Physical Fatigue, Fitness, and Muscle Function in Patients With Antineutrophil Cytoplasmic Antibody-Associated Vasculitis

ObjectiveThis study investigated differences in cardiorespiratory fitness, muscular function, perceived exertion, and anxiety/depression between patients and healthy controls (HCs) and assessed which of these variables may account for the fatigue experienced by patients. MethodsFatigue was measured in 48 antineutrophil cytoplasmic antibody-associated vasculitis patients and 41 healthy controls using the Multidimensional Fatigue Inventory (MFI-20), focusing on the physical component. Quality of life, anxiety/depression, and sleep quality were assessed by validated questionnaires. Muscle mass was measured by dual-energy x-ray absorptiometry scan, strength as the maximal voluntary contraction (MVC) force, and endurance as sustained isometric contraction at 50% MVC of the quadriceps. Voluntary activation was assessed by superimposed electrical stimulation. Cardiorespiratory fitness ( Vo2max and oxygen pulse [O-2 pulse]) and perceived exertion (Borg scale) were measured during progressive submaximal exercise. ResultsPatients reported elevated physical fatigue scores compared to HCs (patients MFI-20 physical 13 [interquartile range (IQR) 8-16], HCs MFI-20 physical 5.5 [IQR 4-8]; P<0.001). Muscle mass was the same in both groups, but MVC and time to failure in the endurance test were lower due to reduced voluntary activation in patients. Estimated Vo2max and O-2 pulse were the same in both groups. For the same relative workload, patients reported higher ratings of perceived exertion, which correlated with reports of MFI-20 physical fatigue (R-2=0.2). Depression (R-2=0.6), anxiety (R-2=0.3), and sleep disturbance (R-2=0.3) were all correlated with MFI-20 physical fatigue. ConclusionThese observations suggest that fatigue in patients is of a central rather than peripheral origin, supported by associations of fatigue with heightened perception of exertion, depression, anxiety, and sleep disturbance but normal muscle and cardiorespiratory function.