A strange pain disorder called Causalgia can occur when the nerves that run from the spinal cord to the limbs are partially damaged, especially when the damage involves the bundles of nerves to the arms called the brachial plexus. Partial damage can come from accidents (that stretch the shoulder away from the neck, literally pulling some nerve fibers apart), from hand surgery, or from war injuries. A more recent name for the disorder is Complex Regional Pain Syndrome.
The pain is burning, constant, and usually involves the hand or foot. Sensory stimulation, such as rubbing the area, can worsen the pain. The pain usually is adjacent to any neurological deficit if there is one. Meaning that the pain may not be located in a region of numbness, but adjacent to it.
What makes this pain syndrome so different from other forms of chronic pain is that changes can occur in the skin and bones of the affected limb. The skin can become swollen and red because of loss of the normal tone of the blood vessels. In fact, comparing the temperature of the affected limb compared to the other normal limb is one aid to making the diagnosis (thermography). Abnormal sweating can also be seen. Interestingly, in some case patchy osteoporosis can be shown on X-rays of the limb in as little as three weeks from onset.
The cause is not really understood, but because of the vascular and skin changes, it has been suggested that the sympathetic nervous system is involved. In severe cases nerve blocks to the sympathetic ganglia in the base of neck can provide enough pain relief so that aggressive physical therapy can be started. Cigarette smokes are at much higher risk for developing the painful disorder than non-smokers.