Complex regional pain syndrome or CRPS is a painful condition that affects the arm and hand or leg and foot. It usually occurs after trauma of some sort, including surgery. Most people think the syndrome occurs after healing is complete. But one theory is that there is ongoing inflammation. The inflammation causes nerve endings to become extra sensitive.
Another theory is that changes in brain function occur. Which area(s) of the brain and why remain unknown. It may be possible that CRPS is a psychiatric problem. Psychosomatic or a mind-body response occurs after trauma leading to a stress-induced condition of pain and other symptoms.
Most recently several studies have been published pointing to nerve damage as the primary cause of CRPS. But the studies were small. The authors themselves said they couldn't be sure the changes measured happened first or after CRPS.
The more we can find out about this condition the sooner effective treatment can be developed. It's entirely possible that there is more than one mechanism involved. The complex interactions between brain, body, and peripheral nervous system support this theory.