Background and aims
Evaluating the degree to which pain has become chronic beyond mere duration poses several problems. The IASP Pain Taxonomy Axis IV employs intensity and duration combined to nine ordered categories. The Chronic Pain Grade links intensity and disability, but only the latter contributes to higher grades. The Mainz Pain Staging System includes temporal and spatial aspects, medication and health care utilization. Their interrelations, scale properties and construct validity are not always known or debatable. The study challenges the generality and homogeneity of the chronicity construct of musculoskeletal pain aiming at necessary and sufficient sub-constructs identified by separable marker clusters. We show chronicity to vary in content and structure with severity and duration and between different populations. This raises the question of validity conditions of general chronicity indices and requires further work on adequate chronicity measures.
Diagnostic entrance data of 185 patients with chronic regional vs. widespread musculoskeletal pain (unspecific back pain, fibromyalgia) from regional pain clinics and 170 active employees in a nationwide prevention program were included in a retrospective cross-sectional analysis of the combined marker sets of the three chronicity indices above. The samples of patients and employees provided intensity, duration and disability degrees over the whole range of the assumed chronicity. Intensity-duration relations were quantified by correlations and frequency distributions of successive duration classes. The dimensional structure of pain and chronicity variables was assessed by factor and cluster analyses.
Pain intensity distributions showed inhomogeneous courses from short to long durations – lowest intensities predominating at longer durations in patients and at shorter in employees. Moreover, pain intensity and duration related nonlinearly to Chronic Pain Grade and Mainz Pain Stage and differently in patients compared to employees, and these indices correlated only moderately to each other. Factor and cluster analyses revealed different dimensions and clusters of chronicity markers for patients and employees. In the former, three dimensions with four clusters were identified with clinical characteristics (intensity, temporal and spatial aspects) separated from direct consequences (disability/interference with activities, medication usage) and chronic development (duration, healthcare utilization). In employees, only two dimensions with three clusters were obtained and clinical pain characteristics clustered with direct consequences both separated from chronic development. Similar differences were shown between unspecific back pain and fibromyalgia but were less well defined.
There appears to be no coherent “chronicity” entity over the entire range of severity and duration for all pain populations with different clinical pictures and social contexts. Statements about chronicity must be differentiated with respect to those aspects relative to patient career.
General indices do not capture the complex and changing composition of chronicity. There is evidence for at least three weakly coupled core domains of chronicity, i.e. the primary clinical characteristics, the direct consequences of current interference with activities, and aspects of the patient history. Hence, multivariate assessment is recommended. The particular syndrome, the diagnostic context and the population under investigation should likewise be considered.
The study is based on data collected in two projects of the Clinical Research Group KFO 107 “Neuroplasticity and Learning in Chronic Pain” (Projects HO 904/11; FL 156/26) of the Deutsche Forschungsgemeinschaft and project “Prevention of Work-related Musculoskeletal Pain” of the German Occupational Health Association, Section Nutrition and Gastronomy Business (Berufsgenossenschaft Nahrungsmittel & Gaststätten, BGN) awarded to RH. The great and congenial support of our colleagues and coworkers of the KFO 107 and the BGN is gratefully acknowledged.
Research funding: The study was funded by the Deutsche Forschungsgemeinschaft (Clinical Research Group 107 “Neuroplasticity and Learning in Chronic Pain”) and the German Occupational Health Association, Section Nutrition and Gastronomy Business (Berufsgenossenschaft Nahrungsmittel & Gaststätten, BGN). Marina Finnern was supported by a PhD grant from the Friedrich Ebert Foundation. None of the funders were involved in the data analysis or manuscript preparation. According to the privacy regulations employee data were collected and managed by the BGN and transferred pseudonomized to the Otto-Selz-Institute, University of Mannheim; pseudonomization documents remained with the BGN.
Conflict of interest: The authors declare that there are no conflicts of interest associated with this manuscript.
Informed consent: All participants were informed of the purpose of research and gave their informed consent.
Ethical approval: The study was approved by the Local Ethics Committee. Patient data were partially acquired in connection with a clinical trial of combined behavioural and cannabinoid treatment for chronic pain (ClinicalTrials.gov Identifier: NCT00176163). The prevention program for the employees was conducted by the nutritional business and gastronomy section of the German employers’ liability insurance association (Berufsgenossenschaft Nahrungsmittel & Gaststätten, BGN) according to legal regulations.
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The online version of this article offers supplementary material (https://doi.org/10.1515/sjpain-2018-0021).
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