Breakthrough in Central Pain Understanding

Painonline has at times been hardpressed to explain why central pain patients have so much muscle pain.  Both kinesthetic dysesthesia (pain with muscle loading) and confinement cramps, aka isometric dysesthesia (the largely spontaneous sensation of cramps in the muscles) are exceedingly common in central pain. Although often glossed over because the patient complains of such agony from burning sensation on the skin, as Beric has pointed out, the muscle pains may be so severe that a person with an intact motor unit may be functionally paralyzed. Muscle pains have been the stepchild of CP.

They also confound cliniicans who often confront ACTUAL musculoskeletal pain in post spinal cord injury patients, which pain is NOT neuropathic. Devising treatment strategies is much more difficult because verbal descriptors are lacking by which the clinician may distinguish between Central Pain in the muscles and the musculoskeletal pains which are to be expected whenever there is alteration anatomically at any motion segment of the spine. The former is neuropathic, and may be utterly resitant to opiates, while the latters is nociceptive and should respond to conventional pain therapies, such as opiates.

In a prior article here, the increasing focus on the posterior nucleus of the thalamus was discussed. This largely ignored nucleus is at the very back of the thalamus. The prior article dealt with connections being uncovered between pain tracts and the posterior nucleus or PoT (also called PO)

Now Masri, Keller and others at the University of Maryland have published a landmark study in the online J. Neurophysiology April edition ** which suggests that the PoT has inhibitory input from the zona incerta. The zona incerta is part of a nucleus which sits BELOW the thalamus, known as the Subthalamic nucleus. The Subthalamic nucleus is itself divided into areas. Nothingi n the brain seems to do just one thing. One neurophysiologist suggested that no area of the brain does just one thing, and that no nucleus has more than thirty percent of its neurons devoted to any one function.

Zona Incerta has always been a mystery as to its function. Its very name means in Latin, the zone of uncertainty, meaning no one had a clue what it did. (If you want a little look at this area, go to Brainmaps.com) Now, the ZI is turning out to be massively important to Central Pain patients. Its failure to inhibit the PoT may be the actual mechanism of CP.

This multifunctional aspect is revealed in the zona incerta. However, proximity often means some RELATION between the various functions should be suspected, or at least looked for. The posterior part of the Zona Incerta is the area of interest. The very most posterior part of the ZI is the area which is sometimes lesioned in Parkinson’s because there are links to the cerebellum and hence the motor functions of the body. (Coordination etc) The forward part of the most posterior region of the ZI was discovered to serve the function of INHIBITING the POT. When Central Pain is present, the inhibitory signal from the ZI is missing.

This leads to the rather logical conclusion that without input from the ZI, the PoT cannot distinguish between something like a breeze on the skin and a burn. (The University of Maryland news release on the discovery called Central Pain a mysterious disease. Perhaps an analogy might be made to color blindness. The color blind cannot distinguish between red and green. Thus, the brain of a person with Central Pain cannot distinguish between a breeze on the skin and a burn)  

The clinical description is hardly surprising, since most of those in the survey here attest to the fact that anything bringing a temperature change, especially a blast of cold air< will evoke burning pain powerfully. However, the more the press refers to Central Pain as “mysterious”, the less mysterious it becomes. This alone is a help to CP subjects. Even more help is the notice that attention and focus must be given to the surprising role of an allegedly ”MOTOR” area, the Zona Incerta, plays in interpretation of pain signal. Perhaps we should say “sensory signal” since most of what causes agony in CP is not inherently a pain stimulus at all.

Carl Saab, noted for identifying an area in the cerebellum which inhibits central pain, ie the vermis, has written here of the unexpected cerebellar link between that structure and pain inhibition. His first paper on the subject was so unexpected that it caused not only an uproar, but anger, at the Ninth World Congress of Pain, when Dr. Saab presented it. However, we now have more backing for the former author at painonline in yet another link between the cerebellum and pain. Namely, the area which connects to the cerebellum, and is a point of interest in Parkinson’s, is immediately adjacent to the pain interepreting area of the posterior region of the ZI which inhibits the PoT.

Look for more material on this, as some Parkinson’s patients have pain. It may be a touch of central pain, given this recent finding. We congratulate the authors (there are a number) and feel this may be the most significant article ever published on Central Pain.

**We are indebted to Mary Simpson for first noting this article when it appeared.