Validation of a questionnaire for central nervous system aspects of joint pain: the CAP questionnaire.

Background: Neuropathic-like pain, fatigue, cognitive difficulty, catastrophizing, anxiety, sleep disturbance, depression and widespread pain associate with a single factor in people with knee pain. We report the Central Aspects of Pain questionnaire (CAP) to characterize this across painful musculoskeletal conditions.

Methods: CAP was derived from the 8-item CAP-Knee questionnaire, and completed by participants with joint pain in the Investigating Musculoskeletal Health and Wellbeing survey. Subgroups had OA, back pain or FM. Acceptability was evaluated by feedback and data missingness. Correlation coefficients informed widespread pain scoring threshold in relation to the other items, and evaluated associations with pain. Factor analysis assessed CAP structure. Intraclass Correlation Coefficient (ICC) between paper and electronic administration assessed reliability. Friedman test assessed score stability over 4 years in people reporting knee OA.

Results: Data were from 3579 participants (58% female, median age 71 years), including subgroups with OA (n = 1158), back pain (n = 1292) or FM (n = 177). Across the three subgroups, ≥10/26 painful sites on the manikin scored widespread pain. Reliability was high [ICC = 0.89 (95% CI 0.84-0.92)] and CAP scores fit to one- and two-factor model, with a total CAP score that was associated with pain severity and quality (r = 0.50-0.72). In people with knee pain, CAP scores were stable over 4 years at the group level, but displayed significant temporal heterogeneity within individual participants.

Conclusions: Central aspects of pain are reliably measured by the CAP questionnaire across a range of painful musculoskeletal conditions, and is a changeable state.