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Persistent Pain: why does pain move and spread?

In another instalment of our persistent pain series, we delve a little deeper into the neuroscience behind it all. Today, we're looking at the commonly-experienced phenomenon of persistent pain moving and spreading.

You've probably experienced this before. For instance - that niggling back ache, which was at first simply an annoyance localised in one spot, has now seemed to have moved and spread up your back, or down your leg, or into your hip.

The pain begins sliding around like jelly on a plate. Most of it in one spot, but now it's splashing up, down, and every which way.

But why does this happen?

To answer this we first need to understand a bit more about the brain, and how we experience sensations (touch, pressure, heat, etc.) on the body.

let's start at the periphery. When we come into contact with a stimulus (e.g. knock our hip against the corner of the table), the sensors in the skin and lower layers send signals up the spinal cord to the brain.

These signals are then processed in the brain in a spot called the Somatosensory Cortex (seen in pink).

The organisation of the Somatosensory cortex (SSc) is what is interesting to us. Each area of our body is mapped topographically on a specific section of the SSc. Our brain being the clever thing that it is, organises the mapping into general areas, and clumps together sections that are anatomically close together.

For instance, the representation of our trunk (torso) is mapped next to the representation of our hip & neck.

Furthermore, the more sensitive an area of the body feels, the more real estate it takes up in the brain. So, paradoxically, the large area of the torso/trunk takes up a relatively small section in the brain. Conversely, the areas on the face hogs a considerable portion of cortical land.

The image below gives a good visual indication of the SSc and it's representations.

So how does this all link to pain that moves and spreads?

This phenomenon centres around the idea of cortical smudging. In an unaffected brain, the areas that represent each body section are functionally distinct, in that they receive information coming from their own specific section in the body, and respond accordingly.

Cortical smudging is something that occurs when someone has been experiencing persistent pain. Smudging refers to changes in the representations of the body in the brain, so that they begin to overlap and smudge together.

Let's use the example of a persistent low back pain sufferer, for instance. To simplify a highly complex phenomenon - once smudging has occurred, signals from the low back that were typically only interpreted by the corresponding low back section in the brain, are now interpreted by the adjacent areas that typically interpret signals from the hip, or the upper back, or the neck.

This overlapping of brain areas causes signals that prompt the experience of pain to be interpreted and acted upon in multiple body areas - thereby causing the pain to feel like it has spread or moved to different areas of the body.

These adjacent areas experience pain, but it does not mean that they have been damaged.

The first step to treating cortical smudging is to understand that the shifting pain does not equal damage to your body. It represents an over sensitivity of the pain system, which is something that can be rectified.

I won't dive into a treatment protocol for cortical smudging in this instalment. As I said, the first step is to understand what is happening and to acknowledge that it is not a physically harmful change. This helps to remove the fear and apprehension from movement, and as we know - movement is medicine.

If you someone who experiences this phenomenon and would like to discuss it further please feel free to reach out. We'd be happy to discuss it further.

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