Spinal cord stimulation: does frequency matter?

243 August 2013 S PINAL cord stimulation was first described in 1967 in an intriguing case report provided by Shealy et al.1 This first case involved the relief from cancerrelated pain using a surgically implanted electrode sutured to the dura to provide stimulation of the dorsal columns. Although the entire treatment was only for approximately 1 day before the patient’s demise, stimulation using 10–50 Hz current provided pain relief to this unfortunate individual. This work followed the slightly earlier publication of Melzac and Wall’s gate control theory2 and might be regarded as one of the earliest clinical applications of that hypothesis. Since that time spinal cord stimulation has become far more sophisticated; clinicians and patients are now able to select several different types of pulse generators and electrode styles. Particularly important advances have been made in the use of multiple electrodes, the design of the contacts, and in the availability of different parameters of current delivery. Likewise, the indications have expanded to include various forms of neuropathic pain, mixed-type pain such as that associated with failed back surgery syndrome, as well as pain from ischemic limbs and myocardium.3–5 Despite the expanding use of this form of therapy, the precise mechanisms underlying the analgesic effects remain unclear, and are probably multiple. Furthermore, many patients, who would otherwise seem to be good candidates for spinal cord stimulation, fail stimulation trials either because the approach does not provide pain relief or because the electrically induced paresthesias overlapping the painful region are found to be unacceptably uncomfortable for the patient. This mix of success and failure of spinal cord stimulation raises the question of whether we are using the optimal stimulation parameters in treating our patients. Using a rat model of spinal cord stimulation, Shechter et al.6 in this month’s ANeStHeSIoLogy demonstrate that the frequency of stimulation may be a critical factor. Recently, reports have begun to appear describing results of the use of high-frequency spinal cord stimulation (1–10 kHz), frequencies well above the firing rates supported by most neurons. The touted benefits of this technology include the lack of uncomfortable paresthesias and perhaps a broader spectrum of analgesic activity. Results of available clinical trials are not entirely clear at this time. For example, a reasonably large european study reported that 74% of patients implanted with high-frequency stimulation systems experienced greater than 50% relief from their back and leg pain as well as improvements in activity levels and analgesic usage.7 However, this was an open-label trial potentially suffering from the biases that accompany this type of study design. At least one randomized blinded study failed to demonstrate benefit of 5 kHz stimulation on low back and leg pain.8 Although having better quality clinical trials that will presumably inform us about the clinical utility of high-frequency spinal cord stimulation, there has existed Spinal Cord Stimulation