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CHAPTER 96  Spinal Cord Stimulation

Also, we cannot reliably predict SCS outcome based on worker’s compensation or litigation status, but some clinicians consider the unresolved possibility of secondary gain a relative contraindication to allowing a patient to undergo a screening trial in clinical practice and an exclusionary factor for clinical trials. For FBSS patients, it seems that the chance of success with SCS decreases as the number of prior surgical procedures and the time since the last surgical procedure increases. The type of surgical procedure the patient has experienced is also important; some ablative procedures, such as rhizotomy, dorsal root gangli­ onectomy, or dorsal root entry zone lesioning, might obliterate the neural substrate on which SCS is thought to depend. 179 For patients with PVD, investigators have reported that 1-year limb salvage is more likely to occur if the baseline su­ pine transcutaneously measured oxygen tension (TcPO 2 ) is . 10 mm Hg, the baseline sitting-supine score is . 17 mm Hg, and the treatment difference is . 4 mm Hg. 180 Some investiga­ tors believe that a 50% improved TcPO 2 score during a 2-week screening period predicts limb salvage 181 regardless of baseline score or disease stage. 182 In the case of diabetic neuropathy, however, these investigators found the stage of neuropathy to be inversely related to SCS treatment success, 183 independent of the stage of the disease. 184 Patient Selection One of the first things we learned about SCS was that achieving a successful outcome required the right patient. In any diagnos­ tic group, the presence of seemingly identical pain syndromes was not enough to predict identical (or even any) SCS success in each individual. A major advantage of SCS, however, is that it is possible to perform a screening trial, which involves tem­ porary (typically 7 to 10 days) placement of an electrode to de­ termine the extent of pain/paresthesia overlap and the resulting therapeutic effect. In some European countries, a trial period of several weeks is required for reimbursement. The screening trial offers the most meaningful prognostic sign of the outcome of SCS, but, of course, not all potential SCS patients proceed to a screening trial. Information to determine a patient’s suitability for a screening trial is gathered from the patient’s history and from the results of a physical examination and imaging studies. Among the important aspects of the patient’s history are the location, intensity, duration, and characteristics of the pain and the patient’s psychological status. Radiating pain that has an objective basis, is distributed in a manner consistent with in­ formation gleaned from a physical examination and imaging studies, and can be labeled with a specific diagnosis is the most straightforward to treat. Imaging studies that are useful for diagnostic purposes can also provide anatomic information, namely, as to spinal canal diameter, that can be helpful or even vital during SCS implantation. In the United States, Medicare and many health insurance companies require SCS candidates to undergo a routine psy­ chological evaluation. Although no psychological test reliably predicts the result of SCS treatment (and physicians rou­ tinely expect new patients to meet reasonable psychological/behavioral standards), patients who have been living with chronic pain often benefit from psychological screening with the attendant possibility of therapy. Any major psychiatric co­ morbidity should be addressed before a patient undergoes an SCS screening trial. Relative contraindications include the following: Unresolved major psychiatric comorbidity Unresolved possibility of secondary gain Inappropriate dependency on pharmaceuticals Inconsistency among the history, pain ratings and descrip­ tion, physical examination, and diagnostic studies

Abnormal pain ratings A predominance of nonorganic signs (e.g., Waddell’s signs) Alternative therapies with a risk–benefit ratio comparable to that of SCS remain to be tried Pregnancy Occupational risk (e.g., of falling) Local or systemic infection Presence of a demand pacemaker or cardioverter defibrillator Foreseeable need for MRI particularly if high field strength will be required and/or the implant will be in or near the area to be imaged. Some contemporary implants allow MRI on a limited basis. Presence of a major comorbid chronic pain syndrome Anticoagulant or antiplatelet therapy Absolute contraindications include the following: Inability to control the device Gross spinal instability at risk for progression, or nerve compression causing a potentially disabling neurologic deficit, amenable to corrective surgery Coagulopathy, immunosuppression, or other condition associated with an unacceptable surgical risk Need for therapeutic diathermy Technique A discussion of techniques used for the various types of SCS screening trials and for SCS implantations is beyond the scope of this chapter. Instead, we refer the reader to previous publications. 185–187 Screening Trial Before the introduction of percutaneous catheter electrodes in the 1970s, physicians used a variety of methods to screen potential SCS candidates, including TENS, which were found to lack prognostic value. The percutaneous electrodes devel­ oped to facilitate a screening trial were immediately adopted for chronic use as well. Today, SCS systems in the United States are generally only implanted in patients who pass a screening trial. In Europe, the screening trial is commonly bypassed in patients with typical angina pectoris, a shortcut that reflects the fact that more than 80% of these patients enjoy significant pain relief with SCS. Beyond indicating that SCS therapy might be successful, the benefits of a screening trial include allowing a patient to expe­ rience the sensation of paresthesia before undergoing implan­ tation of the expensive pulse generator. The trial also provides important information that will dictate the choice of a perma­ nent electrode and pulse generator and the optimum stimu­ lating configuration. A successful screening trial commonly is defined by at least 50% patient-reported pain relief despite ap­ propriate (provocative) physical activity and stable or reduced analgesic consumption. Patient satisfaction is another vital out­ come. Medicare and many third-party payers require a success­ ful screening trial before implantation. SCREENING ELECTRODE CHOICE A percutaneous catheter electrode placed under fluoroscopy provides easy access to multiple target spinal levels and facili­ tates mapping of pain/paresthesia overlap to determine the op­ timal longitudinal level for the electrode. An insulated surgical plate/paddle electrode inserted by laminectomy limits access to spinal levels and mapping but is required if a percutaneous catheter electrode does not access the epidural space satisfactorily. Screening with a surgical plate/ paddle electrode is also necessary in some patients to eliminate uncomfortable extraneous stimulation or provide sufficient pain/paresthesia overlap.