Pain Does Not Match the Size of the Injury

Pain Does Not Match the Size of the Injury

The same knock is nothing on one day and unusually painful on another. Long after the injury, with the wound healed, the place still aches. The image shows the damage is not large, and the degree of pain does not agree with that.

At such times I say this to the person in front of me. Pain does not transmit the size of the injury as it is.

Pain Is a Relay of Damage, Not a Copy

We commonly take pain to be an honest gauge of damage — hurt a lot, feel a lot; hurt a little, feel a little. Half of that is right. But the body does not copy damage across as it is. It senses damage, converts it into an electrical signal, and sends that signal up to the brain having amplified or reduced it on the way.

So pain is not a photograph of the injured place but the body's interpretation of the signal that came up from it. The power to sense and the power to amplify or dampen — these two can move independently of the amount of damage. That is why the size of the injury and the size of the pain fall out of step.

With the Volume Up, a Small Sound Is Loud

I explain this with sound and volume.

Damage is the sound; the sensitivity with which pain is felt is the volume. A loud sound is of course heard loudly. But with the volume up, a small sound is also heard loudly. The speaker has not broken, and the sound has not grown. The volume simply has not come back down.

There are two places in the body where this volume goes up. One is near the injured place. The sensory endings around the wound become sensitive, and send signals even at a light brush that would not ordinarily hurt. The other is the spinal cord and brain. When the same signal comes up repeatedly, the relay stations amplify it more and more. A stimulus small at first snowballs with repetition.

When both places move toward turning the volume up, the situation arises in which the damage has already healed and only the pain remains. The sound has stopped and the volume has not moved, so it is quiet and still sounds loud. This state, in which the threshold for feeling pain has come down, is called sensitisation, and when that amplification occurs in the spinal cord and brain it is called central sensitisation.

Pain Does Not Appear in an Image

Images and tests look at damage — the sound. Whether the bone has knitted, whether the wound has closed. But volume is not the shape of tissue; it is the setting of the nerves, so it does not remain in an image. Hence, "it has all healed, so why does it hurt?" What has healed is the sound; what remains is the volume. The two are different things.

Up to here this is well known among those who study pain. Why one particular body turns the volume up so readily and brings it down so poorly is not yet fully worked out, and it is a part I watch closely in the consulting room.

A Raised Volume Can Be Brought Down

The good fortune is that the body also has a device for lowering the volume.

Among the signals descending from brain to spinal cord is a branch that presses pain down. This is called descending pain inhibition. Sleeping well, feeling safe, and moving the place moderately all give this lowering device strength. So with such a person I do not look only at the sore place; I look at how much they are resting and how tense the body is. Lowering the volume matters as much as silencing the sound.

Of course, pain that has come on suddenly, pain that worsens enough to wake you at night, or pain accompanied by fever or weight loss is a different story. Then a test to check the sound itself comes first.

That the size of the injury and the size of the pain do not match does not make the pain false.

If the test says "the damage is not large" and you are in pain, it is not malingering. The volume has simply not come down yet, and volume can be brought down.