Introduction

Electrical stimulation for the treatment of pain dates back to 46 ad when Scribonius Largus described the use of torpedoes, a fishlike animal capable of emitting an electric discharge, placed over the area of pain for relief from intractable headache and arthritis. In 1745 the Leyden jar allowed physicians to control electrical current, and its use spread rapidly. Electrical stimulation of the brain was noted in 1950. Shealy, Mortimer, and Reswick¹ reported on the use of cardiac pacemaker technology to deliver electric current to the spinal cord via surgically implanted electrodes in 1967. Remarkable surgical and technological advances over the ensuing half century have resulted in various types of percutaneous and surgically implanted paddle leads capable of delivering thousands of different stimulating patterns using totally internalized, radiofrequency coupled, or rechargeable pulse generators. This technological flexibility has dramatically broadened the horizon of clinical application. The emphasis in this chapter is on the use of electrical stimulation in the treatment of chronic pain.

Descartes’ explanation of pain mechanisms and processing² put forth in his 1664 book Treatise of Man held sway from the 1600s until Melzack and Wall presented their gate control theory in 1965.³ The latter theory, which has now undergone many revisions, allowed for the role of psychological factors in the modulation of pain. The ingenious model of chronic pain put forth by Apkarian, Baliki, and Geha⁴ provides for even greater clarification of the role of psychological factors. Spawned by the revealing work on pain mechanisms and system reorganization,⁵ pain processing,^6,7 and neuroimaging,^8,9 the model is neurophysiological in nature. It highlights the brain as a “…dynamical network wherein detailed connectivity is consistently modified by the instantaneous experience of the organism.”^4(p95) Although the involvement of the nociceptive transmission system (i.e., spinal thalamic pathways) is acknowledged, activity at the cortical level is central to the theory.

Along with changes in our understanding of the neurophysiological aspects of “pain” (I use pain inside quotation marks because its definition and our understanding of its nature [i.e., disease vs. symptom vs. syndrome vs. emergent phenomenon] continue to evolve), the psychological/psychiatric conceptualization has changed as well. Psychoanalytical theories of pain based on the work of Sigmund Freud were popular in the psychiatric and psychological communities in the 1960s.¹⁰ The psychodynamic approach of Freud held pain to be a means of controlling the expression of unwanted and unconscious desires or motivations. Engel¹¹ followed by detailing the “pain-prone personality.” The essential features of the psychodynamic approach included (a) pain as a common conversion system, often with symbolic meaning; (b) unpleasant affect, usually guilt, hostility, resentment, or conflict is converted to bodily pain; (c) the choice of the symptom is determined by precipitating events; and (d) frequently there is a hereditary influence, most always a physical substrate, if only muscular.^10,12

Although largely replaced by psychological approaches based on learning theory, the legacy of psychodynamic theory lives on in the classifications of Somatization Disorder and Pain Disorder found in the revised fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) of the American Psychiatric Association.¹³ Because of the tendency of such terms to continue to invigorate the notion of pain as a psychogenic phenomenon, Merskey¹⁴ has called for their removal from the DSM. In fact, the term Complex Somatic Symptom Disorder has been proposed to replace Somatization Disorder, Pain Disorder, Hypochondriasis, and Undifferentiated Somatoform Disorder in the upcoming DSM-V^13a and eliminate “medically unexplained symptoms” as a diagnostic criterion (www.dsm5.org/ProposedRevisions). The DSM Working Group is hopeful that this change will eliminate any unintended reference to mind-body dualism and believe that it is more in keeping with the identification of somatic symptoms and cognitive distortions as shared features among the four existing disorders listed.

Early on, C. Norman Shealy¹⁵ recognized the importance of psychological factors in the treatment of pain as illustrated by his recommendation of (a) the absence of elevations on the Minnesota Multiphasic Personality Inventory (MMPI), except on the depression scale; (b) emotional stability; and (c) involvement in a rehabilitation program such as patient selection criteria in the application of spinal cord stimulation (SCS). Indeed, the psychological status of the patient was noted by Long¹⁶ to be the most common reason for failure of stimulation therapy. One would be hard pressed to find any reputable publication on the subject that does not put forth psychological factors as important to the outcome of SCS therapy for the treatment of pain. In fact, the National Institute for Health and Clinical Excellence (NICE)¹⁷ strongly recommends that SCS therapy for chronic pain be carried out within a multidisciplinary setting.

On surveying the outcome studies from the 1970s to the1980s, Bedder¹⁸ concluded an estimated 40% success rate for SCS therapy. In a 2005 article Taylor, van Buyten, and Bucher¹⁹ noted a significant pain reduction in 62% of patients with 40% returning to work, 53% discontinuing their use of opioids, and 70% expressing satisfaction with SCS therapy. Any concern over significant activity restrictions in patients with a properly secured SCS device appears resolved by reports, including one in 2009, documenting the return to active duty in combat zones of military personnel following internalization of SCS.²⁰

Less encouraging are the results of a prospective controlled study that compared SCS therapy, pain clinic, and usual care for failed back surgery syndrome (FBSS) in patients under worker compensation. By way of summarizing their data, the authors concluded that “…the SCS group did not differ from the other groups at 12 or 24 months on any outcome, including leg pain intensity, physical function, back pain intensity, and mental health. Outcomes were poor in all groups … fewer than 6% of patients achieved success on the primary outcome (a composite index of improvement in pain, function, and medication use); fewer than 10% were working; and more than twice as many patients reported a decline as report improvement in ability to perform everyday tasks.”^21(p23)

May, Banks, and Thomson²² reported that, of the 100% of patients reporting success 16 months after implantation, only 59% did so at 58 months. Disease progression, adaptation, or tolerance to the stimulation; “a regression to the mean”; misinterpretation of the screening trial; and psychological variables may be contributing factors. These very disparate results combined with the loss of effect over time suggests that factors other than those related to surgical technique and device function are contributing to therapeutic outcomes. Indeed, North and Shipley emphasized that, “Technical success, however, is not sufficient to ensure clinical success.”^23(pS202)

Psychological Evaluation

In performing a psychological evaluation, it is important to consider several basic assumptions of pain management. First, persistent pain, regardless of its associated physical pathology (e.g., malignant tumor, degenerative disc disease, or osteoarthritis), is multidimensional and therefore influenced by psychosocial factors. Second, these psychosocial factors can influence the outcomes of pain relief–oriented therapies; in addition, co-use of behavior/psychological therapies along with somatic therapies can enhance pain relief and functioning. Finally, the prominence and priority of psychosocial factors can vary according to the type and degree of pain and pain-related pathology.²⁴

Several years ago my colleagues and I examined the literature and, along with our own experience, listed a number of hypothesized positive patient characteristics thought to indicate an appropriate patient to proceed to a screening trial.²⁵ These positive indicators included (a) generally stable psychologically; (b) cautious, sufficiently defensive, self-confident; (c) self-efficacy, ability to cope with setbacks without responding in emergent fashion; (d) realistic concerns regarding illness often associated with a congruent mild depression; (e) generally optimistic regarding outcome, with the patient and significant other having appropriate expectations; (f) comprehends instructions and has a demonstrated history of compliance with previous treatments; (g) appropriately educated regarding procedure and device, supportive and treatment-educated family/support member(s); (h) behavior and complaints consistent with pathology and a behavioral/psychological evaluation consistent with the patient’s complaints and reported psychosocial status; and (i) able to tolerate electrical stimulation, perhaps evidenced by a trial of transcutaneous electrical nerve stimulation (TENS).

Patient beliefs which were thought to be associated with a less positive outcome include (a) pain is a purely physical phenomenon, (b) psychosocial factors play little role in pain and treatment outcome, (c) chronic pain means loss of productive life, (d) pain can only be relieved if the medical cause (e.g., arthritis, scar tissue) is eliminated, and (e) medical technology holds the solution. Those appearing to correlate with a positive outcome are: (a) pain is multidimensional and multi-factorial, (b) attitudes and behaviors can affect treatment outcomes, (c) coping skill (e.g., relaxation, distraction, goal setting) can be helpful, (d) an active participant in therapeutic decision, (e) support systems that reinforce positive behavioral change are useful, and (f) proper expectations influence outcomes.

In addition, we proposed a set of clinician beliefs and attitudes thought to be more closely linked to a negative or positive outcome:

Assessing and ensuring that the perspective SCS patient has proper, realistic, and appropriate expectations is seen as part of the evaluation process. The term expectation is often used in somewhat of a parochial manner, perhaps because its clinical impact may tend to be underestimated. Imaging studies have examined the effect of both positive and negative expectations. Expectancies have been demonstrated to affect the activity of the pain modulatory system, including the anterior cingulated cortex (ACC), thalamus, prefrontal cortex, and insula cortex.^26,27 Expectations related to anticipated good or bad motor performance influenced the outcome of subthalamic nucleus stimulation in patients undergoing deep brain stimulation.²⁸

Furthermore, expectations contribute to the placebo effect. Price, Finiss, and Benedetti²⁹ noted the average placebo effect to be 2 U on a 0 to 10 pain scale in their total study population. However, it was as high as 5 U in the placebo responders (which varied from 20% to 55%, depending on the study). Expectations accounted for 49% of the variance. A series of studies by Dolce and associates^30–32 reported the impact of self-efficacy and expectancy on pain and exercise tolerance. It might be of interest to study the relationship between patient expectations and the reported loss of analgesia over time, despite a functioning SCS device. The author is unaware of any validated psychological instrument designed to evaluate expectations, particularly as they related to neuromodulation. However, the meeting of patient expectations has been shown to be central to good clinical outcomes.³³ This highlights the importance of appropriate expectations. Here again one encounters the necessity of the clinical interview.

How might the awareness of the patient’s expectations and the possibility of the placebo effect influence the screening trial? For one, the clinical interview could obtain information regarding the level of pain acceptable to the patient, allowing him or her to be more active (i.e., the functional pain level). The patient could then be encouraged to outline their functional goals (e.g., sitting longer, walking further, traveling more), which can be individualized for each patient, depending on his or her particular anatomical limitations. Second, the screening trial might then examine both cognitive or perceptual effects of SCS (i.e., pain ratings) and functional changes. The pleasure gained from being able to perform reinforcing activities again and the accolades from others may be important to long-term improvement. It is likely to be more difficult for a patient to perceive himself or herself as more functional, especially if one involves and solicits observations from a significant other in the screening process, than it is to experience a reduction in pain. In a sense the preimplant screening trial can function as an extension of the interview. Agreed on therapeutic goals can be addressed during the trial and used as a means to determine the desirability or appropriateness of proceeding to implantation. Logic would suggest that the more closely the trial circumstances mimic the final outcome, the less chance there is of a false-positive trial. This type of a screening trial mimics the N-of-1 approach illustrated by Cepeda and colleagues.³⁴

North and Shipley²³ published what might arguably be one of the most comprehensive reviews of the SCS literature. Over 20 participating experts reviewed some 300 articles spanning 40 years from 1967 to 2007. The document summarized some psychologically relevant information. Regarding psychological predictors, it was noted that, “We lack sufficient information to predict SCS outcome from the result of a pretreatment psychological evaluation, but SCS, as is the case for every interventional pain treatment, is reserved for patients with no evident unresolved major psychiatric co-morbidity.”^23(pS233) Concerning the benefits of a psychological evaluation, they stated that it “…provides patient selection information by identifying the small percentage of patients who might benefit from psychological treatment before undergoing SCS therapy or in whom SCS therapy might be complicated by psychological factors.”^23(pS234) The literature was interpreted to suggest that the psychological evaluation be conducted before the screening trial when a surgical lead was being used, before anchoring if percutaneous leads are used, and before internalization. The various tests that had been used in the studies reviewed included the MMPI with Wiggins content scales^35–37; Symptom Checklist-90-R³⁸; Derogatis Affects Balance Scale³⁹; Chronic Illness Problem Inventory⁴⁰; Spielberger State-Trait Anxiety Inventory (STAI) Scale and State-Trait Anger Scale^41–44 ; Beck Depression Inventory (BDI)^45–47; Locus of Control Scale^48–49; Absorption Scale⁵⁰; McGill Pain Questionnaire (MPQ)^51–52; Social Support Questionnaire⁵³; Sickness Impact Profile (SIP)⁵⁴; Oswestry Disability Index (ODI)⁵⁵; Roland Morris Questionnaire⁵⁶; and Fear-Avoidance Beliefs Questionnaire⁵⁷(Table 4-1). Interestingly, the authors noted Conversion Disorder to be a condition that could escape detection.

Table 4-1 Advantages and Disadvantages of Various Psychological Tests