123 Spinal Cord, Peripheral Nerve, and Page 1 Peripheral Nerve Field Stimulation for the Treatment of Pain Timothy R. Deer, M.D. Charleston, West Virginia INTRODUCTION: Stimulation of the nervous system is an option to treat many medical conditions including cardiac arrhythmias, tremor, and depression. This section will focus on the neuromodulation of the nervous system to change neural transmission and perception relating to pain. Electrical current can be applied at the level of the brain, spinal cord (SCS), peripheral nerves (PNS), and peripheral nerve fields (PNFS). HISTORY OF ELECTRICAL CURRENT FOR PAIN TREATMENT: The first recorded use of electrical current to treat pain was in 15 AD when the torpedo fish was discovered to treat the symptoms of gout in a patient exposed to the eel like creature. Gilbert, a 17th century scientist, advanced the use of electricity when he published the use of lodestone, which is a piece of magnetic iron ore possessing polarity like a magnetic needle, as a treatment for headaches, mental disorders and marital infidelity. In 1745, Musschenbroek described the ability to store electricity in a capacitance device called a Leyden jar. This jar was used by Jallabert to stimulate muscle and increase blood flow. Benjamin Franklin advanced these ideas by using electrical current to treat many painful disorders such as headaches, arthritis and back pain. Franklin concluded that the use of high voltage electricity caused more pain than it relieved. The use of electricity in humans was then out of favor until several decades later when Volta explained the chemical interactions that occurred in the body with low voltage electrical exposure. This revelation led to the work that is still relevant in modern day patient care with low voltage current. These concepts were studied by Ampere, Faraday, and Clarke, but the use of current to treat pain was still limited and poorly understood. In 1840, the work of Guillaume Duchenne took the use of electricity to the next level in human application. Duchenne used electropuncture, or application of electricity to small needles inserted directly into muscles to observe that closing of the circuit caused contraction of specific muscles allowing for exact mapping of muscle function. Duchenne summarized his thoughts in the book “De L’electrisation Localise” where he described direct muscle stimulation and indirect nerve stimulation. This work led to the development of early prostheses that used surface electrodes to move the body part. The first successful use of these same concepts for pain did not occur until over one hundred years later when Norman Shealy described the use of Spinal Cord Stimulation for the treatment of pain. In 1968, FDA approval was achieved to treat pain with these devices. The devices were crude bipolar devices with little ability to be individualized to meet patient needs. In the next four decades major advances have been made and the use of stimulation is now entering an exciting time in its history in regard to patient application, technology and science. SPINAL CORD STIMULATION (SCS): SCIENTIFIC PRINCIPLE: Spinal cord stimulation involves the placement of a lead over a neural target in the spine achieved by epidural placement. Once the lead is targeted to the appropriate fibers a power source is attached to deliver energy to the leads to produce an electrical field. By using a programmable generator the amplitude, rate, frequency and shape of the electrical field can be manipulated to create pain relief. The mechanism of neural effect has been theorized to change the balance of inhibitory to excitatory fiber activity by the gate control process, by manipulating the number and position of cathodes and anodes on the lead. INDICATIONS FOR STIMULATION: In the United States, the Food and Drug Administration has approved this therapy for the treatment of moderate to severe pain in the trunk or limbs. The specific indications for which these devices are most commonly used have been well defined. The most common indication for spinal cord stimulation is failed back surgery syndrome. Other common reasons patients undergo these surgeries include cervical and lumbar radiculitis, complex regional pain syndrome, peripheral neuropathies, post herpetic neuralgia, ischemic limb pain, angina, pelvic pain and other neuropathic and visceral pain syndromes. 123 Page 2 PATIENT SELECTION: Patients are appropriate for spinal cord stimulation when they have failed more conservative treatment or when these measures are not acceptable. The patient should be mentally stable, have no untreated drug addiction, have no untreated clotting disorders, and have no local infection at the procedure site or untreated systemic infections. The patient should undergo a trial of stimulation and significant relief during that time period. The trial can vary in length from a few hours to several weeks with most lasting three to seven days. PAIN TREATMENT ALGORITHM: In the past, spinal cord stimulation was relegated to an end of care option for those that have failed all other options. Several factors have changed this antiquated thinking. These changes include the simplification of stimulation trialing which now can be done via a percutaneous skin puncture with no incisions, monitored care and same day discharge. Other major factors include a recent randomized study by North that has shown that spinal cord stimulation is superior to a second back surgery in regard to pain relief, satisfaction, and need to have the other therapy. Recent analyses of long acting opioids for chronic non-cancer pain have shown poor long term outcomes regarding pain relief. These factors coupled with increasing national trends of addiction and abuse has led many clinicians to choose spinal cord stimulation as an earlier option. Recently, a consensus group of experts met to review the evidence for use of these technologies. The recommendation was to advance stimulation to an earlier position in the algorithm for pain of spinal origin. Figure one shows the antiquated algorithm, and Figure two, the algorithm for 2007 and the near future. As the figures indicate the use of stimulation should be considered much earlier in those with spinal related pain syndromes. The device should be used prior to the second or subsequent surgeries unless neurological compromise is a risk. Spinal cord stimulation should be an option instead of the initial spine surgery when surgery isn’t imminent or emergent. Chronic long acting opioids have been recently questioned as to efficacy in improving function or long term pain relief. Figure 1. Old Continuum The old continuum placed SCS at the end of the algorithm equal with intrathecal drug delivery and Neuroablative surgeries. This placement was based on the initial approval of SCS when it always involved a surgical laminotomy. Recent percutaneous techniques have made these therapies much less invasive. Figure 2. New Continuum The new continuum advances SCS to a more appropriate place in the algorithm. SCS should be considered after nerve blocks, physical therapy and other conservative measures fail. Psychological evaluation should occur prior to permanent SCS placement and spine surgery should be considered if loss of neurological function is a risk. 123 Page 3 SPINAL CORD STIMULATION: THE PROCEDURE GENERAL PRINCIPLES: The patient should be prepped and draped widely prior to starting the procedure. The prep solution should be known to kill the common pathogens of your institution. Intravenous antibiotics should be given prior to incision. In most settings, the antibiotic of choice is a third generation cephalosporin with broad coverage including the cerebral spinal fluid. In high risk patients such as those with immunosuppression the use of vancomycin is often recommended. Intraoperative antibiotic irrigation commonly includes bacitracin or kanamycin. Postoperative antibiotics are controversial, but most clinicians prefer cephalosporins. Positioning should optimize spinal exposure, provide comfort to the patient and maintain sterility. The patient can be sedated, but should remain conversant and responsive to report paresthesias. NEEDLE PLACEMENT: The angle of the needle is important to improve the ability to drive the lead to the proper neural target. An angle of 30 degrees is ideal for epidural entry. Steeper angles increase the risk of wet tap and nerve injury. A paramedian approach is preferred, with a skin entry site two levels below the desired entry space. The needle entry should be two to three levels below the final lead target. In the cervical spine the needle entry at or below the C7-T1 space. LEAD PLACEMENT: The lead should be driven to the posterior epidural space at the desired level of stimulation. If the patient reports pain or paresthesia during the placement the lead should be retracted and repositioned. The lead can be guided using a straight or curved stylet. Fluoroscopic guidance should show good placement on both antero- posterior and lateral views. LEAD TARGET: The physician should understand the target for the led to achieve proper stimulation. Table one provides general targets for spinal cord stimulation. Table 1. Lead Placement for Anatomical Stimulation Region Position Target Cervical C2 Lateral Mandible, Neck C2-3 Shoulder C4-6 Arm C7 Anterior Shoulder, Chest Thoracic T1-3 Angina T4-6 Visceral Abdomen T7-9 Axial Back T10 Knee, Hip T11-12 Leg Lumbar L1 Foot L5-S1 Foot Nerve Root Sacral S2-4 Pelvis, Rectum, Perineum LEAD PROGRAMMING: Current is driven into the cord based on the presence of a cathode. The shape of the effect of stimulation on the spinal cord is based on the cathode and anode array, and the number of cathodes and anodes present. 123 Page 4 LEAD ANCHORING: Once the lead is in good position it should be secured to the tissue to avoid later displacement. The desired tissue is ligament and fascia. Prior to securing the anchor all adipose tissue should be débrided. Non-absorbable suture is then used to attach the lead to the spine with an appropriate silastic anchor. Once the lead is anchored a strain relief loop should be placed in the wound. The best way to anchor the lead has been debated. Many modern anchors have mechanical components that assist with keeping the lead in position. The highest risks of migration occur in the first six weeks post implant. POCKET FORMATION: The position of the pocket is based on patient preference and body habitus. Options include the buttock, abdominal wall, flank, and chest wall. The depth of the pocket should be appropriate to avoid skin erosion, but should also assure good communication with telemetry. When creating a pocket the physician should consider the belt line and bony landmarks. COMPLICATIONS OF SCS: The most common complications of spinal cord stimulation include lead migration, superficial infection, and connection abnormalities, wet tap, and nerve irritation. More serious problems include epidural hematoma, epidural abscess, paraplegia, and death. The number of complications can be reduced by sterile technique, preoperative assessment, and proper selection. PERIPHERAL NERVE STIMULATION (PNS) AND PERIPHERAL NERVE FIELD STIMULATION (PNFS) SCIENTIFIC PRINCIPLE: In some disease states the nerve that is involved in the generation of pain is easily stimulated in the periphery. This may be performed by placing the lead directly over the nerve or by stimulating the fibers of the nerve as it courses in the tissue. The theory of this technique is that the device can effect the transmission of pain signals via A delta and C fibers. PNS: Peripheral nerve stimulation is performed by identifying the nerve involved in the pain transmission and directly applying current to the structure. In order to perform PNS, the surgeon has to dissect and identify the nerve. At this point a fascial graft is placed over the nerve to insulate the fibers from direct stimulation. This technique is technically challenging and fraught with problems. In most clinical disease states the use on PNFS has become more common. PNFS: The occipital nerve, ilioinguinal nerve, cluneal nerve, and intercostal nerves are receptive to stimulation of their peripheral fibers in lieu of stimulating the entire nerve. In order to perform this technique, the nerve field is mapped out by exam, the tissue is prepped and draped, local anesthesia is applied and the needle is placed just below the dermis in the subcutaneous tissue. If the needle is too superficial the lead can erode. If the lead is too deep, the nerve fibers are missed. Table two shows the potential targets for peripheral nerve stimulation. COMPLICATIONS OF PNS AND PFNS: The risks of these techniques are limited. They include cellulitis, peripheral nerve injury and mechanical dysfunction of the leads and generator. 123 Page 5 Table 2. Targets for PNS and PNFS Placement Disease Nerve Target Occipital Neuralgia C2 Fibers at the Occiput Neuritis of the Face Supraorbital, Infraorbital Temporal-Auricular, Trigeminal Upper Extremity Pain Median, Ulnar, Radial, Axillary Pain of Torso Intercostal and Thoracoabdominal Pain of Pelvis Ilioinguinal, Iliohypogastric Pain of Lower Extremity Common Peroneal, Superficial Peroneal, Lateral Femoral Cutaneus, Tibial, Saphenous NEUROMODULATION AS A DISEASE MANAGEMENT STRATEGY: Many physicians and patients are excited about the use of neuromodulation to change patient’s function and improve the quality of life. Many uses of this technology are already clinically available, but many more therapies are on the horizon. Figure three below illustrates the current state of these technologies. Targets include the motor cortex, thalamus, peripheral nerves, cranial nerves and new areas of the spinal cord. Figure 3. Approved and Future Neuromodulation Indications 123 Page 6 SUGGESGTED READING: 1. North R, Kidd D, Farraokhi F, Piantadosi S. Spinal cord stimulation versus repeated lumbosacral spine surgery for chronic pain: a randomized, controlled trial. Neurosurgery. 2005;56(1):98-106. 2. Mekhail N, Aeshbach A, Stanton-Hicks M. Cost Benefit analysis of neurostimulation for chronic pain. Clin J Pain. 2004 Nov-Dec:20(6): 462-8. 3. Taylor R, Van Buyten J, Buscher E. Spinal cord stimulation for complex regional pain syndrome: a systematic review of the clinical and cost-effectiveness literature and assessment of prognostic factors. Eur J Pain. 2006 Feb;10(2):91-101 4. Melvin E, Jordan F, Weiner R, Primm D. Using peripheral stimulation to reduce the pain of c2- mediated occipital headaches: a preliminary report. Pain Physician. 2007 May;10(3):453-60. 5. Levy R. Deep brain stimulation for the treatment of intractable pain. Neurosurg Clin N Am. 2003 Jul;14(3):389-99, vi. 6. Deer T. Current and future trends in spinal cord stimulation for chronic pain. Current pain and headache reports 2001, (5): 503-509. 7. Deer T, Levy R, Feler C, Krames E, Hassenbusch S, Racz G, Kim C, Bowman R, Caraway D, Staats P, Raushbaum R, Burton A, Sitzman T, et al. A consensus of the use of Spinal Cord Stimulation for the treatment of pain in pain of spinal origin. Consensus conference on spinal cord stimulation. December 2006. Dallas, Texas.
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