Does pain cause the perception of fatigue in patients with chronic pain? Findings from studies for management of diabetic peripheral neuropathic pain with duloxetine.

A major issue in pain literature is whether an etiological association between pain, sleep, and vitality exists. We utilized data from clinical trials of duloxetine for management of diabetic peripheral neuropathic pain (DPNP) to investigate these associations. Data were pooled from 3 double-blind, randomized, placebo-controlled, 12-week trials of patients without mood disorder (N = 1,139). After excluding 442 patients who reported maximum vitality at baseline, experienced treatment-emergent somnolence, asthenia or fatigue, or were taking sedating concomitant medications or duloxetine 20 mg/day, 697 were included in the analysis. Efficacy measures included weekly mean scores for average daily and night pain, pain interference with sleep, and vitality. Baseline to end point mean improvements in daily and night pain, Brief Pain Inventory sleep interference, and vitality were significantly superior for duloxetine compared with placebo (P < or = 0.001). Correlations between changes in daily and night pain, and sleep interference with vitality changes were -0.34, -0.32, and -0.28, respectively (P < 0.001). The direct effect of treatment on change in vitality was statistically significant (68%, P = 0.010) when assessed in an indirect manner through change in sleep interference alone but not in daily or night pain solely. Path analyses suggested vitality improvement in patients with chronic pain may be secondary to improvement in pain by duloxetine. Results do not prove pain causes fatigue, but indicate in DPNP patients with fatigue that treatment of pain can improve perception of improvement in fatigue. Thus, improvement of pain may be important in the context of trying to improve fatigue in DPNP patients.