Functioning in chronic pain: a call for a global definition

Introduction There is an overwhelming number of instruments to assess the functioning of persons living with chronic pain (PLWCP), but none are based on the same agreed definition of “functioning.” A global consensus on a meaningful definition of functioning is long overdue. Without this, we jeopardize the evolution of chronic pain management, risk unnecessary interventions, and hinder the development and widespread application of meaningful treatments. This article discusses the fundamental importance of measuring functioning and what functioning might mean to different people. We consider the International Classification of Functioning Disability and Health's (ICF's) approach to functioning and why, although important, it does not provide an actual definition. 2. Why measuring functioning in persons living with chronic pain is so important to stakeholders in health care All stakeholders in the management of PLWCP must measure functioning so that individuals can access appropriate treatment, shared decisions can be justified, healthcare resources can be planned and costed, and societal expectations are met. However, PLWCP, clinicians, industry, regulators, guideline makers, healthcare and research funders, and society may have different, albeit overlapping perspectives on what “functioning” means (Fig. 1). For example, some clinical specialists focus on functioning as freedom from psychosocial difficulties arising from chronic pain; regulators may view functioning in terms of measurable physical abilities, eg, the walking distance test.Figure 1.: Examples of major stakeholders in the care of pain with overlapping perspectives on functioning.In the ideal interdisciplinary approach to PLWCP, clinicians collaborate to enable patients to reach their own “functioning” goals.2,25 The interdisciplinary team's overall aim maps to all components of the ICF—body functions and structures, and activities and participation, including domains of interpersonal interactions, relationships, and community life. Within the team, the focus on functioning varies by discipline (Fig. 2).22Figure 2.: Examples* of different stakeholders' perspectives on the meaning of functioning in patients and PLWCP. *The figure is not intended to provide an exhaustive list of stakeholders or their reasons for needing a definition of functioning. PLWCP, persons living with chronic pain.Industry and independent research groups strive to maximize the relevance and value of clinical trials. The multimodal management of PLWCP means that functioning is relevant to a range of interventions, not just pharmacological treatments and surgery, eg, exercise, education, and psychological therapies. For pharmacotherapies, there is a particular need to satisfy requirements for drug approval. The US Food and Drug Administration (FDA) requires labelling claims to be supported by measures of functioning.33 For the European Medicines Agency (EMA), measurements of treatment efficacy for chronic pain include physical and emotional functioning, and activities of daily living.6 Physical and emotional functioning are core domains for chronic pain clinical trials recommended by the Initiative on Methods, Measurement and Pain Assessment in Clinical Trials (IMMPACT).32 Clinical guidelines for managing chronic pain are informed by functional outcomes in clinical trials and recommend assessments of function as part of patient care. For example, European and American guidelines on the use of opioids for chronic noncancer pain emphasize the need for therapeutic objectives that include functioning and/or quality of life.31 Healthcare payers subject to societal, economic, and political forces determine the reality of what health care is available to most PLWCP. Health technology assessments (HTAs) are the foundation for reimbursement and pricing. The European Network for Health Technology Assessment asserts that patients' bodily functions and ability to return to work should always be considered in HTAs.12 3. The meaning of functioning for the individual living with chronic pain Chronic pain can invade all aspects of life—sleep, self-care, household chores, social connections, travelling, sport, exercise, sexual activities, and working.9,17,18 Individuals have their own, personally meaningful functional goals. In an international European survey, patients most frequently selected treatment goals of pain reduction, participation in family and social activities, and performance of household tasks.17 In the Netherlands, patients selected fatigue, social life, cramped muscles, sleeping, housekeeping, concentration, not being understood, and control over pain as the most important attributes of chronic pain to them.28 Psychological and emotional factors add to the complexity, eg, patient expectations of treatment outcomes.23 The World Health Organization's (WHO's) patient-centric perspective on functioning and disability is based on a biopsychosocial model underpinning the ICF.21,36 The ICF is regarded by some as part of a rethinking of health that brings together biological health and lived health.4 Functioning is described by the ICF as “an umbrella term for body functions, body structures, activities and participation. It denotes the positive aspects of the interaction between an individual (with a health condition) and that individual's contextual factors (environmental and personal factors).”37 The ICF's comprehensive approach embraces the heterogeneity between individuals but arguably takes us no closer to a single, meaningful, and quantifiable definition of functioning. Its “definition” describes what functioning does and does not encompass without actually stating what it is. A Global Stakeholders' Initiative by Cochrane Rehabilitation has built upon the ICF's description.24 Functioning was broadly agreed to result from the dynamic interaction of the body's function and structures, the simple and complex activities that a person performs, and the extent the person can participate in life situations in the overall context of their environment.24 4. Is there a measure that can be recommended as a global “gold standard”? Currently, there is no measure recommended as a global gold standard for measuring functioning.22 We argue that this is despite, or perhaps because of, a multitude of PROMs and physical function tests to evaluate the impact of chronic pain in clinical trials. These encompass all aspects of physical, social, and emotional well-being, both perceived and actual, relating to chronic pain or the totality of an individual's physical and mental status.8,15,16,26,27,34,35 A broad diversity of functioning is also measured in clinical practice. For example, 8596 concepts were identified in clinical assessments in one Australian Multidisciplinary Pain Center.1 The electronic platform painDETECT used by clinical specialists in Germany comprises more than 30 validated questionnaires for typical outcomes in chronic pain.13,14 Different groups have proposed the most important domains for functioning, but there is no universally agreed set of measures. A joint initiative by IMMPACT and Outcome Measures in Rheumatology (OMERACT) recommended the use of both PROMs and performance tests in clinical trials.30 A Swiss research group recommended a minimum set of 6 instruments to measure rehabilitation in patients with chronic low back pain.3 The WHO has developed ICF-Core sets for chronic widespread pain.20 A fundamental limitation of existing PROMs for functionality in PLWCP is a mismatch between what is measured and what is meaningful to the individual. In 14 functional PROMs used for chronic pain, 261 items mapped to ICF second-level categories,19 of which only 29% were relevant to the 8 most important attributes of living with chronic pain that patients identified in a separate study.28 A 49% rate of discrepancy between patient-reported outcomes of pain rehabilitation and physiatrist-rated outcomes was partly attributed by patients to irrelevant outcome measures in the PROM.5 Another limitation of PROMs is that a substantial number do not meet all regulatory criteria for proven validity, reliability, and ability to detect change in a representative population.27,33 With or without validation, instruments are being used to support different definitions of functioning, employing a wide range of different and overlapping variables. None of these instruments are truly patient centered and based on individualized and meaningful functioning. Moreover, none are clearly definition based. 5. Moving towards a gold standard measure of functioning The premise for identifying a “gold standard” measure(s) of functioning in chronic pain is a shared definition of functioning; this is currently lacking. Perhaps, we have simply been too focused on measuring functioning to address the need for a definition. Functioning appears to be defined by whatever measure is selected. Darnall and Sullivan cite Inigo Montoya in William Goldman's novel The Princess Bride: “You use that word. I do not think it means what you think it means.”7 It will be critical to consult PLWCP and people with lived experience of pain in the development of a definition of functioning. A definition must precede reevaluation of PROMs and function tests, with the aim of proposing a gold standard instrument(s) for research and clinical practice, incorporating technological advances where appropriate. This work should include alignment by the WHO on the definition of pain itself, as currently the ICF and the International Classification of Health Interventions use a different definition from the International Association for the Study of Pain (IASP).29 6. Reaching a definition of functioning in chronic pain Reaching a definition of functioning in chronic pain might be an initiative driven by the International Classification of Functioning Task Force under the auspices of IASP. Other parties might be drawn from the WHO, academia, clinical healthcare specialties, professional medical and allied healthcare associations, regulatory agencies (eg, the FDA, EMA), major national and international research centers, and patient advocacy groups.10,11 The route to consensus must be patient-centered, practical, goal-oriented, representative, authoritative, and evidence-based with a clear remit and timeframe. Cultural differences that might impact the definition of functioning should also be addressed in the consensus processes. The healthcare community should not be content to “muddle” along without a clear and consented global definition of functioning. It is too important to sidestep any longer and incumbent on us to improve patient care with a universal and workable definition (Audio Abstract, https://links.lww.com/PAIN/C77). Conflict of interest statement The authors have no conflicts of interest to declare. Supplemental audio content An audio abstract associated with this article can be found online at https://links.lww.com/PAIN/C77.