Biopsychosocial Pre-screening for Spinal Cord and Peripheral Nerve Stimulation Devices

Overview

Pain is a complex and serious medical condition, with estimates from the Centers for Disease Control and Prevention in 2016 showing 50 million (20.4%) United States adults experiencing chronic pain, among whom 19.6 million experience high impact pain that limits life or work activities. Beyond the element of human suffering, there are enormous economic costs associated with chronic pain in terms of lost productivity and medical care. Among individuals who continue to work, common pain conditions alone (back pain, headache, arthritis, and musculoskeletal pain) may account for up to $61 billion in lost productivity annually in the United States alone. According to statistics from the Institute of Medicine (2011), the societal cost of all chronic pain and related disability in the United States is $560–635 billion each year. Gaskin and Richard note that these annual costs for pain were greater than the annual costs for heart disease, cancer, and diabetes. The Institute of Medicine Report, “ Relieving Pain in America ” has highlighted the urgent need to develop more cost effective approaches to pain management because the ever-increasing costs associated with current treatment approaches cannot be sustained. In parallel with trends showing increasing chronic pain, there has been considerable concern regarding the use of opioids to treat chronic pain, given their widespread use and potentially lethal side effects. Therefore there is still an urgent need to develop opioid-sparing and cost effective methods for managing chronic pain, especially when it becomes intractable.

Over the past two decades, there has been an expanding role for neuromodulation procedures such as spinal cord stimulation (SCS) and peripheral nerve stimulation (PNS) as treatment options for intractable chronic pain. Traditional neuromodulation has been presumptively operated by Melzack and Wall’s gate control theory of pain , which proposes that the activation of low-threshold afferent nerve fibers decreases the response of dorsal horn neurons to unmyelinated nociceptors, thereby “closing the gate” to pain transmission from the spinal cord. Shealy, Mortimer and Rewick were the first to apply this when they stimulated the dorsal columns to treat chronic, intractable cancer pain. Since that time, level 1 evidence has accumulated in support of implantable dorsal column stimulation to treat axial back pain, lumbar radiculopathy or neuralgia, and complex regional pain syndrome. Proposed mechanisms for SCS depend on the modality but have included action at the dorsal column, dorsal horn, dorsal root ganglion, and supraspinal sites. ^,

Neuromodulation of targets not amenable to neuraxial stimulation has focused on the use of semi-permanent and implantable PNS with a history nearly as long as that of dorsal column stimulation. ^, Nerve regions that are more easily accessed by PNS include the trigeminal nerve, occipital nerve, and subcutaneous peripheral nerves. Conditions for which PNS may be indicated include trigeminal neuropathic pain, occipital neuralgia, supraorbital neuralgia, residual limb, and phantom limb pain, and inguinal neuralgia (although emerging technologies such as dorsal root ganglion stimulation may be appropriate for inguinal chronic post-surgical pain). Headache disorders, including migraines and cluster headaches, might also benefit from cranial forms of PNS. ^,

Successful results with SCS and PNS have been defined variously in clinical trials. Some investigators and practice guidelines identified greater than 50% relief of pain 12-months following implantation as a potential benchmark. ^, Although success rates vary widely from 40%–80%, conditions commonly treated by SCS include failed back surgery syndrome, painful peripheral vascular disease, neuropathic pain, multiple sclerosis (MS), and complex regional pain syndrome (CRPS, previously known as reflex sympathetic dystrophy). The initial expense of these implantable therapies has been shown to be potentially offset in the long term by the benefits and resultant reduction in treatment expenses. Early work by Kumar, Malik, and Demeria evaluated costs for patients who received SCS and compared them to the costs for treatment by conventional pain therapies (CPT). While the cost for SCS was significantly higher than for CPT in the first 2.5 years, after that time the cost of treating patients with SCS not only became less than the cost for CPT, but it also remained less expensive during the remainder of the five-year follow up period. Another analysis of SCS compared to conventional therapies has found SCS to potentially create cost and healthcare resource utilization savings, though perhaps at the risk of higher complication rates.

Adverse events associated with spinal cord stimulators have been several, as have reasons for SCS therapeutic failure because of ineffective pain relief. Adverse events are typically separated into either mechanical (e.g. lead migration, lead or implanted pulse generator failure, lead fracture) or biologic problems (e.g. surgical site infection or loss of efficacy). The incidence of complications of any type varies from 30%–40% in contemporary series. The rate of explant for any reason has been shown to be approximately 8% per year in a large European multicenter cohort study. The study consisted primarily of patients with post-laminectomy syndrome, approximately half of these devices were removed because of inadequate pain relief. In a multicenter retrospective United States based study of SCS explant primarily among patients treated with SCS for post-laminectomy syndrome, Pope and colleagues found that the most common reasons for device explant consisted of loss of efficacy (43.9% of explanted devices) followed by mechanical complications or infection (20.2%). These findings, among ­others, highlight the need for patient selection to hopefully reduce the rate of explanation for lost efficacy.

Efforts to improve patient selection for the surgical treatment of back pain, including post-laminectomy pain treatment with SCS, have a five-decade history. In an attempt to determine both the positive and negative outcome indicators for surgical procedures, Spengler and Freeman initially performed a retrospective analysis of 30 patients who had unsuccessful outcomes from various surgical procedures for either low back pain or sciatica or both. Among their findings, the most commonly reported cause of the poor results was a poor initial candidate-selection process, despite indications for the surgical procedures. A more focused investigation identified instances of drug abuse, alcoholism, marital discord, and personality factors that were felt to have played a role in the patients’ post-surgical success or failure. Therefore Spengler and colleagues recommended pre-surgical psychological evaluation in order to reduce the likelihood of unsuccessful procedures. Following these reports, Long and colleagues performed a retrospective analysis of their own patients, who received surgically-implanted dorsal column SCS between 1970 and 1973. At that time, the only method for identifying candidates for this procedure was continued self-report of pain after failing all other treatments. They found that only 33% of patients (12 of 36) achieved adequate pain relief over the seven-year follow up. Approximately one-half of those patients originally selected for the procedure would have been rejected using updated inclusion criteria for psychosocial factors, including substance use disorders.

Given the substantial burden of chronic pain, the numerous indications for neuromodulation therapy, high initial costs for implantation of neuromodulation systems, and the relatively high rate of explantation for loss of efficacy, there is a substantial value for improved patient selection before use of this therapy. This chapter will review the tools that have been developed to facilitate patient selection for surgical treatment of spinal pathology and its pain-related conditions treated with SCS. Specific attention will be devoted to initial research on patient selection for spine surgery, followed by the extension of these approaches to SCS. Before reviewing current research on this topic, a discussion of the biopsychosocial approach to assessment is warranted.

The Biopsychosocial Approach to Assessment

The biopsychosocial approach is recognized as the most comprehensive and heuristic approach to the evaluation of medical disorders. The biopsychosocial model focuses on the complex interaction among biologic, psychologic, and medicolegal variables that patients encounter when coping with a persistent, distressing medical condition such as chronic pain. A range of psychological, social, and economic factors interact with the physical pathology to modulate a patient’s discomfort and disability associated with the condition. This complex interaction accounts for the likelihood that a patient’s life will be adversely affected in various ways by his or her medical condition, thus requiring a comprehensive assessment and treatment approach designed to address the biologic, psychological, and social aspects of care. This approach contrasts with the traditional biomedical reductionist approach, which assumes that most medical conditions, including chronic pain, can be separated into distinct, independent physical and psychosocial components.

A demonstration of this model and highlights how individuals significantly differ in the frequency of reporting physical symptoms, tendency to visit physicians when experiencing identical symptoms, and in their responses to the same treatment. These, in turn, are not fully predicted by the physical findings. A comprehensive assessment of a patient proceeds from a global biopsychosocial diagnosis of the disorder in question to a more detailed evaluation of the most important interactive factors needed for the diagnosis. ^, For example, for a patient reporting low back pain, a comprehensive physical examination would initially be conducted to assess the bio-component of the equation. However, this becomes less valuable when it is non-specific or demonstrates pain inhibition, leading one to question the contribution from other elements in the biopsychosocial model. As such, contemporary models of treatment, particularly for back pain, now recognize that psychological and social influences may contribute substantially to symptoms, disability, and response to treatment.

Thus, a comprehensive biopsychosocial assessment of each patient is needed before the development of a treatment plan. Current surgical risk assessment techniques for spine surgery and neuromodulation seek to account for both medical risk factors as well as the psychosocial attributes that may predict the likelihood of a successful outcome or, as it is often framed in the literature, association with a poorer outcome. Therefore any successful assessment of patients will need to evaluate these various factors comprehensively. The following discussion will focus primarily on the psychosocial evaluation for this process.

The Biopsychosocial Pre-screening Process