The ANS, a chain of ganglia (nerve bundles) lying in close proximity to the spinal cord, is responsible for constant subconscious control of basic life functions like breathing and blood vessel dilatation. Painful stimuli can, under certain but poorly defined circumstances, cause abnormally vigorous and sustained constriction and altered control of blood supply to a hand or foot. This in turn can lead to altered temperature, color, or sweating in the affected extremity. If not controlled immediately and aggressively, this can lead to a painful, blue or red, hot or cold, sweaty, swollen extremity. All of these features need not always be present. Usually starting distally and spreading proximally, this "dystrophic" response can sometimes involve the whole extremity, or in certain severe cases, even spread, via contaminated spinal cord neurons (nerve cells), to other extremities. Occasional very unfortunate patients can develop RSD in their entire bodies.

The most common causes are injuries, which can be trivial. Other causes can be major trauma, fractures, burns, electrical shocks, heart attacks, frozen shoulder, or certain other medical illnesses. In my experience, the presence of insulin-dependent diabetes can worsen the clinical picture.

Treatment is not always effective, but is more so if instituted early. Most literature suggests that treatment should be started within the first six months to be effective, and in my experience can be dramatically so if started in the first days or weeks. Treatments consist of steroids and blood pressure drugs in the first stages, along with aggressive physical therapy, and painkillers to allow such therapy to be employed. Various types of nerve blocks or even surgery to sever sympathetic nerves may also be necessary. Treatments will vary according to individual circumstances.