1992 Bonica Lecture. Advances in chronic pain management since gate control.

OBJECTIVE

Two pain treatment systems that developed soon after the publication of the gate theory are probably a direct result of its publication: neuraxial opiate administration and electrical stimulation of the spinal cord and peripheral nerves and receptors. Although the use of these modalities has become widespread in managing chronic pain, there is disagreement about their long-term efficacy. This presentation will attempt to review the data regarding the mechanisms of action of these modalities and their efficacy in treating chronic pain of malignant and nonmalignant origin.

DATA SOURCES

Data were derived almost entirely from original articles reporting experimental data from both animal and human studies and from series of patients undergoing treatment with the modalities reviewed.

STUDY SELECTION

Where possible, controlled studies were selected. However, much of the available data regarding treatment results are uncontrolled.

DATA EXTRACTION AND SYNTHESIS

Selected data from studies that were felt to be reasonably well conducted are presented or summarized. Because of the lack of control groups in many of the clinical trials, meta-analyses were not carried out.

CONCLUSIONS

Long-term spinal opiate administration has been shown to be more effective than systemic opiates in some patients with cancer pain, but often must be combined with local anesthetics to provide satisfactory pain relief. Loss of effect over time is a significant problem. Since the identification of spinal opiate receptors and the introduction of spinally administered narcotics, a number of other receptors that are important in both sensitization and suppression of pain projection systems have been characterized. Agonists and antagonists to many of these receptors are being developed, and a few are available for clinical trials. Long-term electrical stimulation of the spinal cord produces substantial analgesia below the stimulated spinal segments in some patients with chronic pain. Although initial results are usually encouraging, long-term efficacy may be disappointing. It is postulated that analgesia associated with spinal stimulation is associated with both stimulation of large fiber ascending tracts and blockade of spinothalamic pathways. Transcutaneous electrical nerve stimulation (TENS) has come into widespread use in managing chronic pain and has had limited trials in cancer pain patients. It is well accepted by patients and physicians, but clinical studies of long-term efficacy have yielded variable results. The analgesic action is probably the result of both large afferent fiber activation and blockade of peripheral nociceptors.