Sensitization to Pain

When your brain is hammered week after week, month after month, and sometimes year after year with the same pain impulses, it becomes very efficient in processing them. It takes less of an impulse from your back to elicit the same response in the brain. Or looked at differently the same impulse causes more neurons in the brain to fire. It is this process that causes patients to complain that their pain is getting much worse in spite of no additional trauma.

The phenomenon was clearly documented in a clinical study done in 2004. Volunteers who had no experience with chronic pain had a pressure applied to their finger. It was slightly painful. The researchers measured the response in the brain with an MRI that is able to track metabolic activity. It is called a “functional MRI”. Researchers consistently identified one small area of the brain that responded to this pressure. They then applied the same pressure stimulus to patients who had experienced chronic pain for more than twelve months. In these patients, five areas of the brain lit up. The difference in the brain’s response was consistent and dramatic.

Water torture provides a crude example of how the brain gets sensitized. A poor prisoner is strapped to a board and water is dripped onto his or her forehead. It is only a drop of water. The intensity of the “source” of the sensation is mild and does not change over a period of time. With the repetition, the nervous system becomes focused on it, and the sensation becomes intolerable. The victim usually goes insane. Granted, there are the added elements of fear and being trapped. Are not those emotions also experienced by those in chronic pain?

I have a good friend of mine, Dennis, who related an interesting experience he had regarding pain sensitization while working as a hod carrier. It occurred when he was in his early twenties.

A hod carrier is the person who sets up the work area for brick masons to enable them to lay blocks or bricks as efficiently as possible. It involves mixing cement, carrying it and the building materials to the next section of wall. It all has to be done fast enough to stay ahead of the masons. The materials also often have to be hauled up scaffolding. The masons are not usually very happy if the hod carrier falls behind. It is an extraordinary intense, physical job.

Part of his job entailed cutting blocks of basalt with a “wet” saw. This is a saw with an industrial diamond blade. To keep the blade from overheating, water from a small sump pump is continuously pumped over the blade. One day he felt a very mild tingling sensation as he placed his hand on the saw handle. As he cut blocks early in the morning, the sensation was not particularly uncomfortable. As the morning progressed the sensation gradually became stronger. By early afternoon, the sensation was so strong that he could no longer comfortably touch the handle. He felt an actual shock. Just the anticipation of the sensation became a problem and he would not touch the handle.

There were three hod carriers on this crew. They watched him get more and more cautious and finally refuse to touch the handle. Yet when they touched the handle they could barely feel anything. Needless to say, they gave him a hard time.

The next day, they rotated and the second guy had the same experience. By mid-afternoon he would no longer touch the handle. The third guy now thought they were “toying” with him. Again, when he touched the handle, the sensation was almost imperceptible.

The third day was his turn and by mid-afternoon he was a believer. The frayed wire was finally fixed on the fourth day.

What is interesting to me about this story is the degree of the difference in sensation over just six to seven hours of repeated exposure. If the initial impulse had been somewhat uncomfortable, the story would have seemed more plausible. However, the sensation went from barely perceptible to an electric shock. Dennis said that even the anticipation of touching the handle became a problem.

There are two situations where pain may increase: 1) either more pain receptors are stimulated, which in turn stimulates more neurons in the brain 2) the central nervous system becomes more sensitive and the same stimulus now recruits more neurons to fire. Remember, the final pain you experience relates only to the number of neurons firing in your brain. The pain is quite literally “in your head”.

Click for Video