Day-to-day fluctuations of fatigue severity in individuals with knee osteoarthritis: an ecological momentary assessment approach.

Objective: The variable course of fatigue adds to the disease burden of patients with OA yet it has been poorly understood. This study aimed to describe within-person fluctuations of fatigue severity and explore its associations with pain, positive affect, negative affect, sleep, and perceived exertion of physical activity. Methods: Individuals with chronic knee pain or a clinical diagnosis of knee OA ≥40 years of age completed daily assessments about fatigue, pain, positive affect, negative affect, sleep, perceived exertion of physical activity (numeric rating scale 0-10), and overwhelming fatigue (yes/no) on a smartphone over 14 days. Within-person fluctuations of fatigue severity were described by the probability of acute changes (PACs) and s.d.s. Associations with pain, positive affect, negative affect, sleep, and perceived exertion of physical activity were explored using multilevel models. Results: Forty-nine individuals were included (mean age 63.4 years; 82% female). PACs and s.d.s of within-person daily fatigue fluctuations ranged from 0.00 to 0.80 and 0.35 to 2.95, respectively. Within-person associations of fatigue severity were moderate for positive affect [β = -0.57 (95% CI -0.67, -0.47)], weak for pain [β = 0.41 (95% CI 0.29, 0.53)] and negative affect [β = 0.40 (95% CI 0.21, 0.58)], and negligible for sleep [β = -0.13 (95% CI -0.18, -0.08)] and perceived exertion of physical activity [β = 0.18 (95% CI 0.09, 0.26)]. Conclusion: Some individuals showed almost stable day-to-day levels of fatigue severity, whereas others experienced a substantial number of clinically relevant fluctuations. To reduce the burden of daily fatigue fluctuations, our results suggest that pain, positive and negative affect rather than sleep and perceived exertion of physical activity should be considered as potential targets.