I had a strange experience recently. My own reaction to it seemed interesting enough to warrant closer examination, and that, in turn, led to a useful insight. Eighteenth century philosophers recognized the limitations of such introspection, but it continues to be a source of psychological insight and hypotheses. I have sometimes learned to understand patients' experiences better by considering my own. In this case, experimental confirmation of what I learned quickly followed.
Last fall I acquired a walking treadmill. I raised my desk and placed the treadmill under it, allowing me to walk while I work. I quickly learned to walk, think, and dictate text to the computer with little cross-task interference. Walking at 2-3 km/h, I could comfortably cover 5 to 10 km a day, get some exercise, burn a few hundred calories and do some writing. This happy outcome lasted for about six weeks.
Then this arrangement fell apart for what seemed like a ridiculous reason. When I walked on the treadmill for progressively shorter periods of time, a powerful aversive sensation would build up in the soles of my feet. With every step, it felt as if I had one or more pieces of small, sharp gravel in my shoes. I kept stopping to inspect my shoes and socks and never found anything that could account for the sensation. Yes, perhaps there were tiny irregularities in the insoles of my shoes or inside my socks, but there was nothing like the sharp gravel my soles were reporting to my brain. I would rate the pain intensity at 3/10 and the unpleasantness at 8/10. The pain would stop a few minutes after I stopped walking on the treadmill, but would return quicker and stronger each time I resumed.
Interestingly, the problem never occurred during ordinary walking. I could easily walk 5 km outdoors without experiencing these aversive sensations in the soles of my feet.
At this point, two months after installing the desk treadmill, some serious catastrophizing set in. I told myself, "I wasted $1200. I'll never be able to use this treadmill. I'll have to go back to sitting at a desk. So much for my fitness plan," and similar discouraged thoughts. When I did attempt to walk on the treadmill while working, I could not concentrate on the work at all: my mind was almost fully occupied with the sensation in my feet and with those catastrophizing thoughts. Realizing this led to a vicious cycle of increased catastrophizing: "What an idiot – I ought to be able to focus on my work!"
After several weeks of this struggle, I finally remembered what should have been obvious to me as a pain researcher and as a person who has observed quantitative sensory testing in both clinical and research settings. I was experiencing neural sensitization to the prolonged and regular repetition of identical physical stimuli produced by walking on the treadmill.
In what has been called the "Chinese water torture" (which is probably not Chinese in origin and was described in Europe in the 16th century), a small drop of water falls at regular intervals on the restrained victim's forehead, becoming agonizing after a while. This is a classic example of sensitization. I realized that putting my feet down in a mechanically regular pattern had the same effect. The feelings in my feet corresponded to the classic phenomena of sensitization including allodynia (interpreting a normal touch as painful) and windup (progressively stronger response to the same stimulus).
With this insight, the cure was obvious: I had to vary the stimulus. This turned out to be easy. When the sharp gravel sensation starts, I kick off my shoes and continue walking in my socks or bare feet; if I am already barefoot when the sensitization begins, I put my socks and shoes back on. I vary the speed setting on the treadmill so that my footfalls occur at different intervals. I vary the way I walk, placing weight on different parts of my feet. Sometimes I just stand for a while. These actions vary the frequency, location, intensity and quality of the stimuli on my feet, thus reducing the sensitization.
Because of these minor changes I can continue my work with little interference. The sensitization still occurs (pain 1/10, unpleasantness 3/10), but it doesn't bother me because I know what to do. In other words, the sensitization is not amplified by catastrophizing and helplessness.
This experience helps me to understand patients whose pain is worsened by sensitization. I don't think this neurophysiological phenomenon is widely understood outside the circles of pain specialists. The combination of sensitization and catastrophizing is dramatically distressing and disabling. Changing either can influence the other.
Coincidentally, the day after I wrote the above, an article by Salomons and colleagues was published online in Pain, providing experimental confirmation of what I learned through direct experience. In this study, repeated thermal stimuli were applied to participants' forearms, producing hyperalgesia. Half the participants were given a cognitive intervention to reduce their stress response to the painful stimuli by identifying negative cognitions and reappraising the situation. In comparison with a control condition, the cognitive intervention led to reduced unpleasantness ratings. The authors conclude, "Reduction in secondary hyperalgesia was associated with reduced pain catastrophizing, suggesting that changes in central sensitization are related to changes in pain-related cognitions. Thus, we demonstrate that central sensitization can be modified volitionally by altering pain-related thoughts."
The take-home message: let's watch out for sensitization and catastrophizing and their mutually reinforcing interaction, in our patients and in ourselves.
Have you experienced sensitization and hyperalgesia in your own life? What did you do, physically and psychologically, to deal with it? Did you learn anything from that experience that informs your research or clinical practice? What other research bears on this relationship between sensitization and catastrophizing?