Mechanisms of sympathetic pain.

In some chronic pain states, notably causalgia and reflex sympathetic dystrophy, activity in sympathetic efferent neurones can exacerbate the pain and sympathectomies relieve it. These patients are said to have sympathetically maintained pain (SMP). In normal tissue, activity in postganglionic sympathetic efferents does not produce pain, nor is it capable of activating nociceptive sensory neurones. It can, however, induce modest firing in some mechanoreceptors. SMP is often held to result from a vicious circle of events which include changes in peripheral and central somatosensory processes, and most importantly a positive feedback element in the form of sympathetic efferent neurones which, by activating sensory neurones in the periphery, completes the vicious circle. Several specific hypotheses have been advanced as to the primary pathophysiological cause of pain in these patients. Suggestions, largely deriving from observations on animal models, include: ephaptic transmission, adrenergic receptors on sensory neurones, indirect coupling of sympathetic and sensory neurones, sensitization of nociceptive afferents, and, in the central nervous system, sensitization of dorsal horn neurones. All these suggestions have some supporting evidence, but none are able to adequately explain all the disturbances seen in patients with SMP.