Definitions, Epidemiology, and Etiological Factors

Abstract

There is no generally and widely accepted definition of fatigue. Despite its increasing importance and awareness, it remains a poorly defined construct that is predominantly found in MS, but also in other diseases such as tumors, strokes, or chronic inflammatory processes. The term most likely refers to “abnormal fatigability,” “abnormal exhaustion,” or “increased exhaustibility.” In any case, MS-related fatigue is quite different from the tiredness that healthy people report and that persons with MS themselves experienced before the onset of the disease: They complain of a lack of drive and energy that is exacerbated or persistent depending on mental and/or physical load and that may affect both, mental and physical performances. Often, fatigue is worsened by increased ambient or body temperature, whether due to heat, fever, or physical exertion (“Uhthoff phenomenon”). Not infrequently, both the quality of life and the ability to work and perform activities of daily living are impaired to such an extent that, particularly in a less adaptive working environment, early retirement is inevitable and those affected increasingly withdraw from social activities (Sterz et al. 2016). Despite the enormous impact on the individual patient, the causes of fatigue have not been conclusively clarified. The following chapter refers to own studies of the author on various etiological factors of MS-related fatigue and discusses them in the context of other studies.