Fatigue and fatigability in neurologic illnesses: proposal for a unified taxonomy

Abstract

Fatigue is commonly reported in many neurologic illnesses, including multiple sclerosis, Parkinson disease, myasthenia gravis, traumatic brain injury, and stroke. Fatigue contributes substantially to decrements in quality of life and disability in these illnesses. Despite the clear impact of fatigue as a disabling symptom, our understanding of fatigue pathophysiology is limited and current treatment options rarely lead to meaningful improvements in fatigue. Progress continues to be hampered by issues related to terminology and assessment. In this article, we propose a unified taxonomy and a novel assessment approach to addressing distinct aspects of fatigue and fatigability in clinical and research settings. This taxonomy is based on our current knowledge of the pathophysiology and phenomenology of fatigue and fatigability. Application of our approach indicates that the assessment and reporting of fatigue can be clarified and improved by utilizing this taxonomy and creating measures to address distinct aspects of fatigue and fatigability. We review the strengths and weaknesses of several common measures of fatigue and suggest, based on our model, that many research questions may be better addressed by using multiple measures. We also provide examples of how to apply and validate the taxonomy and suggest directions for future research.

Figure. Diagram of major factors contributing to the 2 domains of fatigue: perceptions of fatigue and fatigability

While separated in this diagram, is should be noted that perceptions of fatigue and performance fatigability have the potential to influence each other. Letters in boxes refer to the following: a) known neuroanatomic sites mediating this factor; b) normal function of this factor; c) pathologic states involving this factor. CFS = chronic fatigue syndrome; CVA = cerebrovascular accident; EC = excitation/contraction; GBS = Guillain-Barré syndrome; GSD = glycogen storage diseases; MG = myasthenia gravis; MS = multiple sclerosis; PD = Parkinson disease; TBI = traumatic brain injury.