It’s easy to imagine people with persistent pain have cause to become depressed. After all, it’s a problem that affects every part of life, not just the bit that got injured.
But like so much to do with persistent pain, it becomes harder to unravel exactly what the relationship is when one digs a little further. For instance, it has been repeatedly shown that people with increased emotional distress going in to surgery use more painkillers and generally fare worse afterwards.
On the other hand, we have also known for a long time that unexplained pain is one of the common features of diagnosable depressive illness.
Which comes first?
The complicated answer is the relationship goes both ways. Having already had depression or anxiety increases the risk of developing persistent pain, but if you develop a chronically painful condition without such a history, you have dramatically increased chances of being diagnosed with depression not many months later.
No two central nervous systems are exactly alike. In the case of vulnerability to both depression and pain, this is important. The term allostatic load is applied to what Hamlet called the “slings and arrows of outrageous fortune”. The more severe the episodes of stress or adversity you have had to face, the more your nervous system adapts to reflect these experiences.
Watch: The difference between depression and sadness, explained. (Post continues after video.)
It so happens that many of these changes occur to brain regions that process bodily pain, mood, sleep, anxiety and motivation. These structures are collectively called the limbic system and are thought to have evolved very soon after mammals split from reptiles.
The relationship between the limbic system and a number of painful conditions has been studied, including irritable bowel syndrome and migraine. Results tend to support the idea that the way these conditions play out is related to the way our particular brain functions.
Fear, anxiety, anticipation of pain and other outputs of the limbic system play an important part in our everyday experience of normal pain as well as persistent pain.
Migraines are an example of chronic pain. Image: iStock
Further evidence that depression and pain are closely linked comes from the observation that a number of very effective pain drugs are also antidepressants. Amitriptyline and the tricyclic antidepressants as well as duloxetine and the SNRI (serotonin and noradrenaline reuptake inhibitors) antidepressants are first-line drugs for nerve pain and also for conditions such as fibromyalgia syndrome, which are characterised by dysfunctional pain regulation.
Interestingly, the SSRI (selective serotonin reuptake inhibitors) group of antidepressants, which increase levels of serotonin but not noradrenaline in the brain and spinal cord, are not effective at all in pain conditions. SSRIs are reasonably effective for depression with features of anxiety, but it appears boosting serotonin levels may not be particularly important for regulating pain signals.