The relationship between fatigue and other clinical features of multiple sclerosis

Background: There has been considerable debate regarding the precise relationships between fatigue in multiple sclerosis (MS) and disease-related factors, such as disability, sleep disturbance, depression, age and sex. Existing studies give conflicting information. Objective: To clarify such relationships in a large cross-sectional study, using a rigorously developed measurement tool which was based on a clear definition of fatigue. Method: A pack containing the Neurological Fatigue Index for MS Summary Scale, the Hospital Anxiety and Depression Scale, the Epworth Sleepiness Scale, the Multiple Sclerosis Impact Scale and questions regarding sleep and demographics was mailed to patients with MS attending two centres in the UK. All scale scores were converted to parametric measures using the Rasch measurement model. Both linear and non-linear relationships were sought. Results: Data from 635 respondents (52% response) were analysed. Fatigue was strongly related to the impact of MS. Fatigue was worse in those with progressive disease and clearly worsened once ambulation was affected. There was only weak correlation with anxiety and depression. Fatigue was not related to disease duration or patient age. There was an intimate but complex relation between fatigue and sleep. Fatigue levels were minimum at a nocturnal sleep duration of 7.5 h. Conclusion: Clear relationships were found between fatigue and disability, disease type and sleep. Further physiological enquiry and trials of drug treatment and sleep modulation might be guided by these clinical relationships.