contribute to the observed changes in pain. Furthermore, future research should examine personal characteristics (gender, age, personality features) and other factors that may moderate the effects of MBIs on pain intensity. This may have implications for matching the appropriate therapy to the client according to his/her profile, as MBI may be more useful for certain individuals and for certain types of pain conditions. Conclusions Results from this review suggest that along with other beneficial effects, MBIs also reduce pain intensity. This is evident from uncontrolled and controlled trials, as well as 239

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