Monday, September 24, 2007
“Pain as input” is the natural way to think. When we injure ourselves it seems that the “pain bit” is something organised at the injury site and pain then goes into the nervous system to warn the brain. This is “pain as input thinking”, but the idea is biologically bankrupt and likely to be troublesome for the management of chronic pain. “Pain as output” is a better and more scientific way to think. There are no pain fibres in the nervous system. When injured, the nerve fibres send “danger signals” not pain signals towards the brain. This is just an increase in amplitude and number of impulses in some fibres (nociception). It is up to the brain to decide if these danger signals are worth listening to. If you have a mild ankle sprain, but you are trying to escape something or score a winning goal, the brain won’t be too bothered listening to the danger signals. However if you have a backache and you have just heard that serious kidney problems can cause back ache and you have a relative with kidney problems, the brain may be particularly attentive to the danger message coming in from the back. Therefore, while the danger messages from damaged and diseased tissue can contribute to pain, they don’t have to. We (Butler and Moseley 2003) believe that a good way to help with this reconceptualisation is to consider the notion of threat. While we all know that pain has an emotional influence, “threat” is a wider and more overarching concept. Threats are held in the brain as old memories (“oh no, I have damaged the back again”), made in the brain as the injury occurs (“how will I work next week”), and are in the environment (workmates, insurance). There are a massive number of potential threats. If the brain perceives threats and adds them up, sometimes including the danger impulses, pain may be constructed. There is research which shows that if a noxious chemical is placed under the skin and you check the brain activation patterns it looks similar to if the noxious chemical was put in muscle (Casey & Bushnell, 2000). This research supports the idea that the tissue injured may not be such a big contributor to pain as most of us thought in the past. Finally, lets never forget phantom limb pain – there are no tissues to create a pain input and what must be the most maddening pain now exists in fresh air. Email me your thoughts. Butler DS, Moseley GL (2003) Explain Pain. Noigroup Publications, Adelaide Casey, K. L., & Bushnell, M. C. (2000). Pain imaging. Pain: Clinical Updates, 8, 1-4.