Normal sympathetic nerve activity in a reflex sympathetic dystrophy with marked skin vasoconstriction

Intraneural recording in skin fascicles of the left ulnar nerve was performed in a 51 year old patient with reflex sympathetic dystrophy (allodynia, marked vasoconstriction in the ulnar part of the left hand) following Vth finger amputation. Skin sympathetic activity showed normal characteristics with a weak discharge at rest whereas bursts of impulses could be evoked by sympatho-excitatory stimuli. Sympathetic bursts induced by painful stimuli were few, also during long-lasting allodynic pain. Sympathetic bursts induced by inspiratory gasps were not associated with excacerbation of pain. The discrepancy between the marked skin vasoconstriction and the low resting sympathetic discharge underlines the fact that sympathetic effector organ hyperactivity (i.e. vasoconstriction and/or hyperhidrosis) cannot be taken as evidence for increased sympathetic discharge. In the present case, the neuropathic pain syndrome cannot be considered as maintained by an increased sympathetic neural discharge to the symptomatic limb.