Sympathectomy in such cases causes classic Cannon phenomenon. This physiological phenomenon refers to the fact that the end organ that is controlled by sympathetic fibers will become uninhibited in it's chemical dysfunction. As a result, even though the sympathetic nerve fibers are not contributing to acetylcholine or norepinephrine secretion at the area of nerve damage, the partially damaged sensory nerves become uninhibited with resultant increase pain input.
In patients who have had sympathectomy, thermography shows an increase iof temperature in the focus of ephatic nerve damage (Cannon phenomenon) with secondary increase of pain and discomfort.
Chronic Pain: Reflex Sympathetic Dystrophy : Prevention and Management
By Hooshang Hooshmand
Published by CRC Press, 1993