Differential impairment of the sudomotor and nociceptor axon-reflex in diabetic peripheral neuropathy

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.

Body temperature regulation in diabetes

The effects of type 1 and type 2 diabetes on the body's physiological response to thermal stress is a relatively new topic in research. Diabetes tends to place individuals at greater risk for heat-related illness during heat waves and physical activity due to an impaired capacity to dissipate heat. Specifically, individuals with diabetes have been reported to have lower skin blood flow and sweating responses during heat exposure and this can have important consequences on cardiovascular regulation and glycemic control. Those who are particularly vulnerable include individuals with poor glycemic control and who are affected by diabetes-related complications. On the other hand, good glycemic control and maintenance of aerobic fitness can often delay the diabetes-related complications and possibly the impairments in heat loss. Despite this, it is alarming to note the lack of information regarding diabetes and heat stress given the vulnerability of this population. In contrast, few studies have examined the effects of cold exposure on individuals with diabetes with the exception of its therapeutic potential, particularly for type 2 diabetes. This review summarizes the current state of knowledge regarding the impact of diabetes on heat and cold exposure with respect to the core temperature regulation, cardiovascular adjustments and glycemic control while also considering the beneficial effects of maintaining aerobic fitness.

Whole-body heat loss during exercise in the heat is not impaired in type 1 diabetes

PURPOSE

The objective of this study is to determine whether individuals with type 1 diabetes exhibit impairments in local and whole-body heat loss responses that could affect core temperature regulation during exercise in the heat compared with matched, nondiabetic individuals.

METHODS

Twelve otherwise healthy individuals with type 1 diabetes (HbA1c = 7.7% ± 0.3%) and 12 controls matched for age, sex, body surface area, and physical fitness cycled continuously for 60 min at a set rate of metabolic heat production (approximately 400 W) in a whole-body direct calorimeter (35°C and 20% relative humidity). Local sweat rate (ventilated capsule) was measured on the back and skin blood flow (laser Doppler velocimetry) on the forearm. Core (rectal and esophageal) and mean skin temperatures and heart rate were measured continuously. Whole-body heat exchange and change in body heat content were measured using simultaneous direct whole-body and indirect calorimetry.

RESULTS

The change (mean ± SE) in body heat content was similar between groups during exercise (diabetes, 409 ± 27 kJ; control, 386 ± 33 kJ; P = 0.584) and recovery (diabetes, -115 ± 16 kJ; control, -93 ± 24 kJ; P = 0.457). Local heat loss responses of sweating (P = 0.783) and skin blood flow (P = 0.078) as well as rectal temperature (diabetes, 37.87°C ± 0.10°C; control, 37.85°C; ± 0.13°C; P = 0.977) and heart rate (diabetes, 130 ± 9 beats·min, vs control, 126 ± 8 beats·min, P = 0.326) were comparable at the end of the exercise period.

CONCLUSION

During light-to-moderate-intensity exercise performed under conditions permitting full sweat evaporation, otherwise healthy type 1 diabetic individuals did not show impaired heat loss responses during heat exposure when compared with matched individuals without diabetes.

Neurovascular function and sudorimetry in health and disease

In this review of thermoregulatory function in health and disease, we review the basic mechanisms controlling skin blood flow of the hairy and glabrous skin and illustrate the major differences in blood flow to glabrous skin, which is, in essence, sympathetically mediated, while hairy skin is dependent upon neuropeptidergic signals, nitric oxide, and prostaglandin, among others. Laser Doppler methods of quantification of blood flow--in response to iontophoresis of acetylcholine or heat--and nociceptor-mediated blood flow have relatively uniformly demonstrated an impaired capacity to increase blood flow to the skin in diabetes and in its forerunners, prediabetes and the metabolic syndrome. This reduced capacity is likely to be a significant contributor to the development of foot ulcerations and amputations in diabetes, and means of increasing blood flow are clearly needed. Understanding the pathogenic mechanisms is likely to provide a means of identifying a valuable therapeutic target. Thermoregulatory control of sweating is intimately linked to the autonomic nervous system via sympathetic C fibers, and sweat glands are richly endowed with a neuropeptidergic innervation. Sweating disturbances are prevalent in diabetes and its precursors, and quantification of sweating may be useful as an index of diagnosis of somatic and, probably, autonomic dysfunction. Moreover, quantifying this disturbance in sweating by various methods may be useful in identifying the risk of progression from prediabetes to diabetes, as well as responses to therapeutic intervention. We now have the technological power to take advantage of this physiological arrangement to better understand, monitor, and treat disorders of small nerve fibers and the somatic and autonomic nervous system (ANS). Newer methods of sudomotor function testing are rapid, noninvasive, not technically demanding, and accessible to the outpatient clinic. Whether the potential applications are screening for diabetes, following poorly controlled diabetes subjects during alteration of their treatment regimen, or simply monitoring somatic and autonomic function throughout the course of treatment, sudorimetry can be an invaluable tool for today's clinicians.

New and developing drugs for the treatment of neuropathic pain in diabetes

A number of agents from diverse pharmacological classes are used to treat neuropathic pain associated with diabetic peripheral neuropathy. Only three of these have regulatory approval for this indication in the U.S. In this focused article, I will discuss selected drugs, newly approved or in development, to treat neuropathic pain in patients with diabetic neuropathy. These will include agonists and antagonists of the transient receptor potential channels, a family of receptor proteins that play a role in the transduction of physical stress; sodium channel isoform specific antagonists; a recently approved dual-action opioid receptor agonist-norepinephrine reuptake inhibitor; gene therapy for neuropathic pain; and anti-nerve growth factor molecules. Mechanisms of action, preclinical supporting data, clinical trial evidence, and adverse effects will be reviewed.

Laser Doppler imaging in the detection of peripheral neuropathy

Small fiber neuropathy is common in a number of systemic diseases and is often challenging to diagnose. Laser Doppler imaging (LDI) is a test of small fiber neurovascular function that can quantify the integrity of the vasomotor C-fiber mediated axon-reflex, but no standardized method of analysis exists. We developed a novel LDI analysis technique and tested it in a human model of small fiber neuropathy. Eighteen healthy subjects (age 24 ± 3 years) underwent LDI testing to assess the axon-mediated flare area in response to 10% acetylcholine iontophoresis. LDI measurements were taken before and longitudinally after a 48-hour application of 0.1% capsaicin (to cause a transient small fiber neuropathy) on the skin of the thigh; placebo cream was placed on the contralateral thigh as a control. We compared our new LDI image analysis technique to two previously published methods. The new LDI analysis technique was the only method to show a consistent difference in axon-reflex area between capsaicin treated and placebo treated skin on all testing days (p<0.05) with maximum attenuation of the flare area immediately post-application (438 ± 298 mm(2) vs. 824 ± 375 mm(2), p<0.05). In conclusion, this study demonstrates that our novel flare area method for LDI analysis can detect neurovascular dysfunction in a model of small fiber neuropathy, is an improvement over existing methods, and may supplement clinical assessment of small fiber neuropathy.

Do heat events pose a greater health risk for individuals with type 2 diabetes?

Chronic medical conditions such as type 2 diabetes may alter the body's normal response to heat. Evidence suggests that the local heat loss response of skin blood flow (SkBF) is affected by diabetes-related impairments in both endothelium-dependent and non-endothelium-dependent mechanisms, resulting in lower elevations in SkBF in response to a heat or pharmacological stimulus. Thermoregulatory sweating may also be diminished by type 2 diabetes, impairing the body's ability to transfer heat from its core to the environment. Diabetes-associated co-morbidities and the medications (particularly those affecting fluid balance) required to treat these conditions may exacerbate the risk of heat-related illness by decreasing SkBF and sweating further. Unfortunately, the majority of studies measure local heat loss responses in the hands and feet and lack measures of core temperature. Therefore, the impact of these impairments on whole-body heat loss remains unknown. This review addresses heat-related vulnerability in individuals with type 2 diabetes by examining the literature related to heat loss responses in this population. Type 2 diabetes, its associated co-morbidities, and the medications required in their treatment may cause dehydration, lower SkBF, and reduced sweating, which could consequently impair thermoregulation. This effect is most evident in individuals with poor blood glucose control. Although type 2 diabetes can be associated with impairments in SkBF and sweating, more physically active individuals requiring fewer medications and having good blood glucose control may be able to tolerate heat as well as those of similar age and body composition.

Capsaicin induces degeneration of cutaneous autonomic nerve fibers

OBJECTIVE

To determine the effects of topical application of capsaicin on cutaneous autonomic nerves.

METHODS

Thirty-two healthy subjects underwent occlusive application of 0.1% capsaicin cream (or placebo) for 48 hours. Subjects were followed for 6 months with serial assessments of sudomotor, vasomotor, pilomotor, and sensory function with simultaneous assessment of innervation through skin biopsies.

RESULTS

There were reductions in sudomotor, vasomotor, pilomotor, and sensory function in capsaicin-treated subjects (p < 0.01 vs. placebo). Sensory function declined more rapidly than autonomic function, reaching a nadir by Day 6, whereas autonomic function reached a nadir by Day 16. There were reductions in sudomotor, vasomotor, pilomotor, and sensory nerve fiber densities in capsaicin-treated subjects (p < 0.01 vs. placebo). Intraepidermal nerve fiber density declined maximally by 6 days, whereas autonomic nerve fiber densities reached maximal degeneration by Day 16. Conversely, autonomic nerves generally regenerated more rapidly than sensory nerves, requiring 40-50 days to return to baseline levels, whereas sensory fibers required 140-150 days to return to baseline.

INTERPRETATION

Topical capsaicin leads to degeneration of sudomotor, vasomotor, and pilomotor nerves accompanied by impairment of sudomotor, vasomotor, and pilomotor function. These results suggest the susceptibility and/or pathophysiologic mechanisms of nerve damage may differ between autonomic and sensory nerve fibers treated with capsaicin and enhances the capsaicin model for the study of disease-modifying agents. The data suggest caution should be taken when topical capsaicin is applied to skin surfaces at risk for ulceration, particularly in neuropathic conditions characterized by sensory and autonomic impairment.

Sweat testing to evaluate autonomic function

Sudomotor dysfunction is common in many subtypes of neuropathy but is one of the earliest detectable neurophysiologic abnormalities in distal small fiber neuropathy. Clinical assessments of sudomotor function include thermoregulatory sweat testing (TST), quantitative sudomotor axon reflex testing (QSART), silicone impressions, the sympathetic skin response (SSR), the acetylcholine sweat-spot test and quantitative direct and indirect axon reflex testing (QDIRT). These testing techniques, when used in combination, can detect and localize pre- and postganglionic lesions, can provide early diagnosis of sudomotor dysfunction and can monitor disease progression or disease recovery. In this article, we describe many of the common clinical tests available for evaluation of sudomotor function with focus on the testing methodology and limitations while providing concrete examples of test results.

The diagnostic workup of patients with neuropathic pain

Neuropathic pain is initiated or caused by damage or dysfunction of the peripheral or central nervous systems in various disorders, each having pain-related symptoms and signs thought secondary to common pain mechanisms. Ancillary testing may demonstrate associated nervous system abnormalities, however its specificity is inadequate at present, as it makes inferential conclusions from indirect data. Symptom assessment and physical findings remain paramount in the diagnosis of neuropathic pain.

Cutaneous blood flow and sweat rate responses to exogenous administration of acetylcholine and methacholine

The aim of this study was to evaluate cutaneous vasodilation and sweating responses to exogenous administration of acetylcholine (ACh) and methacholine (MCh), which have different sensitivities to endogenous cholinesterase. Four intradermal microdialysis probes were placed in dorsal forearm skin: two sites were perfused with ACh (1 x 10(-7)-1 M) and the other two with the same molar concentrations of MCh. Sweat rate (SR) and cutaneous blood flow were simultaneously assessed directly over each microdialysis membrane. Dose-response curves were constructed, and the effective concentration of the drug resulting in 50% of the maximal response (EC(50)) was identified. For SR and cutaneous vascular conductance (CVC), there were no significant differences in EC(50) between sites receiving the same drug: -1.52 +/- 0.18 and -1.19 +/- 0.09 log-molar concentration of ACh at distal and proximal sites, respectively, and -2.35 +/- 0.24 and -2.42 +/- 0.23 log-molar concentration of MCh at distal and proximal sites, respectively, for SR (P > 0.05) and -3.87 +/- 0.32 and -3.97 +/- 0.27 log-molar concentration of ACh at distal and proximal sites, respectively, and -4.78 +/- 0.17 and -4.46 +/- 0.16 log-molar concentration of MCh at distal and proximal sites, respectively, for CVC (P > 0.05). However, the EC(50) for CVC and SR was significantly lower at the MCh than at the ACh sites. A second procedure was performed to confirm that differences in responses between ACh and MCh could be attributed to different cholinesterase sensitivities. Similarly, four microdialysis membranes were placed in dorsal forearm skin: two sites were perfused with ACh and other two with MCh. However, one of each of the ACh and MCh sites was also perfused with 10 microM neostigmine (an acetylcholinesterase inhibitor). Neostigmine at the ACh site induced a leftward shift (i.e., lower EC(50)) of the SR and CVC dose-response curves compared with the site treated with ACh alone, resulting in no difference in the EC(50) for SR and CVC between the ACh + neostigmine and the MCh site. These results suggest that elevations in SR and CVC occur earlier with MCh than with ACh treatment because of differences in cholinesterase susceptibility between these drugs.

The diagnostic workup of patients with neuropathic pain

Determining the causes of neuropathic pain is more than an epistemological exercise. At its essence, it is a quest to delineate mechanisms of dysfunction through which treatment strategies can be created that are effective in reducing, ameliorating, or eliminating symptomatology. To date, predictors of which patients will develop neuropathic pain or who will respond to specific therapies are lacking, and present therapies have been developed mainly through trial and error. Our current inability to make therapeutically meaningful decisions based on ancillary test data is illustrated by the following: In a study specifically designed to assess the response of patients with painful distal sensory neuropathies to the 5% lidocaine patch, no relationship between treatment response and distal leg skin biopsy, QST, or sensory nerve conduction study results could be established. From a mechanistic perspective, the hypothesis that the lidocaine patch would be most effective in patients with relatively intact epidermal innervation, whose neuropathic pain is presumed attributable to "irritable nociceptors," and least effective in patients with few surviving epidermal nociceptors, presumably with "deafferentation pain," was unproven. The possible explanations are multiple and outside the scope of this review. However, these findings, coupled with the disparity in C-fiber subtype involvement in diabetic small-fiber neuropathy, and the recently reported inability of enzyme replacement therapy in Fabry disease to influence intraepidermal innervation density, while having mixed effects on cold and warm QST thresholds, and beneficial effects on sudomotor findings, when therapeutic benefit was demonstrated, lead one to conclude that the specificity of ancillary testing in neuropathic pain is inadequate at present, and reinforce the aforementioned caveats about inferential conclusions from indirect data. The diagnosis of neuropathic pain mechanisms is in its nascent stages and ancillary testing remains "subordinate," "subsidiary," and "auxiliary" as defined in Webster's Third New International Dictionary. As a consequence of these difficulties, the recent approach by Bennett and his colleagues may have merit. They have hypothesized (and provide data in support) that chronic pain can be more or less neuropathic on a spectrum between "likely," "possible," and "unlikely," based on patient responses on validated neuropathic pain symptom scales, when compared with specialist pain physician certainty of the presence of neuropathic pain on a 100-mm visual analog scale. The symptoms most associated with neuropathic pain were dysesthesias, evoked pain, paroxysmal pain, thermal pain, autonomic complaints, and descriptions of the pain as being sharp, hot, or cold, with high sensitivity. Higher scores for these symptoms correlated with greater clinician certainty of the presence of neuropathic pain mechanisms. Considering each individual patient's chronic pain as being somewhere on a continuum between "purely nociceptive" and "purely neuropathic" may have diagnostic and therapeutic relevance by enhancing specificity, but this requires clinical confirmation. Thus, symptom assessment remains indispensable in the evaluation of neuropathic pain, ancillary testing notwithstanding