The aim of this study was to examine the longitudinal association between the impact of fatigue and health-related quality of life and to determine if potential confounders distorted this association. Baseline, 3-, 6- and 12-week assessments of a randomized clinical trial were used. Quality of life was assessed with the Parkinson’s Disease Questionnaire-39 and the Multidimensional Fatigue Inventory was used to measure fatigue. Time-independent and time-dependent factors were investigated for their bivariate association with quality of life by applying random coefficient analysis. Candidate confounders were successively added to the longitudinal association model to determine if the relationship between quality of life and fatigue was distorted. A change beyond 15% of found regression coefficient of the Multidimensional Fatigue Inventory was thought to be significant. One hundred and fifty-three patients were included. Impact of fatigue was significantly associated with poorer quality of life (β = 0.24, 95% confidence interval = 0.18 to 0.30). This association was significantly distorted by depression (30.0%) and anxiety (24.1%). No distortion was found for other factors. After controlling for confounders, fatigue remained significantly associated with quality of life (β = 0.12, 95% confidence interval = 0.06 to 0.18, r2 = 2.3%).
Theresults of this study indicate that patients who experience fatigue tend to report lower levels of quality of life, Although, this longitudinal relationship is confounded by depression and anxiety and suggests that the unique contribution of fatigue to overall quality of life is rather small.