Computer models

Holsheimer et al.19 have performed computer modelling of electrode designs and suggested that optimal lead configuration is a transverse tripole with a central cathode with longer lateral anodes separated by 2.5-3 mm. A narrow bipole or tripole is optimal for dorsal column stimulation (Figures 20.2 and 20.3), as this keeps the stimulation over the posterior spinal cord better with anodal effects forcing the stimulus to penetrate more over the physiological midline. Many of the leads with large intercontact distances lead to dorsal root stimulation first, as the anode is not close enough to the cathode to exert

Table 20.1 Conductivity of intraspinal elements.

Tissue

Conductivity

Gray matter White matter Longitudinal Transverse Cerebrospinal fluid Epidural fat Dura mater Vertebral bone Electrode insulation

0.23

0.08

0.04

0.03

0.02

0.002

Reprinted with permission from Oakley JC, Prager JP. Spinal cord stimulation. Mechanisms of action. Spine. 2002; 27: 2574-83.

Surround layer

Vertebral bone

Epidural fat

Vertebral bone

Surround layer

Vertebral bone

Epidural fat

Vertebral bone

Ventral

Ventral any significant shaping of the vertical or horizontal extent of the electrical field. The threshold for dorsal column to dorsal root stimulation will increase, the greater the distance between the lead and the spinal cord. The highest threshold occurs at T4-T7 (Figure 20.4).19 Optimization of the usage range (avoiding nerve root stimulation) often requires anodal guarding, which will establish field boundaries around the primary stimulating cathode, and produce more penetration of the dorsal spinal cord in a longitudinal guarded cathode. A transversely oriented guarded cathode (which may require using adjacent parallel leads) with a central cathode and flanking anodes as envisioned by Holsheimer et al.19 allows for greater paresthesia steering to better cover the "sweet spot.''

PROGRAMMING

The concept of multiple electrodes placed in parallel has become popular with many authors (Figure 20.5),20 although others have suggested that a single electrode at the physiologic midline is not inferior to dual lead configurations for initial paresthesia coverage of the axial low back.2 Newer electrodes may have independently variable voltages that allow the voltage ratio to be blended between the midline cathode and lateral anodes to better treat complex pain presentations.18 Barolat and colleagues21 carried out initial mapping of where various cathodal stimulations would produce paresthesias. In general, the areas of stimulation are several levels above the target dermatome, e.g. a foot paresthesia requires targeting the extremely low thoracic spine.

Initial targets for lead placement for the treatment of extremity pain are approximately a C3/4 location for stimulation of upper extremities, and T9/10 for lower

Figure 20.2 Computer model of the low thoracic region demonstrating an electrode in the dorsal epidural space. Spinal stimulation requires that the electrode conducts the distance through the layer of cerebrospinal fluid to reach the dorsal spinal cord. Redrawn with permission from Alo KM, Holsheimer J. New trends in neuromodulation for the management of neuropathic pain. Neurosurgery. 2002; 50: 690-703.

extremity stimulation paresthesia coverage. Narrow contact spacing of bipoles or tripoles produces optimized coverage of painful areas, and maximal paresthesia coverage of the painful area has been a long-standing fundamental concept of spinal stimulation.18

SCS for spinal pain syndromes

The multiple operated back, often termed "failed back surgery syndrome'' (FBSS) or postlaminectomy syndrome is currently the most common indication for spinal stimulation.22 The care of the multiple operated back pain patients is difficult, and requires a multimodal approach that includes physical modalities, cognitive-behavioral therapies, procedural and pharmacologic therapies. Patients with ongoing postsurgical pain have complex pain patterns that often include both back and leg pains of various types (nociceptive, mixed, and neuropathic pains). Causes vary, but include cauda equina syndrome, arachnoiditis, epidural scarring or fibrosis, chronic radicular pain, and many others. Choosing the correct diagnosis is extremely important, as is the proper therapy for each diagnosis. Many patients simply want resolution and will repeatedly opt for more surgery in spite of little evidence of efficacy. An algorithmic approach within an established spinal care network can be useful. Several retrospective and prospective long-term studies have established SCS as a viable choice for these patients.23 [III], 24[II] There are few randomized trials for FBSS. However, a recent trial by North and colleagues of a series of 50 patients randomized to either SCS or reoperation is important. Patients were all thought to be operative candidates by a staff surgeon, due to recurrent or persistent pain in a radicular pattern. Patients were allowed

Figure 20.3 Stimulation may be (a) unipolar, (b) bipolar, (c) tripolar, or (d) narrow tripolar. The computer model predicts narrow tripolar stimulation as optimal. Redrawn with permission from Alo KM, Holsheimer J. New trends in neuromodulation for the management of neuropathic pain. Neurosurgery. 2002; 50: 690-703.

Figure 20.3 Stimulation may be (a) unipolar, (b) bipolar, (c) tripolar, or (d) narrow tripolar. The computer model predicts narrow tripolar stimulation as optimal. Redrawn with permission from Alo KM, Holsheimer J. New trends in neuromodulation for the management of neuropathic pain. Neurosurgery. 2002; 50: 690-703.

to crossover to the other group as warranted. In the SCS group, 9 of 19 patients were successful versus only 3 of 26 in the reoperation group. Patients randomized to reoperation were much more likely to crossover to SCS (14/26) from which an additional 6/14 patients were successful.24[II]

Although extremity neuropathic pain has long been thought to be the best indication for SCS, it is only recently that low back pain coverage for these patients has been achievable. The problem in years past has been that the approximate areas of stimulation required for low back coverage are from about L2 to L5. Stimulation of these areas is prone to also stimulate the anterior thighs, a less common area of pain for many patients (Figure 20.6). As the stimulation amplitude is increased, the abdomen will also be stimulated.25 The sensory homun-culus for the low back is quite small compared to the extremities (Figure 20.7), and low back fibers are not uniformly situated within the dorsal columns. Thus, the problem is one of penetrating the stimulus deeply enough into the spinal cord without activating the more prevalent fibers that are not desirably stimulated. Wide pulse widths may help to some extent with this problem.25 Both Barolat et a/.26[III] and North et a/.2[II] have recently published trials demonstrating efficacy in low back pain patients.

Figure 20.4 The distance from the spinal cord surface to the epidural electrode (d-CSF) is highest in the mid-thoracic spinal cord. High d-CSF decreases the usage range between stimulation threshold and uncomfortable dorsal root stimulation. Redrawn with permission from Alo KM, Holsheimer J. New trends in neuromodulation for the management of neuropathic pain. Neurosurgery. 2002; 50: 690-703.

Vertebral level

Figure 20.4 The distance from the spinal cord surface to the epidural electrode (d-CSF) is highest in the mid-thoracic spinal cord. High d-CSF decreases the usage range between stimulation threshold and uncomfortable dorsal root stimulation. Redrawn with permission from Alo KM, Holsheimer J. New trends in neuromodulation for the management of neuropathic pain. Neurosurgery. 2002; 50: 690-703.

Figure 20.5 A dual-octapolar contact lead array spanning the eighth and ninth vertebral bodies in an anteroposterior projection. Note the cephalad aspect of the rightward lead is beginning to move too far lateral and might produce thoracic root stimulation.

SCS in ischemic syndromes

The pain of intractable cardiac ischemia and ischemic peripheral vascular disease is often difficult to control despite attempts at revascularization, multimodal phar-macologic therapy, and other techniques. Ischemic disease results in increased morbidity, frequent hospitalizations,

* Sacral fibers □ Lumbar fibers • Thoracic fibers

Figure 20.6 Low back stimulation coverage is often difficult and requires stimulation at thoracic levels where the lumbar roots from, e.g. L2, have moved centrally nearer the physiologic midline. At lower thoracic and upper lumbar levels, excessive stimulation paresthesias will be noted by patients in the anterior thighs and abdomen, because of the more lateral location of the representative L2 fibers. L2 fibers depicted in squares. Redrawn with permission from Oakley JC. Spinal cord stimulation in axial low back pain: Solving the dilemma. Pain Medicine. 2006; 7: S58-63.

emergency room visits, and overall poor quality of life. Many patients are not candidates for revasculariza-tion procedures, such as bypass procedures, and it is unclear which of many newer therapies constitute "best practice.''

SCS has emerged as one potentially beneficial therapy with previous prospective trials, retrospective case series and meta-analysis suggesting benefit of stimulation for several ischemic conditions. Unfortunately, little class I evidence for the technique exists because of the difficulty in performing randomized controlled trials for spinal and peripheral stimulation techniques in general.3 Patients must be able to feel somatotopically correct stimulation paresthesia to verify appropriate coverage for all stimulation techniques.

Figure 20.7 The sensory homunculus has a small representation of low back neuronal targets relative to the larger fields devoted to the feet and toes. Redrawn with permission from Oakley JC. Spinal cord stimulation in axial low back pain: Solving the dilemma. Pain Medicine. 2006; 7: S58-63.

SCS in refractory angina pectoris

SCS was first applied two decades ago for the treatment of angina pectoris.27 Since that time, several authors have noted significant improvement in a variety of clinical indices of ischemia. Spinal stimulation has enjoyed great popularity, particularly in Europe, but some resistance as well.28 Reasons for this ongoing controversy are varied, but may include cardiologists' unfamiliarity with the technology, an incomplete mechanistic understanding of SCS, and the required utilization of noncardiac practitioners for ongoing therapy. Therefore, spinal stimulation has competed with other therapies, such as (1) percutaneous laser revascularization, (2) enhanced external counter pulsation (EECP), and (3) gene therapies. A recent comparison study of SCS and percutaneous myo-cardial laser revascularization is typical of the literature, with no difference demonstrated for the primary outcome measure (exercise treadmill time) between the groups. Several shortcomings are apparent with this type of comparative study, however, as the trial did not even mention the lead type, contact spacing, or programming sequences for what the authors admitted was their group's initial use of stimulation techniques. Furthermore, because the stimulation technology used is always changing, the implants were the older, nonrechargeable

units.29

Despite these detractors, there is ample evidence of the efficacy of spinal stimulation for angina pectoris, as well as a potentially large number of patients who may qualify for this therapy. Many of these patients are not candidates for coronary bypass grafts or coronary stenting procedures. Some are at maximal medical therapy with optimized beta-blocker therapy, nitrates, vasodilators, and other pharmacologic agents. Many patients have non-atherosclerotic coronary disease, such as syndrome X, which is attributable to incomplete endothelial relaxation caused by dysfunctional adenosine receptors or other causes.30 Pathologically, cardiac ischemia represents an imbalance of myocardial oxygen supply and demand. Numerous basic science and clinical studies have attempted to explain the mechanism of SCS beneficial effects, potentially attributable to redistribution of myocardial blood flow.

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