​Pain mechanisms: is consensus on terminology and definition possible?

There is no doubt that musculoskeletal pain is a complex multidimensional phenomenon. At present, musculoskeletal pain diagnosis is based primarily on signs and symptoms, sometimes combined with evidence of disease, structural damage, or injury. However, this diagnosis provides limited information regarding the mechanisms underlying the pain experience of the individual patient that may guide choice of treatment. It has been suggested that pain diagnosis and management should be mechanism-based (Gifford and Butler, 1998; Smart et al., 2008; Vardeh et al., 2016; Woolf et al., 1998).

Nowadays, there is no agreement on how pain mechanisms can be classified and discriminated in people with musculoskeletal pain. Although the IASP has provided definitions for 3 types of pain (i.e. nociceptive, neuropathic, nociplastic) (IASP, 2017), multiple other classifications using different terms and definitions for pain mechanisms have been proposed. The question that arises here is: Are all these (pain) mechanism-based classification systems interchangeable? Is there any possibility to reach consensus within the “pain world” for using the same terminology and definitions for pain mechanisms?

A recent systematic review by Shraim et al (2020) has shed light on these questions aiming to synthesize and summarize the scientific literature related to descriptions of mechanism-based classifications for musculoskeletal pain. In addition, convergence and divergence between terminologies and descriptions of each mechanism-based classification have been analysed. What is also really interesting, is that authors, based on thematic analysis of the data, summarized the following topics for each pain mechanism: (1) underlying neurobiology/pathology; (2) aggravating and easing factors and response to treatment; and (3) pain characteristics.

Some important conclusions can be extracted from this systematic review:

• Defining the term “pain mechanism” is in itself challenging. Authors proposed this definition: “Grouping of neurobiological processes involved and dominating the pain experience”.
• A total of 174 different terms were used in the scientific literature to identify pain-mechanisms!! Nevertheless, most pain mechanisms classifications were broadly aligned with the 3 major groups proposed by the IASP: nociceptive, neuropathic and nociplastic pain. In addition to these 3 pain categories, authors identified three more groups or categories namely disputed pain (including sympathetic and psychogenic), mixed pain and “other pain”. Therefore, all the pain mechanisms classifications were converged to a total of 6 defined pain categories.
• A limited number of papers included in the review addressing mechanism-based classifications for musculoskeletal pain involved a consensus approach. Indeed, most classifications were proposed or stated in narrative reviews.
• There are some clinical entities where no consensus exists of the dominant pain mechanism such as CRPS or fibromyalgia.
• A major challenge regarding pain mechanisms is validation as there is no objective gold standard for each pain mechanism against criteria can be compared. Moreover, many diagnostic tools for pain mechanisms have poor reliability and validity.
• Some authors state that pain characteristic features cannot identify mechanisms as they are not specific.
• Overlap among underlying pain mechanisms is common. Multiple mechanisms may be present in one patient despite one may predominantly contribute to the pain experience.

In conclusion, this systematic review has shown that there is considerable agreement amongst the literature when referring to pain mechanisms but some inconsistency and evolution of terminology was found. The lack of consensus-driven criteria for pain mechanisms still is a major issue so this could be the topic of further research.

Enrique Lluch

2020 Pain in Motion

References and further reading

Gifford LS, Butler DS. The integration of pain sciences into clinical practice. J Hand Ther. 1997;10:86–95. 8.

International Association for the Study of Pain. Task Force on Taxonomy. IASP Terminology. 2017. Available at: http://web. archive.org/web/20200130092932/https://www.iasp-pain.org/ Education/Content.aspx?ItemNumber=1698. Accessed January 30, 2020.

Shraim MA, Massé-Alarie H, Hall LM, Hodges PW. Systematic Review and Synthesis of Mechanism-based Classification Systems for Pain Experienced in the Musculoskeletal System. Clin J Pain. 2020;36(10):793-812.

Smart KM, O’Connell NE, Doody C. Towards a mechanismsbased classification of pain in musculoskeletal physiotherapy? Phys Ther Rev. 2008;13:1–10.

Vardeh D, Mannion RJ, Woolf CJ. Toward a Mechanism-Based Approach to Pain Diagnosis. J Pain. 2016;17(9 Suppl):T50-T69.

Woolf CJ, Bennett GJ, Doherty M, et al. Towards a mechanism-based classification of pain?. Pain. 1998;77(3):227-229.