8Causal mechanisms in the clinical course and treatment of back pain
Introduction
Experiencing back pain without knowing its cause can be highly distressing for the patient. Disappointingly, research so far has failed to provide robust causal explanations to mitigate these uncertainties. Poor understanding of causal factors has stifled the development of effective treatments. This is because treatment targets have only been informed by assumptions or implicit theories rather than empirical evidence for well-defined causal mechanisms. For the past 16 years, landmark papers in the field have repeatedly highlighted the paucity of evidence on the causes of low back pain [1], [2], [3]. This is despite back pain researchers ranking research into the mechanisms and causes of low back pain as one of the highest research priorities to advance the field [4].
By understanding causal mechanisms in the clinical course of back pain, we can identify important factors that can be targeted in treatment. Although pain and disability are important outcomes for the patient, they are not the typical targets of treatments. Treatments are devised to target hypothesised intermediary factors (or mediators) that are causally linked to patient-relevant outcomes such as pain and disability. In other words, treatments generally exert their effects on pain and disability through indirect pathways, which we refer to as ‘causal mechanisms’. In a recent mechanism evaluation study, Fordham et al. showed that cognitive behavioural therapy (exposure) reduced disability (outcome) through changes in self-efficacy, fear avoidance and physical functioning (mediators) but not through improvements in mental functioning [5]. This information gained from a mechanism evaluation is particularly useful in understanding how treatments work or why they fail.
By understanding causal mechanisms, treatments can be adapted and refined to improve their clinical efficacy and facilitate implementation. Recently, there has been increasing interest in generating evidence for causal mechanisms in the back pain field [6]. The aim of this article is to provide an overview of current understanding of causal mechanisms in the clinical development and treatment of back pain. This article is divided into five sections (1. Key concepts and terminology; 2. Overview of mechanism evaluations in the field; 3. Application to clinical practice and implementation; 4. Methodological considerations; 5. Future directions).
Section snippets
Key concepts and terminology
Many empirical studies in health research aim to establish whether an exposure causes an outcome [7]. For example, a randomised trial might aim to establish whether an exercise programme (exposure) causes less disability (outcome). However, merely establishing a single causal association is often insufficient to explain how or why an exposure causes an outcome [8]. This approach to understanding causation is often criticised as the ‘black-box’ approach because the underlying causal mechanisms
Causal mechanisms in the clinical course of low back pain
The clinical course of back pain typically follows a pattern where most patients experience significant improvements in pain and disability in the first 6 weeks from acute onset [18], [19]. Patients who continue to experience pain beyond 6 weeks are likely to suffer from significant levels of disability that adds to the burden of back pain [20]. Mediation analysis can be used to identify the processes that lead to prolonged disability in patients with back pain. For example, mediation analyses
Application to clinical practice and implementation
In clinical practice, it is useful to know which treatment targets will lead to improvements in patient-related outcomes. By knowing these targets, clinicians can specifically tailor their treatments to selectively target appropriate mechanisms. A systematic review of cohort studies showed that psychological mechanisms including increased self-efficacy, reduced fear and reduced distress can serve as treatment targets. As shown by Mansell et al. [6] and more recent mechanism evaluations of
Methodological considerations
Both systematic reviews of mediation studies in the field *[6], *[23] and subsequent work [40], [41] have noted that most mechanism studies are of poor methodological quality. Of the 36 mediation models in cohort designs, 59% used cross-sectional designs that violate the assumption of temporal precedence necessary for causal inference [23]. To overcome this problem, longitudinal studies should include at least three waves of measurement points to determine the temporal sequence of the exposure,
Future directions
Effective treatment of low back pain is often stifled by limited understanding of targets that are based on implicit theories about their causal mechanisms. Future research should describe and test these implicit theories for common treatments *[6], [30], *[47]. To do so, a comprehensive programme of work would be required. This might begin with qualitative evaluations and cohort studies to generate new theories for biologically plausible treatment targets. This could be followed by a priori
Conflicts of interest
HL, JHM, SJK, GLM, LW, and CMW receive funding from the Australian National Health and Medical Research Council. GLM has received support from: Pfizer, Kaiser Permanente USA, Results Physiotherapy USA, Agile Physiotherapy USA, workers compensation boards in Australia, North America and Europe, the International Olympic Committee and the Port Adelaide Football Club. He receives royalties for books on pain and rehabilitation, and speaker fees for lectures on pain and rehabilitation.
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