Is vasodilatation following dorsal column stimulation mediated by antidromic activation of small diameter afferents?

Spinal Cord Stimulation 50 Years Later: Clinical Outcomes of Spinal Cord Stimulation Based on Randomized Clinical Trials-A Systematic Review

To assess the efficacy of spinal cord stimulation (SCS) for each indication, one must critically assess each specific clinical outcome to identify outcomes that benefit from SCS therapy. To date, a comprehensive review of clinically relevant outcome-specific evidence regarding SCS has not been published. We aimed to assess all randomized controlled trials from the world literature for the purpose of evaluating the clinical outcome-specific efficacy of SCS for the following outcomes: perceived pain relief or change pain score, quality of life, functional status, psychological impact, analgesic medication utilization, patient satisfaction, and health care cost and utilization. Interventions were SCS, without limitation to the type of controls or the type of SCS in the active arms. For each study analyzed, a quality assessment was performed using a validated scale that assesses reporting, external validity, bias, confounding, and power. Each outcome was assessed specific to its indication, and the primary measure of each abovementioned outcome was a summary of the level of evidence. Twenty-one randomized controlled trials were analyzed (7 for trunk and limb pain, inclusive of failed back surgery syndrome; 8 for refractory angina pectoris; 1 for cardiac X syndrome; 3 for critical limb ischemia; 2 for complex regional pain syndrome; and 2 for painful diabetic neuropathy). Evidence assessments for each outcome for each indication were depicted in tabular format. Outcome-specific evidence scores were established for each of the abovementioned indications, providing both physicians and patients with a summary of evidence to assist in choosing the optimal evidence-based intervention. The evidence presented herein has broad applicability as it encompasses a breadth of patient populations, variations of SCS therapy, and comparable controls that, together, reflect comprehensive clinical decision making.

And yet it moves: Recovery of volitional control after spinal cord injury

Preclinical and clinical neurophysiological and neurorehabilitation research has generated rather surprising levels of recovery of volitional sensory-motor function in persons with chronic motor paralysis following a spinal cord injury. The key factor in this recovery is largely activity-dependent plasticity of spinal and supraspinal networks. This key factor can be triggered by neuromodulation of these networks with electrical and pharmacological interventions. This review addresses some of the systems-level physiological mechanisms that might explain the effects of electrical modulation and how repetitive training facilitates the recovery of volitional motor control. In particular, we substantiate the hypotheses that: (1) in the majority of spinal lesions, a critical number and type of neurons in the region of the injury survive, but cannot conduct action potentials, and thus are electrically non-responsive; (2) these neuronal networks within the lesioned area can be neuromodulated to a transformed state of electrical competency; (3) these two factors enable the potential for extensive activity-dependent reorganization of neuronal networks in the spinal cord and brain, and (4) propriospinal networks play a critical role in driving this activity-dependent reorganization after injury. Real-time proprioceptive input to spinal networks provides the template for reorganization of spinal networks that play a leading role in the level of coordination of motor pools required to perform a given functional task. Repetitive exposure of multi-segmental sensory-motor networks to the dynamics of task-specific sensory input as occurs with repetitive training can functionally reshape spinal and supraspinal connectivity thus re-enabling one to perform complex motor tasks, even years post injury.

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.

A Comprehensive Outcome-Specific Review of the Use of Spinal Cord Stimulation for Complex Regional Pain Syndrome

BACKGROUND

Complex regional pain syndrome (CRPS) is a painful, debilitating affliction that is often difficult to treat. It has become common international practice to use spinal cord stimulation (SCS) for the treatment of CRPS as other therapies fail to provide adequate relief, quality of life, or improvement in function. This comprehensive outcome-specific systematic review of the use of SCS for CRPS was performed to elucidate the available evidence with focus on clinically relevant patient-specific outcomes.

METHODS

A systematic review of the literature was conducted to evaluate the effects of SCS on patients with CRPS for the following outcomes and provide summary levels of evidence in regard to each outcome: perceived pain relief, pain score, resolution of CRPS signs, functional status, quality of life, psychological impact, sleep hygiene, analgesic medication utilization, and patient satisfaction with SCS therapy. Search terms included "complex regional pain syndrome," "spinal cord stimulation," and "reflex sympathetic dystrophy," without restriction of language, date, or type of publication, albeit only original data were included in analyses. Of 30 studies selected, seven systematic reviews were excluded, as were four studies reporting combination therapy that included SCS and other therapies (ie, concurrent peripheral nerve stimulation, intrathecal therapy) without clear delineation to the effect of SCS alone on outcomes. A total of 19 manuscripts were evaluated.

RESULTS

Perceived pain relief, pain score improvement, quality of life, and satisfaction with SCS were all rated 1B+, reflecting positive high-level (randomized controlled trial) evidence favoring SCS use for the treatment of CRPS. Evidence for functional status improvements and psychological effects of SCS was inconclusive, albeit emanating from a randomized controlled trial (evidence level 2B±), and outcomes evidence for both sleep hygiene and resolution of CRPS signs was either nonexistent or of too low quality from which to draw conclusions (evidence level 0). An analgesic sparing effect was observed in nonrandomized reports, reflecting an evidence level of 2C+.

CONCLUSIONS

Spinal cord stimulation remains a favorable and effective modality for treating CRPS with high-level evidence (1B+) supporting its role in improving CRPS patients' perceived pain relief, pain score, and quality of life. A paucity of evidence for functional improvements, resolution of CRPS signs, sleep hygiene, psychological impact, and analgesic sparing effects mandate further investigation before conclusions can be drawn for these specific outcomes.

Spinal cord stimulation as treatment for complex regional pain syndrome should be considered earlier than last resort therapy

BACKGROUND

Spinal cord stimulation (SCS), by virtue of its historically described up-front costs and level of invasiveness, has been relegated by several complex regional pain syndrome (CRPS) treatment algorithms to a therapy of last resort. Newer information regarding safety, cost, and efficacy leads us to believe that SCS for the treatment of CRPS should be implemented earlier in a treatment algorithm using a more comprehensive approach.

METHODS

We reviewed the literature on pain care algorithmic thinking and applied the safety, appropriateness, fiscal or cost neutrality, and efficacy (S.A.F.E.) principles to establish an appropriate position for SCS in an algorithm of pain care.

RESULTS AND CONCLUSION

Based on literature-contingent considerations of safety, efficacy, cost efficacy, and cost neutrality, we conclude that SCS should not be considered a therapy of last resort for CRPS but rather should be applied earlier (e.g., three months) as soon as more conservative therapies have failed.

Physiology of spinal cord stimulation: review and update

Spinal cord stimulation (SCS) was an outgrowth of the well-known gate control theory presented by Melzack and Wall in 1965. Although the method has been used to treat chronic severe pain for more than three decades, very little was known about the physiological and biochemical mechanisms behind the beneficial effects until recently. We now know that SCS activates several different mechanisms to treat different types of pain such as neuropathic and ischemic. In general, these mechanisms seem most dependent on activation of only a few segments of the spinal cord. However, both animal studies and human observations have indicated that supraspinal circuits may contribute as well. In the treatment of neuropathic pain, intermittent SCS may give several hours of pain relief after cessation of the stimulation. This protracted effect indicates long-lasting modulation of neural activity involving changes in the local transmitter systems in the dorsal horns. In ischemic pain, animal experiments demonstrate that inhibition of afferent activity in the spinothalamic tracts, long-term suppression of sympathetic activity, and antidromic effects on peripheral reflex circuits may take part in the pain alleviation. Moderate SCS intensities seem to evoke sympathetic inhibition, but higher stimulation intensities may induce antidromically mediated release of vasoactive substances, eg, the calcitonin gene-related peptide (CGRP), resulting in peripheral vasodilation. The anti-ischemic effect of SCS in angina pectoris due to intermittent coronary ischemia probably occurs because application of SCS appears to result in a redistribution of cardiac blood supply, as well as a decrease in tissue oxygen demand. Recent studies indicate that SCS modulates the activity of cardiac intrinsic neurons thereby restricting the arrythmogenic consequences of intermittent local coronary ischemia. The present state of knowledge is briefly reviewed and recent research directions outlined.

Future indications for sacral nerve stimulation

AIM

The aim of this article was to determine the effect of sacral nerve stimulation (SNS) on the treatment of faecal incontinence, constipation, irritable bowel syndrome, mixed urinary and bowel disorders, spinal injury and neurodegenerative disease, pain syndromes, and sexual dysfunction.

METHOD

A Medline search was performed including the keywords and/or MeSH headings of 'sacral nerve stimulation', 'neuromodulation', 'artificial pacemaker', 'faecal incontinence', 'constipation' and 'anal pain'. Further studies were identified by cross-referencing from relevant articles and by appraisal of recent peer-reviewed conference abstracts and proceedings.

RESULTS

SNS has been used for the treatment of urinary, bowel and sexual dysfunction, as well as pain resulting from such disorders, and dysfunction arising from nerve injury and degenerative disease. There is a paucity of high quality evidence to support the use of SNS for the majority of novel indications at present.

CONCLUSION

Good quality prospective, cross-over studies are required to determine the true benefits of SNS. Further research into patient selection, operative technique and stimulation parameters for existing indications will ensure a place for SNS in the future treatment algorithm of functional pelvic floor disorders.

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.

Spinal cord stimulation: an update

Spinal cord stimulation has been used in the treatment of many chronic pain disorders since 1967. In this update, the indications for spinal cord stimulation are reviewed with attention to recent publications. A focused review of the literature on abdominal and visceral pain syndromes is also provided. Furthermore, the technology has evolved from the use of monopolar electrodes to complex electrode arrays. Similarly, the power source has changed from a radio frequency-driven system to a rechargeable impulse generator. These topics are covered, along with a short discussion of implant technique. Finally, we include a review of complications of such therapy. SCS as a technology and therapy continues to evolve.

Spinal cord stimulation in the treatment of chronic critical limb ischemia

This paper reviews the clinical experience and proposed working mechanisms of spinal cord stimulation (SCS) in the treatment of chronic critical limb ischemia (CCLI). SCS appears to provide a significant long-term relief of ischemic pain and to improve healing of small ulcers, most likely due to effects on the nutritional skin blood flow. Despite these observations, randomized trials were not able to show limb salvage. Assessment of the microcirculatory skin blood flow, by means of transcutaneous oxygen pressure measurements and videocapillaromicroscopy, is necessary to evaluate the remaining microcirculatory reserve capacity likely to be exploited by SCS and to help identify patients that will benefit most from this treatment and in whom stimulation could lead to limb salvage.

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.

Interventional cancer pain therapies

Interventional pain therapies play a critical role in palliation of severe cancer pain. Anesthesiologists specializing in cancer pain management have developed minimally invasive techniques to: (1) optimize pain control; (2) minimize side effects, adverse outcomes, and costs; (3) enhance functional abilities and physical and psychological well-being; and (4) enhance the quality of life for cancer patients. A thorough understanding of the pathophysiology of the cancer pain is needed to implement interventional therapies. It is also important to understand the prognosis of the patient, associated comorbidities, and expectations of the patient and family. Interventional pain therapies are minimally invasive techniques that can be divided into direct drug delivery, neuroablation and neural blockade, and neurostimulation.

Implantable devices for pain control: spinal cord stimulation and intrathecal therapies

Untreated chronic pain is costly to society and to the individual suffering from it. The treatment of chronic pain, a multidimensional disease, should rely on the expertise of varying health care providers and should focus not only on the neurobiological mechanisms of the process but also on the psychosocial aspects of the disease. Implantable devices are costly and invasive, and such efficacious therapies should be used only when more conservative and less costly therapies have failed to provide relief of pain and suffering. Spinal cord stimulation provides neuromodulation of neuropathic, but not nociceptive, pain signals and when used for appropriate indications in the right individuals provides approximately 60-80% long-term pain relief in 60-80% of patients trialled for efficacy. Intrathecal therapies with opioids such as morphine, fentanyl, sufentanil or meperidine--or non-opioids such as clonidine or bupivacaine--provide analgesia in patients with nociceptive or neuropathic pain syndromes. Baclofen, intrathecally, provides profound relief of muscle spasticity due to multiple sclerosis, spinal cord injuries, brain injuries or cerebral palsy.

Spinal cord stimulation: mechanisms of action

STUDY DESIGN

A literature review and synthesis were performed.

OBJECTIVE

To present the current understanding of the mechanisms of spinal cord stimulation in relation to the physiology of pain.

SUMMARY OF BACKGROUND DATA

Spinal cord stimulation has been used for more than 30 years in the armamentarium of the interventional pain specialist to treat a variety of pain syndromes. Traditionally used for persisting leg pain after lumbar spinal surgery, it has been applied successfully in the treatment of angina pectoris, ischemic pain in the extremity, complex regional pain syndrome Types 1 and 2, and a variety of other pain states. This review presents the current status of what is known concerning how electrical stimulation of the spinal cord may achieve pain relief.

METHODS

A literature review was conducted.

RESULTS

The literature supports the theory that the mechanism of spinal cord stimulation cannot be completely explained by one model. It is likely that multiple mechanisms operate sequentially or simultaneously.

CONCLUSION

Some clinical or experimental support can be found in the literature for 10 specific mechanisms or proposed mechanisms of spinal cord stimulation.

Spinal Cord Stimulation: Indications, Mechanism of Action, and Efficacy

Unrelieved pain is costly to the economic fabric of our society; its direct costs to patients and their families is staggering. Spinal cord stimulation for the treatment of chronic pain is cost-effective when used in the context of a pain treatment continuum. Many theories on the mechanism of action of spinal cord stimulation have been suggested, including activation of gate control mechanisms, conductance blockade of the spinothalamic tracts, activation of supraspinal mechanisms, blockade of supraspinal sympathetic mechanisms, and activation or release of putative neuromodulators. Whatever theory or theories of mechanism are correct, spinal cord stimulation has efficacy in the treatment of failed back surgery syndrome, complex regional pain syndromes, intractable pain due to peripheral vascular disease, and intractable pain due to angina.

Interventional pain management. Appropriate when less invasive therapies fail to provide adequate analgesia

Unrelieved chronic pain is costly to patients and society. Noninvasive and less costly therapies should be used before more invasive and more costly therapies. Therapies for pain control should be used according to a pain treatment continuum. Nerve-blocking techniques, neurolytic techniques, and implantable neuromodulatory technologies, such as SCC and spinal delivery of analgesics, are cost-effective when less invasive therapies fail to provide adequate analgesia.

Improvement of limb circulation in peripheral vascular disease using epidural spinal cord stimulation: a prospective study

Spinal cord stimulation was used in 46 patients for pain associated with lower extremity ischemic vascular disease that was considered to be nonreconstructable. Thirty-nine patients who had a follow-up examination between 2 and 36 months after the procedure form the basis of this report. Thirty (77%) of 39 cases were considered successful. Clinical endpoints indicating failure included amputation, vascular reconstruction, poor pain relief, or hardware malfunction. The transcutaneous partial pressure of oxygen (TcPO2) increased in both target and control feet. In patients with good outcome with a preimplantation TcPO2 of less than 30 mm Hg, TcPO2 increased significantly (p < 0.05). Pulse volume recording improved significantly (p < 0.05) at the thigh, metatarsal, and great toe levels in successfully treated patients. Peak blood flow velocity also showed a significant increase in patients with good outcome (p < 0.05). Patients with a TcPO2 of less than 10 mm Hg following stimulation tended to undergo amputation within the first 3 months. Improvement in pain control, combined with an increase in TcPO2 values that was greater than 10 mm Hg, were significant early predictors of long-term success. An initial increase in peak blood flow velocities (measured in Doppler studies) of greater than 10 mm also signified a good long-term outcome. Spinal cord stimulation appears to be a useful therapeutic modality for controlling pain and improving perfusion in a select group of patients with end-stage ischemic vascular disease considered nonreconstructable. The best results were seen in patients with severe claudication and rest pain without trophic changes in the foot. The mechanism of this beneficial effect is not yet completely understood.

Sympathetic mediation of peripheral vasodilation induced by spinal cord stimulation: animal studies of the role of cholinergic and adrenergic receptor subtypes

Electric spinal cord stimulation (SCS) is widely used as a treatment modality for ischemic pain in peripheral arterial insufficiency. The background for the therapeutic effect may be a temporary inhibition of sympathetically maintained peripheral vasoconstriction. In this series of experiments, the involvement of different types of cholinergic and adrenergic receptor subclasses in the vasodilatory effect was explored in anesthetized rats. The microcirculation in hindlimb skin and hamstring muscle was studied by the laser Doppler technique. The ganglionic blocker hexamethonium as well as the nicotinic receptor antagonist chlorisondamine abolished the effect in both vascular beds, whereas the muscarinic receptor antagonists pirenzepine and atropine were ineffective. Among the adrenergic receptor active compounds, phentolamine, prazosine (an alpha 1-receptor antagonist), and clonidine in high doses suppressed the SCS-induced vasodilation. Yohimbine (an alpha 2-receptor antagonist) did not alter the effect. The beta-adrenergic compounds had a differential effect on muscle and skin perfusion. Atenolol, a beta 1-receptor antagonist, inhibited SCS-induced vasodilation only in the skin, whereas the beta 2-receptor antagonist butoxamine selectively depressed the muscle response. The vasodilatory effect of SCS in the animal model used here seems to a large extent to be mediated by an inhibitory effect on peripheral vasoconstriction maintained via efferent sympathetic activity involving nicotinic transmission in the ganglia and the postganglionic alpha 1-adrenoreceptors. The involvement of beta-receptors seems to be different in skin and muscle, beta 1 being more important for the changes in the skin and beta 2 being more important for those in muscle. The high-intensity antidromic response, earlier believed to explain how SCS exerted its vasodilatory effect, was resistant to cholinergic and adrenergic manipulations and seems to depend on entirely different mechanisms.

Dorsal column stimulation induces release of serotonin and substance P in the cat dorsal horn

The neurohumoral mechanisms behind the pain-suppressing effect of dorsal column stimulation (DCS) still remain obscure. Experimental observations have indicated an inhibitory role for serotonin and, under certain conditions, also for substance P (SP), on nociceptive transmission in the spinal cord. Furthermore, some observations suggest that these substances might be involved in the effect of DCS. The present series of experiments was undertaken to investigate whether serotonin and SP are released in the dorsal horn by DCS. Twenty-one adult cats, in some experiments anesthetized, in others decerebrated at the midcollicular level, were used. Microdialysis probes were implanted bilaterally in lumbar dorsal horns (L5-L7) and perfused with Krebs' solution. Dialysates were analyzed for serotonin by high-performance liquid chromatography or for SP by radioimmunoassay. DCS was applied at the thoracolumbar junction with current parameters similar to those used clinically in humans. DCS induced a significant release of serotonin in the dorsal horn of decerebrated animals (173 +/- 83% increase; mean +/- standard error; n = 4; P less than 0.01), whereas the levels of the metabolite 5-hydroxyindoleacetic acid were not significantly influenced. In contrast, no release of SP could be recorded in response to DCS in the decerebrated preparation, although peripheral nociceptive stimulation (pinch) and noxious electric dorsal root stimulation induced an elevation of the SP levels. However, in intact animals DCS provoked a marked SP release in the dorsal horn (190 +/- 92% increase; n = 7; P less than 0.01). The release of serotonin and SP after DCS may indicate that these substances participate in the mediation of the pain alleviating effect of DCS.(ABSTRACT TRUNCATED AT 250 WORDS)

The vasodilating effect of spinal dorsal column stimulation is mediated by sympathetic nerves

Spinal dorsal column stimulation has been used in the treatment of a patient with a painful vasospastic condition in the right arm following surgical sympathectomy on the left side. After sympathectomy the left arm became constantly dry and warm and consistently lacked skin vasomotor (laser Doppler flowmetry) responses to arousing stimuli, indicating a complete loss of sympathetic vasomotor innervation. The return of minimal sudomotor (skin resistance) responses to mental stress 2 years after sympathectomy indicated a partial reinnervation of sweat glands. Immediately after sympathectomy, the contralateral right arm became increasingly cold and cyanotic and the patient complained of chronic painful coldness and severe cold-intolerance in the right arm. Attempts to pharmacologically vasodilate the arm with felodipine did not affect the local vasoconstriction and pain. Dorsal column stimulation (associated with symmetrical paraesthesia in both arms) induced an immediate and marked (ten-fold) increase in skin blood flow in the right arm (and in the leg), whereas skin blood flow in the left arm remained unaffected. The lack of vasomotor response in the left arm was not due to maximal vasodilatation at rest, since skin blood flow in the left arm showed a normal capacity for axon reflex vasodilatation following antidromic activation of sensory afferents. The results suggest that the marked vasodilatation induced by dorsal column stimulation is mediated by sympathetic vasomotor fibres, via modulation of central neuronal circuits involved in the regulation of skin sympathetic discharge.