I was told at one time that the vasodilation/vasoconstriction that occurs in RSD has to do with the tiny nerves on blood vessels. The nerves make it possible for a blood vessel to react, to either constrict or to open up automatically under cold or warmer circumstances - it's a very finetuned system that can instantaneously react. The RSD makes these nerves act totally out of control. That's why you get constantly opened up blood vessels in first stage RSD, then the nerves sort of don't know what to do anymore in stage 2 and they start to vary between constriction and dilation, then in stage 3 it goes the other way and they cannot keep the blood vessels open enough anymore. That's why in late stage RSD often vasodilators are given as meds, to make the blood vessels open up more, to allow for more blood flow and to warm up the limb from the inside out.