For many who suffer depression, chronic pain is a frequent fellow traveler. As many as half of those with chronic pain or with neuropathic pain disorders, such as fibromyalgia, have depression as well.
That the two are so often bound together suggests a complex relationship, and the brain's shared circuitry for social and physical pain may lie at its heart.
Depression and chronic pain are distinct but similar disorders. But both may arise from some faulty wiring in their shared neural circuitry, researchers say. In both disorders, pain continues long after some initial insult has healed, disappeared or moved on, and the experience of social rejection or physical pain persists, feeds on itself and becomes chronic. Both disorders often seem mysterious in that their origins are hard to pinpoint. And both signal their presence with a subtle but powerful physiological marker: systemic inflammation.
Genes, temperament or early experience — including childhood trauma, neglect or abuse — may each play a role in predisposing someone to both disorders. And both are sustained and worsened by hypersensitivity: Those with depression are far more likely than those without it to perceive rejection, as they tend to interpret social interactions in ways that conform to their low self-esteem; those with chronic pain have nerves, spinal cords and brains that are on high alert for any pain signals, and they send an outsized response when any such signal is detected.
Given the shared brain circuitry, some scientists suggest that depression's emotional pain may get confused with signals of physical pain, effectively "tricking" someone with depression into experiencing chronic pain (or vice versa).
Or maybe the simplest explanation reigns: that when the nerve disturbance of chronic pain becomes entrenched, the sufferer's preoccupation divides him (or her) from friends and family, interferes with the pleasure of his usual hobbies and prompts him to question whether he has done something to deserve his pain and isolation.
Perhaps, experts say, all of these possible links are at work.
University of Toronto psychologist Geoff MacDonald says that both depression and pain are often exacerbated by doctors' skepticism and our own loved ones' occasional unwillingness to acknowledge how crushing a burden psychic or physical pain can be.
When physicians are dismissive of diffuse and inexplicable pain symptoms, or family members wave off depression as a trumped-up case of the blues, patients feel more isolated and rejected. That, MacDonald says, "adds insult to injury."