The Fatigue Scale for Motor and Cognitive Functions: a recently validated instrument to assess MS-related fatigue

Introduction: Fatigue is estimated to affect 75 to 95% of patients with MS. Assessment, however, is difficult as it is experienced subjectively and has to be differentiated from other disease factors. Instruments available today do not differentiate sufficiently between cognitive and physical aspects of fatigue and show methodological limitations.

Objective: To develop a new fatigue instrument that provides reliable assessment of the different fatigue components.

Methods: The Fatigue Scale for Motor and Cognitive Functions (FSMC) is a 20-item scale that can be administered in 5 minutes. It consists of a cognitive (FSMCc) and a physical subscale (FSMCm), each comprising 10 items. In a multi-centre design, 357 MS patients and 158 controls were recruited to test the scale against several external criteria: fatigue rated by neurologist, two common fatigue scales (FSS, MFIS), cognitive performance (BRB-N, FST, MSNQ), depression (BDI), physical functionality (MSFC), personality traits (NEO-FFI), motivation (HAKEMP-90), and quality of life (SF-36, FAMS).

Results: A t-test for independent samples showed a significant differentiation between patients and controls. In the patient cohort, Cronbach's Alpha ranged between 0.93 and 0.91 for the two subscales. A logistic regression and ROC analysis revealed high specificity (83.7) and sensitivity (88.7) values. Bivariate correlations showed that the FSMCm correlated equally high with FSS and MFIS motor scale, while the FSMCc correlated highest with the MFIS cognitive scale. Depression correlated with both FSMC scales. Interestingly, these correlations were weaker than those between BDI and MFIS. Disease course was only related to FSMCm. Out of the different cognitive test measures only MSNQ, SDMT and PASAT correlated with fatigue. Neuroticism and extraversion as well as motivational aspects significantly correlated with the FSMC. Quality of life highly correlated with both fatigue components.

Discussion: The results of the validation study show that the FSMC is able to differentiate patients from healthy controls, provides two clear subscales for motor and cognitive fatigue, and shows high specificity, sensitivity and reliability. Besides, fatigue seems to be closer related to personality and motivational aspects than to cognitive functioning per se. PASAT and SDMT, however, seem to be sensitive cognitive outcome measures for cognitive fatigue. As depression was less correlated with FSMC than with MFIS, the new scale might be less susceptible to confounding by depression.