Reflex sympathetic dystrophy: the significance of differing plasma catecholamine concentrations in affected and unaffected limbs.

In 26 patients with features of reflex sympathetic dystrophy, venous blood was collected from painful and unaffected limbs. Levels of plasma adrenaline, noradrenaline and its intracellular metabolite, 3,4-dihydroxyphenylethyleneglycol (DHPG), were measured by combined gas chromatography/mass spectrometry. Plasma DHPG was lower on the painful side. Concentration of plasma noradrenaline was also lower on the painful side in patients with widespread allodynia, and in those with hyperhidrosis in the affected hand or foot. These findings do not support the widely held view that autonomic disturbances in reflex sympathetic dystrophy are due to sympathetic overactivity. Rather, they suggest that sweating and changes in peripheral blood flow result from supersensitivity to sympathetic neurotransmitters. After injury, supersensitivity to noradrenaline may also contribute to spontaneous pain and allodynia by disrupting efferent sympathetic modulation of sensation. This would explain why pain and allodynia are relieved by sympathetic blockade, and why noradrenaline rekindles pain in sympathectomized skin.