The dimensionality of fatigue in Parkinson's disease

Background: Fatigue is a common problem among individuals with Parkinson's disease (PD). It may occur before the overt symptoms of bradykinesia, rigidity and tremor. Little is understood about how to measure fatigue in PD. Here we determined the dimensionality of the constructs of fatigue. Methods: Four recommended scales, the Fatigue Severity Scale, Functional Assessment of Chronic Illness Therapy-Fatigue, Parkinson Fatigue Scale and Visual Analog Fatigue Scale (VAFS) were tested against quality of life measures including cognition, depression, sleep, life orientation, physical activity and PD symptoms in 22 PD subjects and 15 caregivers. Results: Fatigue was associated with many quality of life variables, with the PDQ-39 summary index showing the strongest association. PD subjects agreed more strongly than caregivers that they experienced higher levels of fatigue. 27% of PD subjects rated fatigue as one of their top three most bothersome symptoms. The constructs of fatigue was captured within one dimension which explained 67% of the total variance, of which the VAFS showed the highest internal consistency. The highest likelihood ratio gave a cut-off score of < 5.5 on the VAFS. The change in scores required to produce a perceptible difference or is grossly observable ranged between 1.4 and 2.2 points respectively. Conclusion: The potential utility of a single measure such as the VAFS in PD that is reliably correlated with quality of life is consistent with the pursuit to develop clinical tests and measurements that are accessible, easy to use and universally interpretable across health science disciplines.