Role of primary afferents in spinal cord stimulation-induced vasodilation: characterization of fiber types

Case report: Favorable outcomes of spinal cord stimulation in complex regional pain syndrome Type II consistent with thermography findings

Background:

Complex regional pain syndrome (CRPS) is a chronic pain disorder that develops as a consequence of trauma to one or more limbs. Despite the availability of multiple modalities to diagnose CRPS, a gold standard technique for definitive diagnosis is lacking. Moreover, there are limited reports describing the use of spinal cord stimulation (SCS) to treat CRPS Type II, given the low prevalence of this condition. Herein, we present the case of a patient with CRPS Type II with novel thermography findings who underwent SCS for pain management after an Achilles tendon repair surgery.

Case Description:

A 38-year-old woman was referred to our institute because of chronic left leg pain after Achilles tendon rupture repair surgery. Her case was diagnosed as CRPS Type II based on the International Association for the Study of Pain diagnostic criteria. After an epidural block, thermography showed a significant increase in the body surface temperature of the foot on the observed side. She was subsequently treated with SCS, following which her pain ameliorated. She reported no pain flare-ups or new neurological deficits over 2 years of postoperative follow-up assessments.

Conclusion:

SCS could be a useful surgical treatment for medication refractory CRPS Type II as supported by our thermography findings. We may refine surgical indication for permanent implantation of SCS with the presented method.

Implantable Electrical Stimulation at Dorsal Root Ganglions Accelerates Osteoporotic Fracture Healing via Calcitonin Gene-Related Peptide

The neuronal engagement of the peripheral nerve system plays a crucial role in regulating fracture healing, but how to modulate the neuronal activity to enhance fracture healing remains unexploited. Here it is shown that electrical stimulation (ES) directly promotes the biosynthesis and release of calcitonin gene-related peptide (CGRP) by activating Ca2+ /CaMKII/CREB signaling pathway and action potential, respectively. To accelerate rat femoral osteoporotic fracture healing which presents with decline of CGRP, soft electrodes are engineered and they are implanted at L3 and L4 dorsal root ganglions (DRGs). ES delivered at DRGs for the first two weeks after fracture increases CGRP expression in both DRGs and fracture callus. It is also identified that CGRP is indispensable for type-H vessel formation, a biological event coupling angiogenesis and osteogenesis, contributing to ES-enhanced osteoporotic fracture healing. This proof-of-concept study shows for the first time that ES at lumbar DRGs can effectively promote femoral fracture healing, offering an innovative strategy using bioelectronic device to enhance bone regeneration.

[Factors of adverse prediction of application of spinal cord stimulation with chronic pain syndrome in patients with critical lower limb ischemia]

AIM

To study the clinical dynamics in the long-term period after spinal cord stimulation (SCS) in patients with chronic pain syndrome and critical lower limb ischemia (CLLI) and to identify factors affecting the prognosis of SCS.

MATERIAL AND METHODS

The clinical dynamics was analyzed in 48 patients with pain syndrome and CLLI 1 year after SCS. Microcirculatory blood flow (MBF) was studied in the affected foot by laser-doppler flowmetry (LDF) (Perfusion Units (PU)) and transcutaneous oximetry (TcpO₂, mmHg.) using an occlusive test before and after SCS. The factors associated with negative clinical dynamics 1 year after SCS were determined.

RESULTS

In 74% of cases, SCS contributes to the improvement of clinical status (reduction of pain syndrome, increase in motor activity, healing of ulcers). After SCS, according to LDF and TcpO₂, the authors observed an increase in MBF and tissue metabolism - from 1.3 (0.7-2.8) to 6.2 (3.8-8.7) PU and from 14.5 (7.5-22.1) to 41.1 (26.4-57.6) mmHg, respectively with normalization of the MBF reserve during the occlusion test. Negative clinical dynamics after SCS is associated with high comorbidity, TcO₂ <10 mmHg and the duration of pain.

CONCLUSION

SCS contributes to the improvement of the clinical status of patients with chronic pain syndrome and CLLI. The negative dynamics is associated with high comorbidity, TcrO₂ <10 mmHg and the duration of pain.

Hypoglossal-facial 'side'-to-side Neurorrhaphy Combined with Electrical Myostimulation for Facial Palsy in Rats

Introduction This study investigated the effect of combining hypoglossal-facial nerve “side”-to-side neurorrhaphy and electrical myostimulation in a rat model of facial palsy.

Methods Rats with facial nerve crush injury were subjected to control condition, monotherapy of either neurorrhaphy or electrical myostimulation, or bitherapy of the two treatments. After 1, 3, and 6 months, rats were performed the facial symmetry evaluation, electrophysiological examination and the retrograde labeling of motor neurons.

Results As early as 3 months after injury, face symmetry significantly improved in rats of the bitherapy group. At 3 or 6 months after injury, either the parameters of electrophysiological examination or the number of labeled motor neurons were significantly increased in the bitherapy group than in any other group.

Discussion The combination of neurorrhaphy and electrical myostimulation effectively promoted the functional recovery after facial nerve crush injury.

Femoral vascular conductance and peroneal muscle sympathetic nerve activity responses to acute epidural spinal cord stimulation in humans

NEW FINDINGS

What is the central question of this research? Does acute spinal cord stimulation increase vascular conductance and decrease muscle sympathetic nerve activity in the lower limbs of humans? What is the main finding and its importance? Acute spinal cord stimulation led to a rapid rise in femoral vascular conductance, and peroneal muscle sympathetic nerve activity demonstrated a delayed reduction that was not associated with the initial increase in femoral vascular conductance. These findings suggest that neural mechanisms in addition to attenuated muscle sympathetic nerve activity might be involved in the initial increase in femoral vascular conductance during acute spinal cord stimulation.

ABSTRACT

Clinical cases have indicated an increase in peripheral blood flow after continuous epidural spinal cord stimulation (SCS) and that reduced muscle sympathetic nerve activity (MSNA) might be a potential mechanism. However, no studies in humans have directly examined the effects of acute SCS (<60 min) on vascular conductance and MSNA. In study 1, we tested the hypothesis that acute SCS (<60 min) of the thoracic spine would lead to increased common femoral vascular conductance, but not brachial vascular conductance, in 11 patients who previously underwent surgical SCS implantation for management of neuropathic pain. Throughout 60 min of SCS, common femoral artery conductance was elevated and significantly different from brachial artery conductance [in millilitres per minute: 15 min, change (Δ) 26 ± 37 versus Δ-2 ± 19%; 30 min, Δ28 ± 45 versus Δ0 ± 26%; 45 min, Δ48 ± 43 versus Δ2 ± 21%; 60 min, Δ36 ± 61 versus Δ1 ± 24%; and 15 min post-SCS, Δ51 ± 64 versus Δ6 ± 33%; P = 0.013]. A similar examination in a patient with cervical SCS revealed minimal changes in vascular conductance. In study 2, we examined whether acute SCS reduces peroneal MSNA in a subset of SCS patients (n = 5). The MSNA burst incidence in response to acute SCS gradually declined and was significantly reduced at 45 and 60 min of SCS (in bursts per 100 heart beats: 15 min, Δ-1 ± 12%; 30 min, Δ-14 ± 12%; 45 min, Δ-19 ± 16%; 60 min, Δ-24 ± 18%; and 15 min post-SCS: Δ-11 ± 7%; P = 0.015). These data demonstrate that acute SCS rapidly increases femoral vascular conductance and reduces peroneal MSNA. The gradual reduction in peroneal MSNA observed during acute SCS suggests that neural mechanisms in addition to attenuated MSNA might be involved in the acute increase in femoral vascular conductance.

[Long-term outcomes of spinal neurostimulation in patients with critical lower limb ischemia]

AIM

To evaluate long-term outcomes of spinal neurostimulation (SNS) in patients with critical lower limb ischemia (CLI).

MATERIAL AND METHODS

Long-term outcomes of SNS were assessed in 52 CLI patients. Changes of clinical status were considered by using of Rutherford R.B. et al. scale. Before and in 12 months after SNS percutaneous oxygen partial pressure (TO₂, mm Hg) was measured at the affected lower limb at rest and in orthostatic test. Ankle-brachial index (ABI) was also determined.

RESULTS

SNS improved clinical status in most cases through following effects: 1) reduced pain syndrome and increased motor activity; 2) skin ulcers healing due to increased TO₂ and improved functional state of microcirculation. No augmentation of TO₂ during orthostatic test at TO₂ <10 mm Hg was associated with negative clinical dynamics after SNS (OR 3.2, CI 2.2-54.1, p=0.002). Coronary artery disease with supra-aortic vessels lesion was associated with reduced ABI after SNS (OR 2.1, CI 1.4-3.8, p=0.001).

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.

The Proper Use of Neurostimulation for Hand Pain

Upper extremity neuropathic pain states greatly impact patient functionality and quality of life, despite appropriate surgical intervention. This article focuses on the advanced therapies that may improve pain care, including advanced treatment strategies that are available. The article also surveys therapies on the immediate horizon, such as spinal cord stimulation, peripheral nerve stimulation, and dorsal root ganglion spinal cord stimulation. As these therapies evolve, so too will their placement within the pain care algorithm grounded by a foundation of evidence to improve patient safety and management of patients with difficult neuropathic pain.

Spinal cord stimulator placement in a patient with complex regional pain syndrome and ankylosing spondylitis: a novel approach with dual benefits

Spinal cord stimulation is a treatment modality used to treat various chronic pain conditions, including complex regional pain syndrome (CRPS). We present a case in which spinal cord stimulation was used for the treatment of lower extremity CRPS in a patient with ankylosing spondylitis. Preoperative imaging demonstrated fusion of the lumbothoracic spine with obliteration of the interlaminar spaces. The sacral hiatus remained open and was used to access the epidural space, facilitating the placement of 2 thoracic epidural electrodes. The resulting stimulation controlled not only the patient's lower extremity CRPS pain but also alleviated his chronic axial pain secondary to ankylosing spondylitis.

Spinal cord stimulation to achieve wound healing in a primary lower limb critical ischaemia referral centre

Critical lower limb ischaemia is a diffuse pathology that could cause claudication, severe ischaemic pain and tissue loss. The common treatment includes modification of risk factors, pharmacological therapy and endovascular or surgical revascularisation of the lower limb to restore a pulsatile flow distally. Spinal cord stimulator is seen as a valid alternative in patients unsuitable for revascularisation after endovascular or surgical revascularisation failure and as adjuvant therapy in the presence of a functioning bypass in patients with extensive tissue loss and gangrene presenting a slow and difficult wound healing. We report our experience on spinal cord stimulation (SCS) indication and implantation in patients with critical lower limb ischaemia, at a high-volume centre for the treatment of peripheral arterial disease.

Spinal cord stimulation for chronic limb ischemia

The treatment of chronic limb ischemia involves the restoration of pulsatile blood flow to the distal extremity. Some patients cannot be treated with endovascular means or with open surgery; some may have medical comorbidities that render them unfit for surgery, while others may have persistent ischemia or pain even in the face of previous attempts at reperfusion. In spinal cord stimulation (SCS), a device with electrodes is implanted in the epidural space to stimulate sensory fibers. This activates cell-signaling molecules that in turn cause the release of vasodilatory molecules, a decrease in vascular resistance, and relaxation of smooth muscle cells. SCS also suppresses sympathetic vasoconstriction and pain transmission. When patient selection is based on microcirculatory parameters, SCS therapy can significantly improve pain relief, halt the progression of ulcers, and potentially achieve limb salvage.

Neural mechanisms of spinal cord stimulation

Neuromodulation, specifically spinal cord stimulation (SCS), relieves pain and improves organ function. This chapter discusses the limited information presently available about the underlying mechanisms that explain the beneficial effects of treating patients with SCS. Where applicable, information is presented about translational research that illustrates the importance of collaboration between clinicians, basic scientists, and engineers. This chapter presents the infant stage of studies that attempt to explain the mechanisms which come into play for treating neuropathic pain, ischemic pain in peripheral vascular disease, and diseases of the visceral organs, specifically the gastrointestinal tract and the heart. The basic science studies will demonstrate how SCS acts on various pain syndromes and diseases via multiple pathways in the central nervous system as well as in somatic structures and visceral organs.

Effects of spinal cord stimulation on peripheral blood circulation in rats with streptozotocin-induced diabetes

Objective.  The aim of this study was to investigate the effects of spinal cord stimulation (SCS) on peripheral circulation in rats with streptozotocin (STZ)-induced diabetes. Materials and Methods.  Four weeks after streptozotocin or vehicle was injected (i.p.) in male Sprague-Dawley rats, SCS-induced vasodilation was examined. Results.  Plasma glucose concentration was significantly higher in diabetic rats than in the control animals. Motor threshold (MT) was significantly higher in diabetic rats than in control rats. SCS-induced vasodilation was attenuated at 90% of the MT, but not at 30% and 60% of MT in diabetic rats when compared to control rats (p < 0.001, N = 13). Furthermore, increasing SCS from 30% to 90% of MT typically produced a progressive increase in blood flow in control rats but not in diabetic rats (p < 0.01, N = 13). Conclusion.  This study suggested that SCS-induced vasodilation improves peripheral blood flow, although the pathways were partially impaired in the diabetic condition.

Managing chronic pain with spinal cord stimulation

Since its introduction as a procedure of last resort in a terminally ill patient with intractable cancer-related pain, spinal cord stimulation has been used to effectively treat chronic pain of varied origins. Spinal cord stimulation is commonly used for control of pain secondary to failed back surgery syndrome and complex regional pain syndrome, as well as pain from angina pectoris, peripheral vascular disease, and other causes. By stimulating one or more electrodes implanted in the posterior epidural space, the patient feels paresthesias in their areas of pain, which reduces the level of pain. Pain is reduced without the side effects associated with analgesic medications. Patients have improved quality of life and improved function, with many returning to work. Spinal cord stimulation has been shown to be cost effective as compared with conservative management alone. There is strong evidence for efficacy and cost effectiveness of spinal cord stimulation in the treatment of pain associated with intractable angina, failed back surgery syndrome, and complex regional pain syndrome. In this article, we review the history and pathophysiology of spinal cord stimulation, and the evidence (or lack thereof) for efficacy in common clinical practice.

What does the mechanism of spinal cord stimulation tell us about complex regional pain syndrome?

Spinal cord stimulation (SCS) can have dramatic effects on painful, vascular, and motor symptoms of complex regional pain syndrome (CRPS), but its precise mechanism of action is unclear. Better understanding of the physiologic effects of SCS may improve understanding not only of this treatment modality but also of CRPS pathophysiology. Effects of SCS on pain perception are likely to occur through activation of inhibitory GABA-ergic and cholinergic spinal interneurons. Increased release of both neurotransmitters has been demonstrated following SCS in animal models of neuropathic pain, with accompanying reductions in pain behaviors. Effects of SCS on vascular symptoms of CRPS are thought to occur through two main mechanisms: antidromic activation of spinal afferent neurons and inhibition of sympathetic efferents. Cutaneous vasodilation following SCS in animal models has been shown to involve antidromic release of calcitonin gene-related peptide and possibly nitric oxide, from small-diameter sensory neurons expressing the transient receptor potential V1 (TRPV1) receptor. The involvement of sympathetic efferents in the effects of SCS has not been studied in animal models of neuropathic pain, but has been demonstrated in models of angina pectoris. In conclusion, SCS is of clinical benefit in CRPS, and although its mechanism of action merits further elucidation, what little we do know is informative and can partially explain some of the pathophysiology of CRPS.

Putative mechanisms behind effects of spinal cord stimulation on vascular diseases: a review of experimental studies

Spinal cord stimulation (SCS) is a widely used clinical technique to treat ischemic pain in peripheral, cardiac and cerebral vascular diseases. The use of this treatment advanced rapidly during the late 80's and 90's, particularly in Europe. Although the clinical benefits of SCS are clear and the success rate remains high, the mechanisms are not yet completely understood. SCS at lumbar spinal segments (L2-L3) produces vasodilation in the lower limbs and feet which is mediated by antidromic activation of sensory fibers and decreased sympathetic outflow. SCS at thoracic spinal segments (T1-T2) induces several benefits including pain relief, reduction in both frequency and severity of angina attacks, and reduced short-acting nitrate intake. The benefits to the heart are not likely due to an increase, or redistribution of local blood flow, rather, they are associated with SCS-induced myocardial protection and normalization of the intrinsic cardiac nervous system. At somewhat lower cervical levels (C3-C6), SCS induces increased blood flow in the upper extremities. SCS at the upper cervical spinal segments (C1-C2) increased cerebral blood flow, which is associated with a decrease in sympathetic activity, an increase in vasomotor center activity and a release of neurohumoral factors. This review will summarize the basic science studies that have contributed to our understanding about mechanisms through which SCS produces beneficial effects when used in the treatment of vascular diseases. Furthermore, this review will particularly focus on the antidromic mechanisms of SCS-induced vasodilation in the lower limbs and feet.

Extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) pathways involved in spinal cord stimulation (SCS)-induced vasodilation

BACKGROUND AND AIMS

SCS is used to improve peripheral circulation in selected patients with ischemia of the extremities. However the mechanisms are not fully understood. The present study investigated whether blockade of ERK and AKT activation modulated SCS-induced vasodilation.

METHODS

A unipolar ball electrode was placed on the left dorsal column at the lumbar 2-3 spinal segments in rats. Cutaneous blood flows from left and right hind foot pads were recorded with laser Doppler flow perfusion monitors. SCS was applied through a ball electrode at 60% or 90% of MT. U0126, an inhibitor of ERK kinase, or LY294002, an inhibitor of PI3K upstream of AKT, was applied to the lumbar 3-5 spinal segments (n=7, each group).

RESULTS

U0126 (100 nM, 5 microM and 250 microM) significantly attenuated SCS-induced vasodilation at 60% (100 nM: P<0.05; 5 microM and 250 microM: P<0.01, respectively) and 90% of MT (100 nM and 5 microM: P<0.05; 250 microM: P<0.01, respectively). LY294002 at 100 microM also attenuated SCS-induced vasodilation at 60% and 90% of MT (P<0.05).

CONCLUSIONS

These data suggest that ERK and AKT pathways are involved in SCS-induced vasodilation.

Permanent sacral nerve stimulation for treatment of functional anorectal pain: report of a case

PURPOSE

Patients with functional anorectal pain in the absence of an organic cause often have symptoms that are resistant to conventional medical and behavioral therapy. This study assessed the use of sacral nerve stimulation in the treatment of this condition.

METHODS

A 56-year-old, female subject with an 18-month history of intermittent severe anorectal pain, in the absence of any evacuatory disorder or gross pathology, underwent temporary then subsequent permanent sacral nerve stimulation. Treatment efficacy was measured by verbal pain scores obtained at baseline, during screening, after screening, and subsequent follow-up.

RESULTS

Temporary sacral nerve stimulation of the left S3 root (3-5 V; 14 Hz; 210 microsec) resulted in total alleviation of the patient's symptoms. A verbal pain score of 10/10 preoperatively was reduced to 0/10 with no adverse effects from stimulation. On completing the trial evaluation, the symptoms of pain returned with a verbal pain score of 10/10. A permanent pulse generator was implanted with a Medtronic 3093 quadripolar electrode lead, placed in the left S3 foramen. Results of chronic stimulation showed that pain symptoms were again abolished with no recurrence of symptoms seen at one-year follow-up (1.3 V; 14 Hz; 210 microsec).

CONCLUSIONS

Sacral nerve stimulation may be of benefit in the treatment of functional anorectal pain resistant to conventional treatments. The mechanism of action is not known. Further prospective evaluation of a series of patients is required using pain scoring, quality of life, and psychologic assessment to aid selection.

Roles of peripheral terminals of transient receptor potential vanilloid-1 containing sensory fibers in spinal cord stimulation-induced peripheral vasodilation

BACKGROUND

Spinal cord stimulation (SCS) is used to relieve ischemic pain and improve peripheral blood flow in selected patients with peripheral arterial diseases. Our previous studies show that antidromic activation of transient receptor potential vanilloid-1 (TRPV1) containing sensory fibers importantly contributes to SCS-induced vasodilation.

OBJECTIVES

To determine whether peripheral terminals of TRPV1 containing sensory fibers produces vasodilation that depends upon the release of calcitonin gene-related peptide (CGRP) and nitric oxide (NO) during SCS.

METHODS

A unipolar ball electrode was placed on the left dorsal column at lumbar spinal cord segments 2-3 in sodium pentobarbital anesthetized, paralyzed and ventilated rats. Cutaneous blood flow from left and right hindpaws was recorded with laser Doppler flow perfusion monitors. SCS was applied through a ball electrode at 30%, 60%, 90% and 300% of motor threshold. Resiniferatoxin (RTX; 2 microg/ml, 100 microl), an ultra potent analog of capsaicin, was injected locally into the left hindpaw to functionally inactivate TRPV-1 containing sensory terminals. In another set of experiments, CGRP(8-37), an antagonist of the CGRP-1 receptor, was injected at 0.06, 0.12 or 0.6 mg/100 microl into the left hindpaw to block CGRP responses; N-omega-nitro-l-arginine methyl ester (L-NAME), a nonselective nitric-oxide synthase (NOS) inhibitor, was injected at 0.02 or 0.2 mg/100 microl into the left hindpaw to block nitric oxide synthesis; (4S)-N-(4-Amino-5[aminoethyl]aminopentyl)-N'-nitroguanidine, TFA, a neuronal NOS inhibitor, was injected at 0.02 or 0.1 mg/100 microl into the left hindpaw to block neuronal nitric oxide synthesis.

RESULTS

SCS at all intensities produced vasodilation in the left hindpaw, but not in the right. RTX administration attenuated SCS-induced vasodilation at all intensities in the left hindpaw (P<0.05, n=7) compared with responses before RTX. CGRP(8-37) administration attenuated SCS-induced vasodilation in the left hindpaw in a dose dependent manner (linear regression, P<0.05) compared with responses before CGRP(8-37). In addition, L-NAME at a high dose, but not (4S)-N-(4-Amino-5[aminoethyl]aminopentyl)-N'-nitroguanidine, TFA, decreased SCS-induced vasodilation (P<0.05, n=5).

CONCLUSION

While TRPV1, CGRP and NO are known to be localized in the same nerve terminals, our data indicate that SCS-induced vasodilation depends on CGRP release, but not NO release. NO, released from endothelial cells, may be associated with vascular smooth muscle relaxation and peripheral blood flow increase in response to SCS.

Sensory fibers containing vanilloid receptor-1 (VR-1) mediate spinal cord stimulation-induced vasodilation

BACKGROUND AND AIMS

Spinal cord stimulation (SCS) is used to improve peripheral blood flow in selected populations of patients with ischemia of the extremities. Previous studies show that antidromic activation of sensory fibers is an important mechanism that contributes to SCS-induced vasodilation. However, the characteristics of sensory fibers involved in vasodilation are not fully known. This study investigated the contribution of vanilloid receptor type 1 (VR-1) containing fibers to SCS-induced vasodilation.

METHODS

A unipolar ball electrode was placed on the left dorsal column at the lumbar 2-3 spinal cord segments (L2-L3) in sodium pentobarbital anesthetized, paralyzed and ventilated rats. Cutaneous blood flows from both ipsilateral (left) and contralateral (right) hind foot pads were recorded with laser Doppler flow perfusion monitors. SCS (50 Hz; 0.2 ms) was applied through the ball electrode at 30%, 60%, 90% and 300% of motor threshold (MT). Resiniferatoxin (RTX), an ultra potent analog of capsaicin and VR-1 receptor agonist, was used to suppress the activities of VR-1 containing sensory fibers.

RESULTS

SCS at 30%, 60%, 90% and also at 300% of MT significantly increased cutaneous blood flow in the ipsilateral foot pad compared to that in the contralateral side. RTX (2 microg/kg, i.v.) significantly attenuated SCS-induced vasodilation of the ipsilateral side (P<0.05, n=7) compared with responses prior to RTX administration. A pledget of cotton soaked with RTX (2 microg/ml) placed on L2-L3 spinal cord significantly decreased SCS-induced vasodilation of the ipsilateral side at 30%, 60%, 90% and 300% of MT (P<0.05, n=7) compared with responses prior to RTX administration. Additionally, topical application of a pledget of cotton soaked with RTX (2 microg/ml) on the sciatic nerve at the middle level of the thigh or on the tibial nerve at the lower level of the lower hindlimb also decreased SCS-induced vasodilation (n=5).

CONCLUSION

SCS-induced vasodilation is predominantly mediated via VR-1 containing sensory fibers.

Mechanisms of sustained cutaneous vasodilation induced by spinal cord stimulation

This study was performed to investigate whether spinal cord stimulation (SCS) at intensities below motor threshold prolongs cutaneous vasodilation and whether sustained vasodilation by SCS is mediated through sympathetic inhibition and/or antidromic activation of sensory fibers. SCS was applied to the dorsal surface of the L2-L3 spinal cord of anesthesized rats with stimulus parameters used clinically (i.e., 50 Hz, 0.2 ms duration, and stimulus intensity at 30%, 60%, or 90% of motor threshold). Peripheral vasodilation induced by 5-min SCS was not attenuated by hexamethonium, an autonomic ganglion-blocking agent, but was abolished by dorsal rhizotomy. SCS at < or = 60% of motor threshold increased cutaneous blood flow to the level similar to that obtained at 90% of motor threshold, but the vasodilation did not last for 5 min. SCS-induced vasodilation at 90% of motor threshold persisted for the entire stimulation period up to 30 min, and the vasodilation was not attenuated by hexamethonium. It is concluded that sustained vasodilation, which is induced by SCS at only 90% of motor threshold, in this study was mediated via antidromic activation of sensory fibers.