Spinal Cord Stimulation
Neurostimulation, defined as “the enhancement of normal function or the reduction of dysfunction by modulating the electrical properties of the nervous system” by the International Neuromodulation Society, has had a long history. Ancient physicians were first to identify the healing powers of the torpedo fish. Dioscorides (A.D. 1559) reported that applying a marine torpedo to the top of the head relieved prolonged headache. It was noted that numbness was the sign of a cure. It wasn’t until the 1600s that electricity was identified as the force responsible for the effect.1 Despite this long history, the first theory proposed to explain the suppression of pain by electrical stimulation did not appear until 1965 when Melzack and Wall proposed the gate control theory.2 This theory proposed that the activation of low-threshold myelinated primary afferent fibers decreases the response of dorsal horn neurons to unmyelinated nociceptors. Shealy et al,3 were the first to apply this theory in practice when they electrically stimulated the dorsal columns to treat chronic, intractable pain. Since the first implant, dorsal column stimulation (or spinal cord stimulation, SCS) has been applied to a wide variety of painful disorders. These include tumors, brachial plexus injuries, cord injury, phantom limb pain, reflex sympathetic dystrophy, ischemic limb pain, multiple sclerosis, peripheral vascular disease, arachnoiditis and pain after failed spine surgery.4-11 It has been estimated that 12,000 SCS systems are sold every year worldwide.12
Although first inspired by the gate control theory, spinal cord stimulation is now linked to several other mechanisms. It has been found to activate spinal pain inhibitory circuits, mainly those concerned with Gamma-Amino-butyric Acid (GABA) and adenosine transmission.13-15 SCS also has been shown to have an effect at the cerebral level.16,17 SCS has been shown to increase regional cerebral blood flow in the left ventrolateral periaqueductal gray, the medial prefrontal cortex, the dorsomedial thalamus bilaterally, the left medial temporal gyrus, the left pulvinar of the thalamus, bilaterally in the posterior caudate nucleus and the posterior cingulate cortex. In addition to central increases in blood flow, studies in animals and humans have shown that SCS can induce peripheral vasodilation. Even though the exact mechanism in which SCS produces its effect remains under debate, it has become an indispensable therapeutic tool for treating many chronic pain conditions.
Spinal cord stimulation systems are relatively simple to implant, with many of the stimulation parameters under patient control. This has lead to its use in a wide array of painful conditions without regard to the etiology or pathophysiology.18 As a result, numerous studies report success rates of fewer than 25%. According to the European Group for the Study and Treatment of Pain, only 23% of the preliminary cases reported long-lasting pain relief using SCS.19 The main reason for this low success has been the diverse group of pain conditions typically treated with SCS and the weak patient selection criteria that have been used. Only recently have more stringent selection criteria been followed. It is now recognized that the most appropriate patients for SCS are those with chronic, nonmalignant pain of neuropathic origin.20 Another important selection criterion is psychological attitude. Patients are now routinely screened to eliminate those with major personality disorders, secondary gain issues or drug abuse problems.21-23
Improved patient selection has increased the success rate of SCS. Some of the indications that have been shown to be successfully treated with SCS include failed back surgery syndrome, CRPS I and II, central or peripheral neuropathic pain disorders, angina pectoris and peripheral vascular disease. Recent reports of the results of a series on failed back surgery syndrome and neuropathic pain of peripheral origin showed good long-term outcome in 50-60% of cases treated.23,24 Kumar et al5 also reported that SCS is an effective therapy for pain syndromes associated with peripheral neuropathy. Numerous reports have shown success when using spinal cord stimulation to treat the pain from reflex sympathetic dystrophy (or complex regional pain syndrome I).25-27 A report by Stanton-Hicks et al28 included neuromodulation in the guidelines for therapy for complex regional pain syndromes. Spinal cord stimulation also has been found to be highly effective in treating the pain from angina and peripheral vascular disease.27,29 Patients with peripheral vascular disease have had success rates of 50-80% for the relief of pain, with evidence of improved circulation.27,30
Two different SCS systems are routinely used, including those systems that use percutaneously placed leads or those that require laminectomies to place the leads. The former involves the percutaneous insertion of the lead into the epidural space (see Figure 1). The lead is then connected to an external generator, allowing a trial period of stimulation, or it may be connected subcutaneously to an implanted radio frequency (RF) controlled receiver or to a totally implanted pulse generator (IPG).
Paddle type leads require implantation into the epidural space via laminectomy (see Figure 2). The leads are then connected subcutaneously to a radio-controlled receiver or an IPG. The RF-controlled receiver is activated by an external battery-powered transmitter, which operates through an antenna placed over the receiver. The IPG contains a battery which supplies power to the electrodes.
To effectively treat pain, a spinal cord stimulation system must have the potential to target the anatomic areas where the patient feels pain. The target area must be provided with pain-relieving paresthesia at tolerable and patient-adjustable intensity levels.
With the patient under local anesthesia, a small puncture is made in the skin with a paramedian approach at an angle of no more than 30-40 degrees. An epidural needle is inserted and confirmed as having entered into the epidural space. Using fluoroscopic guidance, a lead blank (a lead without any electrodes) is inserted through the needle into the dorsal epidural space, and is manipulated to establish an appropriate pathway. The lead is then introduced into the epidural space, either through the needle or through the use of a lead introducer. Proper lead placement is verified through intraoperative trial stimulation, in which paresthesia is experienced by the patient. Upon verification of proper lead placement, the lead is secured using a lead anchor and sutures.