Variation in fatigue may be poorly explained by pain: results from a longitudinal, exploratory study.

It is frequently claimed that fatigue reflects pain and that strategies for alleviating fatigue in patients with ankylosing spondylitis (AS) should focus on pain management. The aim of this study was twofold: to investigate the correlation between fatigue and pain over time and to investigate the temporal relationship between fatigue and pain in patients with AS. Fatigue and pain were assessed twice a week for 35 weeks in 23 patients with AS. Data were reported with text messages on mobile phones, constituting around 70 repeated measurements per patient. To estimate correlation over time, the correlation coefficient within individuals was estimated. When estimating the temporal relationship, we lagged the independent variable and performed individual linear regression. In 16 (70 %) of the patients, ≤ 36 % of the variance in fatigue was explained by pain. The association between fatigue and pain was synchronous in time in 13 (57 %) patients, while 5 (22 %) patients reported that fatigue precedes pain by 1 week and 5 (22 %) that pain precedes fatigue by 1 week. Fatigue and pain may be two separate and independent symptoms in some patients with AS. The clinical implication is that the two symptoms should be targeted separately because it cannot always be expected that an improvement in one is followed by an improvement in the other.