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Systematic reviews can provide evidence that informs clinical practice. However, in order to allow valid comparisons, researchers, reviewers and clinicians need to know what was done, and how similar or different study populations were (clinical heterogeneity). The importance of considering clinical heterogeneity when comparing data sets becomes evident when considering the topical issue of central sensitisation in tendinopathy. Central sensitisation refers to the increased responsiveness of the central nervous system.1 Central sensitisation encompasses such features as altered sensory processing in the brain, malfunctioning of descending antinociceptive mechanisms, and increased activity of pain facilitatory pathways, among others.2 Central sensitisation is frequently present in various chronic musculoskeletal pain disorders, including chronic low back pain,3 and rheumatoid arthritis.4 A recent systematic review reported the presence of central sensitisation in tendinopathy; however, these findings were based mainly on upper limb tendons, so data cannot be generalised to the lower limb.5 In fact, it is often argued that unlike upper limb tendinopathy, individuals with lower limb tendinopathy do not display evidence of central sensitisation. There is evidence to support this assertion, with studies appearing to confirm the absence of central sensitisation in the lower limb tendinopathy.6 In contrast there is also evidence to suggest that people with Achilles tendinopathy displayed altered central pain modulation.7 Therefore, the question arises—are pain mechanisms really different in upper and lower limb tendinopathies, or are these studies confounded by differences in the populations samples?
Have studies considered the role of confounding variables?
Most studies investigating sensitisation in upper limb tendinopathy have been conducted in older populations, with low or unreported physical activity levels (online supplementary file 1). Regular physical activity has been shown to prevent the development of pain through modulation of supraspinal mechanisms.8 Furthermore, while age itself is not directly associated with the development of central sensitisation, increasing age is often associated with other risk factors associated with the development of central sensitisation, such as sleep disturbances, fatigue, as well as comorbidities such as obesity or metabolic disorders.9 In contrast, most lower limb tendinopathy studies have focused on younger and/or more active populations (online supplementary file 1). Therefore, the observed differences in central sensitisation between different tendinopathies could simply reflect differences relating to confounding variables (eg, age, comorbidities, activity level). Consequently, the apparent paradox in the current literature could be summarised as:
Supplementary file 1
Central sensitisation is an uncommon feature of lower limb tendinopathy among young, athletic populations who have no comorbid health complaints or other painful regions.
or:
Central sensitisation is a common feature of upper limb tendinopathy in older non-athletic populations with comorbid health complaints or other painful regions.
Implications for clinical practice and research
Start by agreeing on the diagnosis of tendinopathy
Diagnostic criteria are generally broad and must reflect the different features of a disease (heterogeneity). This is particularly difficult in the case of tendinopathy where there is substantial variation in diagnostic criteria. Pain on palpation, pain with specific provocation tests, subjective reports and abnormal imaging are among the diagnostic inclusion criteria reported in the literature. To illustrate this variability , some studies use pain provocation tests, while some do not. There is also large variation in diagnostic criteria across various tendon locations for tendinopathy, and substantial variability exists even within the same tendon (online supplementary file 1).
Consider the role of confounding variables
The potential roles of confounding variables linked to central sensitisation (eg, obesity, multiple pain sites and comorbid health complaints such as low mood, poor sleep and fatigue) are often overlooked and undocumented when comparing tendinopathy studies. In addition, some studies exclude participants with systemic comorbidities such as diabetes or other musculoskeletal complaints while others do not (online supplementary file 1). Appropriate documentation, and consideration, of potential confounders in addition to standardised diagnostic criteria is critical to allow valid comparisons across data sets. Currently, it is not possible to say with sufficient confidence that those labelled with ‘tendinopathy’ across studies are similar enough to allow clinical utility or meaningful statistical analysis.
Describing populations clearly
Tendinopathy affects a wide variety of individuals, young and old, athletic and non-athletic. Therefore, even within a single diagnostic condition such as tendinopathy, there may be considerable variation between patients. Without a clear, detailed understanding of participant characteristics, it is impossible for clinicians to know if this study is representative of the patient in front of them. Better describing tendinopathy participants across a wide range of relevant characteristics may enable more detailed analyses, facilitating a better understanding of how and why people with tendinopathy recover (or not).
The next steps?
In conclusion, the topic of sensitisation in tendinopathy may have important implications for both clinicians and researchers. Confusion surrounding the topic highlights the need for clarity. A first step in achieving clarity may involve a consensus statement outlining the minimum reporting standards for tendinopathy, incorporating clearly defined (1) diagnostic criteria and (2) population descriptors. A second step may involve undertaking appropriate research studies comparing pain sensitisation across a range of clinical populations with tendinopathy, by accounting for potential confounding variables. Minimum reporting standards have been published for other painful musculoskeletal conditions10; perhaps the time for minimum reporting standards in tendinopathy is now!
Note on accompanying supplementary file
The accompanying table in the supplementary file summarises many of the cross-sectional studies to date which have investigated the concept of sensitisation in tendinopathy. Although this is a comprehensive list of studies and was performed by searching academic databases, it is not intended to be a systematic review of the literature. Its intended purpose is to illustrate and support many points raised in the editorial by outlining the various methodologies and demographics of the studies in the field. The table outlines the population sample investigated, body site investigated, severity and duration of symptoms, outcome measures used and results of the study.
Footnotes
Funding The authors have not declared a specific grant for this research from anyfunding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent Not required.
Provenance and peer review Not commissioned; externally peer reviewed.