Drug-enhanced spinal stimulation for pain: a new strategy

The role of endogenous opioid neuropeptides in neurostimulation-driven analgesia

Due to the prevalence of chronic pain worldwide, there is an urgent need to improve pain management strategies. While opioid drugs have long been used to treat chronic pain, their use is severely limited by adverse effects and abuse liability. Neurostimulation techniques have emerged as a promising option for chronic pain that is refractory to other treatments. While different neurostimulation strategies have been applied to many neural structures implicated in pain processing, there is variability in efficacy between patients, underscoring the need to optimize neurostimulation techniques for use in pain management. This optimization requires a deeper understanding of the mechanisms underlying neurostimulation-induced pain relief. Here, we discuss the most commonly used neurostimulation techniques for treating chronic pain. We present evidence that neurostimulation-induced analgesia is in part driven by the release of endogenous opioids and that this endogenous opioid release is a common endpoint between different methods of neurostimulation. Finally, we introduce technological and clinical innovations that are being explored to optimize neurostimulation techniques for the treatment of pain, including multidisciplinary efforts between neuroscience research and clinical treatment that may refine the efficacy of neurostimulation based on its underlying mechanisms.

Spinal stimulation for pain: future applications

With continuous progress and rapid technological advancement of neuromodulation it is conceivable that within next decade or so, our approach to the electrical stimulation of the spinal cord used in treatment of chronic pain will change radically. The currently used spinal cord stimulation (SCS), with its procedural invasiveness, bulky devices, simplistic stimulation paradigms, and frustrating decline in effectiveness over time will be replaced by much more refined and individually tailored modality. Better understanding of underlying mechanism of action will allow us to use SCS in a more rational way, selecting patient-specific targets and techniques that properly fit each patient with chronic pain based on pain characteristics, distribution, and cause. Based on the information available today, this article will summarize emerging applications of SCS in the treatment of pain and theorize on further developments that may be introduced in the foreseeable future. An overview of clinical and technological innovations will serve as a basis for better understanding of SCS landscape for the next several years.

Mechanisms and models of spinal cord stimulation for the treatment of neuropathic pain

Spinal cord stimulation (SCS) is an established and cost-effective therapy for treating severe chronic pain. However, despite over 40 years of clinical practice and the development of novel electrode designs and treatment protocols, increases in clinical success, defined as the proportion of patients that experience 50% or greater self-reported pain relief, have stalled. An incomplete knowledge of the neural circuits and systems underlying chronic pain and the interaction of SCS with these circuits may underlie this plateau in clinical efficacy. This review summarizes prior work and identifies gaps in our knowledge regarding the neural circuits related to pain and SCS in the dorsal horn, supraspinal structures, and the Pain Matrix. In addition, this review discusses and critiques current experimental and computational models used to investigate and optimize SCS. Further research into the interactions between SCS and pain pathways in the nervous system using animal and computational models is a fruitful approach to improve this promising therapy.

Conventional and kilohertz-frequency spinal cord stimulation produces intensity- and frequency-dependent inhibition of mechanical hypersensitivity in a rat model of neuropathic pain

BACKGROUND

Spinal cord stimulation (SCS) is a useful neuromodulatory technique for treatment of certain neuropathic pain conditions. However, the optimal stimulation parameters remain unclear.

METHODS

In rats after L5 spinal nerve ligation, the authors compared the inhibitory effects on mechanical hypersensitivity from bipolar SCS of different intensities (20, 40, and 80% motor threshold) and frequencies (50, 1 kHz, and 10 kHz). The authors then compared the effects of 1 and 50 Hz dorsal column stimulation at high- and low-stimulus intensities on conduction properties of afferent Aα/β-fibers and spinal wide-dynamic-range neuronal excitability.

RESULTS

Three consecutive daily SCS at different frequencies progressively inhibited mechanical hypersensitivity in an intensity-dependent manner. At 80% motor threshold, the ipsilateral paw withdrawal threshold (% preinjury) increased significantly from pre-SCS measures, beginning with the first day of SCS at the frequencies of 1 kHz (50.2 ± 5.7% from 23.9 ± 2.6%, n = 19, mean ± SEM) and 10 kHz (50.8 ± 4.4% from 27.9 ± 2.3%, n = 17), whereas it was significantly increased beginning on the second day in the 50 Hz group (38.9 ± 4.6% from 23.8 ± 2.1%, n = 17). At high intensity, both 1 and 50 Hz dorsal column stimulation reduced Aα/β-compound action potential size recorded at the sciatic nerve, but only 1 kHz stimulation was partially effective at the lower intensity. The number of actions potentials in C-fiber component of wide-dynamic-range neuronal response to windup-inducing stimulation was significantly decreased after 50 Hz (147.4 ± 23.6 from 228.1 ± 39.0, n = 13), but not 1 kHz (n = 15), dorsal column stimulation.

CONCLUSIONS

Kilohertz SCS attenuated mechanical hypersensitivity in a time course and amplitude that differed from conventional 50 Hz SCS, and may involve different peripheral and spinal segmental mechanisms.

Spinal cord stimulation and intrathecal baclofen therapy: combined neuromodulation for treatment of advanced complex regional pain syndrome

OBJECTIVE

Spinal cord stimulation (SCS) is one of the widely used procedures for the treatment of complex regional pain syndrome (CRPS). However, complete pain relief is rarely achieved, and the SCS effect diminishes over time. Recently intrathecal baclofen (ITB) therapy is reported to be a modality for treating fixed dystonia related to CRPS. Other reports have suggested that ITB therapy can enhance the effect of SCS in patients with neuropathic pain. We report the effectiveness of combined SCS and ITB therapy, focusing on the role of ITB therapy as an adjunctive therapy for controlling symptoms of advanced CRPS.

METHODS

Five affected extremities of 4 patients with CRPS (2 male; mean age, 32.5 years) refractory to conservative treatment were evaluated retrospectively. Three patients underwent SCS implantation first, with ITB pumps being implanted a few years later. Bolus ITB injection was administered under temporary percutaneous SCS in 1 patient. Pain intensity was evaluated using the visual analogue scale (VAS) before and after ITB administration.

RESULTS

Pain relief of more than 50% was observed in the upper extremity of 1 patient and one of more than 30% was observed in 2 patients. The mean pain reduction rate in all 4 patients was 28.9% before and 43.8% after treatment. All patients, including those without any improvements in VAS score, showed decreased postural abnormalities after combined SCS and ITB therapy. Improvement in postural abnormalities, such as fixed dystonia or paroxysmal tremor-like movements, resulted in overall pain relief by reducing pain fluctuations.

CONCLUSIONS

Combined SCS and ITB neuromodulation decreases pain intensity in refractory CRPS cases or improves associated abnormal dystonic posture and movement disorders and reduces pain fluctuations.

The use of intrathecal baclofen in pain management

SUMMARY Intrathecal therapy involves the subarachnoid delivery of medications where the administered agents have enhanced potency and diminished systemic exposure compared with oral administration. Baclofen exerts its therapeutic action by enhancing the inhibition produced by the endogenous neurotransmitter GABA. While well-established within the armamentarium of tools for spasticity management, the role of intrathecal baclofen for the management of chronic pain is less certain. The purpose of this review is to describe the pharmacology of intrathecal baclofen, relate the role of intrathecal baclofen in neurologic and non-neurologic conditions, and report upon the potential utility of coadministering baclofen with other agents via the intrathecal route of delivery.

Dorsal column stimulator applications

Background:

Spinal cord stimulation (SCS) has been used to treat neuropathic pain since 1967. Following that, technological progress, among other advances, helped SCS become an effective tool to reduce pain.

Methods:

This article is a non-systematic review of the mechanism of action, indications, results, programming parameters, complications, and cost-effectiveness of SCS.

Results:

In spite of the existence of several studies that try to prove the mechanism of action of SCS, it still remains unknown. The mechanism of action of SCS would be based on the antidromic activation of the dorsal column fibers, which activate the inhibitory interneurons within the dorsal horn. At present, the indications of SCS are being revised constantly, while new applications are being proposed and researched worldwide. Failed back surgery syndrome (FBSS) is the most common indication for SCS, whereas, the complex regional pain syndrome (CRPS) is the second one. Also, this technique is useful in patients with refractory angina and critical limb ischemia, in whom surgical or endovascular treatment cannot be performed. Further indications may be phantom limb pain, chronic intractable pain located in the head, face, neck, or upper extremities, spinal lumbar stenosis in patients who are not surgical candidates, and others.

Conclusion:

Spinal cord stimulation is a useful tool for neuromodulation, if an accurate patient selection is carried out prior, which should include a trial period. Undoubtedly, this proper selection and a better knowledge of its underlying mechanisms of action, will allow this cutting edge technique to be more acceptable among pain physicians.

Spinal cord stimulation: neurophysiological and neurochemical mechanisms of action

Chronic neuropathic pain can significantly reduce quality of life and place an economic burden on individuals and society. Spinal cord stimulation (SCS) is an alternative approach to the treatment of neuropathic pain when standard pharmacological agents have failed. However, an improved understanding of the mechanisms by which SCS inhibits pain is needed to enhance its clinical utility. This review summarizes important findings from recent studies of SCS in animal models of neuropathic pain, highlights current understanding of the spinal neurophysiological and neurochemical mechanisms by which SCS produces an analgesic effect, and discusses the potential clinical applicability of these findings and future directions for research.

Polyanalgesic Consensus Conference--2012: recommendations on trialing for intrathecal (intraspinal) drug delivery: report of an interdisciplinary expert panel

INTRODUCTION

Trialing for intrathecal pump placement is an essential part of the decision-making process in placing a permanent device. In both the United States and the international community, the proper method for trialing is ill defined.

METHODS

The Polyanalgesic Consensus Conference (PACC) is a group of well-published experienced practitioners who meet to update the state of care for intrathecal therapies on the basis of current knowledge in the literature and clinical experience. Anexhaustive search is performed to create a base of information that the panel considers when making recommendations for best clinical practices. This literature, coupled with clinical experience, is the basis for recommendations and for identification of gaps in the base of knowledge regarding trialing for intrathecal pump placement.

RESULTS

The panel has made recommendations for the proper methods of trialing for long-term intrathecal drug delivery.

CONCLUSION

The use of intrathecal drug delivery is an important part of the treatment algorithm for moderate to severe chronic pain. It has become common practice to perform a temporary neuroaxial infusion before permanent device implantation. On the basis of current knowledge, the PACC has developed recommendations to improve care. The need to update these recommendations will be very important as new literature is published.

Decreased intracellular GABA levels contribute to spinal cord stimulation-induced analgesia in rats suffering from painful peripheral neuropathy: the role of KCC2 and GABA(A) receptor-mediated inhibition

Elevated spinal extracellular γ-aminobutyric acid (GABA) levels have been described during spinal cord stimulation (SCS)-induced analgesia in experimental chronic peripheral neuropathy. Interestingly, these increased GABA levels strongly exceeded the time frame of SCS-induced analgesia. In line with the former, pharmacologically-enhanced extracellular GABA levels by GABA(B) receptor agonists in combination with SCS in non-responders to SCS solely could convert these non-responders into responders. However, similar treatment with GABA(A) receptor agonists and SCS is known to be less efficient. Since K⁺ Cl⁻ cotransporter 2 (KCC2) functionality strongly determines proper GABA(A) receptor-mediated inhibition, both decreased numbers of GABA(A) receptors as well as reduced KCC2 protein expression might play a pivotal role in this loss of GABA(A) receptor-mediated inhibition in non-responders. Here, we explored the mechanisms underlying both changes in extracellular GABA levels and impaired GABA(A) receptor-mediated inhibition after 30 min of SCS in rats suffering from partial sciatic nerve ligation (PSNL). Immediately after cessation of SCS, a decreased spinal intracellular dorsal horn GABA-immunoreactivity was observed in responders when compared to non-responders or sham SCS rats. One hour later however, GABA-immunoreactivity was already increased to similar levels as those observed in non-responder or sham SCS rats. These changes did not coincide with alterations in the number of GABA-immunoreactive cells. C-Fos/GABA double-fluorescence clearly confirmed a SCS-induced activation of GABA-immunoreactive cells in responders immediately after SCS. Differences in spinal dorsal horn GABA(A) receptor-immunoreactivity and KCC2 protein levels were absent between all SCS groups. However, KCC2 protein levels were significantly decreased compared to sham PSNL animals. In conclusion, reduced intracellular GABA levels are only present during the time frame of SCS in responders and strongly point to a SCS-mediated on/off GABAergic release mechanism. Furthermore, a KCC2-dependent impaired GABA(A) receptor-mediated inhibition seems to be present both in responders and non-responders to SCS due to similar KCC2 and GABA(A) receptor levels.

Pharmacologically enhanced spinal cord stimulation for pain: an evolving strategy

SUMMARY Spinal cord stimulation (SCS) as treatment for chronic neuropathic pain has developed into an important therapeutic strategy. However, several studies indicate that as many as 30–50% of patients do not respond sufficiently to technically well-functioning SCS. Experimental studies have revealed some of the possible neuronal systems and transmitters involved in SCS. Based on such data, a new strategy has been suggested: “pharmacologically enhanced spinal cord stimulation” using receptor active drugs to improve the therapeutic effect. The present article reviews the animal data on which clinical trials have been based and summarizes the clinical experience up to the present. Relevant data exist for intrathecal baclofen as an adjuvant to SCS, but trials with clonidine and adenosine have also been performed. Available basic studies indicate that other substances might also prove useful in future trials. The present data thus only announce the beginning of ‘drug-enhanced spinal stimulation’.

Pain-relieving prospects for adenosine receptors and ectonucleotidases

Adenosine receptor agonists have potent antinociceptive effects in diverse preclinical models of chronic pain. By contrast, the efficacy of adenosine and adenosine receptor agonists in treating pain in humans is unclear. Two ectonucleotidases that generate adenosine in nociceptive neurons were recently identified. When injected spinally, these enzymes have long-lasting adenosine A(1) receptor-dependent antinociceptive effects in inflammatory and neuropathic pain models. Furthermore, recent findings indicate that spinal adenosine A(2A) receptor activation can enduringly inhibit neuropathic pain symptoms. Collectively, these studies suggest the possibility of treating chronic pain in humans by targeting specific adenosine receptor subtypes in anatomically defined regions with agonists or with ectonucleotidases that generate adenosine.

Intrathecal administration of GABA agonists in the vegetative state

Gamma aminobutyric acid (GABA) is an inhibitor neurotransmitter that plays many important roles in the central nervous system. Because the half-life time of GABA is very short in vivo, GABA itself is not used for clinical practice. An analogue of GABA, baclofen, is an agonist of GABA-B receptor, and has very strong antispastic effect by acting to the posterior horn of the spinal cord. However, baclofen poorly crosses through the blood brain barrier, and the antispastic effect is modest when administered orally. Therefore, direct continuous infusion of small doses of baclofen into the cerebrospinal fluid (intrathecal baclofen therapy, ITB) has become an established treatment for control of otherwise intractable severe spasticity. Spasticity is clinically defined as hypertonic state of the muscles with increased tendon reflexes, muscles spasm, spasm pain, abnormal posture, and limitation of involuntary movements. Spasticity is a common symptom after damage mainly to the pyramidal tract system in the brain or the spinal cord. Such damage is caused by traumatic brain injury, stroke, spinal cord injury, multiple sclerosis, and so on. Patients in persistent vegetative state (PVS) usually have diffuse and widespread damage to the brain, spasticity is generally seen in such patients. Control of spasticity may become important in the management of PVS patients in terms of nursing care, pain relief, and hygiene, and ITB may be indicated. Among PVS patients who had ITB to control spasticity, sporadic cases of dramatic recovery from PVS after ITB have been reported worldwide. The mechanism of such recovery of consciousness is poorly understood, and it may simply be a coincidence. On the other hand, electrical spinal cord stimulation (SCS) has been tried for many years in many patients in PVS, and some positive effects on recovery of consciousness have been reported. SCS is usually indicated for control of neuropathic pain, but it has also antispastic effect. The mechanism of SCS on pain is known to be mediated through the spinal GABA neuronal system. Thus, ITB and SCS have a common background, spinal GABA neuronal mechanism. The effect of GABA agonists on recovery of consciousness is not yet established, but review of such case studies becomes a clue to solve problems in PVS, and there may be hidden serendipity.