Pain and autonomic nerve blocks

Abstract

The sympathetic nervous system was, for a long time, the target for pain relieving techniques, but they are now used much less frequently than in the past because the mechanisms of pain are better understood. The role of the sympathetic nervous system in the generation and maintenance of pain may have been overemphasized previously (Schott 1998) so the rational for sympathetic block in chronic pain management is being re-evaluated. Systemic medication, stimulation techniques, physical therapy, and psychological techniques are all used much more widely than previously. When it is used, sympathetic block can be produced in one of three ways: 1. Direct injections of the sympathetic chain or related ganglia using local anaesthetic or neurolytic solutions; 2. Venous injection, with the circulation excluded by tourniquet, of drugs which deplete adrenergic transmitters—intravenous regional sympathetic block (IVRSB); and 3. Systemic administration (by intravenous infusion) of α-adrenergic antagonist drugs. The analgesic effect of most nerve block techniques (e.g. an epidural in labour or coeliac plexus for cancer pain) is a result of the interruption of transmission in afferent nociceptive fibres, but the effects of sympathetic nerve block can be more complex. The sympathetic nervous system is directly involved in the pathophysiology of some painful states (‘sympathetically maintained pain’), and the analgesic action is due primarily to block of sympathetic efferent activity. There may also be disruption of reflex control systems so that peripheral or central sensory processing is altered. Finally, peripheral vasodilatation can relieve pain due to ischaemia.