Prevalence rates of depression among patients in pain clinics have varied widely depending on the method of assessment and the population assessed. Rates as low as 10% and as high as 100% have been reported. The reason for the wide variability may be attributable to several factors, including the methods used to diagnose depression (e.g., interview, self-report instruments), the criteria used (e.g., DSM-IV, cutoff scores on self-report instruments), the set of disorders included in the diagnosis of depression (e.g., presence of depressive symptoms, major depression), and referral bias (e.g., higher reported prevalence of depression in studies conducted in psychiatry clinics compared with rehabilitation clinics). Most studies reported depression in more than 50% of the chronic pain patients sampled.

Studies of primary care populations (in which generalization is less problematic) have identified some factors that appear to increase the likelihood of depression in patients with chronic pain. Dworkin et al. (1990) reported that patients with two or more pain complaints were much more likely to be depressed than those with a single pain complaint. Dworkin and colleagues also found that the number of pain conditions reported was a better predictor of major depression than pain severity or pain persistence. When dysfunctional primary care patients with back pain were followed up for a year, those whose back pain improved also showed improvement of depressive symptoms to normal levels.

These epidemiological studies provide solid evidence for a strong association between chronic pain and depression but do not address whether chronic pain causes depression or depression causes chronic pain. As indicated above, this question has more importance in medicolegal contexts than in clinical contexts. But because it is a perennial question, some attempt to answer it should be made. Prospective studies of patients with chronic musculoskeletal pain have suggested that chronic pain can cause depression, that depression can cause chronic pain, and that they exist in a mutually reinforcing relationship. A recent review of the literature suggested that a predisposition to depression interacts with the onset of chronic pain to produce a depressive episode.

One fact often raised to support the idea that pain causes depression is that the current depressive episode often began after the onset of the pain problem. Most studies appear to support this contention. However, it has been documented that many patients with chronic pain (especially those disabled patients seen in pain clinics) have often had past episodes of depression that predated their pain problem by years. This has led some investigators to propose that there may exist a common trait of susceptibility to dysphoric physical symptoms (including pain) and to negative psychological symptoms (including anxiety as well as depression). Von Korff and Simon (1996) concluded that “pain and psychological illness should be viewed as having reciprocal psychological and behavioral effects involving both processes of illness expression and adaptation”. Pharmacological and/or psychotherapeutic depression treatment is often an essential component of successful chronic pain treatment and is described in the following section.