Does clinical depression bring about chronic pain? Or does pain lead to depression? Because these two conditions frequently co-exist--30 to 54 percent of patients with major depressive disorder also suffer persistent physical pain--there has been much speculation about whether one causes the other or whether a common underlying factor provokes both. Results of studies into the precise nature of this relationship, however, have been inconsistent.
To gain a clearer understanding of the depression-pain connection, researchers affiliated with the University of Michigan and the University of Cologne, Germany, focused on the underlying mechanisms in the perception of pain, physical and emotional: the brain. Their findings, featured in the May 2005 issue of Arthritis & Rheumatism, challenge existing notions on the interplay of emotion and sensation and have important implications for treating depression and pain as separate conditions, even when they occur simultaneously.
The study focused on 53 patients, 33 women and 20 men, with fibromyalgia (FM). This symdrome is characterized by intense widespread pain and tenderness to touch and is often accompanied by depression. Using this patient population, the research team set out to evaluate whether higher levels of symptoms of depression are associated with increased sensitivity to pressure-induced pain, as well as to determine which regions of the brain are involved in processing acute pain, chronic pain, and depressive symptoms. 42 healthy controls, 20 women and 22 men, were also included in the study. The mean age was 42 for the FM patients and 38 for the controls.
Conducted at Georgetown University's General Clinical Research Center, the study began by assessing the severity of chronic pain and depression in FM patients, through a combination of interviews, questionnaires, and measurement scales. The following day, all subjects, both FM patients and controls, participated in pressure-pain sensitivity experiments, involving the application of pressure to a thumbnail. To get a clear picture of the brain's response to painful stimuli, all subjects underwent magnetic resonance imagining (MRI) scans, before, during, and after the pressure-sensitivity sessions. FM patients were required to discontinue antidepressant medications 4 weeks prior to the study, as well as refrain from using any drugs for pain, including over-the-counter analgesics, starting 3 days before the study.
Based on the MRI results, the researchers found that FM patients required significantly less applied pressure than healthy controls to activate neurons associated with acute pain in the brain's sensory domain. This heightened sensitivity applied to FM patients in general, regardless of whether they had been diagnosed with major depressive disorder or reported any depressive symptoms. Furthermore, the researchers found only a weak correlation between the sensory regions of the brain associated with chronic pain and the affective or emotional regions of the brain associated with depression.
"Much has been made of the overlap and similarities between pain and symptoms of depression, but these and other data suggest it is also important to identify pain-processing mechanisms that are independent of mood," notes the study's leading author, Thorsten Giesecke, M.D. "The notion that sensory and affective aspects of pain may be independently processed is not just of theoretical interest," he adds.
"Evaluation of these sensory and affective dimensions in patients with chronic pain is likely to improve diagnosis, choice of treatment, and treatment efficacy." As this study affirms, prescribing a standard antidepressant medication will not necessarily relieve the suffering of a depressed patient whose pain is not only real but also intensely physical.