CRPS is complicated, and there's not much known about it. The symptoms will vary between people, and what works to treat one person will not work to help another.
CRPS was first discovered during the Civil War. Doctors on the field were faced with bullet wounds that, although healed, still caused soldiers severe, burning pain disproportional to the injury. It was originally termed Causalgia, meaning "burning pain." In fact, some still use this term today.
CRPS has gone through a number of name changes during it's history. Some of them include:
Complex Regional Pain Syndrome (CRPS Types I and II)
Reflex Sympathetic Dystrophy (RSD)
Reflex Sympathetic Dystrophy Syndrome (RSDS)
Causalgia
Sympathetic Maintained Pain Syndrome
CRPS is characterized by a burning pain that is dispropotional to the injury that caused it. The pain can be increased with stress, motion, touch (even the slightest touch can cause severe flare ups of CRPS), breezes, cold, etc.
Common symptoms of CRPS include, but are not limited to:
Burning and/or gripping pain
Skin changes (often the affected limb will because a deep red, blue, or purple color. Also, extremely dry, darker patches of skin often times crop up on the affected limb.)
Increased hair growth on the affected limb
Nails becomming brittle and breaking more easily
Limbs feeling cold to the touch
Changes in bone growth
Secondary symptoms include, but aren't limited to:
Depression
Insomnia
So what is causing the pain? This is a subject that's up for debate. One possible explanation is as follows: There are many types of nerves in the human body. Sympathetic nerves carry signals toward the brain, whereas somatic nerves carry signals away form the brain. My doctor explained it to me by saying that at some point the body gets confused. The Somatic nerves somehow get rewired and begin to send signals to the brain. The brain doesn't know how to comprehend these signals, so i translates them into pain. The sympathetic nerves then continue the pain cycle, and it never ends.
I have also heard theories relating to electrolyte balance in the body.
No matter what it is that's creating the pain, there is no physical reason for the pain to be there. It's a matter of the sympathetic nervous going haywire in some way.