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Spinal—extradural neuromodulation
Introduction
Neuromodulation and specifically epidural spinal cord stimulation (SCS) represented a fundamental change in the thought process behind pain control given that primarily irreversible ablative procedures had been used historically. The benefits of SCS include being reversible and adjustable. Advances in technology over the prior 2 decades have led to significant improvements in device design, overall effectiveness, and a broadening of its indications. The most common indications now include postlaminectomy syndrome, and complex regional pain syndrome (CRPS), but have also expanded to include ischemic limb pain, refractory angina, visceral/abdominal pain, cervical neuritis pain, spinal cord injury pain, postherpetic neuralgia, and various neuropathies. Moreover, some studies are trying to optimize promising results in using it to treat movement disorders and loss of function following cord damage and paralysis. In this introductory text, we will focus on the basics of epidural stimulation for pain, which is the most commonly applied treatment in all neuromodulation.
The gate control theory for pain control by Melzack and Wall gave further merit to the consideration of trying to stimulate circuits within the spinal cord itself. The general concept was that neuropathic pain could be blocked using stimulation but acute nociceptive pain, carried by different fibers from the periphery could still travel through “the gate” to the brain. The first attempt to apply a device to take advantage of this theory was implanted in 1967 by Shealy for a patient with terminal metastatic cancer. Although the mechanism of action and thought processes of SCS has moved on from this simpler gate concept, the premise has largely remained the same. Electrodes are implanted in a variety of techniques including percutaneous and surgical procedures via laminectomy. Furthermore, there has been an adoption of this therapy by several specialties, including anesthesiologists, rehabilitation medicine, neurosurgery, orthopedic surgery, and others specializing in pain management.
The implantable devices have changed over the years with improving technology. Initially, these systems were only radio-frequency-driven passive receivers, unlike the implantable pulse generator (IPG) powered by lithium batteries of the present day. Electrodes were initially unipolar or rudimentary bipolar arrays. Electrode configurations are now complex arrays of up to 32 channels, available in percutaneous and paddle options. Arrays are available in various configurations and numbers of contacts. IPG can be primary cell/nonchargeable, as well as transcutaneously charged rechargeable devices. Interestingly, the most recent advancements have stepped away from improvements in hardware and have focused on software and stimulation paradigms such as waveforms, various programming options, and multistimulation algorithms.
Relevant anatomy
Understanding the somatotopy of the spinal cord is paramount to achieving success with SCS. A basic tenet of most epidural SCS is to create an overlapping of paresthesia from stimulation and the painful region. Mapping of spinal segments that most often capture various anatomical locations for the patient were first concisely described by work from Barolat and target areas were developed to generate appropriate sensory responses based on the painful region of interest. The spinal cord typically terminates between T12 and L1-2 levels. It is generally accepted that lower back and hip coverage is better covered between T7 and T9 levels, while distal lower extremity and reliable foot coverage obtained closer to the levels of T11 and T12. This may vary between individuals based on the level of the conus and other less predictable factors. In addition, with newer technologies and waveforms, the T9-10 interspace specifically has come of interest for capturing lower back and lower extremities. High cervical regions such as C2, as well as peripheral nerve stimulation of the occipital nerve can cover the posterior occipital region. Stimulation at C3 and C4 will provide coverage of the shoulder while stimulation in the lower cervical region such as C5-6 will provide for the entire hand.
Technology has broadened over time to consider a more focused approach as well. Rather than trying to capture as many fibers in the dorsal column of the cord as possible, only specific segments are targeted by stimulating over the nerve root near the dorsal root ganglion (DRG). This more focal coverage, as well as various forms of peripheral nerve stimulation, has been used to capture more focal pain patterns such as foot or knee pain.
SCS generally affects large afferent myelinated fibers that can be found within the dorsal columns, dorsal roots, and the dorsal root entry zone. Coverage and stimulation paradigms may also vary based on midline versus lateral placements. With the advent of paresthesia-free platforms and various waveforms, mechanisms for achieving pain relief may also vary but are largely still being studied.
Indications
The most common indications for SCS include postlaminectomy syndrome, and CRPS, but can also include phantom limb pain, ischemic disease, angina, spinal cord injury pain, abdominal or visceral pain, and various neuropathies. In the past, success rates of 50% pain relief in 50% of individuals were generally acceptable, but advancements have led to significantly higher success of greater than 80% responder rates (>50% pain benefit) with many patients achieving greater than 80% relief.
Complex regional pain syndrome
CRPS has become a common diagnosis that is treated by SCS. It is usually utilized once conservative measures such as medications and physical therapy have not succeeded. Although a challenging disease process, SCS, and recently DRG stimulation, have both proven very effective. Case studies existed early on, but in 2000, Kemler et al. published on a series of 54 patients who either underwent randomization to SCS with physical therapy or physical therapy alone. In the SCS group, 67% of patients experienced significant pain relief which persisted at 6 months. Multiple prospective studies have since been performed with significant improvements.
We have seen great improvements and success with SCS in the treatment of CRPS. This has extended into the use of DRG stimulation specifically for its use on CRPS types I and II. The Accurate study, which directly compared DRG stimulation to traditional SCS for CRPS demonstrated 93% responder rates in the implant group that held consistent at one-year follow-up.
Postlaminectomy syndrome (also called failed back surgery syndrome—FBSS)
Postlaminectomy syndrome is the most common diagnosis treated with SCS. This is perhaps true because the term has become a bucket that many otherwise dissimilar patients are placed in as long as they have had spinal surgery in the past. It is vaguely defined for diagnosis, but also on presenting features. Presentation can include leg pain, back pain, or both. In some scenarios, the pain can be attributed to their prior spinal intervention, while in others it may not be related. Various etiologies can include arachnoiditis, epidural fibrosis, and recurrent disk herniations.
A hallmark study for SCS was performed demonstrating that SCS was superior to repeated spinal surgery on the lumbar spine. This was a cross over study that also demonstrated the majority of spinal surgery patients crossing over to SCS and obtaining relief, while only a very small percentage crossed from SCS to surgery. A systematic review of the literature was conducted by Turner in 1995 demonstrating 50%–60% of patients with postlaminectomy obtained greater than 50% pain relief.
These early studies demonstrating the effectiveness of SCS for postlaminectomy syndrome and were fundamental in supporting more widespread use of SCS. Advancements in technology, as well as newer waveforms and stimulation paradigms, have led to further improvement in outcomes. The Senza study utilizing high-frequency stimulation, as well as the Sunburst study with BurstDR waveforms, has demonstrated further improvements in outcome, obtaining “superiority” labeling. Most newer therapies now approach responder rates of 80% with greater than 50% of those responders achieving greater than 80% relief.
Evidence by stimulation paradigm
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