"For many people, their pain takes over their entire life. It affects their work life, their family life, their social life," Cope says.
It is a personal tragedy to which Toussaint can attest.
"It destroyed every relationship in my life except for my relationship with my partner, John," she says. "He stood by me, but my entire family left me behind.
"The emotional pain becomes more serious than the physical pain."
Little surprise, then, that chronic pain is regularly tied to depression.
"Imagine that you develop chronic pain and in time you are no longer able to work," Christo says. "That can be very disruptive and lead to a loss of self-esteem and self-worth. At the same time, it can change the nature of your relationship with your family and friends.
"If the patient has lost their sense of self-worth and become depressed, that in turn can lead to social isolation."
Smith agrees. "[Patients] get the feeling that they really can't control the pain; they feel helpless about it... In some cases these patients can go to full-blown clinical depression."
The roots of this depression even appear to transcend emotions alone. On Monday, scientists revealed additional biological clues as to why pain and depression may be so closely linked. A team of researchers led by Irina Strigo of the University of California San Diego compared brain scans of people with depression to those of 15 people who were not depressed while these subjects anticipated or experienced a painful sensation.
What the researchers found was that those with depression showed a higher level of activity in the areas of their brain that processed emotions. Moreover, the regions of the brain that are normally involved in mitigating pain were less active in the depressed subjects -- a hint that chronic pain may have a propensity to feedback on itself.
Toussaint says the depression that accompanied her chronic pain was nearly too much for her to bear.
"I suffered very, very extreme depression," she says. "All of us with pain do. We're being tortured 24/7, we're not believed, and our lives are upside-down.
"I had a plan of suicide -- not because of the physical pain but because everyone I knew had abandoned me."
Fortunately, pain experts say, the majority of those battling mysterious chronic pain can find answers if they search long enough.
"Most pain does have a label," Cope says. "If you can find some general idea of what's causing your pain, you can tailor treatment toward that."
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