Our blood vessels can alter the amount of blood flowing to any part of the body. In health this is great and normal - so in cold weather (in a healthy person) the blood is directed away from the skin so that you retain heat and warmth in the body. This has the effect of making fingers and toes and noses go a bit blue. In hot weather, or during exercise, the body wants to get rid of excess heat - so it sends blood to the skin so the heat can radiate away and you get the effect of looking a bit 'red'.

In RSD the mechanisms that control blood flow are not working normally - so the RSD patient can suddenly have the experience of a leg or arm that becomes red and hot, or cool and bluish, for no apparent reason. It's not really dangerous because there is plenty of blood moving around in there; but it's not very nice either. In my son's case his leg will suddenly go 'dusky' and cool to touch in comparison with the other leg. With some of the SKIP kids - and the adults - this colour/heat change can be very dramatic; and they may get swelling and inflammation as well (not forgetting the pain as an further bonus!). We have lots of nerves that activate all manner of bodily changes - pain, touch, pressure, blood vessel tone, sweat glands and so on. In RSD they are not regulated properly - so you get pain when there is no reason, changes in blood supply, unregulated sweating and so on.

One pain specialist recently gave me a nice explanation: when you get a sudden big scare or fright - you go hot first, then pale. This is the body's response to what is known as 'Fright, Flight and Fight'. Much the same is happening in RSD, but in just one place - the site where the pain is concentrated.
Dave Barton - Group Organiser of SKIP (Supporting Kids In Pain)