Any discussion of the relationship between major depressive disorder (MDD) and chronic pain encounters an obstacle immediately: Neither has a singular pathophysiology. Furthermore, MDD and, to a significant extent, chronic pain are defined more by their symptoms than by a presumed etiology and pathogenesis.
Why does this matter to a busy clinician?
Explicitly or implicitly, we often align our treatment approaches with what we assume is the underlying pathophysiology of the conditions we are addressing. An overview of shared pathophysiology of chronic pain conditions and MDD therefore can be useful in practice.
What is chronic pain? Defined as “pain that persists past the healing phase following an injury,”1 chronic pain often is subdivided into 4 types2,3:
- nociceptive (caused by a lesion or potential tissue damage)
- inflammatory
- neuropathic (spontaneous pain or hypersensitivity to pain related to neurologic illness or injury)
- functional (hypersensitivity to pain due to abnormal central processing of a normal input).
Although fibromyalgia often is categorized as a dysfunctional pain syndrome, persons who suffer from it, much like those who suffer neuropathic pain, commonly report hyperalgesia (augmented sensitivity to painful stimuli), allodynia (abnormal pain response to non-noxious stimuli), and paresthesias. These shared clinical features of fibromyalgia and neuropathic pain are consistent with central sensitization, which suggests overlapping pathophysiology.4
Comorbidity between depression and pain is common. A 30% to 60% co-occurrence rate of MDD and chronic pain has been reported.5 Some subtypes of chronic pain, such as fibromyalgia, are so commonly comorbid with psychiatric conditions that they have spawned a scientific debate as to whether the conditions are most parsimoniously considered (1) separate illnesses with high comorbidity or (2) different symptomatic manifestations of a single underlying condition.6 Moreover, cumulative evidence suggests that chronic pain and depression do not just co-occur; each one facilitates development of the other, such that chronic pain is a strong predictor of subsequent onset of MDD, and vice versa.
When pain and depression are comorbid, they also tend to make treatment of each condition more difficult. For example, pain presents (1) a major obstacle to achieving remission when treating depression7,8 and (2) significant risk of relapse.9 A 3-year longitudinal study showed that painful symptoms substantially reduced the chance of recovery in a group of older depressed patients (n = 327). A substantially greater percentage of patients with MDD alone attained recovery (47%), compared with only 9% in whom MDD and painful symptoms were comorbid.10 Furthermore, a higher level of pain can delay remission when treating MDD,11 thus reducing the likelihood of an optimal outcome.12
Understanding shared processes. Recent developments in neuroscience and psycho-immunology point to the fact that comorbid pain and depression might be driven by overlapping pathophysiological processes in the brain and body. In the 2 parts of this article, we (1) review scientific understanding of these shared processes and (2) demonstrate how recent advances in the epidemiology, phenomenology, and etiology of chronic pain and MDD provide important clues for more effective diagnosis (Part 1) and treatment (Part 2, March 2016)—and, therefore, better outcomes. Our focus is primarily on the relationship between MDD and the best-studied comorbid chronic pain conditions: fibromyalgia, neuropathic pain, chronic back pain, and rheumatoid arthritis.
The societal burden of chronic pain conditions is enormous
A recent epidemiological study13 projected that as many as 100 million people in the United States—30.7% of the population—suffer some form of chronic pain, including arthritis and joint pain. A World Health Organization survey yielded a similar (and staggering) 37% prevalence of chronic pain in the population of 10 developed countries.14
Estimates are that various forms of neuropathic pain, including diabetic neuropathy, postherpetic neuralgia, trigeminal neuralgia, spinal cord injury, and radiculopathy, alone afflict as many as 26 million people worldwide, including approximately 1.5% of the U.S. population.15,16
Chronic low back pain is epidemic. With a projected point prevalence of 30%, the condition is the most common cause of activity limitation among people age <45, and the most frequent reason in the United States for visiting a physician.1
Functional somatic syndromes, including fibromyalgia and irritable bowel syndrome, impose an astounding strain on health care: These syndromes account for 25% to 50% of all outpatient visits, or approximately 400 million clinic visits annually in the United States.17
Why should you care about these numbers? The answer is that comorbidity among chronic pain, mood disorders, anxiety disorders, sleep disorders, cognitive impairment, fatigue, and chronic stress presents an enormous clinical challenge because it not only complicates the diagnosis of these conditions but also compromises treatment outcomes and imposes severe limitations on daily functioning and quality of life of those afflicted.5,17-24