The nature of fatigue in multiple sclerosis

Abstract

The aim of this cross-sectional descriptive study was to determine whether people with multiple sclerosis differentially experience physical and mental fatigue, and whether the pattern of fatigue is influenced by mood, disease duration, or disease course. Seventy-eight consecutive out-patients with multiple sclerosis were asked to complete the Fatigue Rating Scale (FRS) and the Hospital Anxiety and Depression Scale (HAD). Sixty-eight (87%) subjects completed the questionnaires. Fifty-eight (85%) subjects in this group scored above the recommended cut-off for fatigue on the FRS scale. Both the mental fatigue score and the total fatigue score were positively correlated with the depresssion and anxiety scores on the HAD scale. There was no significant correlation between the physical fatigue score and either of the HAD subscale scores. There was no significant association between duration of disease or disease course and the total scores or subscale scores of the FRS and HAD. This is the first reported study to differentiate between mental and physical fatigue in multiple sclerosis and to demonstrate a significant correlation between fatigue and mood level. This has important implications for the effective treatment of fatigue in multiple sclerosis.

Introduction

Fatigue is known to be a particular problem for people with multiple sclerosis (MS) [1]. It occurs in those with mild disease and may be the first presenting symptom [2]. It is also a very frequent symptom; fatigue following ordinary exertion has been reported in 78% of people with multiple sclerosis in a community-based population, with 66% of the group experiencing fatigue on a daily basis [3]. In previous studies, a significant number of patients have described fatigue as their most severe symptom 2, 4 and, similarly, two of the current investigators (H.F. and M.J.) have found, in structured interviews with 32 MS patients, that 28% described fatigue as their most troublesome symptom. Also, in focus group sessions during the development of a disease-specific quality-of-life measure for people with MS, fatigue was reported to have a major influence on quality of life [5]. Despite the studies mentioned, the nature of fatigue in MS has been poorly characterized.

The mechanism or precise etiology of fatigue in MS remains unresolved [6]. The presence of fatigue is not correlated with the degree of neurological impairment or disability 2, 7, 8, or with depression [9]. In studies of MS, various self-report instruments have been used to measure the fatigue. These include the Fatigue Severity Scale [9], the Fatigue Impact Scale [7], and the Fatigue Assessment Instrument [10]. The Fatigue Severity Scale and the Fatigue Impact Scale measure the effect of fatigue on functioning. The Fatigue Assessment Instrument measures fatigue severity, whether fatigue is situation specific, the consequences of fatigue, and the response of fatigue symptoms to rest or sleep. These instruments do not identify distinct mental and physical fatigue dimensions. A fatigue index (FI) has been defined that quantitatively measures fatigue using maximal voluntary muscle contraction. The FI was found to correlate with the presence of pyramidal tract signs [11]. Studies of other clinical populations, however, have suggested that fatigue should be viewed as a continuous dimension rather than a discrete variable, with both physical and mental components. For example, in their community-based study of fatigue, Chalder and colleagues found good evidence to support making a distinction between the two constructs of physical and mental fatigue [12]. This was an important new approach in view of the finding that an objective inability to sustain power is not necessarily related to the subjective sensation of fatigue [13], and because existing fatigue scales had a preponderance of items relating to the behavioral consequences of specifically physical fatigue.

The fact that none of the previous studies in MS have attempted to differentiate between physical and mental fatigue is important, as it may be that some MS patients suffer severe mental fatigue without equivalent levels of physical fatigue. This problem may then not be addressed as it is not detected by the usual methods employed. It may be clinically useful to consider these various aspects of fatigue in the management of patients, as different therapeutic interventions may be effective for the different components of a patient’s fatigue.

We carried out a cross-sectional descriptive study to examine whether patients with MS differentially experience physical and mental fatigue, and whether the pattern of fatigue is associated with mood level or clinical features of MS. Our main a priori hypotheses were:

• That it is possible to identify separate features of fatigue (of both a “physical” and “mental” nature) in patients with MS.

• That the “mental” fatigue symptoms are associated with indicators of mood level, although the “physical” fatigue symptoms may not be.