Fatigue of Cancer is Distinctly Different than the Fatigue of Chronic Kidney Disease.

Introduction: It remains unclear whether the mechanisms of fatigue in cancer and chronic kidney disease (CKD) are different. The assessment of self-reported fatigue by standard scales, muscle fatigue by the objective reduction in finger tapping speed, and the cost to the brain of the exercise by the rating of perceived effort (RPE) help to compare the phenotypes of fatigue between the two conditions and explore the mechanisms of fatigue.

Methods: We initially assessed the correlations between fatigue, measured by the Brief Fatigue Inventory (BFI); muscle fatigue (measured by the finger tapping speed) and RPE in 30 patients with cancer and subsequently 30 patients with stage 3B-5 CKD. Exclusion criteria for both groups included movement disorders, history of stroke, and depression. Other exclusions for patients with CKD were ejection fraction < 40%, history of cancer, hemoglobin < 10 g/dL, and recent hospitalization. The Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) scale was also assessed for CKD patients.

Results: In cancer patients, fatigue showed no correlation with muscle fatigue but had a strong correlation with RPE (r = 0.75, p < 0.01). Conversely, in CKD patients, fatigue did not correlate with RPE but was significantly correlated with muscle fatigue at the 30-s tapping interval (r = 0.68, p < 0.01). Additionally, BFI scores were highly correlated with FACIT-F scores in CKD patients (r = 0.89, p < 0.001).

Conclusions: Cancer-related fatigue differs from CKD-related fatigue, based on a protocol that includes the assessment of subjective fatigue, muscle fatigue by the finger tapping speed, and RPE. Whereas cancer-related fatigue correlates with higher perceived effort, CKD-related fatigue correlates with reduced motor performance. These findings support the use of condition-specific fatigue assessments and highlight the value of integrating both subjective and objective measures in the understanding of fatigue.