Sympathetic nerve blocks: in search of a role

The role of sympathetic blocks in pain therapy is examined in the light of changing concepts of pain pathophysiology. A critical review of the literature also sought to develop an evidence-based analysis of outcome studies to provide recommendations for appropriate applications of sympathetic blocks, together with ideas for further clinically based research.

A focus on the pathophysiology of neuropathic and inflammatory pain disorders was used to help redefine what contribution, if any, was provided by the sympathetic system, to chronic pain states. Validation of nerve block therapies based on historical practices and these newer concepts and outcome determinations has then been used to present an overview of clinical nerve block therapies as applied to the sympathetic nervous system.

1. Pain Diagnosis: A reclassification of reflex sympathetic dystrophy (RSD) to the new taxonomy of complex regional pain syndromes (CRPS) is supported, with evidence that only a questionable sympathetic contribution at the dorsal root ganglion level can be ascribed etiologically to this group of disorders. Sympathetic blocks can establish whether pains may be nonresponsive or variably responsive to such blocks, but are considered inappropriate in determining a clinical diagnosis. 2. Neuropathic Pain Therapy: (a) A critical review of the literature regarding the use of sympathetic blocks in the treatment of acute herpes zoster pain and in the treatment of postherpetic neuralgia found little support for the widely held view that sympathetic blocks reduced either the incidence of long-term reduction of pain in these disorders. Further attempts to reduce PHN by the combination of blocks with aggressive drug therapies during acute herpes infection are suggested. (b) CRPS (RSD) treatments are seen as evolutionary at present, with the role of sympathetic blocks being only part of a balanced pain treatment strategy aimed at getting patients activated under cover of good analgesia and improved function. These proposals come as consensus recommendations but are not substantiated by outcome studies. 3. Ischemic Pain: Permanent sympathetic block with neurolytic or thermocoagulation techniques provides up to 50% long-term improved blood flow and reduction of pain and ulceration for patients with advanced peripheral vascular disease. This is particularly appropriate at lumbar levels in which percutaneous techniques are safe when conducted with real time imaging control.

Changes in the understanding of CRPS disorders and the role of the sympathetic nervous system in neuropathic pain has changed both the diagnostic and management strategies for these pain states. The sensitivity and specificity of response to sympathetic blocks in establishing their value at diagnostic aids will not be fully established without further clinical study. Further use of intravenous regional blocks or diagnostic intravenous infusions remains questionable. Preventive and therapeutic use of sympathetic blocks in herpes zoster pain remains open to well-controlled study.