DISCUSSION – SPINAL CORD STIMULATORS:
Spinal cord stimulation uses an electrical current to treat chronic pain. A small pulse generator is implanted in a subcutaneous pocket in the abdomen. It sends electrical pulses to the spinal cord via lead wires to the areas of the spinal cord involved in pain sensation. The electrical impulses interfere with the transmission of pain signals to the brain and theoretically lead to pain relief by stimulating the nerve fibers near the spinal cord masking the sensation of pain.
Prior to the implantation of a spinal cord stimulator device on a permanent basis a trial of stimulation is done to make sure there is a good response to stimulation before a commitment is made to a permanent device and a more invasive surgical procedure is performed. There are programmable systems and patient-controlled devices that allow adjustment in stimulation to respond to changes in the location or severity of pain. A temporary percutaneous lead is used and is connected to an external pulse generator. The trial is generally from three to seven days. If the patient has at least 50% improvement in pain during the trial the patient is considered a candidate for the permanent unit.
Specific criteria have been developed to determine who might be a candidate for a spinal cord stimulator (SCS). Indications generally accepted for SCS implantation include radicular pain syndrome, failed back surgery syndrome with neuropathic pain, chronic sciatic pain due to epidural fibrosis or aseptic adhesive arachnoiditis, Complex Regional Pain Syndrome (Reflex Sympathetic Dystrophy), stump pain, angina and peripheral vascular disease.
SCS is not usually a treatment option considered until after conservative, non-invasive treatment modalities have failed. Such modalities would include but not be limited to pharmacologic treatment with non-steroidal medications, opioids, antidepressants and topical preparations; physical modalities such as ultrasound, TENS unit, diathermy, epidural/spinal blocks; chronic pain rehabilitation; and physical therapy. SCS is not considered a treatment choice if there is a history of poor compliance with treatment, alcohol or drug addiction or psychological concerns.
In regards to cost effectiveness one study compared spinal cord stimulation with medical treatment/conventional pain therapy over a five year period. The actual costs incurred in diagnostic imaging, professional fees paid to physicians, implantation, nursing visits for maintenance of the stimulators, physiotherapy, chiropractic treatments, massage therapy and hospitalization for treatment of breakthrough pain were considered. The mean cumulative cost for spinal cord stimulator therapy for the five-year period was $29,123 compared with $38,029 for the conventional pain therapy. However, in most cases, SCS was implemented only after many of the costs of traditional pain management had already been incurred.
With a goal of pain reduction, the long-term efficacy of SCS is still being debated. Richard North, M.D. of the Department of Neurosurgery at The Johns Hopkins University School of Medicine has authored several articles relating to the efficacy of SCS use.
In the first study performed a series of 50 patients with failed back surgery syndrome who underwent SCS implantation were interviewed at mean follow-up intervals of 2.2 years and 5.0 years. Successful outcome, defined as at least 50 percent sustained relief of pain and patient satisfaction was recorded in 53% of patients at 2.2 years and in 47 percent of patients at 5.0 years. There was no attempt to judge change or improvement in functional status.
A long-term follow-up study conducted by North (mean follow-up seven years) performed several years later reviewed 320 patients treated with SCS devices between 1972 and 1990. Of a final sampling of 171 patients, 52% reported at least 50% relief of pain and 60% stated they would undergo the procedure again even though they continued to have some pain. In the same study by North 22% reported that they had never experienced as much as 50% relief of pain and seven percent claimed never to have experienced any relief at all. Six months after permanent implant, the fraction of patients reporting less than 50% pain relief was 40%.
A literature review by Turner concluded that 59% of patients who had undergone SCS for low back pain had at least a 50% reduction in pain at a mean follow-up of 16 months. At one year the average across the studies reviewed showed 62% of patients as “successes” (defined as a greater than 50% reduction of pain) at one year, 64% at two years and 53% at five years. Only one study reviewed reported success at 10 years and they were reported as 35%. The author cautioned that there were no randomized trials among the studies therefore “no conclusions may be drawn at this time concerning the efficacy of SCS for failed back surgery syndrome relative to other pain treatments, placebo treatments, or no treatment.” In these studies there was no measurement of functional status; no placebo group control was employed.
A second literature review performed by Turner found that pain relief with SCS appeared to decrease over time and that in one randomized trial there was no improvement in patient functioning. This second review concluded that there remained inadequate information to make definitive statements about efficacy of SCS in reducing physical disability, work disability and medication consumption. There was some evidence that SCS plus physical therapy was more effective than therapy alone for patients with Complex Regional Pain Syndrome Type I in relieving pain at six months and 12 months.
Another literature search reported by Mailis-Gagnon to assess the efficacy and effectiveness of spinal cord stimulation in treating Complex Regional Pain Syndrome type I (CRPS) and failed back surgery syndrome after follow-up periods from six to twelve months concluded there was insufficient evidence to assess the benefits and harms of SCS for the relief of chronic pain; more trials were needed.
A prospective study conducted by Burchiel one year following implantation of SCS in 182 patients reported a 55% reduction in pain for the 70 patients on whom data was available at the end of one year. The pain severity, as reported on a visual analog scale range of 0-10, though, was only 18.6%, which would not reach the criterion of clinical significance deemed to be 33%.
The overall success rate when all factors were considered (with success defined as at least 50% pain relief and patient assessment of the procedure as fully or partially beneficial) was 40% after one year. In addition there was only a nine percent difference in work status at the end of the study period. Overall there was deemed to be no significant difference due to the SCS, in particular when functional status was reviewed.
A clinical study performed by Kavar that compared patients’ pain scores before and after insertion of a SCS with a mean follow-up period of 18.5 months demonstrated a statistically improvement in pain scores as well as in sleep, sitting, travel and home duties. There was no significant improvement in their ability to participate in social activities, occupation, self-care or sexual activity. The number of participants who obtained definite pain control, however, was only 26% and more than half continued to require narcotic analgesia. In addition, the failures occurred in individuals over 45 years old with 61.5% over the age of 50.
Kemler reported that a five-year follow-up analysis of patients treated with a SCS for CRPS had a reduction in the characteristic severe burning pain associated with the syndrome by more than 50% but that the effect diminished over time and was no longer significant after three years. Changes in other pain measures or quality of life tests were not statistically significant. The decreased effectiveness was explained as being related to a pain increase over time, exaggerated effect of treatment in the trial phase prior to implantation and spontaneous improvement. It was concluded that SCS does not produce a statistically significant improvement in Complex Regional Pain Syndrome.
According to the ACOEM Occupational Treatment Guidelines spinal cord stimulators have been used for treatment in numerous painful conditions with evidence of short-term efficacy. Among these conditions are chronic low-back pain, failed back surgeries including post laminectomy syndrome, radicular syndromes, peripheral vascular ischemia and peripheral neuropathy.
The theory behind spinal cord stimulators is to deliver electrical pulses to the spinal cord through leads that are implanted percutaneously. The idea behind it is what is called the gate control theory in which stimulated neuro fibers close to paths of pain conduction interfere with the pain pathways. This mechanism is little understood and not proven.
Overall complication rates have ranged from 20% to 40%. The most recent recommendations for spinal cord stimulation came out of the Occupational Medicine Practice Guidelines, 2nd Edition. A Cochran review concluded that there was limited evidence to favor a spinal cord stimulator for failed back surgery. There is a report from the American Society of International Pain Physicians that indicated while there is evidence for spinal cord stimulation efficacy for failed back surgery in the short term, no such data exists for the intermediate and long term situations. Even in those studies, however, the literature that was used was case based series. There were no data series with prospective or controlled trials. The authors were not able to support the use of spinal cord stimulation with any affirmative conclusions.
References available upon request.