This is why you never should use ice on RSD affected limb:
1) Some of the symptoms of CRPS, such as pain, discoloration, and spasms, are due to a constriction of the blood vessels. This causes a reduced blood flow to the extremities. many of you have seen the color show your feet/hands, legs/arms have put on where they turn dusky, purple, etc. and have felt the pain of the constriction as if your limb was being strangled, have felt the extreme coldness due to lack of blood flow, etc. These are all partly due to constriction of the blood vessels.
Now when you apply ice to that same affected limb you are constricting, shrinking, those blood vessels and reducing the flow of blood to your limb even further.
2) The second and more important aspect to the application of ice is the damage done to the myelin sheath (fatty tissue insulating the large nerve fibers) . The best explanation I have seen written on it was done by Dr Hooshmand many years ago;
"As the ice freezes the large nerve fibers, causing freeze damage to the myelinated nerves, the patient develops sensory loss and pain due to permanent damage to the large sensory nerve fibers. This aggravates the RSD by adding sensory nerve pain of non-sympathetic origin to the initial thermal sensory pain of sympathetic origin.
As a result, Ice provides total anesthesia and relief of pain for several minute the same way as the hand becomes numb being exposed to snowballs in the winter. However, a few hours after the cessation of ice exposure, the pain recurs with vengeance due to reactive enlargement of blood vessels after the constriction of blood vessels due to exposure to ice.
This phenomenon causes excellent relief of pain with ice treatment followed by not only aggravation of pain, but damage to the nerve fibers adding sympathetic independent pain (SIP) to the original sympathetic mediated pain (SMP).
The end result is aggravation of the RSD and SIP resulting in failure of nerve blocks and then the patient is told, "You do not have RSD anymore because the nerve block did not help you and the phentolamine test proved that you do not have SMP or RSD".
In most RSD patients ice makes the condition worse and can cause denial of diagnosis and treatment for the patient."