Phenotypic characteristics of patients with chronic widespread pain and fibromyalgia: a cross-sectional cluster analysis

OBJECTIVE

This study aimed to explore whether phenotypic characteristics of patients with chronic widespread pain (CWP) and fibromyalgia (FM) can be aggregated into definable clusters that may help to tailor treatments.

METHOD

Baseline variables (sex, age, education, marital/employment status, pain duration, prior CWP/FM diagnosis, concomitant rheumatic disease, analgesics, tender point count, and disease variables derived from standardized questionnaires) collected from 1099 patients (93.4% females, mean age 44.6 years) with a confirmed CWP or FM diagnosis were evaluated by hierarchical cluster analysis. The number of clusters was based on coefficients in the agglomeration schedule, supported by dendrograms and silhouette plots. Simple and multiple regression analyses using all variables as independent predictors were used to assess the likelihood of cluster assignment, reported as odds ratios (ORs) with 95% confidence intervals (CIs).

RESULTS

Only one cluster emerged (Cluster 1: 455 patients). Participants in this cluster were characterized as working (OR 66.67, 95% CI 7.14 to 500.00), with a medium-term/higher education (OR 16.80, 95% CI 1.94 to 145.41), married/cohabiting (OR 14.29, 95% CI 1.26 to 166.67), and using mild analgesics (OR 25.64, 95% CI 0.58 to > 999.99). The odds of being an individual in Cluster 1 were lower when having a worse score on the PDQ (score ≥ 18) (OR < 0.001, 95% CI < 0.001 to 0.02).

CONCLUSION

We identified one cluster, where participants were characterized by a potentially favourable clinical profile. More studies are needed to evaluate whether these characteristics could be used to guide the management of patients with CWP and FM.

Validity of the Brief Illness Perception Questionnaire in people with knee pain: a Rasch analysis

OBJECTIVE

The Brief Illness Perception Questionnaire (B-IPQ) is a frequently used measure of illness perception (IP). The aim of this study was to explore the psychometric properties of the questionnaire when used in elderly people with knee pain.

METHOD

Based on data from the Frederiksberg Cohort on elderly people reporting knee pain (N = 836), the psychometric properties of the eight B-IPQ items (1 'Consequences', 2 'Timeline', 3 'Personal control', 4 'Treatment control', 5 'Identity', 6 'Concern', 7 'Coherence', and 8 'Emotions') were analysed using Rasch analysis to establish whether the questionnaire provides reliable and valid measures of IP.

RESULTS

Threshold disordering was found on the 1-10 rating scale in all items. When rescaling to a 0-2 rating scale, disordering was resolved in six items. Item goodness-of-fit analyses revealed that two items displayed underfit misfit and four items overfit misfit; hence, the B-IPQ does not present unidimensionality. The person separation index indicated that items separate respondents into only two IP levels. Finally, floor and ceiling effects were found, indicating that the B-IPQ may not fully describe the extent of IP in this population.

CONCLUSION

The 1-10 rating scale used in the Danish B-IPQ targeting knee pain is suboptimal, while a 0-2 scale improves the psychometric properties of the scale. The B-IPQ does not present unidimensionality and the use of a sum score is therefore not recommended when assessing IP. The B-IPQ may not cover the extent of IP in elderly people with knee pain.