Role of the sympathetic nervous system in pain

Some chronic pain conditions appear to be sympathetically mediated because interruption of the sympathetic supply results in pain relief. Examples include: complex regional pain syndrome type I and II; visceral pain due to abdominal and pelvic cancers; ischaemic pain or tissue viability from peripheral vascular disease, arterial spasm or frostbite; ischaemic heart disease; post-herpetic neuralgia; erythromelalgia. The sympathetic ganglion can be blocked at several locations. The stellate (cervicothoracic) ganglion can be blocked to reduce pain in the head and upper limb. The lumbar sympathetic chain can be blocked to reduce pain in the lower limb, and the thoracic sympathetic chain to block thoracic pain. The coeliac plexus can be blocked to reduce pain in the upper gastrointestinal tract (e.g. pancreas, liver cancer). The superior hypogastric can be blocked to reduce pain in the pelvic area (e.g. bladder, uterine cancer). Ganglion impar can be blocked to reduce perineal pain such as pelvic floor cancer. Blocks are usually performed using local anaesthetic, preferably a long acting one to maximize duration of effect. Neurolytic blocks can also be carried out using phenol (6% or ethanol 50-100%), most commonly with coeliac and lumbar sympathectomy. An intravenous regional sympathetic block (Bier's block with guanethidine) can be carried out to reduce pain in the upper and lower limb. Guanethidine is injected into an occluded vein, where it is absorbed into the perivascular sympathetic plexus and induces release of noradrenaline. It then prevents the reabsorption of noradrenaline by being preferentially absorbed into the sympathetic nerve itself, so depleting the vesicles of transmitter. A systemic infusion of phentolamine (alpha antagonist) can be used for diagnosis.