Spinal cord stimulation should no longer be considered the treatment of “last resort.” Long-term success rates reach 85% if SCS is performed within 2 years of symptom onset.
No contemporary discussion on the management of chronic pain is complete without consideration of spinal cord stimulation (SCS). SCS has become an attractive addition to the pain management armamentarium because of its unique blend of efficacy supported by Level I evidence,1-6 cost-effectiveness, 7 and minimally invasive implantation technique. Moreover, its effects are completely reversible.
SCS involves placement of one or more lead(s), consisting of a longitudinal array of contacts (electrodes), into the dorsal epidural space using either a percutaneous technique or through a small laminotomy. The leads are powered by a battery (implantable pulse generator [IPG]), which delivers pulsed electrical energy. This electrical stimulation produces analgesia by inhibiting nociceptive transmission. A multitude of lead types with varying number of contact points and spacing are now available, enabling precise targeting of the area of pain, while avoiding undesirable stimulation.
The flexibility of the device is further augmented by improved programming capabilities, which allow adjustment of stimulation parameters such as pulse-width, frequency, anode/cathode contact configuration, and amplitude. These parameters are optimized by a neuromodulator and can be further modified by the patient within a specified range through a hand-held programmer.
The Food and Drug Administration (FDA) has approved SCS for treatment of chronic, intractable trunk and limb pain. There is widespread agreement among experts that patients who do not respond to conventional medical management by 12 to 16 weeks should be offered a trial of SCS.8
The mechanism by which SCS produces pain relief has yet to be fully elucidated. Initially, the analgesic properties of SCS were ascribed to Melzack and Wall’s Gate Control Theory. However, this model fails to explain key clinical observations, including the differential success of SCS in neuropathic versus nociceptive pain, lack of efficacy in acute pain, ability of pain relief to outlast stimulation, and the finding that activation of large afferent fibers can at times generate pain. 9-11
Current evidence suggests that the mechanism of action of SCS is complex and multifactorial, implicating both spinal and supraspinal pathways (Table 1). 9-11
The first step is to identify the etiology of chronic pain that is amenable to SCS. Common indications include failed back surgery syndrome (FBSS), complex regional pain syndrome (CRPS), peripheral vascular disease (PVD), refractory angina pectoris (RAP), post-herpetic neuralgia, cervical radiculopathy following disc surgery, and painful diabetic neuropathy. Novel indications are emerging and include occipital neuralgia, migraine, visceral pain, as well as ilioinguinal, genitofermoral, and intercostal neuralgias.
As noted, SCS should be considered after failure of 12 to 16 weeks of an interdisciplinary, multimodal medical management trial. Treatment should be performed under the supervision of a pain clinic and should include physiotherapy with psychological evaluation and treatment, if indicated. Clinicians should encourage patients to enroll in randomized control trials (RCTs) if eligibility criteria are met. SCS is also utilized in non-pain disorders such as incontinence and visceral motility dysfunction.
Patient selection is central to SCS outcomes. Although some degree of psychological suffering is expected in patients with chronic pain, any psychological concerns should be addressed prior to implantation. Clinicians must adopt a functional restoration approach, stressing the importance of patients’ returning to active domestic life and gainful employment. Realistic expectations must be fostered—patients must accept that SCS offers symptomatic improvement and does not correct any underlying anatomic deficit or remove the original pathology; pain control is the goal, total abolition of pain is unrealistic.
Contraindications to SCS include untreated severe psychiatric or psychological morbidity, including somatization; non-organic etiology of pain; unwillingness to stop improper drug use; inability to give informed consent or operate equipment; associated coagulopathy, site infection, or sepsis; as well as pending active litigation.
In our opinion, SCS should be considered a widely accepted therapy and no longer a treatment of “last resort.” Therefore, early consideration of SCS is warranted. The efficacy of SCS is time-sensitive. There is an inverse relationship between implantation delay and long-term therapeutic success. Long-term success—defined as ≥ 50% reduction in baseline pain—is attainable in 85% of cases if implantation is achieved within 2 years of symptom onset, but declines to 9% if implantation delay is ≥ 20 years. Today, the average wait time for SCS implantation is 5.45 years. This has generated long-term success-rates of ~50%.12
A neuromodulation team—consisting of a neuromodulator, pain physician, and nurse—can optimize preoperative care and long-term follow-up.
The degree of pain control from SCS is difficult to predict based on clinical characteristics. Therefore, all patients being considered for SCS first must undergo a trial procedure. About 15% to 18% of patients fail an SCS trial.12
Figure 1 provides images of a SCS trial in a patient. The procedure can be performed in an outpatient setting under conscious sedation using cylindrical leads placed under fluoroscopic guidance. The lead to be used can be a temporary lead or a permanent lead with an extension; these leads are connected to an external pulse generator. Placement of the leads are as follows:
During intraoperative testing, stimulation-induced paresthesia should cover 80% of the dermatomal distribution of pain, otherwise outcomes are less than optimal. During the trial phase (which averages 1 week), the patient should achieve ≥50% decrease in pain from baseline as proof of efficacy. The patient may adjust stimulation parameters with a hand-held programmer to maximize pain control.
If a patient achieves the desired pain control, he/she goes onto permanent implanation of SCS. If permanent leads were used for trial, they can now be internalized; temporary leads, however, must be discarded. Internalization of the leads is accomplished by anchoring leads to the supraspinous fascia and tunneling them to an IPG, which is surgically placed in the gluteal region or abdominal wall. Both rechargeable and non-rechargeable IPGs are available. The trial log documents power consumption needed to achieve pain control—a strategy that may aid selection of a rechargeable versus non-rechargeable IPG.
For more information on the field of anesthesiology, visit the American Society of Anesthesiologists online at asahq.org. To learn more about the role physician anesthesiologists play in ensuring patient safety, visit asahq.org/WhenSecondsCount. Join the ANESTHESIOLOGYTM 2013 social conversation today. Like ASA on Facebook, follow ASALifeline on Twitter and use the hashtag #ANES2013.