Idenfifying Pain Areas in the Thalamus

Dr. David Bowsher, of Liverpool, one of the great clinical observers of the Central Pains has found certain areas of the thalamus are more likely to cause allodynia of one type or the other.

“Somatotopic” means nerve fibers are arranged according the different and specific parts of the body. For example, pain fibers coming up from the legs may bunch together nearer the center of the cord and the fibers from the arm may be layered over the top. This arrangement continues right on up to the thalamus, which is somatotopically arranged. Here in this article, we describe another kind of arrangement based on the nature of the sensation. When we can identify an arrangement that corresponds to some area of the body, we say the fibers have been somatotopically grouped. There is no term as yet in common use for arrangement according to the type of pain. Thanks to work by Dr. David Bowsher, we may soon have one.

Dr. David Bowsher, in Liverpool, has been distinguished by many years of careful observation of what central pain is really like. Unlike many, he has refused to lump everything together, but as a good clinician should, he has given names to the individual aspects of the central pains. This allows us to speak rationally of diagnosis and therapy. It is the traditional way one developed the art of clinical medicine. This art is in danger of vanishing in the current demand to rush patients through in managed care situations. The doctor can hardly spend time to distinguish central from peripheral pains, let alone sort out the different Central Pains. Thus, much valuable clinical information is wasted. In this morass of clinical vagueness, Dr. Bowsher stands out as one who has paid the price to learn about the disease.

Dr. Francis Crick, who has contributed to this site, was famous for disdaining experimentation in favor of theorizing. In other words, he wanted to THINK. Medical progress requires both types of skills. The hard worker in the animal lab is awesome in his arena. Occasionally, however, we need pure theoreticians, as in the manner of Dr. Crick, to assemble or synthesize what the benchtop workers have found and try to make meaning of it. We appreciate in Dr. Bowsher the same willingness to pay attention to detail while he makes associations that many others miss.

Particularly welcome is his latest study in J Pain. 2005 Nov;6(11):736-40.

Here, Dr. Bowsher discusses central post stroke pain which leads to allodynia. Based on brain imaging, there is a tendency for those with lesions in the forward part of the thalamaus to develop allodynia (pain from innocuous stimulus) from movement, while those with lesions located more toward the rear of the thalamus develop allodynia to thermal stimulus, particularly cold. Dr. Bowsher mentions touch, temperature, and movement as the three main evocative stimuli to the central pains. He also agrees with us that the movement pain of CP comes from the stretch receptors, which we have elsewhere termed, “the sensory arm of the gamma motor system, or the fusofugal apparatus”. We are talking about the same thing Dr. Bowshwer is speaking of when he says, “stretch receptors”.

We appreciate the painstaking work of this great pain scientist, and look forward to his continued contribution in the area. He did not identify a specific lesion site for touch allodynia. Since this is easily the most common of the central pains, we theorize that nearly any lesion is capable of causing dysesthetic burning. Remember, it is LIGHT touch which evokes the burning in Central Pain.

Heavy pressure sometimes relieves the allodynic pain from the stretch receptors so very few people with central pain report evocation from heavy pressure or heavy touch, as it were. The exception of course, is occlusive touch whether heavy or light, but the evocation here probably comes from a thermal effect on the occluded skin, and is therefore not truly a touch evocation. This is a great paradox, probably deriving from characteristics of the very sensitive fibers which respond to light touch. What those characteristics are has not yet been determined.