'Sympathetically maintained pain.' I. Phentolamine block questions the concept

Abstract

Patients with "reflex sympathetic dystrophy" or "causalgia" underwent sympathetic blocks. In protocol A (77 patients), we infused placebo (saline) for 30 minutes followed by phentolamine (35 mg). In protocol B (23 patients), the saline phase was followed by double-blind infusion of phentolamine or phenylephrine (500 micrograms), a second phase of saline, and then the other active drug. We assessed magnitudes of pain and mechanical hyperalgesias on a 0-to-10 pain scale and monitored sensory and sympathetic effects. With protocol A, pain diminished significantly (> or = 50%) during placebo in 22 patients (28.9%) and during phentolamine in seven (9.2%). With protocol B, four patients (17.3%) had relief of pain during placebo, four (17.3%) during phenylephrine, and two (8.7%) during phentolamine. All "phentolamine responders" had progressive pain relief from placebo. Two patients expressed relief during phenylephrine and worsening during phentolamine. Most patients did not respond significantly to saline or drugs. Thus, pharmacologic manipulation of the alpha-1 adrenergic receptor by either agonist or antagonist drug does not influence neuropathic pains. These results raise questions about the existence of "sympathetically maintained pain," as diagnosed by sympathetic blocks improperly controlled for placebo.