Treating Phantom Limb Pain Using Continuous Peripheral Nerve Blocks: A Department of Defense Funded Multicenter Study

Overview

When a limb is traumatically severed, pain perceived in the part of the body that no longer exists often develops. This is called "phantom limb" pain, and is different from "stump" pain, which is pain within the part of the limb that remains intact. Unfortunately, phantom pain resolves in only 16% of people, with the rest experiencing this pain for the remainder of the lives. There is currently no reliable treatment for phantom limb pain. The exact reason that phantom limb pain occurs is unclear, but when a nerve is cut-as happens with a traumatic amputation-changes occur in the brain and spinal cord that actually worsen with increasing phantom pain. These abnormal changes may often be corrected by putting local anesthetic-termed a "peripheral nerve block"-on the injured nerve, keeping any "bad signals" from reaching the brain, with resolution of the phantom limb pain. However, when the nerve block ends after a few hours, the phantom pain returns. But, this demonstrates that the brain abnormalities-and phantom pain-that occur with an amputation may be dependent upon the "bad" signals being sent from the injured nerve(s), suggesting that a very long peripheral nerve block-lasting many days rather than hours-may permanently reverse the abnormal changes in the brain, and provide lasting relief from phantom pain. Until recently, extending a peripheral nerve block beyond 16 hours was unrealistic. However, a treatment option called a "continuous peripheral nerve block" is now available. This technique involves the placement of a tiny tube-smaller than a piece of spaghetti-through the skin and next to the nerves supplying the amputated limb. The tiny tube may be placed with minimal discomfort in about 15 minutes. Numbing medicine called local anesthetic is then infused through the tube, blocking any signals that the injured nerve sends to the spinal cord and brain. Using a small, portable infusion pump, this prolonged nerve block may be provided in individuals' own homes. The ultimate objective of the proposed research study is to determine if a 6-day continuous peripheral nerve block provided at home is an effective treatment for persistent phantom limb pain following a traumatic limb amputation. The primary hypothesis (what the researchers predict) is that phantom limb pain intensity will be significantly decreased 4 weeks following treatment with a 6-day continuous peripheral nerve block.

Background. The combination of increased munitions force, use of improvised explosive devices, and casualty survival rates has resulted in a dramatic increase in the percentage of injured combat veterans living with a traumatic amputation. Of American Veteran amputees, 35-98% develop chronic, intractable pain perceived as being from the missing limb, a phenomenon termed "phantom limb pain." This pain resolves in only 16% of afflicted individuals, and there is currently no reliable treatment. The etiology of phantom pain remains unclear, but evidence suggests that severing a nerve provokes changes in the spinal cord, thalamus, and cerebral cortex. When neural input from an amputated limb is blocked with local anesthetic (a peripheral nerve block), cortical abnormalities and phantom pain frequently resolve. However, when the single-injection nerve block resolves after a few hours, the phantom pain returns. These findings demonstrate that cortical abnormalities and phantom pain may be maintained from abnormal peripheral input, suggesting that a peripheral nerve block of extended duration-lasting many days rather than hours-may permanently reorganize cortical pain mapping, thus providing lasting relief from phantom pain. A "continuous peripheral nerve block" (CPNB) involves the percutaneous insertion of a catheter directly adjacent to the peripheral nerves supplying an affected limb. Local anesthetic infused via the catheter(s) induces a completely insensate extremity for as long as desired without any systemic side effects. Additionally, CPNB may be provided on an ambulatory basis using a small, portable pump to infuse the local anesthetic. Objective. To determine if ambulatory CPNB is an effective treatment for intractable phantom limb pain following a traumatic limb amputation. Previously-published small, uncontrolled series describe patients immediately following surgical amputation whose phantom limb pain dramatically decreased or completely resolved with CPNB. We have data from a randomized, double-masked, placebo-controlled, crossover pilot study suggesting great promise treating intractable phantom limb pain with ambulatory CPNB: participants (n=3) experienced no change in their phantom pain following a 6-day infusion of normal saline; however, with a 6-day CPNB of potent local anesthetic administered 4 months later, subjects (n=2; one patient returned to duty before crossover infusion) experienced complete resolution of their phantom limb pain. Within the 12-week follow-up period, one subject experienced no phantom pain recurrence; and the other subject reported mild pain occurring once each week of just a small fraction of his original pain. Specific Aims. The primary hypothesis is that phantom limb pain intensity will be significantly decreased 4 weeks following an ambulatory CPNB (as measured by the Numeric Rating Scale of the Brief Pain Inventory). Study Design. We propose a multicenter, randomized, double-masked, placebo-controlled, simultaneous parallel and crossover, human-subjects clinical trial. We will include subjects with an existing upper or lower amputation who experience phantom limb pain at least daily for the previous 4 weeks. Catheter site(s) will be determined by amputation location. Subjects will be randomized to receive one of two study solutions in a double-masked manner: either a local anesthetic (ropivacaine 0.5%) or placebo (normal saline). Catheters will be removed after 6 days of at-home infusion. Although not required, each subject has the option to return for the alternative treatment four weeks later (crossover infusion). The primary endpoint will be the difference in average phantom pain intensity at baseline and 4 weeks following the initial infusion as measured with the Numeric Rating Scale between treatment groups for the initial infusion. Major secondary endpoints will involve intra- and inter-subject comparisons of additional measures of pain and health-related quality-of-life. Clinical Impact. From 2001-2006, over 70% of all U.S. military casualties endured a major limb injury, with an amputation rate of 28% within Operation Enduring Freedom alone. Previous conflicts have left tens-of-thousands of United States Armed Forces Veterans with missing limbs. CPNB are now relatively ubiquitous within the United States, but applied nearly exclusively to provide acute post-injury/surgical analgesia. If the proposed study demonstrates that ambulatory CPNB is a reliable treatment for intractable phantom limb pain, the resulting impact in treating the consequences of traumatic amputation will be immediate and profound, as healthcare providers within the United States Armed Forces and Veterans Affairs Medical Centers already have expertise placing and managing perineural catheters. Currently, CPNB is provided exclusively in the acute setting-to treat pain immediately following a battlefield injury or surgery-and not chronic, intractable phantom limb pain. However, because there is little technical difference in providing CPNB for acute versus chronic pain, the thousands of U.S. Veterans and active duty personnel suffering from intractable phantom pain could be treated relatively easily, rapidly, and with negligible additional costs.