What is chronic pain?
Everyone feels pain differently and nobody can know what somebody else's pain is like. This can make it difficult to define and describe pain.
The definition of pain devised by the International Association for the Study of Pain (IASP) is 'pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage'. This is a widely used and accepted definition of both short-term acute pain and chronic pain.
Dr Macrae, a Consultant Anaesthetist and Pain Specialist explained that acute pain is the body's warning system which tells the body to move away from danger and to protect itself from further injury, and that with chronic pain there are changes in the nervous system and brain so that the pain system no longer functions as a warning system.
So what we have is pain receptors out on the periphery and nerves which relay this to your spinal cord, which is like a relay station except it's more active than that, and then it goes up to your brain and the brain sort of sorts out what's going on here and makes you pay attention to it and tells you what's wrong and makes you take action.
So acute pain acts as a sort of warning system and tells the animal either to avoid something or that there's been damage so to remove itself and if you can think in terms of designing some sort of robot or something so that it wouldn't injure itself any further.
What you'd want to do would be to have this acute warning system to say something's gone wrong, get out of the way, and if you've injured a part, stop using it. And you'd want to stop using that part, say you'd broken your wrist you'd want to stop using your arm and your hand until the wrist had healed and then once it had all healed you'd want to be able to use it again.
So what happens is the human nervous system changes during the time of that injury and during the first part of healing so that pain signals are amplified and made much worse and that makes the animal reluctant to use that so it will keep it still and it won't hurt.
Then once the healing process has happened the nervous system goes back to normal and hopefully the person won't suffer pain anymore. The problem is what happens if it doesn't go back to normal after healing. And we think that this wind up, sensitisation or whatever you like to call it in the nervous system in some people doesn't go back to normal after healing so some patients are left with a sensitised nervous system so the slightest thing causes pain and it doesn't resolve after the healing process which it should have done.
The word chronic, however, refers to the persistence of pain. The IASP defines chronic pain as 'pain which has persisted beyond normal tissue healing time'. Although there is no formally accepted time for this, any pain lasting for more three months is generally considered to be chronic pain. Most people's experience of chronic pain though will have lasted longer than three months.
The information about pain occurring in the body is carried in nerves from all parts of the body through the spinal cord to the brain. There are at least two main pathways that carry information to the brain, one which goes to an area of the brain that tells us how much pain and where it is coming from (the sensory cortex) and another which goes to the emotional areas of the brain.
The first system the nerve delivers it up to the spinal cord and then one pathway goes up into the sensory part of the brain through an area called the thalamus which acts as a sort of relay station in the brain and that system tells you you've injured your finger, it feels like a pinprick or a burn or whatever and it tells you in other words the discriminatory aspects. Where has been hurt and what sort of injury it is. But it doesn't really make you do anything about it.
The other pathway goes from the spinal cord through a different route in the brain up into the emotional centres in the brain around the limbic system and other areas and that grabs your attention, you cannot ignore this.
And this makes pain different from other things like say vision or sound. These pain messages go straight into the attention mechanisms so you can't just say 'Oh that's interesting. I've got my hand in a fire.' It makes you do something about. It grabs your attention and says 'You must remove your hand'. It also tells you there's something very unpleasant going on.
So the combination of the two, you've got one system saying 'Pay attention. Something really nasty's going on' and then you've got another bit of your brain which says 'Oh, it's your finger and you've stuck it in a fire' and the two go well together and enables us to take avoiding action and to then get out of that situation that's causing the damage.
When the body is injured chemicals are released that make the pain system more sensitive and, as the injury heals, this sensitisation normally switches off. Dr Macrae explains that with chronic pain the sensitisation is not switched off and can actually become more sensitive.
The next day you take a shower. Well what's going to happen? Your left shoulder which is normal it's just going to be fine. It's just going to be warm water. But your right shoulder, the one that was sunburnt, it's going to be agony. Now why is that when it's the same stimulus on the two shoulders? Now what's happening here is that the nerves that go to this shoulder have been changed by nasty chemicals released by the damage to the skin. It isn't the damage to the skin itself that causes the sensitisation and if you think, that's logical.
Damaging things doesn't make them work better, it's a result of that damage. When you damage the skin, nasty chemicals called inflammatory mediators, one of them is prostaglandin gets out from all these tissues onto the nerves and that sensitises the nerves and for example brings down the threshold at which that fibre would fire for pain. Normally this would be about 45 degrees. So on this shoulder you would have normal skin with normal nerve endings which would tell you it's painful at 45 degrees. On this shoulder the threshold might have gone down to 38 degrees.
So you go on a shower at 40 degrees, this shoulder is 5 degrees below the pain threshold, that's fine. But this shoulder at 40 degrees that water is 2 degrees above the pain threshold so it says 'That's very painful'. The lesson here is the stimulus is the same. It's the nervous system that's changed and whether or not someone feels pain doesn't just depend on the stimulus. It depends on the state of your nervous system as well.
If we pursue the sunburn analogy a little further you imagine the day after you've had the sunburn things are at their worst. You've damaged the skin, everything is all wound up. Over the next few days things gradually heal. The skin will heal and gradually come back to normal, and the nervous system will also re-adjust itself gradually back to normal, so that after a few days that change in threshold which you'll remember came down from about 45 to maybe 38 degrees, re-adjusts back up again until it's at 45 degrees.
The problem is that we think in chronic pain some people don't re-adjust after the injury so their nervous system is left in that sensitised state. Their thresholds are permanently lowered and they also get amplification, like turning up a volume control within the nervous system, so the whole thing is made worse by this sensitisation and amplification of the problem within the nervous system.
And we believe that in many people who suffer chronic pain after injuries and maybe after other sorts of illnesses, like viral illnesses, it may be that the nervous system has changed and everything's been stirred up and sensitised but instead of re-adjusting to normal after a few days after healing takes place, they are left with this chronically sensitised nervous system that gives them a chronic pain syndrome.
People can develop chronic pain following a wide variety of injuries and illnesses. We talked to people that had chronic pain following an accident, nerve damage (following surgery), arthritis, Lupus, Multiple Sclerosis and ME (chronic fatigue syndrome). These are only some of the causes of chronic pain.
Sometimes it is not easy to identify how the pain started and what caused it. For example, we talked to several people who had unexplained pelvic or abdominal pain. It is often difficult for people to understand why they are still experiencing pain.
Some of the people we talked to suspected that there was ongoing damage in their bodies. However, others had been told that their injuries had healed. Dr Macrae explains some of the different processes that occur with chronic pain from arthritis, pain from nerve injury or disease that affect the nerves and pain where there has been an injury to a disc in the spinal cord.
And we talked about more chronic pains like back pain or the pain, say, of post-herpetic neuralgia, the neuralgia that people get after shingles that can go on for months and years. Nowadays we tend to think of pain more in terms of what was the mechanism that caused it and one of the most important differences is where you have something like a tissue injury, for example the damaged knee when you fall over and bang your knee, you injure your knee, your knee hurts and the nerves pick that up and relay it back to the nervous system.
The pain I mentioned from the shingles and the post-herpetic neuralgia, the nervous system itself is actually damaged there. Shingles is caused by the chicken pox virus and the virus actually attacks the nerves around the spine and damages those nerves, so these are abnormal nerves transmitting unpleasant signals even though in fact nothing may be happening at the far end of that nerve.
The problem lies within the nervous system itself and we call that a neuropathic pain. And there are several other examples of this trigeminal neuralgia in the face, sciatica where people get pain down their leg from a disc, all these are pains where the nerve itself is the origin of the pain and they need different treatments from these pains where for example, if you damaged your knee and it's all inflamed you might need an inflammatory, an anti-inflammatory painkiller or an ordinary sort of painkiller like paracetamol or something a bit stronger like morphine but these drugs aren't so good for neuropathic pains and in fact we use completely different sorts of drugs that work more on the nervous system such as the tricyclic anti-depressants like amitriptyline and the anti-convulsants like gabapentin.
Inflammatory pain for example, rheumatoid arthritis may be different again and we know that in many painful arthritic conditions the nerves in the joint actually change and nerves that normally are silent can wake up so-called silent nociceptors and these nerve endings become extremely sensitive and they have a very low threshold. That means they respond to the slightest thing by giving you pain and it may be this change within the nerves in the joint that actually make conditions like rheumatoid arthritis so painful rather than the actual joint damage itself.
So there are many different sorts of pain and they may need different sorts of treatment and I think that as we learn more about pain and further research is done, we'll probably find that there are other sorts of pain as well and we also know clinically that people often have mixed pain syndromes and that their pain may have some elements of the normal, what we call nociceptive type of pain and other elements may be neuropathic. So it's a complicated business.
Most of the people that we talked to were aware that the level of pain they experienced could be influenced by how they were feeling emotionally, for example whether they were happy, angry or anxious. There is some evidence that psychological distress can lead to developing chronic pain as well as making it worse. A doctor explains how the pain messages can be altered by what we think and feel.
But we have to realise that even at the sort of hardware level of the nervous system there are other aspects of pain. There are different pathways that go to other parts of the brain and there are pathways that go directly from your spinal cord up into the emotional centres of the brain and one of the recent bits of research from Tony Dickinson in London has shown that you actually have a pathway that goes down from the emotional centres to within the spinal cord and actually amplifies the pain at the spinal cord level.
This is not the way I would have assumed that it would have worked so that the emotional centres as it were go across the brain from the emotional side to the more sense, purely sensory side. They actually go right down to the spinal cord and turns things up at that level and then that fires up to the sensory side so the interplay of the sensory amount of pain that you feel and your emotions and all the other factors going on in your life are very complex but very real.
And we know that what is going on in your life in terms of emotions, feelings, all these sorts of things have a direct action within the nervous system which can turn up your pain and perhaps if we are lucky can also turn down people's pain. And that is why it is so useful to have Clinical psychologists and others working with us in the field of chronic pain to try and help with these problems.
Last reviewed May 2015.
Last updated May 2015.