Sensitization and its reversal

After experiencing persistent pain, the body and brain can become so sensitized that even the touch of a feather is painful!

Sensitization can be displayed graphically by plotting a person’s pain rating (from 0 to 10) as a noxious stimulus is increased.

Two key measures of sensitization are allodynia, reduction in pain threshold, and hyperalgesia, increased response to noxious stimulus.

This figure shows what happens in chronic pain. The black curve shows how the body typically works. As a noxious stimulus is increased (think temperature of a block your hand is on or the force of a rod pressing against your leg) no pain is sensed until some threshold above which the pain increases. This helpful, nociceptive, pain is designed to minimize tissue damage.

The dashed curve (Chronic Pain) shows what goes wrong after the brain is sensitized by the prolonged experience of pain. A great story about how bad sensitization can get is in a YouTube video by Stanford physician Elliot Krane, “The mystery of chronic pain.” The video describes the sensitization to even the touch of a feather (as shown above), and continues with an important story concerning the agony of a young girl who became so sensitized that even a light touch caused excruciating, burning pain. It ends with her performing a cartwheel, free from pain, as a result of the work of desensitization he describes.

Researchers have begun to investigate why sensitization happens for some people, leading to chronic pain, while habituation happens for other people. That is, some people become more and more sensitized to repeated noxious stimulus while others just seem to adapt to it. One investigation concluded: “Personality should be seen as a context for modulating pain experience, and as such, a comprehensive evaluation of clinical pain patients should include ways to understand relevant personal styles and traits of an individual patient.” [Nakamura et al] Hopefully future research on this topic will lead to an understanding of optimal methods for retraining the brain away from pain depending on personality.

Some people develop chronic pain and some don’t. The colored regions associated with the experience of pain were detected with functional Magnetic Resonance Imaging (fMRI). The top row has averaged results from patients whose pain on visit 1 evolved into chronic pain by visit 4 about a year later. The bottom row has averaged results from patients whose pain on visit 1 did not evolve into chronic pain as shown by the lack of colored regions in the lower right panel. [Hashmi, et al.]

The top right panel is important. The colored regions are the regions generating the experience of chronic pain. Those colored regions are the dragon we need to slay in order to overcome chronic pain.