A cross-sectional study of the prevalence and correlates of post-traumatic stress among people attending acceptance and commitment therapy for chronic pain

Abstract

Objective

Experiences of trauma are common among people with chronic pain, although estimates of comorbid post-traumatic stress vary widely across contexts. Acceptance and commitment therapy (ACT), which targets psychological flexibility to improve pain-related functioning, is increasingly delivered for people with pain. To inform treatment delivery, understanding of the prevalence and correlates of post-traumatic stress among those attending ACT-based treatment for pain is needed. This study aimed to identify the prevalence of post-traumatic stress and its associations with key pain-related outcomes and other relevant psychosocial factors at the start of an ACT-based treatment.

Methods

This was a cross-sectional study using data collected within routine clinical care at a speciality pain management service in the UK. Four hundred and fifty-one participants completed standardized self-report questionnaires of post-traumatic stress symptoms (PTSS), pain-related outcomes (pain intensity and interference, work and social adjustment, and depression symptoms), psychological flexibility/inflexibility, and stigma at the start of an interdisciplinary ACT-based treatment for chronic pain.

Results

Forty-seven percent of patients (n = 213) reported a traumatic exposure and 30% of the sample reported clinically meaningful PTSS. Greater severity of PTSS and probable post-traumatic stress disorder (PTSD) were associated with significantly worse pain-related outcomes, psychological flexibility/inflexibility, and stigma in bivariate analyses. PTSS severity and probable PTSD showed significant unique associations with poorer pain outcomes in regression models controlling for covariates and psychological flexibility/inflexibility.

Conclusions

Post-traumatic stress is common among people engaging in ACT-based treatment for pain and associated with adverse pain outcomes. Trauma-informed practice is therefore important when providing ACT-based treatment for pain.

Introduction

Chronic pain is often comorbid with mental health problems, particularly depression and anxiety. ¹ There is also recognition of the relevance of trauma among people with chronic pain. ² For some, exposure to extremely threatening experiences leads to post-traumatic stress disorder (PTSD), characterized by re-living, avoidance of reminders of the trauma, altered arousal/reactivity, and negative cognitions and mood. ³

One meta-analysis found a 10% PTSD point prevalence in people with chronic pain, ⁴ in contrast to a 3.5% 12-month prevalence in the general population. ⁵ Similarly, a large proportion of those with PTSD experience chronic pain, with particularly high rates for veterans (around 50%) and refugees (20–80%).^6,7 Importantly, post-traumatic stress is associated with increased pain severity, pain-related disability, and depression.^4,8–10 Moreover, chronic pain and post-traumatic stress are, individually, associated with stigma,^11,12 but research is needed to understand whether comorbid post-traumatic stress is associated with greater stigma among people with pain.

Taken together, many people seeking treatment for pain present with post-traumatic stress.^13,14 Multidisciplinary pain treatments are now widely based on cognitive-behavioural principles.^15,16 However, limited research has investigated the prevalence and correlates of post-traumatic stress among people engaging in these treatments and post-traumatic stress may be particularly relevant for those presenting to pain services. A Swedish study of patients assessed at a pain rehabilitation unit found that 72% reported a traumatic exposure, and 29% met diagnostic criteria for current PTSD. ⁸ Research is needed to investigate the generalizability of these findings to patients seeking cognitive-behavioural pain treatment in other settings and among socio-demographically diverse groups. This can identify the degree to which trauma-informed and trauma-focused treatments are needed as part of, or in addition to, existing evidence-based cognitive-behavioural pain treatments.

Shared vulnerability and mutual maintenance models have been proposed to understand the pain-trauma link. ¹⁷ Within these, people with increased sensitivity to threat-related stimuli and a tendency to focus on pain- or trauma-related cues are more likely to interpret bodily sensations (like pain or arousal) as dangerous. This contributes to fear, hypervigilance, and avoidance that maintain PTSD and chronic pain. ¹⁷

More recent research highlights the psychological flexibility model to understand the pain-PTSD link. ¹⁸ Psychological flexibility is the ability to persist with or change behaviour depending on one’s goals and values, whilst remaining open to and aware of difficult experiences in the present. ¹⁹ This involves six processes: acceptance (willingness to have difficult experiences), cognitive defusion (separating thoughts from behaviour), present-moment awareness (focusing on the now), self-as-context (noticing one is separate from internal experiences), values (identifying what is important), and committed action (acting in line with values). ¹⁹ Psychological flexibility is associated with improved pain outcomes. ²⁰ One study in a sample attending traditional cognitive-behavioural treatment for pain found that those with probable PTSD had lower psychological flexibility than those who did not, as well as higher depression, pain interference and severity. ¹⁸ Pain acceptance, committed action, and cognitive fusion also mediated between trauma symptoms and pain interference. ¹⁸ However, there is limited understanding of how all six psychological flexibility facets together relate to post-traumatic stress and pain-related outcomes.

The links between psychological flexibility, post-traumatic stress, and pain are relevant given that pain management treatments focused on psychological flexibility are increasingly implemented. Acceptance and commitment therapy (ACT) is a third-wave cognitive-behavioural intervention that targets psychological flexibility to improve quality of life with pain, with growing evidence.^21,22 However, existing trauma-pain research has largely been conducted in the context of traditional cognitive-behavioural therapy. As such, there is a need to understand trauma prevalence, and how post-traumatic stress relates to pain outcomes and psychological flexibility for those engaging in ACT.

This study investigated the prevalence and correlates of post-traumatic stress among people attending ACT-based treatment for pain. We hypothesized that those with probable PTSD would report poorer pain outcomes and less psychological flexibility than those with subthreshold PTSS or no trauma exposure. The association between post-traumatic stress and pain-related outcomes controlling for demographic factors and psychological flexibility/inflexibility was also investigated. An exploratory analysis of the bivariate relationship between stigma and post-traumatic stress was also undertaken.

Methods

Design

A cross-sectional observational study was conducted using routine clinical data collected between January 2023 and April 2025. The study had ethical approval (South Central Oxford NHS Research Committee, 22/SC/0388).

Participants

Participants were adults with chronic pain attending a pain management program within the UK’s National Health Service (NHS). Participants were eligible for the program if they were ≥18 years old and had pain ≥3 months that was associated with significant distress and/or disability. Participants were willing to complete an English-language group treatment focused on improving quality of life in the presence of pain rather than pain reduction. Those undergoing medical investigations, unable to move independently, or with significant psychological complexities judged as likely to interfere with treatment (e.g. active psychosis, active suicidal ideation, and substance abuse) were excluded. Eligibility was assessed by a physiotherapist and psychologist/occupational therapist. All patients were invited to complete standardized questionnaires as part of routine care. Patients who consented to their data being used for research were included in this study.

Treatment context

Although this study presents baseline data only, brief details of the treatment are provided for context. The treatment is a three-week residential (four days per week) ACT-based ²¹ multidisciplinary pain management program. The program involves group-delivered psychology, physiotherapy, occupational therapy, and nursing sessions. The program aims to improve quality of life with pain by targeting psychological flexibility. ²⁰ Experiential exercises, metaphors, mindful movement, and values-based goal-setting are used to this end.

Measures

Participants completed standardized self-report measures at the start of treatment.

Clinical and demographic information

Questions asked about pain history (location/duration) and demographic information (age, gender, ethnicity, education, and work). Burden of comorbid physical and mental health conditions was captured using the Multimorbidity Treatment Burden Questionnaire (MTBQ) for descriptive purposes. ²³

Trauma exposure and post-traumatic stress symptom severity

The PTSD Checklist-5 (PCL-5) ²⁴ was used to measure post-traumatic stress symptoms. Trauma exposure was assessed using an adapted PCL-5 criterion A assessment. As self-report measures were completed without clinical support, the clinical team judged it inappropriate for patients to recount extensive details of their trauma histories. Therefore, instead of asking participants to identify/describe the very stressful experience, the adapted version involved one yes/no question asking, ‘Did your stressful experience(s) involve actual or threatened death, serious injury, or sexual violence?’ Those responding ‘Yes’ completed the PCL-5. The rest of the Criterion A assessment was unchanged.

The PCL-5 has 20-items assessing the Diagnostic and Statistical Manual of Mental Illnesses (DSM-5) ³ symptoms of PTSD over the past month. Items are rated from 0 (not at all) to 4 (extremely), with a total score ≥31 indicating probable PTSD. ²⁴ Probable PTSD can also be determined by symptom cluster: cluster B (intrusive symptoms: items 1–5), C (avoidance: items 6–7), D (negative cognition/mood: items 8–14), and E (altered arousal/reactivity: items 15–20). Those scoring ≥2 for one B item, one C item, two D and two E items met criteria for probable PTSD. ²⁴ The PCL-5 is a well-validated and internally reliable measure. ²⁵

Key pain-related outcome variables

Based on the IMMPACT recommendations for core chronic pain outcomes, ²⁶ the following variables were measured:

Pain intensity

A single item rating scale was used to indicate average pain intensity in the last week, ranging from 0 (no pain) to 10 (pain as bad as you can imagine). This item has been shown to have good validity and reliability for assessing pain intensity. ²⁷

Pain interference

Pain interference was measured using the Brief Pain Inventory – Interference Scale (BPI-I). ²⁸ This consisted of 7 items asking about how much, over the past week, pain interfered with daily life, including mood, life enjoyment, sleep, general activity, work, relationships, and walking. Items were assessed from 0 (does not interfere) to 10 (completely interferes). The average of the items was used, with higher scores indicating greater interference. The BPI-I is widely used and validated among people with chronic pain. ²⁹

Impact on functioning

The impact of pain on functioning was further assessed using the Work and Social Adjustment Scale (WSAS). ³⁰ The five items assess the impact of health conditions on work, home, relationships, and social activities. Items were rated from 0 (not at all) to 8 (very severely). Higher total scores indicate greater functional impairment. The WSAS has high internal reliability and has been validated in people with chronic pain. ³¹

Depression

The Patient Health Questionnaire-9 (PHQ-9) ³² was used to measure depression symptoms with nine items, including low mood, sleep disturbance and concentration difficulties, over the past 2 weeks. Items were rated from 0 (not at all) to 3 (nearly every day), with higher total scores indicating more severe depression symptoms. The PHQ-9 has shown reliability and validity as a measure of depressive symptoms across chronic pain samples. ³²

Other psychosocial variables

Psychological flexibility and inflexibility

The Multidimensional Psychological Flexibility Inventory (MPFI) ³³ is a 60-item measure assessing each process of psychological flexibility and inflexibility (i.e. acceptance/experiential avoidance, present-moment awareness/lack of present-moment awareness, self-as-context/self-as-content, cognitive defusion/fusion, contact with values/lack of contact with values, committed action/inaction). Items are rated with respect to the previous 2 weeks, from 1 (never true) to 6 (always true). Despite being strongly related, psychological flexibility and psychological inflexibility are partially distinct and not simply the inverse of one another. ³⁴ Flexibility emphasizes adaptive and values-oriented behaviour, whereas inflexibility captures more rigid and avoidant behaviour. ³⁵ Average scores for each individual subscale were used for bivariate correlations. The overall flexibility and inflexibility subscales were used for all other analyses to reduce the number of variables in the models. Higher average scores indicate greater levels of flexibility or inflexibility. The MPFI has shown strong reliability and validity in people with pain. ³⁴

Stigma

Stigma was measured given previous research showing its relevance for people with pain ¹¹ and, separately, for people with PTSD. ¹² Stigma was therefore included for exploratory purposes to examine its bivariate associations with post-traumatic stress in our sample with chronic pain. ¹¹ The eight-item Stigma Scale for Chronic Illness (SSCI-8) ³⁶ was used to measure internalized and enacted stigma in relation to one’s illness. Items were rated from 1 (never) to 5 (always), with higher total scores indicating higher stigma. The SSCI-8 is internally consistent and valid for use with people with chronic pain. ³⁷

Procedure

To minimize bias, patients attending the program were consecutively sampled. All patients starting treatment were invited to complete standardized and validated questionnaires online before the first day of treatment, or a paper-copy on the first day if they could not complete online. Participants provided consent for their data to be used for research.

Statistical analysis

Mean imputation was used for participants with ≤10% missing data on individual questionnaires. To calculate a valid psychological flexibility/inflexibility score, participants needed a score for all individual subscales. Trauma groups were categorized using the PCL-5 total. ²⁵ Previous research found no differences in pain outcomes for those without trauma exposure and those with trauma exposure and subthreshold PTSS ⁸ ; therefore, the groups were collapsed into a ‘no trauma exposure/subthreshold PTSS’ group and a ‘probable PTSD’ group. This allowed comparison between patients with and without clinically meaningful PTSS. ¹⁸ Probable PTSD was also calculated using the cluster-based criteria. ²⁴ When calculating cluster-based scores for the PCL-5, missing data were not imputed, given that the specific item value was key to meeting the criteria.

Chi-square tests were used to examine group differences for categorical variables. Categorical variables were dichotomized to ensure sufficient numbers per group: gender (male/female), ethnicity (ethnically minoritized/white), working status (employed including part time and student/unemployed or retired). One-way ANOVAs were conducted for parametric continuous variables between groups. Kruskal–Wallis tests were used for non-parametric data, and Welch tests reported where distributions were non-homogenous.

Among participants who reported a traumatic exposure and completed the PCL-5, bivariate Pearson’s correlations were conducted to examine associations between variables. Hierarchical multiple linear regressions were then used to examine the association between PTSS and pain-related outcomes (pain intensity and interference, work and social adjustment, depression), controlling for gender, ethnicity, and age, based on prior research. ³⁸ Individual models controlled for additional demographic variables that showed significant associations with the dependent variable or PCL-5 scores. Pain intensity was included in the second block of each model (except the model predicting pain intensity). Separate models were conducted for psychological flexibility and inflexibility, given potential multicollinearity. ³⁴ Stigma was only included as an exploratory variable for bivariate analyses and was not the main focus of the study. As such, it was not included in regressions to keep the models more parsimonious given the range of variables already included. Sensitivity analyses using trauma as a categorical variable (i.e. no exposure/subthreshold PTSS vs probable PTSD) were conducted using the same outcome variables and covariates.

Normality assumptions for multiple regressions were assessed using histograms, q-q plots and partial regression plots. Where assumptions were not met, standardized residuals with values │2.5│ were identified as potential outliers, and models rerun with these excluded. If exclusion of outliers did not alter model findings or normality, residuals were retained and the original model reported. Analyses were conducted on SPSS (Version 27). The sample size was determined pragmatically based on the number of participants recruited within the specified dates and was deemed sufficient given previous similar research.^8,18

Results

Five hundred and three participants completed questionnaires. Of these, 451 (90%) consented for their data to be used for research. Sample characteristics are in Table 1.

Table 1.

Sample characteristics.

Variable	Mean (SD) or frequency (%)
Agea	50.25 (13.41)
Gender
Female	366 (81.2)
Male	76 (16.9)
Prefer not to say	7 (1.6)
Missing	2 (0.4)
Ethnicity
Asian	29 (6.4)
Black	77 (17.1)
Mixed	33 (7.3)
Other	17 (3.8)
White	294 (65.2)
Missing	1 (0.2)
Pain duration in yearsb	14.39 (13.17)
Pain all over body
Yes	315 (69.8)
No	126 (27.9)
Missing	10 (2.2)
Main pain location
Head, face or mouth	12 (2.7)
Neck region	15 (3.3)
Upper shoulder or upper limbs	38 (8.4)
Chest region	5 (1.1)
Abdominal region	10 (2.2)
Lower back, lumbar spine, sacrum, and coccyx	213 (47.2)
Lower limbs	72 (16)
Pelvic region	18 (4)
Anal or genital region	2 (.4)
Missing	66 (14.6)
Comorbid physical health condition
Yes	374 (82.9)
No	69 (15.3)
Missing	8 (1.8)
Comorbid mental health condition
Yes	251 (55.7)
No	190 (42.1)
Missing	10 (2.2)
Health effortc	27.03 (9.59)
Working status
Employed	191 (42.4)
Unemployed	202 (44.8)
Student	7 (1.5)
Retired	44 (9.8)
Missing	7 (1.5)
Traumatic exposure
Yes	213 (47.2)
No	218 (48.3)
Missing	20 (4.4)
Years since traumatic exposured	19.27 (15.58)
Experience of traumatic exposuree
Directly	163 (78.4)
Witnessed	40 (20.1)
Learnt about happening to close family/friend	39 (19.9)
Repeatedly exposed to details via job	5 (2.6)
Other	7 (3.6)
Cause of death for traumatic exposures involving deathe
Accident/violence	36 (18.7)
Natural causes	35 (18.1)
Not applicable	122 (63.2)

Note. N = 451 unless otherwise specified. ^a = N = 409, ^b = N = 420, ^c = N = 442, ^d = N = 187, ^e= N = 213 (those who completed the Criterion A assessment; multiple exposures could be selected), SD = standard deviation.

Prevalence of post-traumatic stress

Two-hundred and thirteen participants (47%) reported a traumatic exposure. One participant who reported a traumatic exposure did not complete the PCL-5 so was excluded from further analysis. Therefore, 212 participants were included in PCL-5 analyses. A slightly larger proportion of participants were categorized as having probable PTSD using the total score (136/212 participants) versus the cluster-based criteria (125/212 participants). Approximately 85% of participants were categorized the same according to both criteria (65 participants met neither criteria, 114 participants met both). Twenty-two participants (10%) were categorized as having probable PTSD according to the total score but did not meet the cluster-based criteria, whereas 11 (5%) were not categorized as having probable PTSD according to the total score, but did meet the cluster-based criteria.

Group comparison of demographic and clinical characteristics

Chi-squared tests found no significant differences between the two groups by gender, ethnicity, working status, or pain location. Average pain intensity was not significantly different between groups. All other variables were significantly higher for the probable PTSD group, except for psychological flexibility, which was significantly lower for this group (Table 2).

Table 2.

Between-group comparisons for continuous self-report measures.

Mean (SD) (n)				Between-group test	Df	F/adjusted F	p-value	Kruskal–Wallis H	Adjusted p-value
Variable	Total sample	No trauma exposure/subthreshold PTSS	Probable PTSD
Pain intensity	7.41 (1.53) 448	7.35 (1.51) 292	7.42 (1.51) 136	ANOVA	1	0.22	.639	​	​
Pain interference	7.57 (1.67) 448	7.38 (1.68) 292	7.94 (1.61) 136	ANOVA	1	10.78	.001	​	​
Impact on functioning	30.94 (7.19) 444	29.73 (7.57) 293	33.22 (5.82) 135	Kruskal–Wallis	1	​	​	22.54	<.001
Depression	16.80 (5.77) 441	15.53 (5.76) 292	19.74 (4.59) 134	Kruskal–Wallis	1	​	​	12.45	<.001
Psychological flexibility	3.28 (.91) 432	3.34 (.93) 279	3.08 (.86) 121	Kruskal–Wallis	1	​	​	49.77	<.001
Psychological inflexibility	3.44 (.89) 431	3.22 (.84) 279	3.89 (.82) 125	ANOVA, Welch	1	7.17	.008	​	​
Stigma	22.23 (6.89) 435	20.48 (6.48) 288	25.56 (6.35) 132	ANOVA	1	56.35	<.001	​	​

Note. SD = standard deviation, n = sample size, Df = degrees of freedom.

Correlations among variables

Except for non-significant correlations with acceptance, present-moment awareness, and experiential avoidance, all variables were significantly associated with PCL-5 scores (Table 3). There were large correlations between PCL-5 scores and depression and psychological inflexibility. Correlations with psychological inflexibility subscales were larger than with psychological flexibility subscales.

Table 3.

Correlations between PCL-5 total scores and other study variables.

Variable	PCL-5 total score
Pain intensity	.20*
Pain interference	.34**
Depression	.63**
Psychological flexibility	−.24*
Acceptance	.11
Present-moment awareness	.01
Self-as-context	−.21*
Defusion	−.27**
Values	−.29**
Committed action	−.32**
Psychological inflexibility	.62**
Experiential avoidance	.11
Lack of contact with present	.31**
Self-as-content	.51**
Fusion	.59**
Lack of contact with values	.47**
Inaction	.62**
Stigma	.43**

Note. * = statistically significant at p < .01, ** = statistically significant at p < .001. PCL-5 = PTSD Checklist for DSM-5.

Multiple regressions

Multiple regression analyses examined whether PTSS severity was associated with pain-related outcomes after controlling for covariates. PTSS was a significant associated factor in all models. Controlling for covariates, PTSS accounted for an additional 4%, 5%, 6% and 44% of the variance in pain intensity, interference, impact on functioning, and depression, respectively. There were small significant effects in the models for pain intensity (n = 181, β = .21, p = .006), interference (n = 181, β = .23, p < .001) and impact on functioning (n = 180, β = .25, p < .001). PTSS was most strongly associated with depression (n = 179, β = .58, p < .001).

Sensitivity analyses for associations between depression symptoms and PCL-5 symptom clusters

Due to item overlap between the PHQ-9 and PCL-5 Cluster D (negative cognitions and mood), sensitivity analyses were conducted for multiple regressions between PHQ-9 scores and each PCL-5 cluster (Supplemental Table 1). Each individual cluster was significantly associated with depression, controlling for demographics and pain intensity. Cluster D accounted for the most additional variance (β = .60, p < .001) compared to other clusters. However, all clusters showed medium to large effects (β = .41 to β = .60).

Regression models including psychological flexibility and inflexibility

Additional models were conducted including psychological flexibility/inflexibility (Tables 4 and 5). Partial regression plots showed that linearity assumptions were not met for some models. Removal of standardized residuals did not alter assumptions or findings, so models with residuals are reported. Psychological flexibility accounted for 0% (pain intensity) to 13% (depression) of the variance in the models. Controlling for psychological flexibility, PCL-5 scores remained significantly associated with all the outcomes. Psychological inflexibility accounted for 0% (pain intensity) to 28% (depression) of the variance in the models. Controlling for psychological inflexibility, PCL-5 total scores remained significantly associated with all the outcomes.

Table 4.

Regression models including psychological flexibility and post-traumatic stress symptom severity as factors associated with pain outcomes.

	Independent variable	F Change	Df	p	R2adj	ΔR2adj	β	Unstandardized B (SE)	p
Model 1 – Dependent: Pain intensity
Step 1	Age	1.49	(4, 187)	.206	.01		.11	0.01	.130
	Gender						.01	0.02	.943
	Ethnicity						.02	0.05	.843
	Pain location						−.10	−0.10	.157
Step 2	Psychological flexibility	0.01	(1, 186)	.920	.01	.00	.04	0.04	.561
Step 3	PCL-5	6.40	(1, 185)	.012	.03	.02	.19	0.19	.012
Model 2 – Dependent: Pain interference
Step 1	Age	1.12	(5, 186)	.352	.00		−.02	−0.00 (.01)	.729
	Gender						−.03	−0.13 (.27)	.637
	Ethnicity						−.03	−0.09 (.21)	.682
	Pain location						−.04	−0.13 (.23)	.570
	Work status						.07	0.23 (.20)	.266
Step 2	Pain intensity	71.35	(1, 185)	<.001	.28	.28	.49	0.55 (.07)	<.001
Step 3	Psychological flexibility	9.15	(1, 184)	.003	.31	.03	−.13	−0.24 (.12)	.036
Step 4	PCL-5	10.47	(1, 183)	.001	.34	.03	.21	.02 (.01)	.001
Model 3 – dependent: Impact on functioning
Step 1	Age	3.69	(5, 186)	.003	.07		.04	0.02 (.04)	.555
	Gender						−.02	−0.41 (1.23)	.739
	Ethnicity						−.06	−0.91 (.98)	.353
	Pain location						−.10	−1.62 (1.03)	.116
	Work status						.19	2.74 (.93)	.004
Step 2	Pain intensity	31.71	(1, 185)	<.001	.20	.13	.33	1.58 (.31)	<.001
Step 3	Psychological flexibility	6.46	(1, 184)	.012	.22	.02	−.10	−0.82 (.52)	.119
Step 4	PCL-5	12.73	(1, 183)	<.001	.27	.05	.24	0.09 (.03)	<.001
Model 4 – dependent: Depression
Step 1	Age	3.33	(5, 186)	.007	.06		−.06	−0.03 (.02)	.249
	Gender						−.05	−0.73 (.82)	.377
	Ethnicity						−.06	−0.75 (.65)	.249
	Pain location						−.16	−2.03 (.69)	.004
	Work status						.05	0.59 (.62)	.347
Step 2	Pain intensity	11.46	(1, 185)	<.001	.11	.05	.14	0.55 (.21)	.008
Step 3	Psychological flexibility	33.00	(1, 184)	<.001	.24	.13	−.23	−1.47 (.35)	<.001
Step 4	PCL-5	86.98	(1, 183)	<.001	.48	.24	.53	0.16 (.02)	<.001

Note. All Standardized β were from final regression models. Gender, ethnicity, pain location, and work status were dichotomized. Reference categories for dichotomized variables were male (gender), white (ethnicity), pain location not all over (pain location), employed including part time and student (work status). Df = degrees of freedom, ΔR²adj = change in R², SE = standard error. PCL-5 = PTSD Checklist for DSM-5.

Table 5.

Regression models including psychological inflexibility and post-traumatic stress symptom severity as factors associated with pain outcomes.

	Independent variable	F Change	Df	p	R2adj	ΔR2adj	β	Unstandardized B (SE)	p
Model 1 – Dependent: Pain intensity
Step 1	Age	1.49	(4, 187)	.206	.01		.11	0.01 (.01)	.158
	Gender						.01	0.04 (.29)	.881
	Ethnicity						.01	0.03 (.24)	.908
	Pain location						−.11	−0.36 (.24)	.146
Step 2	Psychological inflexibility	0.83	(1, 186)	.362	.01	.00	−.08	−0.13 (.16)	.407
Step 3	PCL-5	5.92	(1, 185)	.016	.04	.03	.23	0.02 (.01)	.016
Model 2 – Dependent: Pain interference
Step 1	Age	1.12	(5, 186)	.352	.00		−.03	−0.00 (.01)	.646
	Gender						−.04	−0.19 (.27)	.489
	Ethnicity						−.03	−0.11 (.22	.609
	Pain location						−.03	−0.10 (.23)	.658
	Work status						.07	0.23 (.21)	.259
Step 2	Pain intensity	71.35	(1, 185)	<.001	.28	.28	.49	0.55 (.07)	<.001
Step 3	Psychological inflexibility	10.65	(1, 184)	.001	.34	.06	.09	0.16 (.15)	.287
Step 4	PCL-5	5.55	(1, 183)	.019	.36	.02	.19	0.02 (.01)	.019
Model 3 - dependent: Impact on functioning
Step 1	Age	3.69	(5, 186)	.003	.07		.04	0.02 (.04)	.580
	Gender						−.03	−0.62 (1.23)	.613
	Ethnicity						−.06	−0.93 (.99)	.347
	Pain location						−.10	−1.52 (1.03)	.140
	Work status						.19	2.75 (.93)	.004
Step 2	Pain intensity	31.71	(1, 185)	<.001	.20	.13	.33	1.58 (.31)	<.001
Step 3	Psychological inflexibility	11.51	(1, 184)	<.001	.24	.04	.09	0.73 (.68)	.286
Step 4	PCL-5	6.34	(1, 183)	.013	.26	.02	.21	0.08 (.03)	.013
Model 4 – dependent: Depression
Step 1	Age	3.33	(5, 186)	.007	.06		−.05	−0.02 (.02)	.344
	Gender						−.07	−1.14 (.82)	.166
	Ethnicity						−.05	−0.62 (.66)	.346
	Pain location						−.15	−1.85 (.69)	.008
	Work status						.05	0.60 (.62)	.346
Step 2	Pain intensity	11.46	(1, 185)	<.001	.11	.05	.15	0.56 (.21)	.007
Step 3	Psychological inflexibility	86.08	(1, 184)	<.001	.39	.28	.29	1.87 (.45)	<.001
Step 4	PCL-5	33.39	(1, 183)	<.001	.48	.09	.40	0.12 (.02)	<.001

Note. All Standardized β were from final regression models. Gender, ethnicity, pain location, and work status were dichotomized. Reference categories for dichotomized variables were male (gender), white (ethnicity), pain location not all over (pain location), employed including part time and student (work status). Df = degrees of freedom, ΔR²adj = change in R², SE = standard error. PCL-5 = PTSD Checklist for DSM-5.

Regressions with trauma as categorical variable

Sensitivity analyses using trauma as a categorical variable were conducted (Supplemental Tables 2-4). Normality assumptions were not met. However, removal of outliers did not change findings or normality deviations, so models with all residuals are presented. Probable PTSD was significantly associated with pain interference (n = 379, β = .16, p < .001), impact on functioning (n = 378, β = .20, p < .001), and depression (n = 377, β = .33, p < .001). Controlling for psychological flexibility or psychological inflexibility, probable PTSD remained significantly associated with all outcome variables, except for pain intensity.

Discussion

In this study within a publicly funded healthcare system in the UK, nearly half of patients beginning intensive ACT-based treatment for pain reported a trauma exposure, and nearly two-thirds of those with a traumatic exposure had probable PTSD. Probable PTSD and more severe PTSS were associated with worse pain-related outcomes, psychological flexibility/inflexibility, and stigma. The results highlight the relevance of considering post-traumatic stress among people engaging in psychologically based treatment for pain.

Thirty percent of the baseline sample in the current study were categorized as having probable PTSD using the PCL-5 total score. This is substantially higher than the 12-month general population prevalence of 3.5%. ⁵ It is also higher than the 10% point prevalence of PTSD in people with chronic pain reported in a previous meta-analysis. ⁴ The prevalence using the cluster method in the current sample was slightly lower (28%). There has been wide variability in prevalence estimates of PTSD among people with pain, attributed to the specific diagnostic classifications and measures used.^4,39 Estimates may also vary as a function of healthcare context and populations assessed, where greater prevalence may be observed in publicly funded systems that are more likely to serve marginalized communities at greater risk of post-traumatic stress. ⁴⁰

Probable PTSD in the current study was similar to a Swedish sample attending their first assessment within a speciality government-funded pain service (29%). ⁸ The current data were collected after patients completed their first multidisciplinary assessment within the pain service. This is notable, as a previous audit within the service identified that nearly one third of patients are discharged following initial assessment. ⁴¹ A frequent reason for discharge following initial assessment is the complexity of psychosocial factors, including post-traumatic stress, judged as likely to interfere with engagement in short-term and group-based ACT for pain. ⁴¹ Therefore, the prevalence of probable PTSD among people presenting for initial assessment within the service is likely to be even higher.

Probable PTSD and PTSS severity were both associated with greater pain interference, functional impact, and depression controlling for covariates and psychological flexibility/inflexibility in regression analyses. This is consistent with research showing adverse associations between post-traumatic stress and pain-related outcomes.^8–10 Despite this cross-sectional evidence, emerging research suggests that pre-treatment PTSD/PTSS may not be associated with worse outcomes following CBT or ACT for pain.^10,42 Importantly, though, one study found that while approximately 30% of trauma-exposed participants experienced a clinically meaningful improvement in PTSS following a CBT-based pain program, 20% reported a meaningful exacerbation in these symptoms. ⁴² Therefore, investigation of adverse effects of ACT-based pain management with respect to trauma symptoms is needed.

The association between PTSS and depression is worth considering. There was a strong bivariate correlation between these variables, consistent with previous findings. ⁸ This may be partially due to overlap between depression and the ‘negative cognitions and mood cluster’ of PTSD. However, sensitivity analyses indicated that each PTSD symptom cluster was uniquely associated with depression. Thus, symptom overlap may not be the only factor accounting for this association. The association between PTSD and depression is complex, and numerous studies have investigated causal relationships and shared vulnerabilities. ⁴³ Traditional cognitive-behavioural approaches for pain have highlighted the role of variables such as pain-related worrying and pain self-efficacy in contributing to depression among people with pain.^44,45 The current finding highlights the importance of targeting trauma-related distress to manage depression for some people with pain.

The psychological flexibility model has more recently been applied to understand the pain-PTSD link. ¹⁸ PTSD/PTSS severity have been associated with lower pain acceptance and committed action, and higher cognitive fusion, but not values-based action. ¹⁸ Although PTSS were not significantly associated with all individual MPFI subscales, there were significant correlations between PTSS scores and the comprehensive psychological flexibility/inflexibility scores in the current study. Interestingly, there was a large correlation between psychological inflexibility and PTSS and a small correlation between flexibility and PTSS. Using the MPFI, a previous study also reported stronger associations between psychological inflexibility versus flexibility and adverse pain-related outcomes. ³⁴ The MPFI inflexibility and PCL-5 items are all negatively framed. As such, the relative strength of their association may reflect a negative response bias. ³⁴ The findings further support psychological flexibility and inflexibility as related but partially distinct constructs.^33,34

The inclusion of psychological flexibility/inflexibility reduced some of the variance in pain-related outcomes accounted for by PTSS/PTSD, although these remained significant associated factors. This is consistent with cross-sectional mediation analyses showing that facets of psychological flexibility/inflexibility mediate between PTSD and pain intensity and interference. ¹⁸ Taken together, emerging evidence suggests that targeting psychological flexibility/inflexibility may have some benefits for post-traumatic stress and chronic pain outcomes. However, the cross-sectional nature of these studies precludes causal interpretation, and the reverse directions of effects are also plausible. Longitudinal and experimental intervention research are needed to directly test the causal nature of relationships between these variables.

The results highlight the importance of trauma-informed practice when delivering interdisciplinary pain care. Trauma-informed practice recognizes the widespread impacts of trauma and uses a strengths-based approach to support service engagement and reduce re-traumatization. ⁴⁶ It includes principles of safety, trustworthiness, choice, collaboration, empowerment, and cultural considerations. ⁴⁶ Ongoing reflection about how to deliver ACT-based pain management based on these principles is needed. The findings highlight the need for future research examining whether outcomes following ACT-based treatment for pain differ for those with clinically significant post-traumatic stress. Notably, people with the most complex PTSD presentations may struggle to access ACT-based pain management. Indeed, people with severe mental illness are often excluded from psychologically based pain treatments. ⁴⁷ Research is needed to understand how to meet the pain management needs of people with severe mental illness, including complex presentations of PTSD. Some patients may benefit from engaging with trauma-focused psychological therapy ⁴⁸ before or in addition to ACT-based pain management. Innovations in delivering pain care, such as up-skilling clinicians in mental health services in pain management, could also help address these disparities. ⁴⁷

Several limitations must be considered. Firstly, we did not conduct an in-depth structured clinical interview to diagnose PTSD. We also did not include an extended Criterion A assessment. We used a brief criterion A assessment, which was deemed appropriate with respect to trauma-informed practice, so that patients were not asked to provide extensive details about their trauma in the absence of appropriate clinical support. However, this may have influenced the estimated prevalence of probable PTSD. Approximately 4% of participants did not complete the traumatic exposure question. Additionally, some participants who did not complete the PCL-5 at baseline did complete this at post-treatment (data not reported here). Therefore, some individuals may not have felt able to reflect on these experiences at pre-treatment, which may have contributed to an underestimation of the baseline prevalence.

There were a relatively smaller number of participants (n = 191) in the analyses examining associations between PTSS severity and pain-related outcomes. However, the ratio of participants to variables in the models is more than double the rule-of-thumb minimum of 10 per variable. ⁴⁹ Furthermore, the cross-sectional design precludes causal interpretations of the associations between variables, as mentioned. Indeed, the reverse associations between post-traumatic stress and pain outcomes are plausible, as highlighted within the mutual maintenance model,^17,50 for example. Rather than identifying causal relationships, the current study should be interpreted as correlational and providing evidence only for the existence of associations between the variables. Finally, we dichotomized ethnicity for our analyses due to the relatively small numbers of participants within certain ethnic groups. While dichotomizing facilitated statistical analyses, doing so limits a more nuanced understanding of potential differences in the studied variables between specific groups.

In conclusion, exposure to trauma and the presence of clinically elevated PTSS are common among people engaging in ACT-based treatment for pain. Post-traumatic stress is associated with adverse pain-related outcomes. Trauma-informed practice is important when assessing suitability for and providing ACT-based treatment for pain. Research is needed to investigate the impact of trauma symptoms on treatment outcomes following ACT-based treatment for pain. Longitudinal and experimental intervention research are needed to disentangle the nature of the causal relationships between post-traumatic stress, psychological flexibility, and pain-related outcomes.

ORCID iD

Whitney Scott https://orcid.org/0000-0002-2529-9083

Ethical considerations

The study had ethical approval (South Central Oxford NHS Research Committee, 22/SC/0388).

Consent to participate

Written informed consent was obtained from all participants for their participation in this study.

Consent for publication

Not applicable as data pertaining to individual participants are not presented.

Data Availability Statement

Sharing of anonymized data for research purposes consistent with the original aims of the overall research database will be considered upon reasonable request.*