Ask your doctor if anything in here will cure post stroke fatigue. The key word there is cure, NOT MANAGE OR ASSESS!
The role of clinical neurophysiology in the definition and assessment of fatigue and fatigability
Keywords
1. Introduction
Chronic, disruptive fatigue is a significant symptom in several chronic diseases, with a myriad of definitions. Since the turn of the century, there has been several attempts to delineate and differentiate different types of fatigue in pathophysiological terms (Chaudhuri and Behan, 2004, Kluger et al., 2013, Kuppuswamy, 2017). Some have focussed on symptoms reported by patients, which may be defined as ‘the state of weariness following a period of exertion, mental or physical, characterized by a decreased capacity for work and reduced efficiency to respond to stimuli,’ (Dittner et al., 2004), ‘a reversible decrease or loss of abilities associated with a heightened sensation of physical or mental strain even without conspicuous effort, due to an overwhelming feeling of exhaustion, which leads to an inability to sustain or difficulty in sustaining even routine activities,’ (Staub and Bogousslavsky, 2001). Others have tried to capture the mechanisms of fatigue such as ‘difficulty in initiation of or sustaining voluntary activities,’ (Chaudhuri and Behan, 2004) and ‘a percept arising primarily from alterations within the activational systems that inform voluntary action,’ (Kuppuswamy, 2017).
These approaches have, at times, been confused by conflation of two distinct phenomena both described as fatigue; the ‘feeling’ or ‘perception’ which can only be described using self-reported measures such as questionnaires, and a reduction in performance over time, measurable by behavioural and neurological indices. This confusion has led to some proposing the use of the term ‘fatigue’ to refer only to the ‘feeling’ and not to performance changes, which are then referred to as ‘fatigability’ (Kluger et al., 2013). We will adopt this nomenclature. A further source of confusion arises from attributing fatigue and fatigability to different underlying neural systems. The terms ‘mental’ and ‘physical’ fatigue have been commonly interpreted as mental fatigue; perception involving higher order cognitive networks (fatigue in accordance with the new proposal); and physical fatigue; performance change related to reduced motor output (fatigability in accordance with the new proposal). However, it is important to note that both ‘fatigue’ and ‘fatigability’ may apply in both motor output and cognitive function.
The possible sites and mechanisms of fatigue are illustrated in Fig. 1.
Fig. 1. Schematic diagram of the sites and possible mechanisms of fatigue. From Patikas et al. (2018).
Both fatigue and fatigability may also have ‘central’ and ‘peripheral’ origins within different systems. Within sensorimotor networks, changes seen in the neuromuscular junction and the muscular end organ are labelled as peripheral fatigability, while changes in neural networks proximal to the neuromuscular junction are labelled central fatigability. (Note: The dichotomisation into central and peripheral does not directly map on to fatigue and fatigability). Both central and peripheral changes are seen within the sensorimotor networks with repetition of activity, and therefore both central and peripheral factors contribute to performance fatigability (Gandevia et al., 1996, Gandevia et al., 1995b). Though the origins of fatigue are still debated, the general consensus is that it must be generated within the brain, and is possibly a result of poor integration between anticipated and real sensory input (Greenhouse-Tucknott et al., 2022, Kuppuswamy, 2022). Evidence from those with various diseases supports the idea that altered attention to a wide range of sensory inputs including proprioceptive, visual and auditory, (De Doncker et al., 2020), in the absence of explicit autonomic dysfunction, may lead to fatigue due to impaired sensory processing in this population.
The application of the concept of central and peripheral fatigue in cognitive systems is somewhat trickier, as the output of the cognitive system converges on the sensorimotor system, such as speech or movement. Metrics such as reaction time, movement speed and accuracy of movement capture the decline in cognitive function. Such motor metrics need to be relied upon as cognitive function does not have any independent effector organs. For purposes of this review, we will focus on fatigue and fatigability within the sensorimotor system.
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