SMP and SIP – What is the Difference between Sympathetically Mediated Pain and Sympathetically Independent pain?

This question was posed most recently by “Suzy Q” on RSDHope Teens and has been asked many times before.

“What is the difference between SMP? and SIP? 

I am wondering is the difference between sympathetically mediated pain (SMP) and sympathetically independently pain (SIP) ; sometimes called are IMP or independently mediated pain.

Suzy was asking this question in regards to Sympathetic Blocks and whether or not they are always effective, if you aren’t sure how they are related I would suggest you read this page first.

My response to her;

I recently answered a similar question 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;  (

“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, sometimes on purpose, sometimes not, but you need to place yourself right in the thick of it. Demand to be part of the process in the most positive way possible. This is your body and CRPS is a disease that you could possibly be dealing with for a number of years so it is only right that you be a part of making these decisions. Yes, you need to get all of their opinions and involve them as well, but in the end they need to understand it is your decision.”

“There is a reason blocks don’t work nearly as well after the first few months, especially after the first twelve months , for MOST patients (always exceptions to the rule). It is explained on the website in more detail but it has to do with the percentage of your CRPS pain that is Sympathetically Mediated Pain (SMP) and the percentage that is sympathetically independently pain (SIP).  

Let’s do a math problem.

I know, argh! But bear with me.

Think of a giant X that is put in a chart format.

I   M                          I
I        M               I
I            M      I
I              I  M
I            I        M
I        I                 M
I    I                         M
I———————————

Line One (SIP) starts in the bottom left corner and progresses up to the top right corner, so that it is increasing in strength as the months pass. That is the Sympathetically INDEPENDENT Pain.

Line Two (SMP) starts in the top left corner and goes to the bottom right corner, so it is decreasing in strength as the months pass. That is the Sympathetically MEDIATED  Pain, the one more likely to respond to nerve blocks. 

Everyone still here or did you all run away at the mention of a math problem?

In the beginning nearly all of your CRPS pain is SMP 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 SIP or sympathetically independent at this point.

As time passes, 3, 6, 9 months, your pain gradually changes. The percentage of SMP reduces and the percentage of SIP increases. Eventually they cross at 50/50, the middle of the giant X, and then the percentage of the SIP keeps growing until it is almost entirely SIP, or Sympathetically Independently Mediated.

At this point, around 12 to 18 months, it is a lot less likely that any sympathetic treatment, such as nerve blocks, etc., are going to have any effect at all and they can actually produce negative effects. There are exceptions of course, this is CRPS after all! And if you have a NEW area where CRPS is starting the process and time-table starts all over. 

In addition, if you go into a CRPS “flare”, where you symptoms kind of go hay-wire for a few days, weeks, or even months sometimes,  there are localized solutions that can be effective in batting that flare down; everything from lidoderm patches and ketamine cream, to blocks and aqua therapy have been reported as being effective by CRPS patients. 

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 SIP term)

“As CRPS progresses, more and more of the pain becomes Sympathetically Independent, that is, it becomes centered or generated in the brain. It actually becomes independent of the injury.

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 CRPS. Once your pain becomes SIP it is much harder to affect/reduce it through a block and even most of the typical pain medications. You are now leaving the acute stage and entering the beginning of the chronic stage. Dealing with the disease, medically and emotionally, becomes much more difficult after the first twelve months. 

So does that help you understand a little more clearly why blocks don’t usually work after a year and explain the difference between SMP and SIP?