Introduction

Neuromodulation achieves analgesia without producing destruction of nerves. Attempts to alter pain perception in the cerebral cortex by neuromodulation can take place via the peripheral nervous or the central nervous system. Neuromodulation within the central nervous system occurs at the spinal cord level or within the brain. All of these sites have been used to decrease the perception of pain and produce analgesia.

The recent clinical focus of neuromodulation techniques in chronic pain medicine center on two main areas: electrical (nondrug) stimulation and intrathecal drug delivery. This chapter looks at electrical stimulation as a neuromodulatory way of producing pain relief in patients suffering from chronic pain syndromes. Specifically, the chapter focuses on percutaneous and surgical techniques of spinal cord stimulation (SCS).

Patient Selection

Several factors are considered when selecting patients for SCS. The type (neuropathic vs. nociceptive) and location (radicular vs. axial) of pain are considered key components in the selection process. However, it is equally important to consider physical characteristics of the patient when deciding if he or she is an appropriate candidate for SCS. This consideration is also important for implanters who only perform percutaneous techniques to a decision about potentially referring the patient for surgical or paddle lead placement.

Patient selection for SCS is reviewed in detail elsewhere in this book. The ideal candidates for percutaneous leads are younger patients who do not have significant degenerative spine disease or pronounced scoliosis and/or kyphosis. The patients depicted in Figs. 6-1 and 6-2 have significant scoliosis and were considered potentially difficult percutaneous placements. Approaching from the convex side of the scoliotic curve, the trials proceeded uneventfully as did the subsequent permanent percutaneously placed epidural leads.

Fig. 6-1 Scoliotic patient undergoing trial spinal cord stimulator lead placement. Note position of needle on convex side of scoliosis. This approach facilitates successful lead placement.

Fig. 6-2 Scoliotic patient with hardware in place from previous surgery. Note approach is from convex side of scoliotic curve.

It can prove difficult to place and anchor percutaneous leads to appropriate fascial tissue in morbidly obese patients. Percutaneously placed trial leads can often be maintained in the obese patient for 1 to 2 weeks without incident.

Migration of percutaneously placed spinal cord stimulator leads has been reported in many studies.^1–3 The reported incidence ranges from 5% to 23% in different series.^1–4 Proper patient selection should help to minimize the likelihood of subsequent migration. As with morbidly obese patients, very thin patients may prove more technically challenging for the percutaneous implanter. This may include finding appropriate space for the generator and anchors and fixation of the leads.

Trialing

Trialing methods for SCS vary from one implanter to the next. No consensus exists for duration of trials, percutaneous or surgically implanted leads, or number of leads used for the trial (commonly, one or two leads).

Most implanters trial from 48 hours to 10 days. The overwhelmingly and single most important goal of a spinal cord stimulator trial attempts to determine the likelihood of a patient achieving clinically meaningful long-term pain relief from the device. No studies have determined the length of time needed for a trial to answer this question.

How much pain relief is necessary before considering a trial successful for subsequent permanent implant? The literature often reports 50% pain relief as an outcome for judging a successful trial.^5–6 No good studies have looked at whether a criterion of 50% pain relief during a trial period predicts long-term success with SCS. It is quite possible that some patients with less than 50% relief may find acceptable relief long term and/or significant improvement in activities of daily living and increased functional abilities. It is known that some patients who report 50% or greater pain relief during a trial do not sustain relief long term and eventually become therapy failures.

Pain relief estimated at 50% often does not correlate with 50% reduction as measured by a numeric pain rating scale (NPRS) (Rauck, unpublished data). It is unclear if implanters who use 50% as a cutoff for determining whether to implant a permanent system should use the patient’s verbal response or an NPRS.

Percutaneously implanted spinal cord stimulator leads can be inserted less invasively than surgically implanted trial leads. Surgically implanted trial leads are commonly sutured to spinal elements. An unsuccessful trial requires a second surgical procedure to remove the leads.

Percutaneously implanted spinal cord stimulator leads can be tunneled and exteriorized. These leads commonly are anchored in the deep fascia tissue of the back at the time of insertion. If the trial is unsuccessful, a second surgical procedure is required to remove the anchors and lead(s).

Many percutaneous implanters place leads through a special epidural needle with no intention of leaving the lead in place for long-term use. The lead or leads are exteriorized and sutured in place against the skin. At the end of the trial the lead(s) can be removed in the office without a surgical procedure or use of fluoroscopy.

The effort to place surgically implanted trial leads or percutaneous leads that are anchored, tunneled, and exteriorized may present a bias for second-stage completion or implantation of the generator. Patients with marginal results during the trial may opt for permanent implantation rather than agreeing to a second surgical procedure for removal. The downside to removing the trial leads at the end of the trial is the risk that subsequently placed permanent lead(s) may not recreate paresthesias exactly as the trial. Patients may report that the permanent system does not perform as well as the trial.

The implanting physician must decide whether to use one or two leads during a trial. Cost, increased procedural time, and increased risk of complications mitigate against placing two leads for all trial purposes. However, placing only one spinal cord stimulator lead for the trial may prove inadequate for producing sufficient paresthesias and analgesia for the patient. Many implanters use two leads for all permanent implants; thus one can argue that two leads should be used at trial to mimic the long-term paresthesias one expects over time.

Risk of infection during spinal cord stimulator trial has been infrequent.^7–8 Meticulous sterile technique should be followed during the trial placement. Literature from other implantable trial catheters suggest that the risk of infection increases with the duration of the trial.⁹ A recent study with intrathecal catheters (in which the risk of serious neuraxial infection would be expected to be greater than epidurally placed spinal cord stimulator leads) reported no infections until week 3 and thereafter an incidence of 16% for catheters placed longer than 2 weeks.⁹

Our policy (RR and JN) has been to implant one percutaneously placed lead for most of our spinal cord stimulator trials. During paresthesia mapping in the fluoroscopy suite, we place a second lead if we are unable to get adequate paresthesias in the area of pain as reported by the patient. This occurs occasionally in our practice.

We inform patients to expect the trial to last 1 week. Patients are followed closely by telephone to make sure that the trial is progressing smoothly. If paresthesias are inadequate, too light, or too strong, patients are brought back to the clinic for reprogramming. Occasionally trials are extended beyond 1 week if patients cannot adequately assess the results of the trial.

If the lead migrates or pulls out (very rare) before 1 week, the results of the trial are assessed and reviewed with the patient. If sufficient pain relief (>50%) is reported and the patient and implanter agree, the decision to go forward with a permanent implantation is often made. Similarly, if no or minimal pain relief has been obtained, no permanent implantation is performed. Occasionally the trial lead is replaced 1 to 2 weeks later if the patient cannot assess the effects of the trial at the time of lead migration.

Finally, after removal of the percutaneous trial lead, we wait 1 to 2 weeks before permanent implantation. This allows for any indolent infection from the trial to manifest before moving forward with a permanent implantation. It is always preferred to find a subcutaneous or other skin infection from the trial and eradicate it before placing the permanent implant.

Positioning the Spinal Cord Stimulator Lead

Ultimately the location of any spinal cord stimulator lead depends on individual patient characteristics. No single location produces the same paresthesias in all patients. The optimal time for finding the location that produces paresthesias in the patient’s pain distribution is during trial lead placement. The implanter and the programmer coordinate efforts along with feedback from the patient to map paresthesias that cover the patient’s complaints of pain. Nevertheless, it is important to mention that it is not always possible to reproduce the paresthesias reported during the trial by implanting the permanent leads in the same radiologically defined location. Often, the permanent lead is implanted in the vicinity of the area where the trial lead was placed.

Patients are most commonly positioned prone for both percutaneous trial and permanent lead placement. If necessary, clippers are preferred over shaving and should be used the day of surgery. Prophylactic intravenous antibiotics should be administered within 1 hour of the permanent implant unless there is a strong contraindication present. We also routinely administer intravenous antibiotics before the percutaneous trial implant, although all implanters do not agree on its efficacy.

After sterile surgical preparation (e.g., chlorhexidine), many implanters use an Ioban drape over the surgical site. Standard sterile surgical techniques are used. Needle entry for percutaneous placement depends in part on anticipated final placement of the lead(s). Common needle entry for the lower extremity and/or axial low back pain is the midlumbar region. Skin entry commonly is marked at L2-3, L3-4, or L4-5. Entry into the epidural space should be as flat as possible, dependent in part on the body habitus of the patient. Entry into the epidural space is either one or often two levels above skin insertion. A paramedian approach should be used to avoid both the forces of the supraspinous and interspinous ligaments and the tendency of the spinous process to fracture a lead placed through a midline approach. The percutaneous implanter should not hesitate to use a longer-than-standard epidural needle to ensure that the angle of approach to the epidural space is shallow (less than 45 degrees whenever possible). A lateral view should be taken to ensure that the lead has not migrated anteriorly in the epidural space or into the dura (Fig. 6-3).

Fig. 6-3 Lateral view of spinal cord stimulator lead positioned erroneously in the anterior epidural space. Note that it is not possible to ascertain that a lead is anterior in the epidural space with a standard anteroposterior view.

Common lead placement for lower-extremity paresthesias vary from T9 to T12 (Figs. 6-4 and 6-5). Lead placement below T12 will not consistently stimulate posterior columns since the spinal cord often terminates at L1 or L2. Stimulation for axial back paresthesias commonly requires placement of the leads at T7 and/or T8. As stated previously, final lead placement should always be individualized to the patient response during intraoperative mapping.

Fig. 6-4 Lead placement for typical spinal cord stimulation of lower extremities and axial back pain.