Elucidating the behavior of physical fatigue and mental fatigue in cancer patients: a review of the literature.

BACKGROUND: Although the multidimensional nature of cancer-related fatigue is widely accepted, it could be questioned whether fatigue dimensions are expressions of one symptom (multidimensional concept) or expressions of several phenomena that are all called fatigue but actually are separate symptoms (multiple-symptom concept).
METHODS: Therefore, we investigated in this review whether physical fatigue and mental fatigue behave differently in cancer patients by studying their intensity in different stages of cancer, their changes in intensity during anti-tumor therapy, the variables to which they are related, and their changes in intensity by interventions on fatigue.
RESULTS: In some studies, physical fatigue and mental fatigue behaved similarly: they were both more intense in cancer patients than in healthy controls, and sometimes, they had the same course during anti-tumor therapy, or both improved during an intervention. On the contrary, there were some studies suggesting that physical fatigue and mental fatigue behaved differently: physical fatigue seemed to be more prominent than mental fatigue in some stages of the disease trajectory; several studies reported changes in physical fatigue not accompanied by changes in mental fatigue during anti-tumor therapy or by interventions aimed to relieve fatigue; and physical fatigue and mental fatigue had different correlates.
CONCLUSIONS: In conclusion, we found some studies in which physical fatigue and mental fatigue behaved differently. These findings might indicate that physical fatigue and mental fatigue are separate phenomena. To prove this multiple-symptom concept, studies on the pathophysiological mechanisms leading either to physical fatigue or to mental fatigue are urgently needed.