I just read a new paper by Lorimer Moseley and Herta Flor called “Targeting Cortical Representations in the Treatment of Chronic Pain: A Review." This is an excellent short review of what is known about the connection between chronic pain and changes to the brain, and how this information might be used by therapists to treat chronic pain. I have already written several articles on related subjects. But this is a very easily digestible review that includes discussion of a few concepts I haven’t touched on before, so I thought I would write a quick post on it. Do yourself a favor and get a full text copy of this paper and put it on your required reading list. Here’s a brief review of the contents with my some of my own thoughts.
“The brain responds to the perceived, not the actual reality”
The paper starts by noting one of the most important and now increasingly familiar ideas that has emerged in pain science, namely that:
Pain emerges from the brain according to the apparent danger to body tissues and the need for concerted response from the individual, not according to activity in nociceptive fibers or the actual state of the tissues.
The authors point out that that the failure to draw a distinction between tissue damage and pain is widespread in the treatment of pain. This confusion is likely to cause more problems in the case of persistent pain, which leads to changes in the brain that make the disconnect between pain and nociception that much greater.
Persistent pain and changes to the brain
Chronic pain is associated with sensitization of the spinal cord and supra-spinal centers, as well as “cortical disinhibition.” Excited neurons tend to excite other neurons, unless the spread of excitement is inhibited. Proper inhibition is what creates a meaningful pattern of neural activity as opposed to an undifferentiated explosion of neural activity such as an epileptic seizure. To use a simple example, imagine you are pricked on the arm by a needle. This will stimulate the cortical sensory maps for that part of the arm. The excitement will tend to spread to other areas unless it is inhibited. Without proper inhibition you would not be able to precisely locate where the needle prick occurred - you would feel it all over the arm.
Because people in chronic pain tend to have cortical disinhibition, they are less able to tell the difference between two different types of tactile stimulation. In other words, sensation (and pain) tend to spread inappropriately.
Telling left from right
The paper also discusses a very interesting relationship between chronic pain and the ability to tell left from right.
One way to measure whether the movement maps of a particular subject have been disrupted by chronic pain is to ask them to engage in a “left/right judgment task.” This means asking the subject to look at a picture of a hand or foot and determine whether the hand or foot is from the left or right side of the body. The determination requires the subject to mentally move his own limbs into the position shown in the picture, which requires use of the body maps. It has been shown that people with chronic pain are slower in performing these tasks, suggesting that their movement maps have been altered in some way as a result of the pain.
Lorimer Moseley and David Butler have developed an interesting therapeutic program called “recognize laterality” which attempts to treat chronic pain patients by having them practice the ability to identify pictures of right or left hands. There is at least some evidence to suggest that this practice can increase skills and reduce pain.
Harnessing neuroplasticity to treat pain
I like this line:
That pain emerges according to the apparent danger to body tissues and the need for concerted action, not according to the true danger or damage at a tissue level, means that anything that is detectable or accessible to the brain and relevant to the evaluation of danger to body tissue has the capacity to modulate pain
Well said. One of the optimistic conclusions that can be drawn about the role of the brain in creating pain is that it opens a wide range of potential treatment options.
One primary strategy for pain reduction is based on cognitive behavioral work. The primary objective here is to reduce feelings of helplessness, establish a sense of control over pain and learn behaviors that reduce the impact of pain on quality of life. Pain education is part of the program here, as it is one way that such control can be given to the patient.
Normalizing sensory representations
The paper then discusses various strategies to correct the abnormalities in sensory maps that tend to occur in patients with chronic pain. I have written a lot about this subject before, arguing that novel movements can provide novel sensory feedback that can fill in gaps in sensory maps and thereby reduce associated pain and dysfunction.
The authors make a very important point in relation to this idea. In order for sensory feedback to result in meaningful changes in cortical sensory maps, the information must have “functional salience.” That is, it must be relevant in some way to the performance of a functional task.
This makes sense. The brain receives a tremendous amount of sensory information each day. Obviously it is not going to completely change all of its sensorimotor maps each time a new piece of information comes to its attention. Instead it will ignore most of the information and only make a change when the information seems particularly interesting - for example, that it can assist the performance of a functional task.
This is illustrated by studies testing the use of sensory discrimination training as a means to reduce pain in patients with CRPS. As discussed above, chronic pain patients such as those with CRPS have a reduced ability to discriminate between two different types of tactile stimuli (for example the difference between a pen cap and a wine cork.) One study found that tactile discrimination training created significant reductions in pain and disability. But touch stimulation alone created no effect in the control group. The authors note that this is exactly what is predicted by the rule of functional salience.
The practical import of this information is that novel sensory stimulus alone is not likely to be therapeutic for pain. The stimulus must also suggest some solution to a functional problem. I see this rationale built into good movement programs such as the Feldenkrais Method, which uses movements not just as a way to entertain the brain with novelty, but to show it a more efficient way to move and solve physical problems.
Normalizing motor representations
The authors discuss a strategy called graded motor imagery, which involves three stages. The first stage is sessions of left/right limb judgments tasks. The second is imagining movements that normally cause pain. The third stage is mirror therapy, which involves moving the non-painful limb in a mirror box, which creates the illusion of non-painful movement in the painful limb. Each stage is essentially a method of activating motor areas without also activating the pain response, and thereby creating some neural differentiation between the two. I think slow gentle pain free movement is a good alternative for doing the same thing.
The paper also discusses the theory that sensory motor incongruence is a cause of chronic pain, which I have written about before. The basic idea is that inconsistency between motor output and sensory feedback is confusing and may even result in pain. The authors point out that evidence in support of this theory is conflicting. Which I find confusing and a little painful, but what can you do?
Well that's about it. Make sure to pass this around and let me know if you have any thoughts in the comments.