1Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access Acceptance and commitment therapy for mild traumatic brain injury (ACTion- mTBI): a quasiexperimental feasibility study Josh Faulkner ,1 Devin Prouty,2 Lucy Devlin,2 Damien Appleton,3 Maree Roche,4 Karen Below,5 John Moffat,1 Deborah Snell ,6 Matt N Williams ,7 Suzanne Barker- Collo,8 Alice Theadom 9 To cite: Faulkner J, Prouty D, Devlin L, et al. Acceptance and commitment therapy for mild traumatic brain injury (ACTion- mTBI): a quasiexperimental feasibility study. BMJ Open 2025;15:e089727. doi:10.1136/ bmjopen-2024-089727 ► Prepublication history and additional supplemental material for this paper are available online. To view these files, please visit the journal online (https://doi.org/10.1136/ bmjopen-2024-089727). Received 08 June 2024 Accepted 31 January 2025 For numbered affiliations see end of article. Correspondence to Josh Faulkner; josh. faulkner@ vuw. ac. nz Original research © Author(s) (or their employer(s)) 2025. Re- use permitted under CC BY- NC. No commercial re- use. See rights and permissions. Published by BMJ Group. ABSTRACT Objectives This study aimed to determine the feasibility of recruiting, implementing and delivering an acceptance and commitment therapy (ACT) intervention for mild traumatic brain injury (mTBI) (ACTion- mTBI) within a multidisciplinary outpatient mTBI rehabilitation services. The study also aimed to conduct a preliminary investigation of group differences between ACTion- mTBI and an equivalent cognitive behavioural therapy (CBT) intervention on various outcome measures and psychological treatment targets. Design A two- arm quasiexperimental feasibility study. Setting Five mTBI rehabilitation clinics throughout New Zealand. Intervention Psychologists working in mTBI rehabilitation clinics throughout New Zealand were trained to deliver ACTion- mTBI or CBT. Eligible participants were assigned to either of these interventions based on the psychologist available at the clinic they were referred to. ACTion- mTBI is a five sessions intervention that incorporates all six components of the ACT model. The CBT intervention is an equivalent intervention and incorporating all four components of the CBT model. Both interventions are adapted for an mTBI context. Primary outcome measures The primary outcomes were related to the feasibility of ACTion- mTBI. This included recruitment, retention and treatment adherence of participants, study procedure and fidelity of treatment delivery. Secondary outcome measures To explore group differences between ACTion- mTBI and CBT on functional disability, postconcussion symptoms, mental health, valued living and psychological flexibility. Results The intervention proved feasible to implement with community- based mTBI rehabilitation services. Attrition rates were comparable between the two psychological interventions and fidelity to the treatments was high. At post- treatment, when covarying pretreatment scores, ACTion- mTBI had a significantly greater improvement in functional disability than CBT (moderate effect). ACTion- mTBI also had a significantly greater reduction in postconcussion symptoms, anxiety and stress. Promisingly, significant improvements in psychological flexibility was also found post- treatment. There were no group differences on depressive symptoms and valued living. Conclusion We conclude that a full clinical trial of ACTion- mTBI for individuals with mTBI is feasible and warranted. Trial registration number ACTRN1262100059482. A mild traumatic brain injury (mTBI) results from a transfer of mechanical energy to the brain from external forces (eg, the head being struck by an object), which results in acute physiological disruption.1 According to the American Congress of Rehabilitation Medicine (ACRM), the term ‘concussion’ is synonymous with mTBI so long as struc- tural abnormalities are neither suspected nor detected on neuroimaging.1 Following mTBI individuals may experience postconcus- sion symptoms (PPCS) which can be somatic symptoms (eg, nausea, dizziness, headache, blurred vision, auditory disturbance and fatigue), cognitive complaints (concentration and memory difficulties), emotional and/ STRENGTHS AND LIMITATIONS OF THIS STUDY ⇒ This is the first study to examine the feasibility of a clinical trial of acceptance and commitment therapy (ACT) in mild traumatic brain injury (mTBI). ⇒ We developed an protocol that can guide the deliv- ery of ACT specifically for individuals with mTBI and compared it to an equivalent cognitive–behavioural therapy intervention. ⇒ The study occurred with an mTBI rehabilitation ser- vice illustrating that it is feasible to conduct a clin- ical trial within current health services; however, to enable this to occur, a quasiexperimental design is needed. ⇒ A consequence of this design, and embedding the trail within health services, is that it was not possi- ble to randomise to each of the intervention arms. ⇒ Conclusions regarding the effectiveness of ACTion- mTBI should not be made; however, preliminary group differences on specific outcome measures justify proceeding with a full clinical trial. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ http://orcid.org/0000-0001-9555-4628 http://orcid.org/0000-0003-1664-0603 http://orcid.org/0000-0002-0571-215X http://orcid.org/0000-0003-0351-6216 https://doi.org/10.1136/bmjopen-2024-089727 https://doi.org/10.1136/bmjopen-2024-089727 http://crossmark.crossref.org/dialog/?doi=10.1136/bmjopen-2024-089727&domain=pdf&date_stamp=2025-02-14 http://bmjopen.bmj.com/ 2 Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access or behavioural (eg, low mood and emotional lability). It is well established that PPCS can persist beyond the acute period following injury;2–5 commonly referred to as persistent PPCS.6 In a recent systematic review and meta- analysis, Cancelliere and colleagues3 concluded that after controlling for participant attrition, one- sixth of adults who present to an emergency department or trauma centre with mTBI report ongoing PPCS 3–6 months later. The burden of PPCS can be substantial resulting in reduced functioning,7 vocational participation,8 increased healthcare utilisation9 and decreased quality of life.9 Psychological factors, when considered alongside demographic variables and clinical indicators of brain injury severity, have consistently emerged as robust predictors of PPCS.10–13 As a result, it has been argued that PPCS have a biopsychosocial conceptualisation.14–17 Such models emphasise the role of preinjury vulnera- bilities, early physiological effects from the mTBI and over time, an increasing role of other factors such as psychological reactions, psychosocial and environmental stressors, coping strategies and medicolegal issues.18–21 As a result, clinical practice guidelines for mTBI under- score the importance of multidisciplinary rehabilitation of PPCS including the provision of mental health treat- ments, such as psychology.22–24 These interventions are particularly important in considering the high prevalence of mental health difficulties in PPCS.12 25 26 For example, a recent review reported prevalence rates of depres- sion and anxiety in adults with PPCS of 44% and 24%, respectively.25 A common approach to address mental health difficul- ties in following mTBI is cognitive–behavioural therapy (CBT).27–29 CBT explores the relationship between thoughts, emotions and behaviours, identifying those that are unhelpful and changing these with various tech- niques.30 31 Applied to mTBI, CBT may focus on identi- fying thoughts or emotions that an individual has about their PPCS or injury (ie, feeling fearful of getting a head- ache or exacerbating symptoms) and supporting indi- viduals to understand how these experiences influence behaviour (ie, avoiding tasks that could trigger a head- ache). A CBT approach would then introduce techniques to challenge these unhelpful cognitions (ie, cognitive restructuring) in the hope that they will result in more adaptive behaviour. Evidence of the effectiveness of CBT in mTBI is mixed.27 28 32 33 For example, in a recent system- atic review, Chen et al27 found that CBT was an effective treatment for anxiety, depression and social integration after mTBI. Additionally, Silverberg et al32 presented preliminary evidence to show that CBT may be beneficial in addressing fear avoidance and endurance behaviours after mTBI. However, two systematic reviews27 34 both found no support for the effectiveness of CBT on PPCS and functional disability. Consequently, there is a need to explore the efficacy of other psychotherapeutic modali- ties for mTBI rehabilitation. Acceptance and commitment therapy (ACT) is based on Relational Frame Theory with philosophical roots in functional contextualism, the idea that the function of behaviour changes based on the context and situation in which it is occurring.35 Hence, it is considered to be a contextual- based CBT and is commonly referred to as part of the ‘third wave cognitive behavioural therapies’.36 The focus of change with ACT is on the context of the distress rather than the content. Emotional problems are acknowledged as an inevitable part of living, and there- fore deemed to be a universal human experience. ACT aims to cultivate psychological flexibility by strengthening the following six core skills: acceptance, cognitive defu- sion, self as context, being present, values and committed action.37 ACT adheres to a health rather than illness model.35 Accordingly, the core goal of ACT is to facili- tate individuals to live a purposeful and meaningful life by focusing and engaging with their values despite their personal circumstances.37 ACT has a well- established evidence base for improving functioning and well- being in a variety of populations with psychological difficul- ties and/or medical problems38–40 (eg, cancer,41 chronic pain,42 tinnitus43 and chronic fatigue44). In the context of mTBI, ACT might be better at improving functional disability because it focuses on acceptance of persistent symptoms, fostering psychological flexibility, encouraging value- driven action even if symptoms are still present and reducing avoidance behaviours. This would have the potential at facilitating a increased activity, behavioural activation and increased mood, supporting habituation and desensitisation which are well known to improve PPCS.22 The evidence pertaining to the effectiveness of ACT in traumatic brain injury (TBI) is limited and has produced mixed results. There is some work that this psychological intervention reduces anxiety and depression,45–47 cogni- tive fusion (being overly attached to and dominated by thoughts), quality of life46 and psychological flexibility.47 Whereas, other studies have shown no improvements in psychological flexibility45 46 or value- driven behaviour.46 In addition, the literature has predominately focused on the effectiveness of ACT in severe TBI or mixed acquired brain injury cohorts. It is important to examine the effects of ACT exclusively in mTBI, given its unique pathophysiology, diversity of symptom presentation in terms of severity and duration, as well as recovery trajec- tories. In addition, the focus of treatment is likely to differ, for mTBI, therapy strives to return an individual to their baseline functioning.48 In more severe injuries, treatment often focuses on adjusting and living with the permanent consequences of an injury.49 Faulkner and colleagues50 found that psychological flexibility (the core focus of the ACT approach) significantly contributed to mTBI outcomes and the development of persistent symptoms and functional disability over time. Thus, given the current paucity of empirical evidence, there is a clear rationale for examining ACT in mTBI. However, to ensure that the treatment is acceptable and feasible to implement within the context of mTBI, a feasibility study is needed before clinical effectiveness can be determined. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 3Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access The present study aimed to achieve this by embedding an ACT informed intervention for mTBI (ACTion- mTBI) into multidisciplinary outpatient mTBI rehabilitation services. As this study occurred within a health service, allocation to a waitlist condition was not appropriate and therefore ACTion- mTBI was compared with an equiv- alent CBT intervention. The primary objective of this study was to determine outcomes related to the feasibility of ACTion- mTBI. This included recruitment, retention and treatment adherence of participants, study proce- dures and fidelity of treatment delivery. The second objective was to explore possible group differences in various outcomes between ACTion- mTBI and CBT post- treatment. As a primary objective of rehabilitation is to improve functional status (ie, engagement in func- tional activities), this was the primary outcome used for this purpose. Group differences on secondary outcome measures post- treatment: PPCS, mental health, valued living and ACT targeted treatment processes (psycholog- ical flexibility) were also explored. METHOD All reporting is in accordance with Consolidated Stan- dards of Reporting Trials (CONSORT).51 Design The study was initially designed as a randomised feasibility trial, with pretest–post groups, within mTBI rehabilita- tion services in NZ (commonly referred to as ‘concussion services’). Embedding the study within current health- care practice, enabled evaluation of the feasibility of delivering ACTion- mTBI as part of real- life routine reha- bilitation practice. This enhances the real- world applica- bility of the study findings, as well as increasing the ease in which the intervention can be rolled out into clinical practice if found to be effective in a future full- scale clin- ical trial. There are, however, ethical and practical consid- erations that need to be considered when adopting such a design. Concussion services in NZ specify that individuals referred for psychological treatment must be seen within 4 weeks. Consequently, it was not appropriate to adopt a waitlist- controlled study design. Instead, the two arms of the study were ACTion- mTBI and CBT to ensure that all participants enrolled in the study were provided with a psychological treatment. Randomisation by random number generation into the ACT or CBT or treatment arms was initially planned to be conducted on a 1:1 basis. This design was adopted as the study was going to be conducted at one clinic with multiple clinicians available to deliver either ACTion- mTBI or CBT. However, due to unforeseen circumstances, it was not plausible to conduct the study at one clinic, as it eventuated that only one clinician at the original site was available to deliver the intervention. Consequently, a quasiexperimental feasibility study with pretest–post- test non- equivalent groups design was adopted. More specif- ically, to ensure adherence to a two- arm design and to support adequate recruitment, four clinics from different geographical locations were added into the study. This enabled additional psychologists within these clinics to deliver the interventions. ACTion- mTBI was offered in two clinics and CBT was offered in three. Thus, partic- ipants were assigned to the intervention offered by the psychologist at the clinic they were referred to. Clinics were similar in that each one was contracted by ACC to provide concussion rehabilitation and therefore service provision occurred under the proviso of these contracts. Differences between clinics occurred based on clinician experience and training in mTBI. However, due to the feasibility objective of this study and the small sample sizes required to meet these objectives, controlling for these nuance differences in clinics was not conducted. Participants and procedure All participants were recruited from Accident Compensa- tion Cooperation (ACC) funded concussion services. In NZ, the no- fault insurer, ACC, contracts specialty clinics to provide mTBI rehabilitation for people post- mTBI. To be eligible for these services, individuals must have sustained an mTBI within the past year, have ongoing PPCS and have at least one additional risk factor (eg, inability to return to work or school for more than a week, have a high functioning job, a pre- existing psychi- atric condition). Referral pathways to this tertiary service are generally from a primary care provider (ie, Hospital ED, Primary Care) and take on average 60 days from time of injury to first engagement with a concussion service provider.52 Consequently, individuals who engage in this service are beyond the acute phase of injury and experi- encing functional impacts due to symptom persistence. These services take a multidisciplinary approach to assess- ment and treatment of mTBI. All individuals referred receive occupational therapy and physiotherapy input, and may also have any of the following: neuropsycho- logical screening assessment, specialist medical review and clinical psychology treatment. The only clinical psychology treatment provided for participants in our study was either ACTion- mTBI or CBT. Recruitment took place between August 2021 and June 2023 and concluded due to study funding requirements. However, the study was paused between January 2022 and August 2022 due to COVID- 19 and the periods of government- imposed lock- downs during this time. Five participants completed the study prior to these lockdowns. Given this small number, we were not able to examine the effects of COVID- 19 on the study outcomes. Individuals engaged in concussion services were informed about the study by a clinician. Potential partic- ipants who consented to be involved were screened for eligibility based on the following criteria: (1) a clinical diagnosis of mTBI by a medical specialist predominately using the ACRM53 or WHO54 criteria which include those with or without CT abnormalities (ie, complicated and uncomplicated), (2) adults between 16 years and 65 years of age, (3) engaged in a multidisciplinary concussion P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 4 Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access rehabilitation service and (4) had elevated levels of PPCS as indicated by a score on the Rivermead Post- Concussion Questionnaire of 16 or greater55 or had elevated distress as indicated by a score in the moderate range or greater on at least one subscale of the Depression Anxiety and Stress Scale (DASS- 21). Participants were excluded if they had an unstable or severe comorbid mental or physical health condition which could affect participation in the intervention or reporting on the outcome measures. Participants engaged in the study continued to receive multidisciplinary input as part of the concussion service rehabilitation. Intervention In many mTBI rehabilitation settings, only a limited number of psychological services are funded and avail- able. For example, in NZ, only 5 hours of psychological services are funded by the national ACC. Consequently, to aid in the implementation of an ACT approach into clinical practice, there is also a need for ACT to fit within current mTBI service provision and be redesigned and tested within a shorter intervention timeframe. As a result, we developed an ACT- based intervention for mTBI (ACTion- mTBI), a five- session treatment which aims to apply the ACT model to improve recovery after mTBI. The ACTion- mTBI and the CBT interventions consisted of five 1 hour sessions delivered by a Registered Psycholo- gist trained in the specific intervention approach. Acceptance and commitment therapy intervention for mild traumatic brain injury The development of the ACTion- mTBI programme was informed by focused ACT56 and modified to fit the time- limit on psychological services within concussion services. The treatment manual was designed in consultation with a trained facilitator and expert in ACT with extensive experience in mTBI (JM). ACTion- mTBI incorporates all six components of the ACT model and adapts experien- tial exercises, metaphors, discussions and homework to an mTBI context. This aims to: (a) cultivate awareness and acceptance of thoughts and emotions about mTBI, (b) recognise the impact that rigid thoughts/beliefs, behaviours and responses to emotions have on mTBI recovery, and create a more flexible response to such experiences and (c) clarify personal values and commit to pursuing meaningful activities aligned with these values while recovering from mTBI. A summary of the ACTion- TBI protocol is presented in table 1. A recent report by the Association for Contextual Behavioural Science (ACBS) task force57 stated: ‘It is unhelpful to allow applied psychological science to remain at the level of extensive intervention protocols, when the spirit of ACBS idiographic functional analysis linked to processes of change requires a more personalized approach’ (p.176). In accordance with this recommendation, ACTion- mTBI was delivered with flexibility based on clinical judgement according to the participant’s presentation. For example, if a central feature was severe fusion with unhelpful cognitions driving the presenting issues, then session 3 ‘unhooking from the mind’ was delivered earlier in the intervention. What is critical in the delivery of ACTion- mTBI is that the therapist adopts an ACT stance, using the ACTion- mTBI manual to guide delivery of the inter- vention and that all aspects of the ACT model are covered within the five sessions. ACTion- mTBI was delivered by JF a board registered Clinical Psychologist and Neuropsy- chologist who has 8 years of clinical experience working with mTBI and advanced- level training in ACT. CBT intervention The CBT protocol was informed by brief CBT thera- peutic intervention protocols.58 The treatment manual (see table 1) was designed in consultation with a clinical neuropsychologist who is trained in CBT with extensive clinical experience with mTBI (LD). The CBT interven- tion protocol incorporates all four components of the CBT model. It aims to: (1) educate participants on how each component of this model is linked and the role these connections have within an mTBI context, (2) iden- tity unhelpful thinking patterns as they pertain to mTBI recovery and (3) learn a range of CBT- informed skills that can mitigate the influence that each component of the CBT model has on mental well- being. Consistent with ACTion- mTBI, the CBT manual was also delivered flex- ibly based on clinical judgement, what is critical in the delivery of this intervention is that the clinician adopts the CBT therapeutic model. It was initially planned that the CBT intervention would be delivered by one clinician in the same manner as the delivery of ACTIon- mTBI. However, due to unforeseen circumstances with clinician availability, the CBT interven- tion was delivered by three clinicians: LD (three partici- pants), DP (17 participants) and DA (three participants). All clinicians are board- registered Clinical Psychologists with over 10 years of experience with mTBI. Patient and public involvement Patients and members of the public were involved throughout the design and implementation of this study. During the development phase, input from individuals with lived experiences of mTBI was sought to ensure that the intervention was relevant and met the needs of patients receiving concussion services. Stakeholder feed- back informed the adaptation of the ACTion- mTBI inter- vention to align with the time constraints and funding limitations of the New Zealand ACC framework. Patients contributed to refining the recruitment mate- rials and processes to ensure clarity and accessibility. During the implementation phase, patient representa- tives were consulted to monitor the appropriateness of the interventions and facilitate iterative improvements where necessary. While patients were not involved in conducting the statistical analyses, their feedback will be sought for interpretation and dissemination of findings in a manner that is meaningful and accessible to individ- uals with mTBI. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 5Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access Outcome measures To assess the group differences between the interven- tions, outcome measures were administered by a research assistant at pretreatment (T1) and immediately post- treatment (T2). Primary clinical outcome Functional status: functional disability was assessed using 12- item WHO Disability Assessment Schedule (WHODAS V.2.0). The WHODAS V.2.0 evaluates disability based on the International Classification of Functioning activity and participation: cognition, self- care, mobility, inter- personal functioning, life activities and participation.59 60 The WHODAS V.2.0 asks respondents how much diffi- culty they have had in the past 30 days in relation to all their health problems for each of the 12 items. The rating scale options are as follows: 0=none, 1=mild, 2=moderate, 3=severe and 4=extreme/cannot do (higher scores repre- sent greater disability). Scores on each item were summed Table 1 A summary of the ACTion- mTBI and CBT manual content Session number ACTion- mTBI protocol CBT manual content Session title ACT therapeutic process Content Session title CBT component Content 1 Dropping the struggle Acceptance contact with the present moment Snapshot of life Identifying the main problem Dropping the struggle/creative hopelessness Opening up and acceptance (dropping anchor) Homework Thinking, feeling and behaving are linked Introduce the link between thoughts, emotions, behaviour and physiology. Focus on physiology. Agenda Snapshot of life Identifying the main problem Concussion psychoeducation Brief introduction to CBT Relaxation technique (diaphragmatic breathing) 2 Moving towards Acceptance contact with the present moment Values committed action Dropping anchor exercise Recap and review of the main problem Values exploration Choice point model Homework Thinking can have errors Education on the link between thoughts, feelings, behaviour and physiology with functional examples. Focus on physiology and thoughts. Agenda Review session 1 Education on the CBT model (with reference to the mood monitoring worksheet) Education on thinking styles Relaxation technique (progressive muscle relaxation) 3 Unhooking from the mind Acceptance contact with the present moment Defusion Mindfulness exercise Recap and review of main problem Functional analysis of choice point model Defusion education and exercise Homework Restructuring Physiology, feelings and thoughts. Relaxation technique Agenda Review session 2 Main focus on thoughts and how this impacts mood Use homework to complete seven part thought record—challenge with alternate thought 4 The noticing self and committing Acceptance contact with the present moment Self as context values committed action Mindfulness/self as context exercise Recap and review of the main problem Self as context education and excise Values bullseye exercise and SMART goals Engaging in what is rewarding Physiology, feelings and behaviour. Relaxation technique Agenda Review session 3 Main focus on behaviour and how this impacts mood Review behaviour activity to facilitate positive event scheduling 5 Review and relapse prevention Acceptance contact with the present moment Diffusion Self as context Values committed action Mindfulness/self as context exercise Review of presentation Recap on intervention and experiential exercises Relapse prevention Discharge planning Review and relapse prevention Physiology, feelings, thoughts and behaviour. Relaxation technique Agenda Review session 4 Review activity schedule— increase activities that bring sense of pleasure and achievement Recap and review intervention Relapse prevention Discharge planning ACTion- mTBI, acceptance and commitment therapy intervention for mild traumatic brain injury; CBT, cognitive behavioural therapy. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 6 Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access and total scores were used. Snell et al61 showed that the 12- item WHODAS V.2.0 had high internal consistency in mTBI (Cronbach’s alpha=0.92). Secondary clinical outcomes PPCS: the Rivermead Post- Concussion Symptom Ques- tionnaire (RPQ) is a 16 item self- report questionnaire that assesses common symptoms following mTBI (Cron- bach’s alpha=0.90).62 The RPQ consists of somatic symp- toms (headaches, dizziness, nausea and vomiting, noise and light sensitivity, sleep disturbance and double vision); cognitive symptoms (forgetfulness/poor memory, poor concentration and taking longer to think) and emotional symptoms (being irritable/easily angered feeling depressed or tearful, feeling frustrated or impa- tient). Participants are required to rate the presence and problem status of these symptoms on a scale of 0–4 (0=not experienced at all; 1=no more of a problem than before injury; 2=a mild problem; 3=a moderate problem; 4=a severe problem). Scores of 1 (‘no more of a problem than before injury’) were recoded to 0 as per the recom- mendation of King et al.62 Depression, anxiety, stress: the Depression, Anxiety and Stress Scale- 21 (DASS- 2163) is a 21 item self- report with three subscales that measure depression, anxiety and stress over the previous week. It uses a 4- point Likert scale with 0=never, 1=sometimes, 2=often and 3=always. Higher scores on this measure are indicative of elevated levels of depression, anxiety and stress symptoms. A score of at least 7 on the depression scale, 6 on the anxiety scale and 10 on the stress scale were classified as moderate in severity and used for participant inclusion. The DASS- 21 has good psychometric properties (Cronbach’s alpha=0.73–0.8163) and is a valid measure of depression, anxiety and stress symptoms in people with ABI.64 Valued living: the Valued Living Questionnaire (VLQ) is a two- part instrument designed to assess valued living.65 In the first part, participants rate the importance of 10 domains of living on a 10- point Likert scale. These domains are family, couples’ relations, parenting, friend- ship, work, education, recreation, spirituality, community life and physical well- being. In the second part, partici- pants rate, using a 10- point Likert scale, how consistently they had lived by the valued behavioural pattern within each domain over the past week. Responses from both parts are used to calculate a valued living composite (VLC), which quantifies the extent to which one is living out particular values in everyday life. An overall VLC is calculated, as well as for each valued domain within the VLQ. The VLQ has demonstrated adequate interitem consistency (α=0.77) and test–retest reliability (r=0.75).65 The VLQ has been used to measure valued living in a range of clinical populations including anxiety, chronic pain, depression and TBI.66 67 Treatment process measures Psychological flexibility: when measuring psycholog- ical flexibility, it has been recommended that both a context- specific and a more generic measure of psycho- logical flexibility should be used.68 Consequently, the Action and Acceptance Questionnaire Acquired Brain Injury (Reactive Avoidance) (AAQ- ABI (RA) and the Acceptance and Action Questionnaire Second Edition (AAQ- II)) were administered. The AAQ- ABI (RA) uses a 5- point Likert scale (0= ‘never true’ to 4 = ‘always true’) with scores ranging from 0 to 36; higher scores indicate greater reactive avoidance asso- ciated with an ABI. The measure has been developed and validated in an undifferentiated sample of ABI (Cronbach’s alpha 0.8969). In the current study, items within the ques- tionnaire were modified by replacing ‘brain injury’ with ‘concussion’. Faulkner and colleagues70 found that the AAQ- ABI (RA) has one distinct underlying factor in mTBI and strong internal consistency (Cronbach’s α=0.87). The Acceptance and Action Questionnaire Second Edition (AAQ- II) is a 7- item questionnaire utilising a 7- point Likert scale (response format, 1 = ‘never true’ to 7 = ‘always true’) with scores ranging from 7 to 49. Higher scores are indicative of greater experimental avoidance a component of psychological flexibility.67 The AAQ- II has been found to have good reliability and validity, with the Confirmatory Factor Analysis of this measure in supporting a one- factor model (Cronbach’s alpha ranging from 0.78 to 0.8871). Statistical analysis A per protocol analysis was conducted and Shapiro- Wilk tests found no evidence against an assumption of normality in the data. Independent- sample t- tests and χ2 tests were first conducted to examine potential differences in the demographic and injury characteristics between the two groups. Descriptive statistics were used to describe the measures at pretreatment and independent- sample t- tests were computed to determine any differences between groups. χ2 was also used to determine any differences in attrition between the two treatment groups. To examine group differences between the two interventions on the primary and secondary outcomes measures, analysis of covariance (ANCOVA) was used. Analyses included post- treatment scores as the outcome variable, pretreatment scores as a covariate and a between- subjects factor (group: CBT or ACTion- mTBI). By assessing the difference in post- test means while accounting for pretest values, ANCOVA provides more statistical power.72 73 Effect sizes (Cohen’s d) were also calculated and interpreted according to standard conventions (<0.50 = small; 0.50–0.80=medium; >0.80 = large74). Alpha levels of 0.05 alpha without correc- tion for multiple comparisons were applied, given that the analyses are exploratory and each outcome is best consid- ered as individual tests rather than disjunction tests.75 Analyses were performed using SPSS V.29.0 (IBM, USA). RESULTS Recruitment and retention feasibility Sixty- one individuals who expressed interest in the study and were deemed eligible provided written informed P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 7Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access consent to participate. As shown in figure 1, of those allo- cated to ACTion- mTBI, 75.0% completed the interven- tion. 79.3% allocated to the CBT intervention completed the intervention, with no significant difference in attri- tion (χ2(1) = 0.01, p=0.95). In the ACTion- mTBI group, 30 participants (92.86%) attended at least one session. Those who did not attend treatment were unable to do so because of illness (n=1) or unknown (n=1). Twenty- four participants (80.0%) attended all five sessions of ACTion- mTBI. Of the six who did not attend all sessions, two participants did not require all treatment sessions and four disengaged from treatment for unknown reasons. This sample size was consistent with sample sizes of 24–35 participants conventionally recommended for pilot and feasibility studies.76–78 In the CBT group, 28 of the 29 participants enrolled in this treatment attended at least one session. Twenty- three participants completed the intervention (82.1%). Of the five who did not attend all sessions, two did not require all treatment sessions and three disengaged from treatment for unknown reasons. Recruitment proved to be feasible with an average of six participants recruited into either treatment arm per month (in the last 8 months of the study where there were no COVID impacts on service access). Treatment fidelity feasibility Fidelity checks of the ACTion- mTBI delivery were completed by JM on 10% of sessions to ensure adher- ence to the ACT model, using the ACT fidelity checklist measure (ACT- M).79 The average rating on the ACT- M was 34.0 out of a total score of 36 indicating high consistency with the ACT model. 10% of CBT sessions by different psychologists were independently assessed to ensure adherence to the CBT therapeutic model using the Comparative Psychotherapy Process Scale- External Rater form (CPPS).80 The CPPS assesses the degree to which a therapist uses techniques of cognitive–behavioural psychotherapy. Fidelity measures were completed by AT a Registered Psychologist with training and expertise in CBT. The average rating on each on the CBT subscale was 5.42 out of 6 indicating high adherence to the CBT therapeutic model.81 Group differences Data on the demographic and clinical characteristics of the groups are summarised in table 2. There were no significant differences in the demographic and clin- ical characteristics of the ACTion- mTBI group and CBT groups except on age; the ACTion- mTBI group were, on average, significantly younger. Scores on the outcome measures at pretreatment assessment were also compared between the ACTion- mTBI and CBT groups. As shown in table 2, there were no significant differences in the outcome measures and treatment processes at pretreat- ment assessment. Figure 2 shows the mean scores for functional disability at pretreatment and post- treatment for each group. Based on an ANCOVA accounting for pretreatment scores, there was a significant difference between the groups (F (1,46) = 9.84, p=0.003) at post- treatment assessment with a medium effect size (d=0.58). More specifically, the ACTion- mTBI group had significantly lower scores on functional disability (M=10.25, SE=1.22) than the CBT group (M=14.91, SE=1.22) at post- treatment. Secondary clinical outcomes were PPCS, depression, anxiety, stress and valued living. Based on an ANCOVA, covarying pretreatment scores, a significant difference between the treatment groups was found in PPCS with Figure 1 Overview of participant involvement in the study. An illustration of participant recruitment into the two arms of the current study. The direction of flow in the illustration depicts participant enrolment in the two intervention conditions, pretreatment assessment, engagement in the intervention and post- treatment assessment. ACtion- mTBI, acceptance and commitment therapy intervention for mild traumatic brain injury; CBT, cognitive–behavioural therapy. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 8 Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access small effect size (F (1,46) = 8.02, p=0.007, anxiety with medium effect size (F (1,46) = 5.35, p=0.025) and stress with small effect size (F(1,46) = 4.24, p=0.045) at post- treatment assessment. The ACTion- mTBI group had significantly lower scores on each of these secondary clin- ical outcome measures than the CBT group (see table 3). No significant group differences were found on post- treatment scores, while covarying pretreatment scores, on depression (F (1,47) = 0.66, p=0.421) and valued living (F (1,47) = 1.07, p=0.307)). Treatment processes included context- specific measures of reactive avoidance (AAQ- ABI(RA)), and a general measure of experiential avoidance (AAQ- II). Table 2 Demographic and injury characteristics of participants at pretreatment assessment ACTion- mTBI (n=24) CBT (n=23) P value Demographic characteristics Age 31.65 (11.84) 39.70 (16.90) 0.04* Gender Male 9 (37.50%) 11 (47.83%) 0.47 Female 15 (62.50%) 12 (52.17%) Ethnicity Māori† 2 (8.33%) 5 (21.74%) 0.14 NZ European 17 (70.83%) 17 (73.91%) Other 5 (20.83%) 1 (4.35%) Education history High school or less 6 (25.00%) 5 (21.74%) 0.79 University/ tertiary 18 (75.00%) 18 (78.26%) Preinjury employment Working 23 (95.83%) 21 (91.30%) 0.53 Not working 1 (4.17%) 2 (8.70%) Medical history Yes 8 (33.33%) 11 (47.83%) 0.25 No 16 (66.67%) 12 (52.17%) Previous concussion(s) Yes 12 (50.0%) 12 (52.17%) 0.88 No 12 (50.0%) 11 (47.83%) Mental health history Yes 15 (62.50%) 12 (52.17%) 0.39 No 9 (37.50%) 11 (47.83%) Injury characteristics Time since injury (weeks) Pretreatment 25.58 (18.83) 25.09 (19.95) 0.47 Post- treatment 33.68 (14.04) 37.86 (22.05) 0.61 Mechanism of injury Motor vehicle accident 2 (8.33%) 4 (17.39%) 0.58 Fall 9 (37.50%) 11 (47.83%) Assault 5 (20.83%) 2 (8.70%) Hit by object 8 (33.33%) 5 (21.74%) Additional injury Yes 13 (54.17%) 16 (69.57%) 0.28 No 11 (45.83%) 7 (30.43%) Continued ACTion- mTBI (n=24) CBT (n=23) P value Pretreatment outcome measures RPQ 34.42 (10.48) 30.35 (13.00) 0.12 DASS- D 9.04 (4.72) 9.78 (9.63) 0.37 DASS- A 6.50 (4.17) 7.00 (4.48) 0.35 DASS- S 11.21 (4.32) 10.35 (5.14) 0.27 WHODAS V.2.0 20.79 (7.98) 19.17 (8.42) 0.25 AAQ- ABI (RA) 18.83 (6.48) 18.65 (6.64) 0.46 AAQ- II 29.17 (7.82) 27.39 (7.80) 0.22 VLQ 41.29 (15.03) 40.87 (14.85) 0.46 *P<0.05. †Māori are the indigenous population of NZ. AAQ- ABI (RA), Action and Acceptance Questionnaire Acquired Brain Injury (Reactive Avoidance); AAQ- II, Action and Acceptance Questionnaire Second Edition; DASS- A, Depression, Anxiety and Stress Scale- 21- Anxiety Subscale; DASS- D, Depression, Anxiety and Stress Scale- 21- Depression Subscale; DASS- S, Depression, Anxiety and Stress Scale- 21- Stress Subscale; RPQ, Rivermed Post Concussion Questionnaire; VLQ, Valued Living Questionnaire; WHODAS V.2.0, WHO Disability Assessment Schedule V.2.0. Table 2 Continued Figure 2 Group differences on self- reported measure of functional disability at pretreatment and post- treatment assessment time points. Line graph describing the changes in functional disability. The X- axis represents pretreatment and post- treatment and the Y- axis represents average scores on the WHO Disability Assessment Schedule (WHODAS V.2.0). The graph shows a decrease in average WHODAS V.2.0 scores from pretreatment to post- treatment across both groups (ACTion- mTBI and CBT) with the most significant decrease in the ACTion- mTBI group. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 9Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access Based on ANCOVA (see table 3) covarying pretreat- ment scores, significant group difference was found on the AAQ- ABI (RA) with medium effect size (F (1,47) = 5.04, p=0.030). More specifically, the ACTion- mTBI had significantly lower levels of context specific reactive avoid- ance (M=9.32, SE=1.02) than the CBT group (M=12.58, SE=1.04). There were no significant group differences at post- treatment on the AAQ- II (F (1,47) = 0.83, p=0.367). Finally, all ANCOVA models were rerun to include age as a covariate given that significant differences in this variable were found between the intervention groups. No significant effects of age were found on the primary, secondary and treatment process post treatment (see online supplemental table 1). DISCUSSION The overall objective of this study was to explore the feasibility and group differences of an ACT- informed intervention for mTBI (ACTion- mTBI) embedded within community- based multidisciplinary rehabilitation services when compared with an equivalent CBT inter- vention. Our first aim was to determine the feasibility of recruitment, retention and treatment adherence of participants, study procedures and fidelity of treatment delivery of ACTion- mTBI within the context of concus- sion services in NZ. A randomised feasibility trial was initially planned to be conducted at one clinic. This approach was not achievable and to ensure adequate recruitment using the procedure specified, additional clinics and treating psychologists were added to the study. Consequently, the study design shifted to a quasiexperi- mental design and participants were allocated to either ACTion or CBT based on the treatment that was avail- able at the clinic the participant was referred to. This research design was feasible to conduct within concussion services. Participants were able to engage in the research and intervention protocol while also receiving multidis- ciplinary rehabilitation. Current consensus guidelines for the treatment of mTBI stipulate the importance of multidisciplinary care to maximise treatment outcomes for this population.22 24 It is promising that we were able to conduct this quasiexperimental feasibility study within this context to replicate best practice guidelines. It also demonstrates the plausibility of conducting a larger- scale clinical trial using this design. We also found that fidelity to the treatments was very high, which further supports the applicability of delivering these psychological inter- ventions within a concussion service. To further support feasibility, when removing the impact of COVID- 19, we were also able to recruit an average of six participants into either treatment arm per month. This enabled us to achieve the sample size of 24–35 partici- pants per arm conventionally recommended for pilot and feasibility studies.76–78 Furthermore, there was very high uptake into the treatment arms as part of the recruitment process. Attrition rates were low and comparable between ACTion and CBT with 80.0% of participants attending all five sessions of ACTion- mTBI and 82.1% attending all five sessions of CBT. This provides confidence that recruitment and data collection can be obtained from an adequate sample that would be needed for full- scale trial. The second aim of this study was to conduct a prelim- inary test of the trends in effects of ACTion- mTBI comparted to CBT. To achieve this, a range of outcome and treatment process measures completed by the ACTion- mTBI group were compared with CBT. We found that, after covarying pretreatment scores, ACTion- mTBI had a significant reduction in functional disability (medium effect size) and PPCS severity (small effect size) compared with CBT. This finding is somewhat consistent with Dino and Colleagues,38 who found that veterans with mTBI participating in a 1 day ACT workshop experienced Table 3 F statistics for post- treatment secondary clinical outcome and treatment process measures from ANCOVA (using the pretreatment scores as a covariate) ACTion- mTBI Mean±SE CBT Mean±SE Mean difference (ACTion- CBT) 95% CI F statistic P value Cohen’s d Secondary outcomes Post- concussion symptoms 15.21±2.23 24.31±2.28 −9.10 −15.58 to −2.63 8.02 0.007 0.47 Depression 4.11±0.66 4.88±0.68 −0.77 −2.68 to 1.14 0.66 0.421 0.21 Anxiety 3.74±0.70 6.06±0.72 −2.32 −4.34 to −0.30 5.35 0.025 0.60 Stress 5.05±0.63 7.35±0.64 −1.84 −3.65 to −0.04 4.24 0.045 0.41 Valued living 49.12±2.54 45.36±2.60 3.63 −3.56 to 11.08 1.07 0.307 0.33 Treatment process measures AAQ- ABI (RA) 9.32±1.02 12.58±1.04 −3.27 −6.21 to −0.33 5.04 0.030 0.52 AAQ- II 21.33±1.22 22.92±1.25 −1.59 −5.12 to 1.93 0.83 0.367 0.12 AAQ- ABI (RA), Action and Acceptance Questionnaire Acquired Brain Injury (Reactive Avoidance); AAQ- II, Acceptance and Action Questionnaire Second Edition; ANCOVA, analysis of covariance; CBT, cognitive behavioural therapy. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as https://dx.doi.org/10.1136/bmjopen-2024-089727 http://bmjopen.bmj.com/ 10 Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access improvements in functioning and reintegration post- workshop compared with the treatment as usual group. ACT adopts a health model, and has an emphasis on enhancing functioning, life satisfaction and the ability to pursue meaningful goals, rather than focusing on symptom reduction.82 83 This approach may resonate with individuals who do not recover naturally from mTBI, and they may therefore benefit from adopting an acceptance- based stance. This includes a strong focus on validating and acknowledging the difficulties associated with their mTBI rather than a focus on reframing their thoughts.84 85 These findings are also consistent with previous evidence suggesting the effects of CBT may be more specific to mental health and do not generalise to improve mTBI outcomes.27 28 34 Although, replication of these findings is needed in a larger sample, this study provides promising preliminary evidence that adopting an ACT approach in mTBI rehabilitation may have cascading positive effects on improving functioning and reducing PPCS severity. In terms of mental health, for depression, no significant differences between the ACTion- mTBI and CBT groups were found post- treatment. Although, significantly lower levels of anxiety (medium effect size) and stress (small effect size) were found in the ACTion- mTBI group post- treatment after covarying pretreatment scores. It is surprising that ACTion- mTBI and CBT both had a similar effect on depression symptomology, whereas ACTion- mTBI appeared to have a greater impact on reducing anxiety and stress symptoms. Brief psychological interven- tions either using a CBT or ACT approach have demon- strated similar effects of reducing a range of mental health presentations.86 87 Given the feasibility nature of this study and limited sample size, replication of this study is needed to confirm the effect that ACTion- mTBI has on specific mental health outcomes. However, these find- ings do add to an emerging evidence base demonstrating potential benefits of ACT on mental health difficulties in neurological conditions, including TBI.45–47 Our study provides initial support for clinicians to adopt a wider repertoire of psychological interventions, such as ACT, in mTBI rehabilitation to address mental health difficulties after mTBI. However, we found no significant difference between ACTion- mTBI and CBT on valued living post- treatment. This was a surprising finding given that there is a focus in ACTion- mTBI to support individuals to identify and increase their engagement in values- based activities. There could be multiple reasons for this finding and further examination in future studies with a larger sample is needed. It may be that as both treatments occurred within a multidisciplinary mTBI where the focus is on a graduated return to activity, that overtime this rehabili- tation inadvertently increases valued living engagement. It should also be noted that previous research on the impact of ACT on values- based participation has revealed mixed findings in TBI,46 47 which may reflect the way in which valued living is assessed in this population. Miller et al88 recently identified validity problems of the valued living questionnaire in a cohort of individuals with ABI. The results of this study could therefore reflect limita- tions in the way the construct is measured for this popula- tion. Future research could address this gap and examine the effects of ACTion- mTBI on valued living using other measures (ie, the Valuing Questionnaire89). The third aim of this study was to explore the effects of ACTion- mTBI on ACT- targeted treatment process. In this study, psychological flexibility was examined in two ways. This included assessing an mTBI context- specific compo- nent of psychological flexibility and a general measure of experiential avoidance. At post- treatment, the ACTion- mTBI group had significantly lower mTBI- specific and general experiential avoidance than CBT. No significant differences were found at post- treatment on the general measure of experiential avoidance. Given that ACTion- mTBI aims to specifically target mTBI- related presenting difficulties, it is not surprising that the effects of this intervention appear to be contained to context- specific treatment processes. The core aim of ACT is to increase psychological flexibility and it is therefore promising that there is initial evidence that ACTion- mTBI is targeting a component of its intended process. Furthermore, given evidence that psychological flexibility influences mTBI outcomes and recovery,90 targeting this process in the ACTion- mTBI intervention may also account for the reduction in PPCS and improvement in functional status observed in this study. However, in this study, only one facet of psychological flexibility was measured and future research would benefit from using a more comprehen- sive measure of this construct (ie, the Multidimensional Psychological Flexibility Inventory.91 The quasiexperimental design of this study enabled feasibility and trends in effect of the intervention to be examined within the clinical environment that it will be delivered in if found to be an effective treatment. However, embedding this study in current practice does have limitations that need to be acknowledged. It is chal- lenging to infer if the effects identified in this study are due to the psychological treatments provided or due to other factors that are less well controlled in this study design. For example, although the influence of engaging in a multidisciplinary rehabilitation service was likely miti- gated by including two treatment groups that were both engaged within such a service, the nature of the reha- bilitation provided may differ across each group due to the different clinics participants were referred to. Conse- quently, future research with a larger sample should include site stratification in the analysis of the effects of the treatment to control for these potential confounding effects. Furthermore, ACTion- mTBI was delivered by one clinician across two clinics; whereas the CBT intervention was delivered by three across three clinics. Due to our small sample size, we also could not feasibly account for clinician or clinic effects in our analyses. It is therefore crit- ical that the next step in the evaluation of ACTion- mTBI is to determine that these initial findings generalise to other clinicians and clinics who deliver this intervention. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://bmjopen.bmj.com/ 11Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access Post- treatment effects were also only measured at one time and therefore any inferences regarding the sustain- ability of these effects over time cannot be determined. Future research will need to ensure serial post- treatment assessments to ensure the effects of the treatment are examined over time. Participants in both treatment groups were also more likely to be female, middle age and of European ethnicity; however, this does not represent the most common characteristics of individuals who expe- rience mTBI in NZ.92 As part of the feasibility analysis of ACTion- mTBI, participants also engaged in an interview post- treatment to ascertain their perception of the inter- vention.93 The feedback demonstrated that the ACTion- mTBI approach was highly valued by Māori (indigenous people of NZ), but may reflect that fewer Māori were referred to or chose to access the service where these psychological interventions were delivered, or were less able to access the service. It is imperative that the efficacy of ACTion- mTBI is established with a more diverse and representative sample of individuals who require psycho- logical treatment as part of their mTBI recovery. In conclusion, we determined feasibility of recruitment and study procedures of a quasiexperimental feasibility study of ACTion- mTBI within concussion services in NZ. ACTion- mTBI was found to significantly reduced func- tional disability, PPCS, anxiety, stress and a component of psychological flexibility. However, there were no observed effects for depression or valued living. The results indi- cate that ACTion- mTBI shows promising preliminary effects in not only addressing mental health difficulties in mTBI, but also in improving mTBI- specific outcomes. However, clinical effectiveness of ACTion- mTBI needs to be determined in a large- scale clinical trial, with a larger, more representative sample, with multiple clinicians delivering the intervention, and tracking the impact of outcomes over time. This study illustrates that a full clin- ical trial is feasible and provides initial evidence that this is warranted for people recovering from mTBI. Author affiliations 1Te Herenga Waka - Victoria University of Wellington, Wellington, New Zealand 2Proactive Rehabilitation, Wellington, New Zealand 3Appleton Psychology, Dunedin, New Zealand 4School of Management, Auckland, New Zealand 5Evolution Healthcare, Wellington, New Zealand 6University of Otago, Dunedin, New Zealand 7Massey University, Palmerston North, New Zealand 8Auckland University, Auckland, New Zealand 9TBI Network, Auckland University of Technology, Auckland, New Zealand X Alice Theadom @atheadom Acknowledgements We thank Proactive Rehabilitation and TBI Health for their support and collaboration with this research. Contributors JF are the guarantor and contributed to conception, design, development of the intervention protocols, delivery of acceptance and commitment therapy intervention for mild traumatic brain injury (ACTion- TBI), data analysis and interpretation, drafting and final approval of the manuscript. DP, LD and DA: development and delivery of the traumatic brain injury intervention, revision of the manuscript and final approval. DS and SB- C: conception, design, data interpretation, final approval of the manuscript. MR and KB: conception, design, cultural supervision and final approval of the manuscript. JM: development of the ACTion- mTBI intervention, treatment fidelity checks and final approval of the manuscript. MW: conception, design, data analysis and interpretation, drafting and final approval of the manuscript. AT: conception, design, treatment fidelity checks, data interpretation and final approval of the manuscript. Funding The study was funded by a Health Research Council of New Zealand Small Project Grant (20/588). Competing interests None declared. Patient and public involvement Patients and/or the public were involved in the design, or conduct, or reporting or dissemination plans of this research. Refer to the Methods section for further details. Patient consent for publication Not applicable. Ethics approval This study involves human participants and was approved by Southern Health and Disability Ethics Committee (21/STH/117) and Auckland University of Technology Ethics Committee (21/159). Participants gave informed consent to participate in the study before taking part. Provenance and peer review Not commissioned; externally peer reviewed. Data availability statement Data are available upon reasonable request. Data available on request due to privacy/ethical restrictions. The data that support the findings of this study are available on request from the corresponding author (JF). The data are not publicly available due to restrictions, for example, their containing information that could compromise the privacy of research participants. Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer- reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise. Open access This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY- NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non- commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non- commercial. See: http://creativecommons.org/licenses/by-nc/4.0/. ORCID iDs Josh Faulkner http://orcid.org/0000-0001-9555-4628 Deborah Snell http://orcid.org/0000-0003-1664-0603 Matt N Williams http://orcid.org/0000-0002-0571-215X Alice Theadom http://orcid.org/0000-0003-0351-6216 REFERENCES 1 Silverberg ND, Iverson GL, ACRM Brain Injury Special Interest Group Mild TBI Task Force members, et al. The American Congress of Rehabilitation Medicine Diagnostic Criteria for Mild Traumatic Brain Injury. Arch Phys Med Rehabil 2023;104:1343–55. 2 Theadom A, Parag V, Dowell T, et al. Persistent problems 1 year after mild traumatic brain injury: a longitudinal population study in New Zealand. Br J Gen Pract 2016;66:e16–23. 3 Cancelliere C, Verville L, Stubbs JL, et al. Post- Concussion Symptoms and Disability in Adults With Mild Traumatic Brain Injury: A Systematic Review and Meta- Analysis. J Neurotrauma 2023;40:1045–59. 4 Machamer J, Temkin N, Dikmen S, et al. Symptom Frequency and Persistence in the First Year after Traumatic Brain Injury: A TRACK- TBI Study. J Neurotrauma 2022;39:358–70. 5 Coffeng SM, Jacobs B, de Koning ME, et al. Patients with mild traumatic brain injury and acute neck pain at the emergency department are a distinct category within the mTBI spectrum: a prospective multicentre cohort study. BMC Neurol 2020;20:315:315:. 6 Ewing- Cobbs L, Cox CS Jr, Clark AE, et al. Persistent Postconcussion Symptoms After Injury. Pediatrics 2018;142:e20180939. 7 Recker R, Alshaikh E, Kaur A, et al. Change in health- related quality of life and functional disability over time post- concussion in youth. Qual Life Res 2023;32:3339–47. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as https://x.com/atheadom http://creativecommons.org/licenses/by-nc/4.0/ http://orcid.org/0000-0001-9555-4628 http://orcid.org/0000-0003-1664-0603 http://orcid.org/0000-0002-0571-215X http://orcid.org/0000-0003-0351-6216 http://dx.doi.org/10.1016/j.apmr.2023.03.036 http://dx.doi.org/10.3399/bjgp16X683161 http://dx.doi.org/10.1089/neu.2022.0185 http://dx.doi.org/10.1089/neu.2021.0348 http://dx.doi.org/10.1186/s12883-020-01887-x http://dx.doi.org/10.1542/peds.2018-0939 http://dx.doi.org/10.1007/s11136-023-03480-4 http://bmjopen.bmj.com/ 12 Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access 8 Theadom A, Starkey N, Barker- Collo S, et al. Population- based cohort study of the impacts of mild traumatic brain injury in adults four years post- injury. PLoS One 2018;13:e0191655. 9 Stroupe KT, Smith BM, Hogan TP, et al. Healthcare utilization and costs of Veterans screened and assessed for traumatic brain injury. J Rehabil Res Dev 2013;50:1047–68. 10 Ponsford J, Nguyen S, Downing M, et al. Factors associated with persistent post- concussion symptoms following mild traumatic brain injury in adults. J Rehabil Med 2019;51:32–9. 11 Silverberg ND, Gardner AJ, Brubacher JR, et al. Systematic review of multivariable prognostic models for mild traumatic brain injury. J Neurotrauma 2015;32:517–26. 12 Broshek DK, De Marco AP, Freeman JR. A review of post- concussion syndrome and psychological factors associated with concussion. Brain Inj 2015;29:228–37. 13 Nelson LD, Temkin NR, Dikmen S, et al. Recovery After Mild Traumatic Brain Injury in Patients Presenting to US Level I Trauma Centers: A Transforming Research and Clinical Knowledge in Traumatic Brain Injury (TRACK- TBI) Study. JAMA Neurol 2019;76:1049–59. 14 Wäljas M, Iverson GL, Lange RT, et al. A prospective biopsychosocial study of the persistent post- concussion symptoms following mild traumatic brain injury. J Neurotrauma 2015;32:534–47. 15 Polinder S, Cnossen MC, Real RGL, et al. A Multidimensional Approach to Post- concussion Symptoms in Mild Traumatic Brain Injury. Front Neurol 2017;9. 16 Faulkner JW, Snell DL. A Framework for Understanding the Contribution of Psychosocial Factors in Biopsychosocial Explanatory Models of Persistent Postconcussion Symptoms. Phys Ther 2023;103:pzac156. 17 Iverson GL. Network Analysis and Precision Rehabilitation for the Post- concussion Syndrome. Front Neurol 2019;10:489. 18 Berrigan L, Marshall S, McCullagh S, et al. Quality of clinical practice guidelines for persons who have sustained mild traumatic brain injury. Brain Inj 2011;25:742–51. 19 Krpan KM, Stuss DT, Anderson ND. Coping behaviour following traumatic brain injury: what makes a planner plan and an avoider avoid? Brain Inj 2011;25:989–96. 20 McCrea M, Iverson GL, McAllister TW, et al. An integrated review of recovery after mild traumatic brain injury (MTBI): implications for clinical management. Clin Neuropsychol 2009;23:1368–90. 21 Rickards TA, Cranston CC, McWhorter J. Persistent post- concussive symptoms: A model of predisposing, precipitating, and perpetuating factors. Appl Neuropsychol Adult 2022;29:284–94. 22 Silverberg ND, Iaccarino MA, Panenka WJ, et al. Management of Concussion and Mild Traumatic Brain Injury: A Synthesis of Practice Guidelines. Arch Phys Med Rehabil 2020;101:382–93. 23 Levin HS, Diaz- Arrastia RR. Diagnosis, prognosis, and clinical management of mild traumatic brain injury. Lancet Neurol 2015;14:506–17. 24 Marshall S, Bayley M, McCullagh S, et al. Updated clinical practice guidelines for concussion/mild traumatic brain injury and persistent symptoms. Brain Inj 2015;29:688–700. 25 Lambert M, Sheldrake E, Deneault A- A, et al. Depressive Symptoms in Individuals With Persistent Postconcussion Symptoms: A Systematic Review and Meta- Analysis. JAMA Netw Open 2022;5:e2248453. 26 Howlett JR, Nelson LD, Stein MB. Mental Health Consequences of Traumatic Brain Injury. Biol Psychiatry 2022;91:413–20. 27 Chen C- L, Lin M- Y, Huda MH, et al. Effects of cognitive behavioral therapy for adults with post- concussion syndrome: A systematic review and meta- analysis of randomized controlled trials. J Psychosom Res 2020;136:110190. 28 Sullivan KA, Kaye S- A, Blaine H, et al. Psychological approaches for the management of persistent postconcussion symptoms after mild traumatic brain injury: a systematic review. Disabil Rehabil 2020;42:2243–51. 29 van Gils A, Stone J, Welch K, et al. Management of mild traumatic brain injury. Pract Neurol 2020;20:213–21. 30 Beck AT. THINKING AND DEPRESSION. I. IDIOSYNCRATIC CONTENT AND COGNITIVE DISTORTIONS. Arch Gen Psychiatry 1963;9:324–33. 31 Beck AT, Dozois DJA. Cognitive therapy: current status and future directions. Annu Rev Med 2011;62:397–409. 32 Silverberg ND, Cairncross M, Brasher PMA, et al. Feasibility of Concussion Rehabilitation Approaches Tailored to Psychological Coping Styles: A Randomized Controlled Trial. Arch Phys Med Rehabil 2022;103:1565–73. 33 Snell DL, Surgenor LJ, Hay- Smith EJC, et al. A systematic review of psychological treatments for mild traumatic brain injury: an update on the evidence. J Clin Exp Neuropsychol 2009;31:20–38. 34 Teo SH, Fong KNK, Chen Z, et al. Cognitive and psychological interventions for the reduction of post- concussion symptoms in patients with mild traumatic brain injury: a systematic review. Brain Inj 2020;34:1305–21. 35 Hayes SC, Strosahl KD, Wilson KG. Acceptance and commitment therapy. 6. New York: Guilford press, 1999. 36 Hayes SC, Hofmann SG. The third wave of cognitive behavioral therapy and the rise of process- based care. World Psychiatry 2017;16:245–6. 37 Hayes SC, Strosahl KD, Wilson KG. Acceptance and commitment therapy: the process and practice of mindful change. Guilford press, 2011. 38 Dindo L, Van Liew JR, Arch JJ. Acceptance and Commitment Therapy: A Transdiagnostic Behavioral Intervention for Mental Health and Medical Conditions. Neurotherapeutics 2017;14:546–53. 39 Gloster AT, Walder N, Levin ME, et al. The empirical status of acceptance and commitment therapy: A review of meta- analyses. J Contextual Behav Sci 2020;18:181–92. 40 Ost L- G. The efficacy of Acceptance and Commitment Therapy: an updated systematic review and meta- analysis. Behav Res Ther 2014;61:105–21. 41 González- Fernández S, Fernández- Rodríguez C. Acceptance and Commitment Therapy in Cancer: Review of Applications and Findings. Behav Med 2019;45:255–69. 42 Hughes LS, Clark J, Colclough JA, et al. Acceptance and Commitment Therapy (ACT) for Chronic Pain. Clin J Pain 2017;33:552–68. 43 Hesser H, Gustafsson T, Lundén C, et al. A randomized controlled trial of Internet- delivered cognitive behavior therapy and acceptance and commitment therapy in the treatment of tinnitus. J Consult Clin Psychol 2012;80:649:649–61:. 44 Jacobsen HB, Kallestad H, Landrø NI, et al. Processes in acceptance and commitment therapy and the rehabilitation of chronic fatigue. Scand J Psychol 2017;58:211–20. 45 Whiting D, Deane F, McLeod H, et al. Can acceptance and commitment therapy facilitate psychological adjustment after a severe traumatic brain injury? A pilot randomized controlled trial. Neuropsychol Rehabil 2020;30:1348–71. 46 Rauwenhoff JCC, Bol Y, Peeters F, et al. Acceptance and commitment therapy for individuals with depressive and anxiety symptoms following acquired brain injury: A non- concurrent multiple baseline design across four cases. Neuropsychol Rehabil 2023;33:1018–48. 47 Sander AM, Clark AN, Arciniegas DB, et al. A randomized controlled trial of acceptance and commitment therapy for psychological distress among persons with traumatic brain injury. Neuropsychol Rehabil 2021;31:1105–29. 48 Risdall JE, Menon DK. Traumatic brain injury. Philos Trans R Soc Lond B Biol Sci 2011;366:241–50. 49 Sveen U, Guldager R, Soberg HL, et al. Rehabilitation interventions after traumatic brain injury: a scoping review. Disabil Rehabil 2022;44:653–60. 50 Faulkner JW, Snell DL, Theadom A, et al. The influence of psychological flexibility on persistent post concussion symptoms and functional status after mild traumatic brain injury. Disabil Rehabil 2023;45:1192–201. 51 Eldridge SM, Chan CL, Campbell MJ, et al. CONSORT 2010 statement: extension to randomised pilot and feasibility trials. BMJ 2016;355:i5239. 52 Bastos Gottgtroy R, Hume PA, Theadom A. Describing the patient journey through healthcare pathways following mild traumatic brain injury in New Zealand using novel Graph analysis. Brain Inj 2023;37:1294–304. 53 Mild Traumatic Brain Injury Committee, A. C. R. M. Definition of mild traumatic brain injury. J Head Trauma Rehabil 1993;8:86–7. 54 Cassidy JD, Carroll L, Peloso P, et al. Incidence, risk factors and prevention of mild traumatic brain injury: results of the who collaborating centre task force on mild traumatic brain injury. J Rehabil Med 2004;36:28–60. 55 Thompson C, Davies P, Herrmann L. Approaches to establishing validated cut- off scores on the rivermead post- concussion symptoms questionnaire (RPQ). In: Brain injury. Philadelphia, PA, USA: Taylor & Francis Inc, 2016: 30. 770–1. 56 Linde T, Strosahl K. Doing ACT briefly: the practice of focused acceptance and commitment therapy. 2014. 57 Hayes SC, Merwin RM, McHugh L, et al. Report of the ACBS Task Force on the strategies and tactics of contextual behavioral science research. J Contextual Behav Sci 2021;20:172–83. 58 Cully JA, Teten AL. A therapist’s guide to brief cognitive behavioral therapy. Houston: Department of Veterans Affairs South Central MIRECC, 2008. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://dx.doi.org/10.1371/journal.pone.0191655 http://dx.doi.org/10.1682/JRRD.2012.06.0107 http://dx.doi.org/10.1682/JRRD.2012.06.0107 http://dx.doi.org/10.2340/16501977-2492 http://dx.doi.org/10.1089/neu.2014.3600 http://dx.doi.org/10.1089/neu.2014.3600 http://dx.doi.org/10.3109/02699052.2014.974674 http://dx.doi.org/10.1001/jamaneurol.2019.1313 http://dx.doi.org/10.1089/neu.2014.3339 http://dx.doi.org/10.3389/fneur.2018.01113 http://dx.doi.org/10.1093/ptj/pzac156 http://dx.doi.org/10.3389/fneur.2019.00489 http://dx.doi.org/10.3109/02699052.2011.580317 http://dx.doi.org/10.3109/02699052.2011.597045 http://dx.doi.org/10.1080/13854040903074652 http://dx.doi.org/10.1080/23279095.2020.1748032 http://dx.doi.org/10.1016/j.apmr.2019.10.179 http://dx.doi.org/10.1016/S1474-4422(15)00002-2 http://dx.doi.org/10.3109/02699052.2015.1004755 http://dx.doi.org/10.1001/jamanetworkopen.2022.48453 http://dx.doi.org/10.1016/j.biopsych.2021.09.024 http://dx.doi.org/10.1016/j.jpsychores.2020.110190 http://dx.doi.org/10.1016/j.jpsychores.2020.110190 http://dx.doi.org/10.1080/09638288.2018.1558292 http://dx.doi.org/10.1136/practneurol-2018-002087 http://dx.doi.org/10.1001/archpsyc.1963.01720160014002 http://dx.doi.org/10.1146/annurev-med-052209-100032 http://dx.doi.org/10.1016/j.apmr.2021.12.005 http://dx.doi.org/10.1016/j.apmr.2021.12.005 http://dx.doi.org/10.1080/13803390801978849 http://dx.doi.org/10.1080/02699052.2020.1802668 http://dx.doi.org/10.1080/02699052.2020.1802668 http://dx.doi.org/10.1002/wps.20442 http://dx.doi.org/10.1007/s13311-017-0521-3 http://dx.doi.org/10.1016/j.jcbs.2020.09.009 http://dx.doi.org/10.1016/j.jcbs.2020.09.009 http://dx.doi.org/10.1016/j.brat.2014.07.018 http://dx.doi.org/10.1080/08964289.2018.1452713 http://dx.doi.org/10.1097/AJP.0000000000000425 http://dx.doi.org/10.1037/a0027021 http://dx.doi.org/10.1037/a0027021 http://dx.doi.org/10.1111/sjop.12363 http://dx.doi.org/10.1080/09602011.2019.1583582 http://dx.doi.org/10.1080/09602011.2022.2053169 http://dx.doi.org/10.1080/09602011.2020.1762670 http://dx.doi.org/10.1080/09602011.2020.1762670 http://dx.doi.org/10.1098/rstb.2010.0230 http://dx.doi.org/10.1098/rstb.2010.0230 http://dx.doi.org/10.1080/09638288.2020.1773940 http://dx.doi.org/10.1080/09638288.2022.2055167 http://dx.doi.org/10.1136/bmj.i5239 http://dx.doi.org/10.1080/02699052.2023.2230878 http://dx.doi.org/10.1097/00001199-199309000-00010 http://dx.doi.org/10.1080/16501960410023732 http://dx.doi.org/10.1080/16501960410023732 http://dx.doi.org/10.1016/j.jcbs.2021.03.007 http://bmjopen.bmj.com/ 13Faulkner J, et al. BMJ Open 2025;15:e089727. doi:10.1136/bmjopen-2024-089727 Open access 59 Ustün TB, Chatterji S, Kostanjsek N, et al. Developing the World Health Organization Disability Assessment Schedule 2.0. Bull World Health Organ 2010;88:815–23. 60 Federici S, Bracalenti M, Meloni F, et al. World Health Organization disability assessment schedule 2.0: An international systematic review. Disabil Rehabil 2017;39:2347–80. 61 Snell DL, Siegert RJ, Silverberg ND. Rasch analysis of the World Health Organization Disability Assessment Schedule 2.0 in a mild traumatic brain injury sample. Brain Inj 2020;34:610–8. 62 King NS, Crawford S, Wenden FJ, et al. The Rivermead Post Concussion Symptoms Questionnaire: a measure of symptoms commonly experienced after head injury and its reliability. J Neurol 1995;242:587–92. 63 Lovibond PF, Lovibond SH. The structure of negative emotional states: comparison of the Depression Anxiety Stress Scales (DASS) with the Beck Depression and Anxiety Inventories. Behav Res Ther 1995;33:335–43. 64 Ownsworth T, Little T, Turner B, et al. Assessing emotional status following acquired brain injury: the clinical potential of the depression, anxiety and stress scales. Brain Inj 2008;22:858–69. 65 Wilson KG, Sandoz EK, Kitchens J, et al. The Valued Living Questionnaire: Defining and Measuring Valued Action within a Behavioral Framework. Psychol Rec 2010;60:249–72. 66 Pais Hons C, Ponsford JL, Gould Clin Neuro KR, et al. Role of valued living and associations with functional outcome following traumatic brain injury. Neuropsychol Rehabil 2019;29:625–37. 67 Michelson SE, Lee JK, Orsillo SM, et al. The role of values- consistent behavior in generalized anxiety disorder. Depress Anxiety 2011;28:358–66. 68 Ong CW, Lee EB, Levin ME, et al. A review of AAQ variants and other context- specific measures of psychological flexibility. J Contextual Behav Sci 2019;12:329–46. 69 Whiting DL, Deane FP, Ciarrochi J, et al. Validating measures of psychological flexibility in a population with acquired brain injury. Psychol Assess 2015;27:415:415–23:. 70 Faulkner JW, Snell DL, Theadom A, et al. Psychological flexibility in mild traumatic brain injury: an evaluation of measures. Brain Inj 2021;35:1103–11. 71 Bond FW, Hayes SC, Baer RA, et al. Preliminary psychometric properties of the Acceptance and Action Questionnaire- II: a revised measure of psychological inflexibility and experiential avoidance. Behav Ther 2011;42:676–88. 72 Clifton L, Clifton DA. The correlation between baseline score and post- intervention score, and its implications for statistical analysis. Trials 2019;20:43. 73 Van Breukelen GJP. ANCOVA versus change from baseline: more power in randomized studies, more bias in nonrandomized studies [corrected]. J Clin Epidemiol 2006;59:920–5. 74 Cohen J. Statistical power analysis for the behavioral sciences. Academic press, 2013. 75 Rubin M. When to adjust alpha during multiple testing: a consideration of disjunction, conjunction, and individual testing. Synthese 2021;199:10969–1000. 76 Teare MD, Dimairo M, Shephard N, et al. Sample size requirements to estimate key design parameters from external pilot randomised controlled trials: a simulation study. Trials 2014;15:1–13. 77 Julious SA. Sample size of 12 per group rule of thumb for a pilot study. Pharm Stat 2005;4:287–91. 78 Whitehead AL, Julious SA, Cooper CL, et al. Estimating the sample size for a pilot randomised trial to minimise the overall trial sample size for the external pilot and main trial for a continuous outcome variable. Stat Methods Med Res 2016;25:1057–73. 79 O’Neill LF. Development of a fidelity measure for acceptance and commitment therapy, doctoral dissertation. University of Leeds, 2018 80 Hilsenroth MJ, Blagys MD, Ackerman SJ, et al. Measuring Psychodynamic- Interpersonal and Cognitive- Behavioral Techniques: Development of the Comparative Psychotherapy Process Scale. Psychother Theory Res Pract Train 2005;42:340–56. 81 Goodyer IM, Reynolds S, Barrett B, et al. Cognitive behavioural therapy and short- term psychoanalytical psychotherapy versus a brief psychosocial intervention in adolescents with unipolar major depressive disorder (IMPACT): a multicentre, pragmatic, observer- blind, randomised controlled superiority trial. Lancet Psychiatry 2017;4:109–19. 82 Hayes SC, Luoma JB, Bond FW, et al. Acceptance and Commitment Therapy: Model, processes and outcomes. Behav Res Ther 2006;44:1–25. 83 Zhang C- Q, Leeming E, Smith P, et al. Acceptance and Commitment Therapy for Health Behavior Change: A Contextually- Driven Approach. Front Psychol 2017;8:2350. 84 Curvis W, Methley A, eds. Acceptance and commitment therapy and brain injury: a practical guide for clinicians. Routledge, 2021. 85 Kangas M, McDonald S. Is it time to act? The potential of acceptance and commitment therapy for psychological problems following acquired brain injury. Neuropsychol Rehabil 2011;21:250–76. 86 Cape J, Whittington C, Buszewicz M, et al. Brief psychological therapies for anxiety and depression in primary care: meta- analysis and meta- regression. BMC Med 2010;8:38:1–13:. 87 Roberts K, Travers- Hill E, Coker S, et al. Brief psychological interventions for anxiety and depression in a secondary care adult mental health service: an evaluation. tCBT 2021;14:e29. 88 Miller H, Lawson D, Power E, et al. How do people with acquired brain injury interpret the Valued Living Questionnaire? A cognitive interviewing study. J Contextual Behav Sci 2022;23:125–36. 89 Smout M, Davies M, Burns N, et al. Development of the Valuing Questionnaire (VQ). J Contextual Behav Sci 2014;3:164–72. 90 Faulkner JW, Snell DL, Theadom A, et al. The role of psychological flexibility in recovery following mild traumatic brain injury. Rehabil Psychol 2021;66:479:479–90:. 91 Rolffs JL, Rogge RD, Wilson KG. Disentangling Components of Flexibility via the Hexaflex Model: Development and Validation of the Multidimensional Psychological Flexibility Inventory (MPFI). Assessment 2018;25:458–82. 92 Feigin VL, Theadom A, Barker- Collo S, et al. Incidence of traumatic brain injury in New Zealand: a population- based study. Lancet Neurol 2013;12:53–64. 93 Faulkner JW, Chua J, Voice- Powell A, et al. Experience of Acceptance and Commitment Therapy for those with mild traumatic brain injury (ACTion mTBI): A qualitative descriptive study. PLoS One 2025;20:e0312940. P ro tected b y co p yrig h t, in clu d in g fo r u ses related to text an d d ata m in in g , A I train in g , an d sim ilar tech n o lo g ies. . b y g u est o n F eb ru ary 25, 2025 h ttp ://b m jo p en .b m j.co m / D o w n lo ad ed fro m 16 F eb ru ary 2025. 10.1136/b m jo p en -2024-089727 o n B M J O p en : first p u b lish ed as http://dx.doi.org/10.2471/BLT.09.067231 http://dx.doi.org/10.2471/BLT.09.067231 http://dx.doi.org/10.1080/09638288.2016.1223177 http://dx.doi.org/10.1080/02699052.2020.1729417 http://dx.doi.org/10.1007/BF00868811 http://dx.doi.org/10.1016/0005-7967(94)00075-u http://dx.doi.org/10.1080/02699050802446697 http://dx.doi.org/10.1007/BF03395706 http://dx.doi.org/10.1080/09602011.2017.1313745 http://dx.doi.org/10.1002/da.20793 http://dx.doi.org/10.1016/j.jcbs.2019.02.007 http://dx.doi.org/10.1016/j.jcbs.2019.02.007 http://dx.doi.org/10.1037/pas0000050 http://dx.doi.org/10.1080/02699052.2021.1959062 http://dx.doi.org/10.1016/j.beth.2011.03.007 http://dx.doi.org/10.1186/s13063-018-3108-3 http://dx.doi.org/10.1016/j.jclinepi.2006.02.007 http://dx.doi.org/10.1007/s11229-021-03276-4 http://dx.doi.org/10.1186/1745-6215-15-264 http://dx.doi.org/10.1002/pst.185 http://dx.doi.org/10.1177/0962280215588241 http://dx.doi.org/10.1037/0033-3204.42.3.340 http://dx.doi.org/10.1016/S2215-0366(16)30378-9 http://dx.doi.org/10.1016/j.brat.2005.06.006 http://dx.doi.org/10.3389/fpsyg.2017.02350 http://dx.doi.org/10.4324/9781003024408 http://dx.doi.org/10.4324/9781003024408 http://dx.doi.org/10.1080/09602011.2010.540920 http://dx.doi.org/10.1186/1741-7015-8-38 http://dx.doi.org/10.1017/S1754470X21000258 http://dx.doi.org/10.1016/j.jcbs.2022.01.003 http://dx.doi.org/10.1016/j.jcbs.2014.06.001 http://dx.doi.org/10.1037/rep0000406 http://dx.doi.org/10.1037/rep0000406 http://dx.doi.org/10.1177/1073191116645905 http://dx.doi.org/10.1016/S1474-4422(12)70262-4 http://dx.doi.org/10.1371/journal.pone.0312940 http://bmjopen.bmj.com/ Acceptance and commitment therapy for mild traumatic brain injury (ACTion-­mTBI): a quasiexperimental feasibility study Abstract Method Design Participants and procedure Intervention Acceptance and commitment therapy intervention for mild traumatic brain injury CBT intervention Patient and public involvement Outcome measures Primary clinical outcome Secondary clinical outcomes Treatment process measures Statistical analysis Results Recruitment and retention feasibility Treatment fidelity feasibility Group differences Discussion References