Why isn’t there increased itching in Central Pain. Except for one possible exception, all of the itch patients have turned out to have peripheral neuropathy. There should be a matching condition in pain of central origin.
As we approach a thousand respondents to the survey, we find not a single person who has itching from injury to the Central Nervous System. This is more surprising than one might think. The reason is that the conventional viewpoint is that there is a continuum of sensations from touch to pain, with itch in between along with what are called metasensations, which is a nice way of saying the person cannot describe them. Itch is said to be one of the alarm sensations to prevent injury or damage. We question the notion of a continuum, based on the surveys.
Everyone has played the child’s game of “rock, paper, and scissors”. No matter what one chooses, something else has the potential to trump it. If there really is a continuum, what then has happened to the itching in nerve injury? It is common knowledge that scratching stops itching. How can this possibly be? If a continuum is the explanation, a noxious event such as scratching should upgrade the firing rate until pain is noted. Indeed, scratching a sensitized area does indeed cause increased pain. Thus, what is sensitized in Central Pain must NOT be sensitized in itching.
Thus, it simply cannot be that simple. If pain covers itch, why does loss of touch cause pain, such as we see in Central Pain, where the dysesthetic burning occurs only where there is diminution in the sense of touch.
it is also noted that opiates INCREASE itch, through disinhibition, which means opiates suppress pain inhibition pathways in the brain, yet we are talking about ITCH here. We are back to rock, paper, and scissors. The paradox is consistent since opiates also cause more pain by disinhibition in the brain, yet they increase itch. The reciprocal reactions are not clear, are not linked in an obvious fashion, and do not account for the pain where loss of touch comes from nerve injury. We think a better explanation would be found by abandoning the idea of a continuum, and thinking more along the “if…then” way of viewing sensation. Pain can take priority over other sensations, and claim whatever space touch vacates with nerve injury, in recruiting via the interneurons, but itch is not part of the pathways which carry burning pain. Other conclusions are possible but must explain why scratch relieves itching.