Towards an understanding of chronic pain mechanisms: the use of psychologic tests and a refined differential spinal block

Methods to discriminate between mechanism-based categories of pain experienced in the musculoskeletal system: a systematic review

ABSTRACT

Mechanism-based classification of pain has been advocated widely to aid tailoring of interventions for individuals experiencing persistent musculoskeletal pain. Three pain mechanism categories (PMCs) are defined by the International Association for the Study of Pain: nociceptive, neuropathic, and nociplastic pain. Discrimination between them remains challenging. This study aimed to build on a framework developed to converge the diverse literature of PMCs to systematically review methods purported to discriminate between them; synthesise and thematically analyse these methods to identify the convergence and divergence of opinion; and report validation, psychometric properties, and strengths/weaknesses of these methods. The search strategy identified articles discussing methods to discriminate between mechanism-based categories of pain experienced in the musculoskeletal system. Studies that assessed the validity of methods to discriminate between categories were assessed for quality. Extraction and thematic analysis were undertaken on 184 articles. Data synthesis identified 200 methods in 5 themes: clinical examination, quantitative sensory testing, imaging, diagnostic and laboratory testing, and pain-type questionnaires. Few methods have been validated for discrimination between PMCs. There was general convergence but some disagreement regarding findings that discriminate between PMCs. A combination of features and methods, rather than a single method, was generally recommended to discriminate between PMCs. Two major limitations were identified: an overlap of findings of methods between categories due to mixed presentations and many methods considered discrimination between 2 PMCs but not others. The results of this review provide a foundation to refine methods to differentiate mechanisms for musculoskeletal pain.

Psychological aspects of interventional therapy

This discussion is not, nor could it hope to be, an exhaustive examination of all of the various interventional therapies. Instead, it is intended to highlight the potential contribution of psychosocial factors. These factors may vary to some degree or another depending on the specific procedure, but clearly play a role whenever the desired outcome involves a reduction in subjective pain, alteration in the adaptiveness with which the patient responds to the experience of pain, and quality of life. Many notables, including Dr. Michael Cousins, have echoed the importance of incorporating interventional therapies into an interdisciplinary approach. Yet, there seems to be a preponderance of "block shops". Even when used for diagnostic or prognostic purposes, the impact of psychosocial variables and the potential relevance of a meaningful behavioral or psychologic evaluation cannot be overstated. It is easy to understand how the reader might conclude that immersing oneself in the minutiae of all these variables could lead to a feeling of intellectual paralysis when it comes to evaluating the data and arriving at a conclusion or diagnosis. However, ignoring these psychosocial variables and their complex interaction does not constitute a solution. This is particularly true in considering discography where, depending on the criteria applied, the percent of "false positives" can vary from 0% to as much as 40%. The implication for the performing of "unnecessary" spine surgery is obvious. The thoughtful practitioner will be mindful of the role of psychosocial variables in so far as they are thought to be relevant in a particular case. The overall contribution of psychosocial variables to the application of interventional therapies for the diagnosis and treatment of pain can be overlooked and ignored, but not denied. A certain percentage of patients will respond in a predictable, desirable or positive fashion purely on a statistical basis. Historically, and there seems to be no reason to believe this will change in the immediate future, the degree to which the psychosocial variables are considered is left up to the interventionalist. Some are content to perform a directed procedure or therapy concerned only, and sometimes to a less than sufficient degree, with the technical adequacy of the procedure. Others will appreciate the role of human factors including those of the practitioner and patient alike, and strive not only for a statistically derived outcome but the best possible outcome for a given patient. Psychosocial factors can sometimes take on the character of "nuisance variables". However, it is hard not to wonder how much care each would want to have given to these factors if one were on the other end of the needle.

Intrathecal normal saline without preservative does not have a local anesthetic effect

The effects of intrathecally administered normal saline (preservative-free) were studied in patients undergoing differential spinal block anesthesia for evaluation of chronic pain below the waist. The injection of 5 ml saline did not significantly change the sensitivity to pinprick measured in the lower back and both lower extremities. Temperature measurement in both the lower extremities and psychogalvanic skin reflex did not show evidence of sympathetic block; the psychogalvanic skin reflex was abandoned because of inconsistent results. These findings refute the earlier conclusion of Urban and McKain who found that normal saline acts as a weak local anesthetic when injected intrathecally. We recommend use of normal saline (preservative-free) in 5 ml doses as a placebo for differential spinal block anesthesia.