Last updated: Wed, Jul 31, 2024
Chronic pain is often accompanied by lack of sleep, difficulty concentrating, and a general lack of enjoyment of life. These problems are shared by others who don't particularly suffer from pain, and who often are diagnosed with depression. Those of us who suffer significant prolonged pain may feel that these problems are clearly caused by the pain, and that talking about depression in this context is like talking about the tides. There is much evidence that chronic pain is closely intertwined with depression, whether or not it makes a lot of sense to the patient to differentiate the two.
Depression, as it is thought of by today's medical and scientific people, is defined by a section in the Diagnostic and Statistical Manual of the American Psychiatric Association. If you've been in a psychologist's or a psychiatrist's office, you've probably seen a copy of it, perhaps close to hand. The manual is commonly referred to as DSM, usually followed by a roman numeral to indicate the version. The definitions are constantly reviewed and revised as scientific and medical thought change. The definition gives us a way to talk about the phenomena and a way to gather together related knowledge and treatment techniques. The fourth version of DSM, labeled DSM-IV, was published in 1994 and revised in 2000.
Experimental evidence seems to indicate that depression changes pain sensitivity, but the results are mixed. Ischemic pain (pain due to lack of circulation in a muscle) created in a laboratory was felt similarly by depressed and non-depressed patients, but depressed patients were more sensitive to laboratory heat pain. In other experiments, depressed patients were less sensitive to heat and electrical stimulation, but more sensitive to ischemic pain. Depressed patients had a lower threshold for laboratory cold. The sensitivity of depressed patients to heat was reduced after they were treated with duloxetine, an anti-depressant that also is used to treat pain.1
A “sad mood” created in a laboratory caused a reduction in the pain threshold in normal subjects and a reduction in pain tolerance in chronic pain patients. Subjects in a “sad mood” who were observed with fMRI had more brain activation when exposed to heat pain, and the activation subsided less quickly.2
This evidence indicates that depression may make people more sensitive to pain. Besides this, depression makes people more likely to see themselves as disabled. Successful treatment for depression appears to reduce interference of pain with working. On the other hand, pain seems to interfere with treatment for depression. Those in pain tend to remain depressed longer.3
Current theories of depression include both the idea that some personalities are prone to depression and the idea that events and circumstances can cause depression. It is of course difficult to untangle causality, and both ideas are reasonable. Depression is so closely related to chronic pain that there are problems applying the DSM-IV definition of a “major depressive disorder” (MDD) to chronic pain patients. Some of the criteria for MDD are affected by pain, including sleep and fatigue problems. One study applied the DSM-IV criteria for MDD against a group of chronic pain patients. When all the DSM criteria for MDD were used, 38% of the chronic pain patients qualified for a diagnosis of MDD. When the pain-related criteria were excluded, only 19% qualified.4 (Regardless of this, successful treatment for depression provides a number of important benefits, included among which may be less perceived pain.)
The question of whether pain causes depression or perhaps the other way around has been of interest to pain researchers as well as to patients. A review of 83 different relevant studies concluded that there is more support for (1) the idea that pain causes depression and (2) that depression facilitates the development of pain with injury than there is for (3) the idea that depression causes pain. More evidence for (1) came from a separate analysis of another 12 studies.5
Treatment of pain with opioids has been found to improve depression, and reduction in pain severity improves the odds that someone with MDD will improve.6 On the other hand, a number of medications used in treating pain unfortunately have depressive effects, including steroids, dopamine-blocking agents, and some muscle relaxants.