Perceived injustice in fibromyalgia: psychometric characteristics of the Injustice Experience Questionnaire and relationship with pain catastrophising and pain acceptance.

OBJECTIVE: To validate a Spanish version of the Injustice Experience Questionnaire (IEQ), a measure of perceived injustice, in a fibromyalgia sample and to examine its relationship with pain catastrophising and pain acceptance.
METHODS: The IEQ was administered along with the Pain Visual Analogue Scale, the Fibromyalgia Impact Questionnaire, the Hospital Anxiety and Depression Scale, the Pain Catastrophizing Scale (PCS) and the Chronic Pain Acceptance Questionnaire (CPAQ) to 250 primary care patients with fibromyalgia.
RESULTS: The IEQ had good test-retest reliability (intraclass correlation coefficient=0.98) and internal consistency (Cronbach's α=0.92). The factor structure obtained was similar to the original validation study. The multiple regression analyses showed that perceived injustice (PI) accounted for significant pain-related outcomes after controlling pain intensity, PCS and CPAQ. Principal component analysis of both the IEQ and the CPAQ taken together showed that the two constructs do not represent opposite extremes of the same dimension.
CONCLUSION: The IEQ is a reliable assessment tool for measuring PI among patients with fibromyalgia. PI seems to be distinct from catastrophising, although the two constructs are very similar. The factor analysis showed that PI and acceptance represent related constructs, and this entails relevant implications for therapy, as acceptance-based interventions would be appropriate.