Physiotherapist and PhD student Cass Macgregor, and Professor Blair H Smith share their thoughts on chronic pain, the WHO/IASP definition of chronic primary pain and the term's use in the NICE guidance.
Chronic pain has been defined in the literature as pain that persists beyond three months (Merksey, 1986), however, the conceptualisation of chronic pain has changed over recent decades. Wall and Melzack (1965) first published a new theory of pain mechanisms, introducing ‘gate control theory’ which proposed that pain is modified by central nervous system processing and that this can result in chronic pain. Further advancement in imaging techniques of the central nervous system enabled better understanding of the neurobiological changes that happen in chronic pain and how this is different from acute pain, and have led to discussion in the literature that chronic pain is a disease state (Tracey and Bushnell, 2009; Lema and Voscopolous, 2010, Tracey et al, 2019). These developments improved conceptualisation of pain being both a symptom of injury or another condition, and of chronic pain as the disease/ condition itself, and enabled, for example, the Scottish Government to recognise chronic pain as a long term condition in its own right (NHS QIS, 2008).
The importance of chronic pain being recognised as a long term condition is that this model can be applied to management, bringing the approach to chronic pain management in line with that of other long term conditions such as asthma and cardiovascular disease. However, Mackie, writing in the preface to Mellor (2018) sums the difficulty well that the clinician and patient face which involves identification of the chronic pain state over time, excluding other possibilities and working together to ‘create a way of managing the pain in a way that works for them.’ He points out that other diseases can be more straightforward to identify and manage. Despite this, Barke et al (2018) report that specialist pain clinicians felt confident in identifying chronic pain conditions as part of field testing for the new World Health Organization (WHO) International Classification of Diseases, 11 th edition (ICD-11).
The proposed version of ICD-11 will provide, for the first time, formal definitions of chronic pain. It will create the new diagnosis of ‘chronic primary pain’ as a disease state, and distinct from ‘chronic secondary pain’, which can be conceived as a symptom of another disease such as arthritis or cancer (Treede et al, 2019). The definition of chronic primary pain was developed by the International Association for the Study of Pain (IASP) task force on the classification of chronic pain: ‘when pain has persisted for more than 3 months and is associated with significant emotional distress and/or functional disability, and the pain is not better accounted for by another condition.’ Sub-categories of chronic primary pain are diverse and include chronic widespread pain (e.g. fibromyalgia), chronic primary orofacial pain, chronic primary visceral pain, complex regional pain syndromes and chronic primary musculoskeletal pain. A diagnosis of chronic primary pain can change with the arrival of new information, or the results of investigations.
Chronic pain is a broad, heterogeneous diagnosis and the ICD-11 definition of chronic primary pain describes a clinically useful sub population, conferring a distinct disease entity for people whose diagnosis might otherwise be in doubt or lack validation. They are substantially impacted by their condition, and require access to the most appropriate healthcare management, just as those diagnosed with chronic secondary pain. This definition includes the crucially important factors of emotional distress and functional disability, which are often overlooked or misinterpreted in medical classification systems. Nicholas et al (2019) affirm the aim of moving chronic primary pain from a biomedical towards a biopsychosocial concept, dispensing with the ‘obsolete dichotomy’ of physical and psychological pain that has existed unhelpfully in previous classification systems. There has long been debate over how to subgroup, stratify and select care for people with chronic pain based on psychological, psychosocial and biological characteristics (Turk, 2005; Williams et al, 2012; Mun et al, 2019, Hill et al, 2011). The ICD-11 classification of chronic primary pain offers the potential to help with clinical selection in order to meet the needs of our patients. The inclusion of disability and distress as part of the diagnosis highlights the need to consider a pain management programme, or similar multi-disciplinary rehabilitative/therapeutic approach to address it.
The National Institute for Health and Care Excellence (NICE) has recently published guidelines for England and Wales which are titled: ‘Chronic pain (primary and secondary) in over 16s: assessment of all chronic pain and management of chronic primary pain’ (NICE, 2021). In contrast to the WHO/IASP definition, NICE have defined chronic primary pain as ‘pain with no clear underlying cause, or pain (or its impact) that is out of proportion to any observable injury or disease.’ This definition does not exist elsewhere, and it is unclear how it was determined by NICE. They state that in distinction from chronic secondary pain, chronic primary pain is classified if the cause of the pain is unclear. Basing pain treatment recommendations on lack of understanding about its underlying cause, or about its relationship to observed injury or disease, is potentially unhelpful given the importance and centrality of validation to the healing journey in chronic pain (Toye et al, 2021). When defining chronic primary pain, NICE have not explicitly recognized the emotional distress and functional disability acknowledged by WHO/IASP, nor the need for investigation results to be (1) available and (2) completed before making their recommendations.
Importantly, the ICD-11 innovative classification will not formally come into effect until 2022. As a new diagnosis, therefore, it cannot have been used to identify and select populations for clinical trials, such as those which are needed for evidence-based recommendations. Along with the crucial and clear differences between ICD-11 and NICE case definitions, it follows that the NICE guidelines cannot apply to the ICD-11 diagnosis of chronic primary pain, and that management of people with this diagnosis should continue as to work to existing recommendations until relevant evidence becomes available. There is no doubt that rehabilitation and therapy options have a great deal to offer to people with chronic pain (Macfarlane et al, 2017; SIGN, 2019) and this should remain a priority.
There is potential for the ICD-11 to help facilitate the paradigm shift needed in the conceptualisation and classification of chronic pain, leading to its acceptance by patients and professionals, and thus better management. Rehabilitation and therapy professionals should be aware of the limitations of the NICE interpretation of case definition, and application of evidence, and consider the ongoing need to offer multimodal treatment to those fulfilling the ICD-11 diagnosis of chronic primary pain. In due course, the clearly defined classification will improve understanding, recording and managing chronic pain in the community, as well as providing a platform for a new generation of targeted clinical trials to inform future evidence-based treatments (Smith et al 2019). Meanwhile, we should assert the biopsychosocial nature of all chronic pain, address this in all aspects of it management, and embrace ‘chronic primary pain’ as a condition whose treatment merits the full range of evidence based options.
Blair H Smith is a GP Clinical Professor at the University of Dundee, Honorary Consultant in Pain Medicine at NHS Tayside and Specialty Advisor for Chronic Pain to the Scottish CMO. He is on Twitter: @blairhsmith1H.
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