NeuroScience of Chronic Pain Part III

By: admin Published: April 18, 2013

by Jay McCallum, PT, DPT, OCS

How Does Chronic Pain Change the Sensory System?

In my last post I tried to outline the super-complex process that the brain goes through to generate the experience of pain, and what I want to try to do today is talk about what changes when the nervous system is bombarded by pain impulses over a long period of time.  This can be a challenging idea to transmit to people who have had pain for may years – they have nearly all heard some refrain on the idea that ‘your pain is in your head,’ with the subtext being ‘your pain is not real’ or, worse yet, ‘you’re faking it.’  I have had patients become angry and leave care, or become tearful or defensive when I try to bring this topic up, which is why I wanted to spend the time writing in my last post about how the experience of pain is generated by the brain.  What we are talking about here is not ‘the subconscious’ or anything like that – it’s about structural changes in the parts of the brain responsible for processing various inputs and generating the output or experience of pain.

There are several ways that the nervous system adapts over time to pain, and unfortunately the idea that we get tougher or desensitized is not at all accurate.  Remember that our nervous system is “plastic”, that it, it changes its structure in response to stimulus.  Generally, the more of a stimulus you give the nervous system, the more ‘resources’  it devotes to that stimulus.  Think of practicing a skill like hitting a golf ball – to break down the details of that, we improve our skill at that because with practice we devote more neurons to the motor program.  Or think of a sommelier who can identify many nuances of a wine — again, more neurons devoted to that process.  With pain, what we see is that the spinal cord itself ‘turns up the gain’ on its pain pathways, devoting more neurons to nociception (pain stimulation) than ‘normal.’  It looks a bit like this diagram, if you think of the vertical axis as being basically loading to a tissue, with the peak being the point at which injury would occur:

Normal Protective Pain

Normal Protective Pain

This is how pain is supposed to work – the person feels pain just a little before the point at which tissue damage occurs.  But when the spinal cord is turning up the gain, it looks more like this:

Abnormally early onset of pain in relation to tissue load

Abnormally early onset of pain in relation to tissue load

In this diagram you see that there has been a change in true tissue tolerance related to the original injury but the nervous system begins to generate pain far before any tissue damage begins to occur.  Let me emphasize here for a minute again that this is TOTALLY REAL PAIN, it just isn’t calibrated well to tissue damage anymore.  This change appears to happen primarily in the spinal cord, and as such is often referred to as “central sensitization.”

Central Facilitation

As if this isn’t enough, the brain itself has to get into the act.  As with other things I’ve described with this series of posts, the changes are complex, but there are two main themes – loss of precision and a process called facilitation.  We’ll start with the loss of precision.  As neurons in the sensory cortex of the brain are continuously bombarded with stimulation the stimulation basically starts to bleed over to neighboring neurons, to the point that brain imaging scans show overlap between areas of activation in the brain with stimulation of different parts of the painful region.  Patients experience a sense that their pain is spreading over time, sometimes even spreading to the opposite side of the body, and simultaneously becoming more difficult to describe.  Rather than pointing at the pain with a finger they wave at a whole area of their body, and they describe a sensation that the pain is ‘moving around.’  This loss of precision is easiest to ‘see’ experimentally in people who have a single extremity involved, say a very painful right arm.   We use our sensory cortex to help us identify whether we are looking a picture of a right arm or a left arm, basically sort of superimposing that picture onto our sensory cortex to see which side it ‘fits.’  Those patients with painful right arms, for example, are significantly slower to identify pictures of right arms than left arms, which tells us that their sensory cortex is not processing information quickly or accurately.

Facilitation basically means ‘priming’, and to understand this remember that the brain is absorbing information about many different things as it creates the experience of pain, and over time it no longer needs the entire set of stimuli to create the experience.  In our course, the analogy was to ask a group of attendees to stand up, then sit, then stand, then sit, etc.  Each time the speaker gestured along with giving a verbal command, until the last time when all he did was gesture.  And everyone stood.  A partial stimulus – had he just walked up to them and gestured initially they would have just stared at him – leading to the output of standing.  With pain, it might be the stimulus of an environment or activity that has historically provoked pain.  A case study of a bicyclist who had chronic pain with hill climbing was presented – she was placed in a setting where screens to either side of her could be tilted to make it look and feel like she was climbing a hill, and her pain could be brought on (and relieved) simply by tilting those screens, with no change in the actual intensity of her riding.  A partial stimulus of hill climbing leading to the experience of pain.  (You can read more about her here).

So…to recap:  Nociception (remember, that’s the name we use for nerves in the body that carry a painful stimulus to the brain) gets amplified by the spinal cord over time.  The sensory cortex of the brain that is responsible for generating the sensation of pain and sending it to our consciousness becomes somewhat sloppy (very like what happens with the motor cortex as I described in my first post on this subject).  And the pain system begins to leap to conclusions based on incomplete data.  Finally, just because I can’t say it enough times, the pain is REAL.  It’s just no longer accurately representing tissue damage like it is supposed to.

I have one more post in the pipeline, which will deal with what we can do in physical therapy to address this process, but the short version is that it is very much a thing that physical therapy can impact because it’s about retraining the brain, and PTs work with changing brain function every day.  Lots of things can change the way the brain functions, including everything from trauma (a head injury or a stroke) to what might amount to an overuse injury to part of the brain (chronic pain), and PTs treat all those things.  So if you have a history of longstanding pain that has not always ‘made sense’ to your medical providers, then consider giving us a call at 928-556-9935 or an email at [email protected].

 

 

The Neuroscience of Chronic Pain

By: admin Published: March 10, 2013

by Jay McCallum, PT, DPT, OCS

This post has been a very long time in the incubator, partly because we’re all so busy and partly because we wanted to be able to give you both some science and some clinical reality.  It started last October when Amy and Jay went to a conference in Portland on the topic of chronic pain.  It was taught by a pair of amazing researchers, Paul Hodges and Lorimer Moseley.  Paul and Lorimer are both physical therapists – ‘physiotherapists’, actually, given that they’re Australians – but they have devoted their professional lives to the study of how pain works and between them have published nearly 300 scholarly articles on the topic.

Because they covered so much ground this is going to be the first of a three part blog.  I’ll talk about Paul’s work today, and then try to cram Lorimer’s material into a second blog, and finally talk about our experiences in the clinic working to implement their findings into actually treating patients.

How Does Chronic Pain Affect How We Move?

This is really what Paul Hodges’ work is about.  He is best known in the world of physical therapy as the researcher who told us all that the transversus abdominus is important in stabilizing the spine, but that’s really not at all fair to the body of his work.  What he has focused on is how the body solves the problem of keeping a stack of bones like the spine stiff enough that it doesn’t collapse, yet mobile enough to move.  And, what he’s found is that people do it in lots of different ways (of course).  There are, however, some patterns once you step back from the detailed specifics, and, more importantly, people who have a history of back pain tend to use a different set of strategies than people who don’t have back pain.  Most of the research here is focused on back pain, but it’s quite likely that similar patterns are at work with other locations of chronic pain, like the neck.

Generally speaking, the muscles of the trunk can be divided into muscles that are closer to the spine and those that are further away, with of course some muscles situated in a bit of a gray zone in between.  And Paul’s work suggests that the ‘normal’ way that we move is that the muscles that are closer to the spine activate to create just the right amount of stiffness to support the loads we put on our spines, while those further from the spine are primarily concerned with generating movement of the spine.

The best known of those deep, stabilizing muscles are the multifidus and the transversus abdominus, and the ones we tend to forget about are those that make up the pelvic floor.    These muscles are relatively small, located right in there close to the spine, and are designed to stay ‘on’ for long periods of time at a relatively low intensity.  They are also supposed to ‘pre-activate’, meaning that before you go to pick up that pencil on the floor the brain tightens them to stabilize the back.

View from the side of the multifidus, pelvic floor and transversus abdominus

What goes wrong?

We get stuck in a pattern.  When a person first injures his or her back, the body’s reaction is to brace it – to tighten the bigger muscles that are further out from the spine and have longer lever arms to pull on it.  Those muscles normally exist to generate movement, but get pressed into service to hold the injured spine in place.  That pattern is supposed to quiet after a while, but if it doesn’t it can become the “new normal,” the primary strategy for keeping the spine stable.  And that’s really a problem, because those muscles are NOT designed to be on for long periods of time, and so they complain about it, and get trigger points, and make the spine feel stiff all the time.  And they use too much force, so the underlying joints have too much pressure on them.  And they just aren’t very good at this new job, so people hurt their back picking up that innocent pencil because the nervous system didn’t stabilize the spine quite right (but they DON’T hurt their back lifting heavy things, because they consciously brace.  Sound familiar?).  And so, the classic pattern goes, people have further episodes of back pain and each one intensifies this pattern.

Hey!  That’s me!  What can I do??

And…there we have the problem.  Obviously, what we’d LIKE to do is turn off those overactive superficial muscles and re-activate the deep ones.  But, it isn’t quite so simple.  These changes in muscle activation are accompanied by changes in brain structure.  Put more simply, most people have had a LOT of practice at this pattern.  We’ll talk about this more in my third post, but a couple of key points is that pain of this type is not just a ‘strengthening’ issue – it’s much more about coordination.  And you have to work at it from a lot of angles, trying to both activate the deep ones (intentional exercise, practice, feedback) and quiet the overactive ones (stretching, motion, relaxation, massage and manual therapy).  Most importantly, treatment needs to be individualized and very attentive to details, which is how we do things at CoreBalance Therapy.

If you want to learn more, give us a call at 928-556-9935 or email us at [email protected].

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