Persistence of pain induced by startle and forehead cooling after sympathetic blockade in patients with complex regional pain syndrome.

BACKGROUND Stimuli arousing sympathetic activity can increase ratings of clinical pain in patients with complex regional pain syndrome (CRPS). OBJECTIVE To determine whether the increase in pain is mediated by peripheral sympathetic activity. METHODS The effect of sympathetic ganglion blockade on pain evoked by a startle stimulus and cooling the forehead was investigated in 36 CRPS patients. RESULTS Loss of vasoconstrictor reflexes and warming of the limb indicated that sympathetic blockade was effective in 26 cases. Before sympathetic blockade, pain increased in 12 of these 26 patients when they were startled. Pain increased in seven of the 12 patients and in another five cases when their forehead was cooled. As expected, pain that increased during sympathetic arousal generally subsided in patients with signs of sympathetic blockade. However, pain still increased in three of 12 of patients after the startle stimulus and in six of 12 of patients during forehead cooling, despite indisputable sympathetic blockade. CONCLUSIONS These findings suggest that stimuli arousing sympathetic activity act by a central process to exacerbate pain in some patients, independent of the peripheral sympathetic nervous system. This may account for the lack of effect of peripheral sympathetic blockade on pain in some CRPS patients.