Capsaicin and the cause of causalgia

Defining the therapeutic role of local anesthetic sympathetic blockade in complex regional pain syndrome: a narrative and systematic review

OBJECTIVE

There is growing controversy on the value of blocking the sympathetic nervous system for the treatment of complex regional pain syndromes (CRPS). The authors sought to evaluate the efficacy of sympathetic blockade with local anesthetic in these syndromes. In addition, they performed a comprehensive review of the pathophysiology and other treatments for CRPS.

DESIGN

Systematic review of the literature was performed. MEDLINE was searched from 1966 through 1999. The authors identified only three randomized controlled trials (RCTs) that evaluated sympathetic blockade with local anesthetic, but because of differences in study design they were unable to pool the study data. The authors therefore included nonrandomized studies and case series.

INTERVENTIONS

Studies were included if local anesthetic sympathetic blockade was used in at least 10 patients. Studies were excluded if continuous infusion techniques, somatic nerve blocks, or combined sympatholytic therapies were evaluated.

OUTCOME MEASURES

Pain relief was classified as full, partial, or absent. The lack of a comparison group in the studies allowed only the calculation of distribution of the response categories, and the sum of the pooled rates does not equal 100%.

RESULTS

Twenty-nine studies were included that evaluated 1,144 patients. Nineteen studies were retrospective, 5 prospective case series, 3 RCTs, and 2 nonrandomized controlled studies. The quality of the publications was generally poor. Twenty-nine percent of patients had full response, 41% had partial response, and 32% had absent response. It was not possible to estimate the duration of pain relief.

CONCLUSIONS

This review raises questions as to the efficacy of local anesthetic sympathetic blockade as treatment of CRPS. Its efficacy is based mainly on case series. Less than one third of patients obtained full pain relief. The absence of control groups in case series leads to an overestimation of the treatment response that can explain the findings.

Sympathectomy for causalgia: experience with military injuries

BACKGROUND

Causalgia is a rare disease in civilian practice, and most reports regarding causalgia in the literature are from major extended wars. To increase awareness of this syndrome, our wartime experience with this disease is presented.

METHODS

The charts of patients with causalgia referred to two university hospitals for treatment from 1985 to 1989 were retrospectively studied. Characteristics of the cause, clinical manifestations, and the response to therapy were evaluated.

RESULTS

Among 1,564 patients with peripheral nerve injuries, there were 54 cases (3.4%) of causalgia. All of the patients, except five, were injured in battle by high-velocity bullets or missiles. Upper extremities were involved in 28 patients (52 %) and lower extremities in 26 patients (48%). The most common presenting symptoms were as follows: burning pain, 100%; wet extremity, 100%; cold extremity, 93%; sensitivity to cold, 89%; paresthesia, 78%; and color changes in the extremities, 55%. In 48 patients (89%), pain was relieved by sympathetic block (3 patients had permanent cure). Six patients had no response to the blocks (11%). Of 45 patients who had temporary relief, all underwent sympathectomy. All of these patients had complete relief of symptoms in the immediate postoperative period and for follow-up from 1 to 6 years.

CONCLUSION

Causalgia is essentially a war casualty disease. The condition is associated with burning pain, hyperesthesia, and symptoms of sympathetic overactivity. Sympathectomy is effective and the treatment of choice, particularly for patients who respond temporarily to sympathetic blocks.

Vanilloid receptors: new insights enhance potential as a therapeutic target

Compounds related to capsaicin and its ultrapotent analog, resiniferatoxin (RTX), collectively referred to as vanilloids, interact at a specific membrane recognition site (vanilloid receptor), expressed almost exclusively by primary sensory neurons involved in nociception and neurogenic inflammation. Desensitization to vanilloids is a promising therapeutic approach to mitigate neuropathic pain and pathological conditions (e.g. vasomotor rhinitis) in which neuropeptides released from primary sensory neurons play a major role. Capsaicin-containing preparations are already commercially available for these purposes. The use of capsaicin, however, is severely limited by its irritancy, and the synthesis of novel vanilloids with an improved pungency/desensitization ratio is an on-going objective. This review highlights the emerging evidence that the vanilloid receptor is not a single receptor but a family of receptors, and that these receptors recognize not simply RTX and capsaicin structural analogs but are broader in their ligand-binding selectivity. We further focus on ligand-induced messenger plasticity, a recently discovered mechanism underlying the analgesic actions of vanilloids. Lastly, we give a brief overview of the current clinical uses of vanilloids and their future therapeutic potential. The possibility is raised that vanilloid receptor subtype-specific drugs may be synthesized, devoid of the undesirable side-effects of capsaicin.