Spinal Cord Stimulation: Fundamentals
Spinal cord stimulation (SCS) has been used in pain management since C. Norman Shealy, MD, PhD, implanted the first neuroaugmentive device in a cancer patient in 1967.1-3 Since then, several studies have examined the long-term effects of SCS in pain management, with variable outcomes and success rates.4-8 As with many novel procedures, initial problems with SCS included poorly designed hardware, inadequate patient selection criteria, and suboptimal surgical techniques.7,9-11
Significant advances in SCS, however, have been made in recent years. Postoperative outcomes of the procedures have shifted to more positive outcomes in the field of neuroaugmentation, especially with respect to such practical factors as return to work, reduction in medication use, reduction in visual analog pain scores, and improvement in activities of daily living. The hardware is more durable, more effective, more maneuverable, and provides a greater range of coverage for the affected area. The devices can be implanted percutaneously under fluoroscopic guidance (especially for the trial leads placement), which allows operator–patient verbal interaction and more accurate positioning of spinal cord stimulator leads for trial and eventual permanent placement. In addition, more than three decades of experience have provided improved patient selection criteria, which is paramount in effecting a positive eventual outcome. The net result is an improved capability to control various chronic pain conditions, especially those that are peripherally referred as opposed to centrally referred.8
This article will discuss the pathophysiology, mechanism of action, and clinical applications of SCS; as well as current clinical results, and potential future trends in SCS, also known as dorsal column neuromodulation.
SCS Mechanism of Action
Although the exact mechanism for pain control from SCS is not entirely understood, it is believed to result from direct or facilitated inhibition of pain transmission.3,5-7,12,13 Table 1 lists the five mechanistic theories for why SCS works.13
The gate control theory motivated Shealy et al to apply SCS as a means to antidromically activate the tactile myelinated A-β fibers through dorsal column stimulation.1,2 Shealy reasoned that sustained stimulation of the dorsal columns would keep the gate closed and provide continuous pain relief. While the theoretical "gate control" model put forth by Melzack and Wall has been shown not to be precisely correct, pain gating or pain control has been shown to exist.5-7,12
Others believe that pain relief from SCS results from direct inhibition of pain pathways in the spinothalamic tracts and not secondary to selective large fiber stimulation.14 This theory has been supported by Hoppenstein, who showed that the posterolateral stimulation of the spinal cord provided effective contralateral pain relief with substantially less current than posterior stimulation.15
Some investigators think that the changes in blood flow and skin temperature from SCS may affect nociception at the peripheral level.16-20 This postulation is further supported in part by data from Marchand et al, who investigated the effects of SCS on chronic pain using noxious thermal stimuli.21 Since it was discovered that SCS causes vasodilation in animal studies, clinicians have used this modality for the treatment of chronic pain due to peripheral vascular disease and is the leading indication for SCS in Europe today.13,15,22-26 The precise action of pain modulation by SCS is still in debate. A better understanding of the pain system may lead to more effective stimulators and allow for even greater success.
Today, the most common indication for SCS is for the treatment of chronic low back and lower extremity pain due to chronic radiculopathy or postlaminectomy lumbar pain syndrome despite adequate surgical intervention.27-32 This population represents the primary indication for SCS in our practice and has provided us with an effective treatment option. Table 2 contains a list of commonly accepted and potential indications, in addition to commonly accepted contraindications.
As noted, proper patient selection is essential to the long-term success of a spinal cord stimulator system.7,9-11 Technical advances leading to improved hardware, coupled with improved patient selection, have improved the rate of long-term efficacy of SCS to approximately 70% today, up from approximately 40% since the 1970s and 1980s.3,4,8
A spinal cord stimulator neuromodulation system should be considered for patients who have failed all reasonable conservative care including appropriate diagnostic, therapeutic, and rehabilitative techniques, and have been given a reasonable period of time to recover from the condition.8 A reasonable time period is at least 6 to 12 months of conservative, pain-relieving, minimally invasive treatments, and/or failure of surgical treatments, with persistent extremity pain greater than axial spine pain.
An ideal patient should be motivated, compliant, and free of drug dependence.33 Psychological screening is recommended but not mandatory to exclude conditions that predispose to failure of the procedure. Diagnoses that are typical indications for this procedure include chronic radiculopathy, perineural fibrosis, neuropathic pain, and complex regional pain syndrome.34-38 In the United States, peripheral vascular disease is not an FDA-approved indication.
When considering pain topography, extremity pain responds better than axial pain, and the more distal the extremity pain the greater the clinical response.27,39 Middle and upper lumbar pain as well as thoracic, cervical, and chest wall pain are difficult to adequately control and maintain long term. Pain due to severe nerve damage superimposed on cutaneous numbness (ie, anesthesia dolorosa) is also difficult to treat with SCS. Central pain syndromes do not respond to SCS and are best treated by other modalities.
The use of an outpatient percutaneous trial of between 3 to 7 days with an SCS system has been proven helpful in determining which patients will respond well enough to warrant a permanent spinal cord stimulator implantation and determine the future permanent implantation levels.27,28,39,40 Absolute criteria that must be present for a patient to have a positive trial include tolerance of paresthesia, >50% to 75% pain relief, and overall patient satisfaction. Relative requirements for a positive trial include improved functional level, reduced usage of pain medication, and reduced reliance on the healthcare system.