Neurogenic Flare Response Research Articles

Neurogenic Flare Response following Image-Guided Focused Ultrasound in the Mouse Peripheral Nervous System in Vivo

Focused ultrasound (FUS) has been used to non-invasively elicit or inhibit motor neuronal activity in the mouse peripheral nervous system in vivo. However, less is known about whether FUS elicits immune system responses associated with peripheral sensory neuronal activity. In this study, we sought to determine that non-invasive ultrasound image-guided FUS can elicit the neurogenic axon reflex of peripheral nerves in the mouse sciatic nerve. The local vasodilation in the plantar view of the hind paw detected with a high-resolution laser Doppler imager indicated neurogenic flare responses after FUS stimulation. The effects of FUS were compared with control groups, where a distinct pattern of blood flow changes was observed only in FUS-elicited neurogenic flare responses. The findings indicate that image-guided FUS elicits local axon reflexes in vivo with a high degree of specificity and penetration depth.

Laser Doppler Assessment of Vasomotor Axon Reflex Responsiveness to Evaluate Neurovascular Function.

The vasomotor axon reflex can be evoked in peripheral epidermal nociceptive C-fibers to induce local vasodilation. This neurogenic flare response is a measure of C-fiber functional integrity and therefore shows impairment in patients with small fiber neuropathy. Laser Doppler flowmetry (LDF) and laser Doppler imaging (LDI) are both techniques to analyze vasomotor small fiber function by quantifying the integrity of the vasomotor-mediated axon reflex. While LDF assesses the flare response following acetylcholine iontophoresis with temporal resolution at a single defined skin point, LDI records flare responses with spatial and temporal resolution, generating a two-dimensional map of superficial blood flow. LDF is characterized by a high intra- and interindividual measurement variability, which is smaller in LDI due to its spatial resolution. Nevertheless, LDI still lacks standardized methods for image analysis. Consequently, use of the technique currently remains on an experimental level. Here, we sought to review the current literature on laser Doppler assessment of vasomotor function and discuss potential future applications of established techniques as well as those that are still experimental.

Differential effect of Incobotulinumtoxin A on pain, neurogenic flare and hyperalgesia in human surrogate models of neurogenic pain.

The effectiveness of Botulinum-neurotoxin A (BoNT/A) to treat pain in human pain models is very divergent. This study was conducted to clarify if the pain models or the route of BoNT/A application might be responsible for these divergent findings. Sixteen healthy subjects (8 males, mean age 27±5years) were included in a first set of experiments consisting of three visits: (1) Visit: Quantitative sensory testing (QST) was performed before and after intradermal capsaicin injection (CAPS, 15μg) on one thigh and electrical current stimulation (ES, 1Hz) on the contralateral thigh. During stimulation pain and the neurogenic flare response (laser-Doppler imaging) were assessed. (2) Four weeks later, BoNT/A (Xeomin® , 25MU) was injected intracutaneously on both sides. (3) Seven days later, the area of BoNT/A application was determined by the iodine-starch staining and the procedure of the (1) visit was exactly repeated. In consequence of these results, 8 healthy subjects (4 males, mean age 26±3years) were included into a second set of experiments. The experimental setting was exactly the same with the exception that stimulation frequency of ES was increased to 4Hz and BoNT/A was injected subcutaneously into the thigh, which was stimulated by capsaicin. BoNT/A reduced the 1Hz ES flare size (p<0.001) and pain ratings (p<0.01), but had no effect on 4Hz ES and capsaicin-induced pain, hyperalgesia, or flare size, regardless of the depth of BoNT/A injection (i.c./s.c). Moreover, i.c. BoNT/A injection significantly increased warm detection and heat pain thresholds in naive skin (WDT, Δ 2.2°C, p<0.001; HPT Δ 1.8°C, p<0.005). BoNT/A has a moderate inhibitory effect on peptidergic and thermal C-fibers in healthy human skin. The study demonstrates that BoNT/A (Incobotulinumtoxin A) has differential effects in human pain models: It reduces the neurogenic flare and had a moderate analgesic effects in low frequency but not high frequency current stimulation of cutaneous afferent fibers at C-fiber strength; BoNT/A had no effect in capsaicin-induced (CAPS) neurogenic flare or pain, or on hyperalgesia to mechanical or heat stimuli in both pain models. Intracutaneous BoNT/A increases warm and heat pain thresholds on naïve skin.

Nociceptors in the skin: fire-raisers to be kept at bay?

Nociceptors in the skin: fire-raisers to be kept at bay?

(317) Increased neurogenic and inflammatory pain in healthy young adults with greater early life stress

Early life stress is a risk factor for chronic pain in adulthood, yet little is known about the underlying mechanisms. The purpose of this experiment was to evaluate whether healthy individuals reporting high levels of early life stress would show enhanced spontaneous pain and neurogenic flare responses to topical capsaicin. Capsaicin temporarily induces localized spontaneous pain and neurogenic inflammation by engaging mechanisms involved in neuropathic and inflammatory pain. A total of 2,625 participants were prescreened for their early life stressful events using the Early Trauma Inventory Self-report-short form (ETISR-SF).

Measurement of cooling detection thresholds for identification of diabetic sensorimotor polyneuropathy in type 1 diabetes.

ObjectiveCompared to recently-studied novel morphological measures, conventional small nerve fiber functional tests have not been systematically studied for identification of diabetic sensorimotor polyneuropathy (DSP). We aimed to determine and compare the diagnostic performance of cooling detection thresholds (CDT) in a cross-sectional type 1 diabetes cohort.Research Design and Methods136 subjects with type 1 diabetes and 52 healthy volunteers underwent clinical and electrophysiological examination for DSP classification concomitantly with the Toronto Clinical Neuropathy Score (TCNS) and three small fiber function tests: CDT, heart rate variability (HRV), and laser doppler imaging of axon-mediated neurogenic flare responses to cutaneous heating (LDIFLARE). Area under the curve (AUC) and optimal thresholds were determined by receiver operating characteristic (ROC) curves in the type 1 diabetes cohort.ResultsType 1 diabetes subjects were 42±17 years of age with mean HbA1c 7.9±1.7%. Fifty-nine (45%) met the case definition for DSP. CDT values were lowest in cases with DSP (18.3±8.4°C) compared to controls without DSP (28.4±3.5°C) and to healthy volunteers (29.6±1.8°C; p-value for both comparisons<0.0001). AUCCDT was 0.863 which was similar to AUCTCNS (0.858, p = 0.24) and AUCHRV (0.788, p = 0.05), but exceeded AUCLDIFLARE (0.750, p = 0.001). The threshold of <25.1°C was equivalent to the lower bound of the healthy volunteer 95% distribution [25.1, 30.8°C] and performed with 83% sensitivity and 82% specificity.ConclusionsAkin to novel small fiber morphological measures, CDT is a functional test that identifies DSP with very good diagnostic performance. These findings support further research that revisits the role of CDT in early DSP detection.

Comparison of electrically induced flare response patterns in human and pig skin

We compared the characteristics of neurogenic flare responses in human and pig skin to establish a translational research animal model. Eight domestic pigs and six male subjects were investigated. Electrical pulses were delivered transcutaneously with increasing current intensities, pulse frequencies and pulse widths. Inflammatory skin responses were recorded by laser Doppler imaging and analyzed by ANOVA and Fisher's (LSD) post hoc test. Transcutaneous stimuli of 5 mA onward induced a significant flare development in humans. In the pig, significantly lower currents of 2.5 mA already induced a flare response. Smaller flare sizes of about 3.5 cm(2) were analyzed. The flare continuously declined despite ongoing stimulation. Lower excitation thresholds and smaller receptive fields of nociceptors can be suggested in pigs. Impaired neuropeptide release, altered vesicle replenishment, different neuropeptide sensitivity, or insufficient peripheral decoding of action potentials may contribute to steadily decreasing flare responses. These attributes may be objectives of pre-clinical anti-hyperalgesic studies and their accurate analysis in pigs reveals a particularly sensitive translational animal model for nociceptor researches.

Neurovascular Factors in Wound Healing in the Foot Skin of Type 2 Diabetic Subjects

Delayed wound healing in diabetic patients without large-vessel disease has been attributed to microvascular dysfunction, neuropathy, and abnormal cellular and inflammatory responses. The role of these abnormalities has mainly been examined in animal models. Few studies have been undertaken in diabetic patients, and those that have are limited due to analysis in wounds from chronic ulcers. In this study, we quantified the rate of wound healing in relation to skin neurovascular function and structure following a dorsal foot skin biopsy in type 2 diabetes. Twelve healthy control subjects and 12 type 2 diabetic subjects with neuropathy but without macrovascular disease were studied. We quantified rate of wound healing and related it to skin microvascular function (laser Doppler imager [LDI](max)), blood vessel density, small nerve fiber function (LDI(flare)) and nerve fiber density, vascular endothelial growth factor (VEGF) and its receptor (FLK1), and hypoxia-inducible factor (HIF)-1alpha expression. The rate of wound closure was identical between control subjects and diabetic patients despite a significant reduction in maximum hyperemia (LDI(max)), epidermal and dermal VEGF-A, and epidermal and dermal blood vessel VEGFR-2 expression as well as the neurogenic flare response (LDI(flare)) and dermal nerve fiber density. There was no significant difference in HIF-1alpha and dermal blood vessel density between control subjects and diabetic patients. In conclusion, the results of this study suggest that wound closure in subjects with type 2 diabetes is not delayed despite significant alterations in neurovascular function and structure.

The LDIflare

The aim of this study was to evaluate a novel method for assessing the axon reflex and to determine its value in detecting neuropathy in type 2 diabetes. The neurogenic flare response to nociceptive stimuli is mediated by an axon reflex involving small unmyelinated C-fibers. We developed a method to assess this reflex involving skin heating to 44 degrees C to evoke the flare followed by scanning the site using a laser Doppler imager (LDI) to measure the area; we termed this method LDIflare. To confirm its neurogenic nature, we examined the LDIflare in eight healthy subjects before and after topical administration of anesthesia. We used this technique to detect C-fiber neuropathy in people with type 2 diabetes. A total of 36 subjects were studied: 12 subjects with neuropathy (group DN), 12 subjects without neuropathy (group DC), and 12 age- and sex-matched control subjects (group NC). For comparison, small-fiber function was also assessed using the Computer Aided Sensory Evaluator-IV (CASE IV) (WR Medical Electronics, Stillwater, MN). In the eight healthy control subjects, LDIflare was markedly reduced after topical administration of anesthesia (1.62 [1.45-1.72] vs. 5.2 cm2 [3.9-5.9], P <0.0001), confirming its neurogenic nature. Similarly, in neuropathic subjects, LDIflare was significantly smaller compared with normal and diabetic control subjects (LDIflare area: DN 1.3 cm2 [0.9-1.8], NC 5.5 cm2 [3.9-5.8], and DC 2.8 cm2 [2.5-3.8]; P <0.0001 and P=0.01, respectively). The group without neuropathy (DC) also demonstrated a reduced flare compared with the NC group (P=0.01). In contrast, C-fiber function assessed by evaluating the quantitative thermal thresholds (CASE IV) did not detect a difference between the latter two groups. This study confirms the neurogenic nature of the LDIflare and clearly demonstrates loss of C-fiber function in neuropathic subjects with type 2 diabetes. Moreover, it demonstrates C-fiber dysfunction before its detection by other currently available methods, including CASE IV. The LDIflare seems to be a simple objective method to detect early neuropathy and may be of value in assessing therapeutic interventions aimed at preventing or reversing C-fiber dysfunction.

Capsaicin-induced flare and vasodilatation in patients with post-herpetic neuralgia

The aim of the present study was to investigate the role of primary afferent fibres with polymodal nociceptors in the various pain symptoms and signs associated with post-herpetic neuralgia (PHN). Forty-four patients with PHN affecting thoracic dermatomes were examined clinically for evidence of sensory disturbance to touch and pinprick and compared to 14 normal subjects and 9 subjects with evidence of past herpes zoster infection but no pain. The patients were then divided into 3 groups on the basis of their clinical symptoms and signs — those with steady burning discomfort only (n = 12), those with burning discomfort, allodynia and hyperalgesia to pinprick (n = 17), and those with burning discomfort, allodynia and hypalgesia to pinprick (n = 15). Indirect measurement of primary afferent fibre function was performed by measuring the neurogenic axon reflex flare to topical capsaicin using Doppler flowmetry in the 5 clinical groups. The 2 groups with allodynia had significantly decreased neurogenic flare responses compared to PHN subjects without allodynia and the 2 control groups. These results suggest that allodynia in patients with post-herpetic neuralgia may be a consequence of disrupted function of primary afferent fibres.

Neurogenic flare responses are heterogeneous in superficial and deep layers of human skin

Infrared thermography and video image analysis were used to compare the development of the neurogenic flare response in deep and superficial skin layers. Neurogenic vasodilatation was induced by injection or iontophoretic application of histamine. Thermograms were recorded every 10 s to evaluate the local warming reaction. Color-video image analysis was used for computerized delineation of the visible flare. Images derived from video analysis were superimposed to the thermograms after linear transformation. The thermal reaction started within 10 s simultaneously at 2–5 distinct spots after histamine application at distances of 4–25 mm from the application site while the flare reaction seemed to spread from the application site when it became visible after a delay often exceeding 20 s. The visible flare but not the warming reaction was suppressed by topically applied local anesthetic (EMLA®). Warming at focal spots was also observed during post-occlusive hyperemia. About 70% of these spots were identical to those activated by histamine. The results indicate that the vascular axon reflex is differently organized in different layers of the skin.

Altered heat pain thresholds and cerebral event-related potentials following painful CO 2 laser stimulation in subjects with fibromyalgia syndrome

A decrease in mechanical pressure pain thresholds, particularly over pre-designated tender points, is one of the defining characteristics of fibromyalgia syndrome (FS); however, changes in thermal pain sensitivity have not been investigated. The present study examined heat pain thresholds and cerebral event-related potentials following CO 2 laser stimulation in 10 subjects with FS and 10 age-matched control volunteers. The results indicate that patients with FS exhibit a significant reduction in heat pain threshold when tested on the dorsal surface of the hand. In accordance with previous research, we also found a decrease in mechanical pain threshold over pre-designated tender points and at control sites as well as a significantly larger mechanically induced neurogenic flare response. These measures were highly correlated with thermal pain threshold even though different anatomical sites were stimulated. Hence, it seems likely that FS patients display a multimodal change in pain sensitivity which is generalized rather than anatomically restricted. Patients with FS also displayed a significant increase in the peak-to-peak amplitude of the cerebral potential evoked by CO 2 laser stimulation at pain threshold intensity and 1.5 times pain threshold intensity. These findings suggest a greater activation of central nervous system (CNS) pathways following noxious input. Putative explanations for the increased CNS response are discussed, including mechanisms of peripheral nociceptor sensitization, altered CNS function and the role of psychological factors.

Cerebral Event Related Potentials and Heat Pain Thresholds in Fibromyalgia Syndrome

Objectives: To assess the pain threshold to heat and subsequent event related cerebral evoked potential in patients with fibromyalgia and controls. Methods: Eleven females with fibromyalgia [mean age 28.3 years] and 11 subjects without symptoms or signs of fibromyalgia [mean age 26.6 years] were identified. Using CO2 laser heat stimulation as controls but pain threshold was significantly reduced. The latency between the onset of the painful heat stimulus and the CNS evoked response was the same in FS subjects and controls, however, FS subjects displayed significant change in peak-to-peak amplitude of the event related CNS evoked response. There was a strong association between neurogenic flare response and the nociceptor evoked cerebral response. Conclusions: This study provides further evidence of an altered pain threshold to a variety of noxious modalities in FS implying a major role for the polymodal nociceptors in the clinical expression of this syndrome.

Chronic nicotine exposure enhances cutaneous axon reflexes in the rat

The effect of chronic nicotine exposure on the cutaneous neurogenic flare response was investigated non-invasively in rats. Axon reflexes were evoked by transdermal iontophoresis of acetylcholine, and resultant changes in skin microvascular blood flux were measured by laser Doppler flowmetry. Rats given five injections of nicotinesulphate (1 mg kg-1 i.p.) daily for 14 days were compared to saline-injected littermates. Axon reflexes were enhanced by 143% after 7 days exposure to nicotine, and by 336% after 14 days exposure. Controls did not display these increases. Axon reflexes measured 7 days after nicotine cessation were similar to pre-nicotine levels. These results may suggest that nicotinic cholinoceptors on skin nociceptors and primary afferents upregulate in response to chronic nicotine exposure, and return to normal following nicotine cessation.

Somesthetic and Electrophysiologic Effects of Topical 0.025% Capsaicin in Man

As a topical agent, capsaicin reportedly blocks pain without otherwise impairing sensation. Ten subjects underwent blinded, multiple-dose application of topical 0.025% capsaicin or control emollient to skin test sites in order to evaluate capsaicin effectiveness and toxicity. Clinical changes in somesthetic perception of pinprick, heat and cold developed, but neurogenic flare responses were unchanged. Clinical and electrophysiologic measures of large fiber and autonomic function were unaffected by capsaicin. This study indicates that 0.025% capsaicin is a safe topical agent with demonstrable clinical effects on small fiber function but not on large fiber or autonomic function.

Somesthetic and electrophysiologic effects of topical 0.025% capsaicin in man.

As a topical agent, capsaicin reportedly blocks pain without otherwise impairing sensation. Ten subjects underwent blinded, multiple-dose application of topical 0.025% capsaicin or control emollient to skin test sites in order to evaluate capsaicin effectiveness and toxicity. Clinical changes in somesthetic perception of pinprick, heat and cold developed, but neurogenic flare responses were unchanged. Clinical and electrophysiologic measures of large fiber and autonomic function were unaffected by capsaicin. This study indicates that 0.025% capsaicin is a safe topical agent with demonstrable clinical effects on small fiber function but not on large fiber or autonomic function.

Assessment of the neurogenic flare response as a measure of nociceptor C fibre function

A modified method for quantitative measurement of nociceptor C fibre function is described. The electrophoresis of acetylcholine stimulates C fibres to produce an axon reflex flare which is measured by laser Doppler flowmetry. The use of the dorsal, rather than the plantar, surface of the foot at a skin temperature of 32-33 degrees C rather than 34-35 degrees C improves the sensitivity of the technique. Acetylcholine produces an earlier and more predictable response than normal saline, which is approximately half maximal at a current of 0.2 mA and maximal at 1.0 mA. The use of different current strengths should allow more detailed assessment of neurogenic inflammation and the effects of drugs and disease states on C fibre function.

Neurogenic flare responses following topical application of capsaicin in humans.

The flare response to noxious stimulation of the skin is mediated by polymodal nociceptors of C fiber primary afferent nerves. Topical application of capsaicin initiates a flare response and burning pain. In this study, the variability of capsaicin-induced flare and pain was assessed in 220 subjects. The major factors in flare response are body site and age; more severe reactions occur in more proximal sites and in younger subjects. Larger flares were shown to be associated with greater pain. Variability is probably due to differences in the structure and reactivity of the neurovascular unit as proposed in the Lewis model of the axon reflex. It may prove possible to assess polymodal nociceptor function using topical capsaicin in disease states that affect the peripheral terminals of primary afferent nerves.