Fatigue

====Uni- or multi-dimensional====
Fatigue can be seen as a uni-dimensional phenomenon that influences different aspects of human life.<ref>{{cite journal |author1=Roald Omdal|author2=Svein Ivar Mellgren|author3= Katrine Brække Norheim|title=Pain and fatigue in primary Sjögren's syndrome |journal=Rheumatology |date=July 2021 |volume=6 |issue=7 |pages=3099–3106 |doi=10.1093/rheumatology/kez027|pmid=30815693 }}</ref><ref>{{Cite journal|title=Importance of fatigue and its measurement in chronic liver disease|first1=Lynn H|last1=Gerber|first2=Ali A|last2=Weinstein|first3=Rohini|last3=Mehta|first4=Zobair M|last4=Younossi|date=July 28, 2019|journal=World Journal of Gastroenterology|volume=25|issue=28|pages=3669–3683|doi=10.3748/wjg.v25.i28.3669|doi-access=free |pmid=31391765|pmc=6676553}}</ref> It can be multi-faceted and broadly defined, making understanding the causes of its manifestations especially difficult in conditions with diverse pathology including autoimmune diseases.<ref name="pmid31447842">{{cite journal | pmid=31447842 | year=2019 | last1=Zielinski | first1=M. R. | last2=Systrom | first2=D. M. | last3=Rose | first3=N. R. | title=Fatigue, Sleep, and Autoimmune and Related Disorders | journal=Frontiers in Immunology | volume=10 | page=1827 | doi=10.3389/fimmu.2019.01827 | pmc=6691096 | doi-access=free }}</ref>

A 2021 review considered that different "types/subsets" of fatigue may exist and that patients normally present with more than one such "type/subset". These different "types/subsets" of fatigue may be different dimensions of the same symptom, and the relative manifestations of each may depend on the relative contribution of different mechanisms. Inflammation may be the root causal mechanism in many cases.<ref name="pmid34599320" />