Table 2.

Fatigue and anhedonia as separate constructs.

Author	Population	Definition of fatigue	Measurement of fatigue	Definition of anhedonia	Measurement of anhedonia	Relationship of the constructs
Ang et al.32	n = 505 healthy (men = 211, 23 = gender undisclosed); online survey	The feeling of exhaustion caused by the exertion of effort, which is unrelated to actual exertion of energy by muscles	MFIS	Is a mood disorder characterized by an inability to derive pleasure	SHAPS	Different subtypes of apathy are predictive of different associations with depression, anhedonia, and fatigue in healthy people. Apathy and anhedonia have a close relationship; however, there are unique aspects of anhedonia not related to apathy
Bennett et al.40	All women (n = 20 CFS, n = 20 post cancer fatigue, n = 16 major depression)	Physical fatigue in chronic fatigue: “difficulty achieving motor tasks,” mental fatigue in chronic fatigue: “difficulty achieving cognitive tasks”	SCIN; SPHERE	“Loss of motivation”, “mood disturbance”	SCIN, SPHERE	Fatigue in women with chronic fatigue syndrome, post-infective fatigue syndrome, post-cancer fatigue, and major depression is an indistinguishable symptom of depression. Anhedonia, however, is unique and a distinguishable symptom of depression. Fatigue–sadness was most associated with an increased risk of major cardiac events
Doyle et al.41	n = 408 hospitalized with acute coronary syndrome (ACS) (men = 326.7)	Classified as vital exhaustion, “tired,” “without energy”	HADS-D; BDI-FS	Not specified	HADS-D; BDI-FS	Anhedonia may be potentially less cardiotoxic than fatigue and sadness, which are strong predictors of cardiovascular prognosis
Drijgers et al.45	n = 25 with PD, 25 healthy controls (AGE matched) female = 2 in each group, 4 total	Not specified	POMS	A mood and motivational symptom	SHAPS	Acute stimulation of methylphenidate (dopamine reuptake inhibitor), but not pramipexole (dopamine 2 receptor agonist), improved anhedonia and vigor in PD patients, implies that dopamine plays a role in mood (anhedonia) but not cognition (fatigue)
Emmert-Aronson and Brown27	N = 2907 anxiety/mood disorder patients	As defined by DSM-IV	ADIS-IV-L-semi-structured interview and dimensionally rated	Correlated with low mood, as defined by DSM-IV	ADIS-IV-L-semi-structured interview and dimensionally rated	“Depressed mood” and “anhedonia” showed higher discrimination of higher depression (alpha = 3.25) than the other symptoms. Fatigue also discriminates better than most symptoms (alpha = 1.97). Anhedonia and depressed mood are required for diagnosis of MDD but could also create a very inclusive and encompassing symptom set for all severities of depression that includes depressed mood, anhedonia, fatigue, and concentration difficulties
Hawkins et al.28	n = 326 HF patients (women = 132)	Physical, somatic symptom of depression	PHQ-9	Behavioral, non-somatic symptom of depression	PHQ-9	Interventions that target a patient’s somatic symptoms (fatigue) may not yield maximum cognitive benefit compared to comprehensive treatment that targets depressed mood, anhedonia, and other non-somatic symptoms
Lapidus et al.30	n = 11 with major depressive disorder (MDD), n = 10 age-/sex-matched controls	Not specified. Noted to have an effect on motivation to participate in enjoyable activities	MFI	Reduced capacity to experience pleasure	QIDS-SR	There is no association between fatigue and glutathione levels (antioxidant) in the MDD group. Anhedonia is negatively correlated with brain occipital glutathione levels, supporting the role of glutathione in oxidative stress and inflammation, specifically in anhedonia in MDD
Leventhal et al.44	n = 187 regular smokers	Fatigue domain in POMS: “Fatigue (e.g., “worn out,” “fatigued”)”	POMS-Fatigue	Anhedonia, a specific factor for depression indicative of reduced interest/motivation, pleasure, and positive affect, and is putatively distinct from anxiety	MASQ-AD; (e.g., “felt like there wasn’t anything interesting or fun to do,” “felt like nothing was very enjoyable”)	Anhedonia, but not fatigue, was associated with only abstinence-induced reductions in positive affect. Used the tripartite model to elucidate the relation of anxiety and depressive symptoms to tobacco withdrawal and found that anhedonia predicts greater smoking relapse risk
McGuire et al.31	n = 323 (women = 73) patients who were admitted because of a cardiac event	Physical or mental weariness, part of somatic symptom cluster of depression	BDI; HRSD	Loss of interest or pleasure in activities, part of cognitive/affective symptom cluster of depression	BDI, HRSD	Cognitive/affective symptom clusters (anhedonia) is important for initial screening and are unique correlates of depression in patients with CHD. However, cognitive/affective symptoms must accompany somatic symptoms (fatigue) before depression can be diagnosed
Olivan-Blazquez et al.42	n = 741 primary care patients	Loss of energy within the context of depression	DSM-V criteria; CIDI v2	Decreased/lack of interest within the context of depression	DSM-V criteria	Out of the nine proposed diagnostic symptoms of depression, anhedonia and depressed mood are essential for the diagnosis. Anhedonia was significant in the first 6–12 months of onset of depression. Fatigue was only significant within the first 12 months of onset of depression
Ritchie et al.35	n = 1000 community-dwelling older adults (age >65 years)	Feeling tired, classified as a physical symptom	BSS	Classified as an affective symptom	BSS	Fatigue was found to be most related to other symptoms and most commonly endorsed when evaluating symptoms in older adults. No conclusion on anhedonia
Sibitz et al.36	n = 290 physically ill general hospital patients	Decreased energy or increased fatigability	PQm (adapted)	Loss of interest of pleasure	PQm (adapted)	Inclusion of “fatigue” does not yield an advantage to just using “depressed mood” and/or “anhedonia” to screen depression. Special attention to be paid to “anhedonia” as a screening symptom of depression
Trincas et al.38	n = 456 participants (women = 328), with dysphoric reactions (low mood to depression)	Physical symptom of depression	CES-D; DSM-V criteria	Predicted by Failure schema in the Young Schema Questionnaire, which is the belief that one is inadequate compared with others	CES-D; DSM-V criteria	Depression is not a unitary phenomenon. Fatigue and other physical symptoms were not predicted by any of the early maladaptive schemas (EMSs). The occurrence of anhedonia was predicted by the EMS Failure, that is, the belief that one is inadequate compared with others

Lists the articles that presented fatigue and anhedonia as distinct constructs.