Except for chemotherapy-induced anemia, the mechanisms responsible for fatigue in people with cancer are not known. Understanding the causes of fatigue in people with cancer is especially challenging because each individual is likely to experience multiple possible causes of fatigue simultaneously. This multifactorial etiologic hypothesis is apparent in the various models that have been proposed for the study of fatigue.[1,2] Energy balance, stress, life demands, sleep, neurophysiologic changes, disruption of circadian rhythms, and neuroimmunologic changes are generally incorporated in these models, based on the rationale that these factors are associated with fatigue in contexts other than cancer.[3]The cancer literature supports some of these variables. Sleep disruption was associated with fatigue in women receiving adjuvant chemotherapy for breast cancer, one study demonstrated variations in energy requirements in people with cancer, and proinflammatory cytokines are elevated in some studies of people experiencing persistent fatigue following cancer treatment.[4,5] In addition, concurrent medications such as analgesics, hypnotics, antidepressants, antiemetics, steroids, or anticonvulsants, many of which act on the central nervous system, can significantly compound the problem of fatigue.

References

1. Miaskowski C, Portenoy RK: Update on the assessment and management of cancer-related fatigue. Principles and Practice of Supportive Oncology Updates 1 (2): 1-10, 1998. 
   
   
2. Morrow GR, Andrews PL, Hickok JT, et al.: Fatigue associated with cancer and its treatment. Support Care Cancer 10 (5): 389-98, 2002.  [PUBMED Abstract]
   
   
3. Aistars J: Fatigue in the cancer patient: a conceptual approach to a clinical problem. Oncol Nurs Forum 14 (6): 25-30, 1987 Nov-Dec.  [PUBMED Abstract]
   
   
4. Kaempfer SH, Lindsey AM: Energy expenditure in cancer: a review. Cancer Nurs 9 (4): 194-199, 1986. 
   
   
5. Ancoli-Israel S, Liu L, Marler MR, et al.: Fatigue, sleep, and circadian rhythms prior to chemotherapy for breast cancer. Support Care Cancer 14 (3): 201-9, 2006.  [PUBMED Abstract]
   
   

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