This question was posed most recently by "Suzy Q" on RSDHope Teens and has been asked many times before. "What is the difference between IMP and SMP? I am wondering why Sympathetic Mediated Pain and Independently Mediated Pain are different and how?" My response follows. 

I recently answered a similar question that was posed by one of our teens on the RSDHope Teen Corner. Angela wrote "The last set of nerve blocks that I had was back in January, and I got 3 done in a week. And again, barely any relief. Sometimes even after the blocks the pain would still get worse. I mean, do you have any suggestions of the next step that I could take to try to help my pain???" My response was, "Angela; Let me first ask you a question it appears no one else over there is, Do you think the blocks are helping you, hurting you, or are simply a waste of time? Or maybe a combination of the above? Remember, you need to be a big part of any treatment program no matter your age. I realize that is often-times easier said than done. Many parents and/or Drs will shut teens and pre-teens out of the treatment option process but you need to place yourself right in the thick of it. Demand it, in the most positive way possible. This is your body and RSD is a disease that you could possibly be dealing with for a number of years, into your twenties and beyond, so it is only right that you be involved in making these decisions. Yes, you need to get all of their opinions but in the end they need to understand it is your decision. There is a reason blocks don't really work after the first few months. It is explained on the website in more detail but it has to do with the percentage of your RSD pain that is Sympathetically-Mediated Pain (SMP) and the percentage that is Independently-Mediated Pain (IMP) . Let's do a math problem...I know, argh! But bear with me.

Everyone still here or did you all run away at the mention of a math problem? In the beginning nearly all of your RSD pain is SM or sympathetically-mediated. This type of pain is very responsive to local pain reduction efforts, most notably sympathetic nerve blocks, but also includes topical creams, massage, etc. Only a very small percentage of your pain is IM or independently-mediated at this point. As time passes, 3, 6, 9 months, your pain gradually changes. The percentage of SMP reduces and the percentage of IMP increases. Eventually they cross at 50/50, the middle of the giant X, and then the percentage of the IMP keeps growing until it is almost entirely IMP. At this point, around 12 to 18 months, it is extremely unlikely that any sympathetic treatment, such as nerve blocks, topical creams, etc., are going to have any effect at all and they can actually produce negative effects. The amount of time it takes to reach this point of course varies depending on the individual, the type of injury, whether it is CRPS Type I or Type II, how it was treated, medical history, etc., but as a general rule it is around 12 to 18 months.

As explained in our MEDICAL TERMS section (look under the IMP term), "As CRPS progresses, more and more of the pain becomes Independently Mediated, that is, it becomes centered or generated in the brain. This explains why localized pain reduction techniques (i.e. sympathetic blocks, creams etc.) no longer work after the first few months of onset of CRPS." This is also why blocks usually work best in the early stages of RSD. Once your pain becomes IMP it is much harder to affect/reduce it.

So Angela, Does that help you understand a little more clearly why blocks don't usually work after a year?" Suzy Q, does that help explain the difference between SMP and IMP?