Burst and Tonic Spinal Cord Stimulation Differentially Activate GABAergic Mechanisms to Attenuate Pain in a Rat Model of Cervical Radiculopathy

Effects of Rate on Analgesia in Kilohertz Frequency Spinal Cord Stimulation: Results of the PROCO Randomized Controlled Trial

OBJECTIVE

The PROCO RCT is a multicenter, double-blind, crossover, randomized controlled trial (RCT) that investigated the effects of rate on analgesia in kilohertz frequency (1-10 kHz) spinal cord stimulation (SCS).

MATERIALS AND METHODS

Patients were implanted with SCS systems and underwent an eight-week search to identify the best location ("sweet spot") of stimulation at 10 kHz within the searched region (T8-T11). An electronic diary (e-diary) prompted patients for pain scores three times per day. Patients who responded to 10 kHz per e-diary numeric rating scale (ED-NRS) pain scores proceeded to double-blind rate randomization. Patients received 1, 4, 7, and 10 kHz SCS at the same sweet spot found for 10 kHz in randomized order (four weeks at each frequency). For each frequency, pulse width and amplitude were titrated to optimize therapy.

RESULTS

All frequencies provided equivalent pain relief as measured by ED-NRS (p ≤ 0.002). However, mean charge per second differed across frequencies, with 1 kHz SCS requiring 60-70% less charge than higher frequencies (p ≤ 0.0002).

CONCLUSIONS

The PROCO RCT provides Level I evidence for equivalent pain relief from 1 to 10 kHz with appropriate titration of pulse width and amplitude. 1 kHz required significantly less charge than higher frequencies.

Burst and high frequency stimulation: underlying mechanism of action

INTRODUCTION

Paresthesia-free spinal cord stimulation (SCS) techniques, such as burst and high-frequency (HF) SCS, have been developed and demonstrated to be successful for treating chronic pain, albeit via different mechanisms of action. The goal of this review is to discuss the mechanisms of action for pain suppression at both the cellular and systems levels for burst and HF SCS. In addition, we also discuss the neuromodulation devices that mimic these paradigms.

AREAS COVERED

The authors performed a literature review to unravel the mechanisms of action for burst and HF SCS coupled with booklets and user manuals from neuromodulation companies to understand the programmable parameters and operating ranges. Burst SCS modulates the medial pathway to suppress pain. On cellular level, burst SCS is independent on activation of γ-aminobutyric acid (GABA) receptors to inhibit neuronal firing. HF SCS blocks large-diameter fibers from producing action potentials with little influence on smaller fibers, increasing pain suppression as frequency increases.

EXPERT COMMENTARY

The neuromodulation industry is in a phase of intense innovation characterized by adaptive stimulation to improve patients' experience and experiment with alternative frequencies and novel stimulation targets.

Conventional-SCS vs. Burst-SCS and the Behavioral Effect on Mechanical Hypersensitivity in a Rat Model of Chronic Neuropathic Pain: Effect of Amplitude

OBJECTIVE

Various spinal cord stimulation (SCS) modes are used in the treatment of chronic neuropathic pain disorders. Conventional (Con) and Burst-SCS are hypothesized to exert analgesic effects through different stimulation-induced mechanisms. Preclinical electrophysiological findings suggest that stimulation intensity is correlated with the effectiveness of Burst-SCS. Therefore, we aimed to investigate the relation between amplitude (charge per second) and behavioral effects in a rat model of chronic neuropathic pain, for both Conventional Spinal Cord Stimulation (Con-SCS) and biphasic Burst-SCS.

MATERIALS AND METHODS

Animals (n = 12 rats) received a unilateral partial sciatic nerve ligation, after which they were implanted with quadripolar electrodes in the epidural space at thoracic level 13. Mechanical hypersensitivity was assessed using paw withdrawal thresholds (WTs) to von Frey monofilaments, at various SCS intensities (amplitudes) and multiple time points during 60 minutes of stimulation and 30 minutes post stimulation.

RESULTS

Increasing amplitude was shown to improve the efficacy of Con-SCS, whereas the efficacy of Burst-SCS showed a non-monotonic relation with amplitude. Con-SCS at 66% MT (n = 5) and Burst-SCS at 50% MT (n = 6) were found to be equally effective in normalizing mechanical hypersensitivity. However, in the assessed time period Burst-SCS required significantly more mean charge per second to do so (p < 0.01). When applied at comparable mean charge per second, Con-SCS resulted in a superior behavioral outcome (p < 0.01), compared with Burst-SCS.

CONCLUSION

Biphasic Burst-SCS requires significantly more mean charge per second in order to achieve similar pain relief, as compared with Con-SCS, in an experimental model of chronic neuropathic pain.

The Advancing Role of Neuromodulation for the Management of Chronic Treatment-Refractory Pain

Neuropathic pain is a common cause of disability and health care utilization. While judicious pharmacotherapy and management of comorbid psychological distress can provide for improved quality of life, some patients with treatment-refractory disease require more invasive therapies. Spinal cord stimulation can provide for improvement in pain and decrease in medication utilization, with level 1 evidence supporting its use across various pain etiologies including persistent postoperative neuropathic pain, complex regional pain syndrome, chronic inoperable limb ischemia, treatment refractory angina, and painful diabetic neuropathy. These procedures can be done with acceptably low morbidity and provide a cost-effective solution for those patients in whom medical therapies have failed. Technological innovation in lead design, implantable pulse generator capability, and stimulation algorithms and parameters may further enhance the success of this therapy. Neuromodulation of distal targets such as dorsal root ganglion may permit greater anatomic specificity of the therapy, whereas subthreshold stimulation with high-frequency or burst energy delivery may eliminate noxious and off-target paresthesiae. Such new technologies should be subject to rigorous evaluation as their mechanisms of action and long-term outcomes remain hitherto undefined.

Spinal Cord Stimulation in Chronic Pain: Mode of Action

STUDY DESIGN

Literature review.

OBJECTIVE

A review of the literature that presents a perspective on mechanisms of actions behind spinal cord stimulation (SCS) therapy for chronic pain.

SUMMARY OF BACKGROUND DATA

SCS is an effective therapeutic alternative for the treatment of intractable chronic pain. Its application has been mostly based on the gate control theory of pain. Computational models have been fundamental on the understanding of clinical observations and the design of therapies that provide optimal neuromodulation. Research has provided insight into the involvement of specific neurotransmitters that support segmental and supraspinal mechanisms of action.

METHODS

A literature review was performed with emphasis on mechanisms of action for SCS including the effects of electrical fields on spinal cord structures based on computational models and preclinical and clinical explorations.

RESULTS

This review provides background on the development of SCS, which has been driven around a paresthesia-based paradigm as a result of the gate control theory. A review of computational models emphasizes their importance on our current understanding of the mechanism of action and clinical optimization of therapy. Electrophysiology and molecular biology have provided a closer, yet narrow, view of the effect of SCS on neurotransmitters and their receptors, which have led to the formulation of segmental and supraspinal mechanisms. Literature supporting the involvement of glial cells in chronic pain and their characteristic response to electrical fields should motivate further investigation of mechanisms involving neuroglia. Finally, a review of recent results paresthesia-free strategies should encourage research on mechanisms of action.

CONCLUSION

The mechanisms of SCS have been extensively studied and several consistent phenomena have emerged. The activation of A-beta fibers to induce paresthesia also involve neurotransmitter release via segmental and supraspinal pathways. Despite advancements, much remains to be understood, particularly as new stimulation strategies are developed.

LEVEL OF EVIDENCE

N /A.

Conventional and Novel Spinal Stimulation Algorithms: Hypothetical Mechanisms of Action and Comments on Outcomes

OBJECTIVE

Spinal cord stimulation (SCS) emerged as a direct clinical spin-off from the Gate Control Theory from 1965. Over the last decade, several new modes of SCS have appeared. This review discusses these novel techniques and their hypothetical mechanisms of action.

MATERIAL AND METHODS

A recent literature search on SCS coupled with the most recent data from poster presentations and congress lectures have been used to illustrate new hypothetical ways of modulating pain.

RESULTS

Several physiological and neurochemical mechanisms for conventional paresthetic SCS have been described in detail. However, much less is known about the novel SCS modes of action. One new algorithm utilizes very high frequencies (up to 10 kHz) intended for direct stimulation of dorsal horns at the T9-T10 level to treat both low back pain and leg pain. Another technique uses bursts of impulses with a high internal frequency delivered to the dorsal spinal cord with a frequency of 40 Hz. Both of these therapies intend to be subparesthetic and effective both for neuropathic and nociceptive pain components. During the last few years, more moderate changes in SCS parameters have been tried in order to increase the amount of electric charge passed from the lead to the neural tissue. This strategy, called "high density SCS," utilizes frequencies up to 1200 Hz or long pulse widths.

CONCLUSIONS

The present SCS therapies have developed beyond the Gate Control Concept. New hypotheses about mechanisms of action are presented and some improved results are discussed.

Are 10 kHz Stimulation and Burst Stimulation Fundamentally the Same?

BACKGROUND

Spinal cord stimulation (SCS) is routinely used for intractable pain syndromes. For SCS to be efficacious the painful area needs to be covered by SCS induced paresthesia symptoms. Recently, novel stimulation designs have been developed for spinal cord stimulation (SCS) that are superior to classical spinal cord stimulation and exert their effects without the mandatory paresthesia. Two such stimulation designs are burst stimulation and 10 kHz stimulation.

OBJECTIVE

Whereas the mechanism of action of burst SCS has been partly elucidated, in that it modulates the medial pain pathway in contrast to tonic stimulation, the mechanism of action of 10 kHz SCS is still enigmatic. The goal of this paper is to provide a perspective or informed opinion on the differences and similarities between burst SCS and 10 kHz stimulation by using a literature search on the two stimulation designs.

DISCUSSION/CONCLUSION

Human clinical data, simulation studies, quantitative sensory testing, cellular investigations, and comparative animal and human studies all point in the same direction, namely that 10 kHz and burst SCS might both modulate the medial pain pathway, and could be fundamentally similar neurostimulation designs.

Developmental Changes in Pain and Spinal Immune Gene Expression after Radicular Trauma in the Rat

Neuropathic pain is chronic pain that develops after nerve injury and is less frequent in infants and children than in adults. Likewise, in animal models of neuropathic pain, allodynia and hyperalgesia are non-existent or attenuated in the infant, with a “switch” during development by which acute nerve injury transitions to chronic pain. Concomitant with the delay in neuropathic pain, there is a parallel delay in the ability of nerve injury to activate the immune system. Models of neuropathic pain in the infant have used various ligation methods and find that neuropathic pain does not occur under after postnatal days 21–28 (PN21–PN28), linked to activation of immune processes and developmental regulation of anti-inflammatory cytokines. We applied a model of neuropathic pain in the adult using a transient compression of the cervical nerve or nerve root in infant rats (injured at 10, 14, 21, or 28 days of age) to define transition periods during which injury results in no change in thermal and mechanical pain sensitivity or in short-term changes in pain. There was little to no hyperalgesia when the injury was imposed at PN10, but significant thermal hyperalgesia and mechanical allodynia 1 day after compression injury when performed at PN14, 21, or 28. Thermal withdrawal latencies returned to near baseline by 7 days postsurgery when the injuries were at PN14, and lasted up to 14 days when the injury was imposed at PN28. There was mechanical allodynia following injury at 1 day postinjury and at 14 days after injury at PN14. Measurements of mRNA from spinal cord at 1, 7, and 14 days postinjury at PN14, 21, and 28 showed that both the magnitude and duration of elevated immune markers and chemokines/cytokines were greater in the older animals, corresponding to the development of hyperalgesia. Thus, we confirm the late onset of neuropathic pain but found no evidence of emergent hyperalgesia if the injury was before PN21. This may be due to the use of a transient, and not sustained, compression ligation model.