1. Background

Chronic pain and depression are each prevalent and often co-occur. 5 Although the underlying mechanism of the interaction between pain and depression is not fully understood, their coexistence has been shown to incur additive adverse effects on patient outcomes, including poorer functioning and reduced response to treatment. 2, 36 The systematic review of Bair et al. 5 revealed wide variation in estimates of the prevalence of depression in patients with chronic pain. Estimates ranged from 4.7% to 22% in population-based studies and from 5.9% to 46% in primary care studies. Variability in estimates was particularly striking in studies of specialist pain populations (1.5%-100%). Most of these studies, however, were conducted in the 1980s and included fewer than 100 participants. More recent studies of depression in specialist pain settings have yielded prevalence estimates ranging from 12.1% to 72%, but sample sizes remain small. 20, 27, 28, 33