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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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Chapter 45 – Implantation of Spinal Drug Delivery Systems

James P. Rathmell, M.D.

PERSPECTIVE

Following thousands of years of empiric use of opioids for treating pain, in 1971 a class of highly specific opioid receptors was identified. Soon thereafter, opioid receptors were localized in the brain and spinal cord. Yaksh developed a model of chronic catheterization of the spinal subarachnoid space in experimental animals; and evidence that direct application of morphine at the spinal cord level produces selective spinal analgesia soon followed. Based on this limited experimental evidence Wang and colleagues (1979) administered bolus intrathecal morphine, and Onofrio (1981) used chronic intrathecal morphine infusions, both working in patients with severe pain associated with advanced cancer.

Since these first, bold clinical experiments, we have witnessed a rapid transition from the laboratory to clinical practice. Intrathecal morphine and other opioids are now widely used as useful adjuncts in the treatment of acute and chronic pain, and a number of agents show promise as analgesic agents with spinal selectivity. Continuous delivery of analgesic agents at the spinal level can be carried out using percutaneous epidural or intrathecal catheters, but vulnerability to infection and the cost of external systems typically limits them to short-term use (less than 6 weeks). Reliable implanted drug delivery systems are available that make long-term delivery of medications to the intrathecal space feasible. These systems are comprised of a drug reservoir/pump implanted in the subcutaneous tissue of the abdominal wall that is refilled periodically through an access port. The pump may be a fixed-rate, constant-flow device or a variable-rate pump that can be programmed using a wireless radiofrequency transmitter similar to those used for implanted cardiac pacemakers.

Patient selection for intraspinal pain therapy is empirical and remains a subject of some debate. In general, intrathecal drug delivery is reserved for patients with severe pain that do not respond to conservative treatment. Most patients with cancer-related pain have ongoing pain despite appropriate oral opioid therapy, or they may have developed intolerable side effects related to these medications. Randomized, controlled trials comparing maximal medical therapy with intrathecal drug delivery for cancer-related pain have demonstrated improved pain control and reduced opioid-related side effects in the studies that used intrathecal pain therapy. Intrathecal drug delivery has also been widely used for noncancer pain, particularly for the treatment of chronic low back pain. Use of this therapy for noncancer pain, however, has not been subject to controlled trials and remains controversial.

Once a patient is selected for intrathecal therapy, a trial is carried out. Most physicians now conduct trials by placing a temporary, percutaneous intrathecal catheter and infusing the analgesic agent over several days to judge the effectiveness of this therapy before a permanent system is implanted. Some carry out the trial of intrathecal therapy using a single dose or a continuous epidural infusion. The most common analgesic agent used for spinal delivery is morphine, which remains the only opioid approved for intrathecal use by the U.S. Food and Drug Administration.

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