Date Published: October 5, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Dominic Hegarty.
The use of neuromodulation for pain relief is among the fastest-growing areas of medicine, involving many diverse specialties and impacting on hundreds of thousands of patients with numerous disorders worldwide. As the evidence of efficacy improves, the interest in spinal cord stimulation (SCS) will increase because it is minimally invasive, safe, and a reversible treatment modality with limited side effect profile. While the mechanism of action evades complete understanding, the technological improvements have been considerable and current neuromodulation developments have been coupled with the rapid growth of the neuromodulation device industry resulting in the development of the next-generation neuromodulation systems. The development, the newest technicaliti and the future for the clinical application of spinal cord stimulation (SCS) are reviewed here.
Neuromodulation is among the fastest-growing areas of medicine, involving many diverse specialties and impacting on hundreds of thousands of patients with numerous disorders worldwide . Historically, electricity, either in the form of the torpedo fish or man-made electrotherapy, has been used to try and cure various ailments . For example, in the middle of the 18th century “electroanalgesia” became advocated for the treatment of angina pectoris, gout, headaches, pleuritic pain, and sciatica. However, by the 20th century the enthusiasm for the medical use of electricity became associated with “quackery”  and was banned from clinical practice. In 1965 Melzack and Wall presented the “Gate Theory” , which postulated that stimulation of nonpainful stimuli can inhibit painful afference, thereby offering the opportunity to align basic research with the clinical application of electricity which has resulted in the development of neuromodulation techniques as we know them today [5–7]. While the mechanism of action evades complete understanding, the technological improvements have been considerable and current neuromodulation developments have been coupled with the rapid growth of the neuromodulation device industry resulting in the development of the next-generation neuromodulation systems. The development, the newest technicalities, and the future for the clinical application of spinal cord stimulation (SCS) are reviewed here.
Essentially there are two components of a fully implanted SCS system: the electrodes (or lead) and an implantable pulse generator (IPG). In SCS the placement of epidural electrodes is generally targeted at the dorsal column of the spinal cord; however, in patients with segmental pain (single dermatome), stimulation is focused at the corresponding dorsal root. This is where the ascending tracts pass without decussation to the gracile and cuneate nuclei of the medulla oblongata. These tracts are composed of a wide range of fiber diameters which are the central processes of the primary afferent neurons located in the spinal ganglia. As the tracts ascend, they receive accession from the dorsal roots, resulting in a somatotopic organization . The recruitment of fibers is correlated directly with the diameter of the fiber and inversely with the distance between the electrode contacts and the fibers . Hence the thickness of the cerebrospinal fluid layers , the individual anatomy, and the electrodes each influence the recruitment of the dorsal column [8–11] and dorsal root fibers .
Remarkable technological advances have been achieved in terms of electrode contact/lead design. Firstly, the new multicontact arrays available in traditional and five-column paddle leads (St. Jude Medical, Inc, USA) have resulted in the ability to provide improved programmable capability and possible treatment outcome. Mathematical modeling has highlighted the potential benefits of tight-electrode spacing in electrode contact design whereby gaps in stimulation are avoided (Boston Scientific Neuromodulation, Valencia, Calif, USA). Indeed, to obtain large paraesthesia coverage, all active contacts (anodes and cathodes at one or more arrays) should be closely spaced.
Originally regarded as just a battery, the IPG has now evolved to become an engineer’s paradise. Long gone are the nickel-cadmium systems which are replaced by lithium-based batteries thereby prolonging the lifespan of the device. With the advent of complex stimulation settings involving the activation of an increasing number of contacts the premature exhaustion of the battery is avoided by using automatic nocturnal, time-cycled, or manual interruption of stimulation. The industry has developed a variety of new generation of compact rechargeable IPGs to meet the new requirements of SCS; thereby, energy consumption becomes less of a problem.
In SCS, the pulse amplitude is usually the focus of stimulation control as it is intuitively understood by clinician and patient alike [26–29]. With advances in SCS technology, particularly rechargeable IPG implantable devices, pulse width (PW) programming ranges of now match that of older radiofrequency systems (with programmability up to 1000 μs). Traditionally PW was only changed when other parameter adjustments fail to achieve therapeutic goals. In neurostimulation the pulse amplitude and width relate directly to the depolarization of the cell membrane and are therefore critical parameters for determining the locus of excited tissue . The value of PW programming was investigated in 19 subjects who had a fully implanted SCS in place for over 3 months to treat chronic intractable low back and/or leg pain. It was shown that the baseline median PW parameter was 295 μs (range 242–326 μs) with a median amplitude of 2.5 mA (1.3–3.3 mA). Following independent modification of the PW, the median PW of all patients’ programs increased to 400 μs, approximately 48% higher (P = 0.01) and showed a significant increase in the paraesthesia-pain overlap (56%, P = 0.04). It was estimated that 10/19 patients appeared to have greater paraesthesia coverage, 7/19 patients selected the new PW programs, and 8/19 patients appeared to display a “caudal shift” of paraesthesia coverage with increased PW .
Although SCS is a recommended treatment for patients with failed back surgery syndrome (FBSS) , if paraesthesia over the lumbar dermatomes cannot be obtained, then axial low back pain is very difficult to treat and clinical results are poor . Ongoing multicentred European prospective trials  using dual octapolar, percutaneous leads placed sequentially near anatomic midline and connected to a rechargeable IPG capable of delivering waveforms with frequencies up to 10 kHz. (Nevro, Menlo Park, Calif) have shown that of 34 cases with full implantation the average back pain VAS decreased by 77% (8.9 cm baseline to 2.0 cm at 6 months, P < .001) and leg pain VAS decreased by 82% at 6-month follow-up. (5.5 cm baseline to 0.7 cm at 6 months, P < .001). In addition the average Oswestry Disability Index score decreased by 36% (from 58 to 37, P < .001). This approach is novel for several reasons: (a) the use of high-frequency stimulation provides sustained analgesia in a previously difficult patient cohort without paraesthesia—thus adequate axial low back pain relief is achieved without the overwhelming leg sensation one would have expected by increasing the frequency using a traditional IPG; (b) anatomical placement of the leads is possible and intraoperative paraesthesia mapping is avoided; (c) it has decreased programming requirements; (d) continued use of the system independent of position including night-time use is possible. To date, no adverse effect of such high-frequency stimulation has been reported however, the clinical outcome in the longer term is awaited. Pre-clinical studies in goats who received 10 days of continuous stimulation at amplitudes up to the sensory/motor threshold showed no difference in the behaviour or spinal cord neural histology between the therapy and control groups. Why such stimulation has this remarkable effect still remains to be understood and may influence our approach to this co-cohort heretofore unsatisfactorily managed with conventional SCS technology. There are several established indications for SCS such as neuropathic back and leg pain, complex regional pain syndrome, spinal cord injury, and ischemic pain (vascular and angina pectoris). While it is beyond the remit of this paper to discuss each clinical indication, there is a growing database of clinical-based evidence to support the use of SCS. The economic evaluation in these areas is limited but the initial costs of SCS is generally both more effective and less costly then conventional management over a period of 3–5 years . Unfortunately SCS is regarded as a last-resort option by many healthcare providers, and the real economic benefits may lie in the earlier introduction of the technique. Modern medicine requires that any treatment modality is based on rational knowledge and well-documented theories; however, some conditions, particularly those involving chronic pain, often remain imprecise. SCS may be one of the few examples of a treatment that has significantly contributed to a change in attitudes and providing satisfactory relief to patients who in the past would have been left untreated. Source: http://doi.org/10.1155/2012/375691