Dermal neurovascular dysfunction in type 2 diabetes.

OBJECTIVE: To review evidence for a relationship between dermal neurovascular dysfunction and other components of the metabolic syndrome of type 2 diabetes. RESEARCH DESIGN AND METHODS: We review and present data supporting concepts relating dermal neurovascular function to prediabetes and the metabolic syndrome. Skin blood flow can be easily measured by laser Doppler techniques. RESULTS: Heat and gravity have been shown to have specific neural, nitrergic, and independent mediators to regulate skin blood flow. We describe data showing that this new tool identifies dermal neurovascular dysfunction in the majority of type 2 diabetic patients. The defect in skin vasodilation is detectable before the development of diabetes and is partially correctable with insulin sensitizers. This defect is associated with C-fiber dysfunction (i.e., the dermal neurovascular unit) and coexists with variables of the insulin resistance syndrome. The defect most likely results from an imbalance among the endogenous vasodilator compound nitric oxide, the vasodilator neuropeptides substance P and calcitonin gene-related peptide, and the vasoconstrictors angiotensin II and endothelin. Hypertension per se increases skin vasodilation and does not impair the responses to gravity, which is opposite to that of diabetes, suggesting that the effects of diabetes override and counteract those of hypertension. CONCLUSIONS: These observations suggest that dermal neurovascular function is largely regulated by peripheral C-fiber neurons and that dysregulation may be a component of the metabolic syndrome associated with type 2 diabetes.

Small fiber neuropathy and neurovascular disturbances in diabetes mellitus.

Functional and organic abnormalities in small unmyelinated C fibers are the hallmark of type 2 diabetes. These may be silent clinically or present with burning feet, neurovascular abnormalities, wherein warm, cold, and heat pain thresholds are disturbed in association with impairment in skin blood flow and loss of PGP 9.5 immunostaining nerves in the skin. There is a dysfunctional phase preceding organic structural damage to the neurovascular unit. It coexists with elements of the metabolic syndrome, particularly insulin resistance (IR), elevated systolic blood pressure, and diabetic dyslipidemia i.e. dysfunction of the neurovascular unit may contribute to IR due to compromised blood flow with decreased delivery of fuels to their target tissues. If this proves to be the case, it will become important to re-focus energies on the defective neuropeptidergic regulation of blood flow as an approach to ameliorating diabetes. Because there is a functional phase that precedes structural damage, reversibility of the defect is achievable.

Methods for evaluation of peripheral neurovascular dysfunction.

Measurement of skin blood flow is a sensitive marker of C-fiber neurovascular dysfunction. It precedes development of abnormalities in diabetes mellitus, correlates with in vivo indices of the metabolic syndrome, and may be a "benchmark" for future studies on agents to improve microvascular dysfunction in diabetes mellitus. Skin blood flow can be measured under basal and stimulated conditions. There are different methods of evaluation. Iontophoresis and microdialysis are novel methods of drug delivery and the latter may be used as a means of extracting analytes in the skin. Theses methods are not invasive (iontophoresis) or minimally invasive (microdialysis). They can be performed repeatedly and safely in most patients. The use of microdialysis may be limited by sampling only water-soluble molecules. An alternative to microdialysis is iontophoresis, which works better with polar molecules. A combination of microdialysis and iontophoresis techniques can be useful in assessment of the pharmacokinetics of polar and nonpolar agents and the physiology and pathophysiology of the skin neurovascular system.

Diabetic neuropathies.

Diabetic neuropathy is a common complication of diabetes that is often associated both with considerable morbidity and mortality. The epidemiology and natural course of diabetic neuropathy is clouded with uncertainty, largely due to confusion regarding the definition and measurement of this disorder. The recent resurgence of interest in the vascular hypothesis, oxidative stress, the neurotrophic hypothesis and the possibility of the role of autoimmunity have opened up new avenues of investigation for therapeutic intervention. Paralleling our increased understanding of the pathogenesis of diabetic neuropathy, there must be refinements in our ability to measure quantitatively the different types of defects that occur in this disorder, so that appropriate therapies can be targeted to specific fibre types. These tests must be validated and standardised to allow comparability between studies and a more meaningful interpretation of study results. Our ability to manage successfully the many different manifestations of diabetic neuropathy depends ultimately on our success in uncovering the pathogenic processes underlying this disorder.

Neurotrophic role of interleukin-6 and soluble interleukin-6 receptors in N1E-115 neuroblastoma cells.

Interleukin 6 (IL-6) plays a role in physiological and pathophysiological processes in neuronal cells. We studied whether IL-6 plays a role in neuroblastoma cells in culture. These studies demonstrate that N1E-115 cells constitutively express IL-6 but not IL-6R. Exogenous IL-6 stimulated neuronal proliferation in a dose-dependent manner. Under serum-free conditions soluble IL-6 receptors (sIL-6R) alone or in combination with IL-6 exerted significant proliferative effects, while IL-6 alone failed to promote cell proliferation. Neutralizing anti-IL-6 antibody caused a 30-40% reduction in IL-6 mediated proliferation. Our results suggest the importance of IL-6/sIL-6R for proliferation and survival of N1E-115 adrenergic neuroblastoma cells.

Primary nociceptive afferents mediate the blood flow dysfunction in non-glabrous (hairy) skin of type 2 diabetes: a new model for the pathogenesis of microvascular dysfunction.

OBJECTIVE: To test the independent contributions of vascular endothelium, sympathetic activation and inhibition, vessel distensibility, and nociceptor-mediated vasodilation in both glabrous and hairy skin circulations. RESEARCH DESIGN AND METHODS: We measured blood flow using laser Doppler techniques in 10 people with type 2 diabetes and 10 age- and BMI-matched healthy control subjects at the pulp of the index finger (glabrous skin) and the dorsum of the hand (hairy skin). A 5-min ischemic block of the arm was used to test vascular endothelium. Warming of the probe site to 45 degrees C tested neurogenic vasodilation in hairy skin only. Vessel distensibility was tested by gravitational pressure. RESULTS: Basal blood flow and reactive hyperemia did not differ between groups at either skin site. The vasodilative response to local warming (P < 0.01) and limb lowering (P < 0.05) were significantly different between groups in hairy skin but not in glabrous skin in the absence of objective measured neuropathy. Nociceptor-mediated flow correlated significantly with the warm thermal threshold (r = -0.50, P < 0.05). Endothelial-mediated blood flow correlated with systolic blood pressure (r = -0.76, P < 0.01), LDL cholesterol (r = -0.62, P < 0.001), C-peptide (r = 0.65, P < 0.05), and triglycerides (r = 0.47, P < 0.05). CONCLUSIONS: These data suggest that neurogenic nociceptor-mediated vasodilation is impaired in subjects with type 2 diabetes when endothelial and sympathetic function are relatively intact. Heat-induced vasodilation may be a specific test of small heat-sensitive C-fiber peripheral neurons and may be an integral part of the metabolic syndrome.

Normal blood flow response and vasomotion in the diabetic Charcot foot.

Vasomotion, the spontaneous rhythmic contraction exhibited by small arteries and arterioles is dysregulated in patients with diabetic neuropathy. We examined the relationship between Charcot arthropathy and vasomotion at the dorsum of the foot. We studied nine diabetic patients with clinically diagnosed neuropathy and Charcot arthropathy in 13 feet (n=13), twelve subjects with diabetic neuropathy and no Charcot deformity (n=12), and 11 healthy controls (n=11). Following neuropathy assessment, blood flow was measured by laser Doppler flowmetry with local skin warming. Fast Fourier transformation was performed to provide an index of vasomotion. Subjects with Charcot osteoarthropathy had more severe somatic neuropathy and higher circulating levels of serum calcium (9.8+/-0.1 versus 9.3+/-0.1 mg/dL). Raising local temperature increased skin blood flow and vasomotion in both control subjects and Charcot subjects, but not in diabetic patients with neuropathy alone (p < 0.05 for blood flow, p < 0.02 for vasomotion). Patterns of peripheral vasomotion and blood flow which are clearly disordered in diabetic neuropathy are intact in patients with a Charcot osteoarthropathy, despite a more severe sensory nerve impairment. These findings suggest that the loss of peripheral blood flow and vasomotion often seen in diabetic neuropathy may actually be protective against Charcot arthropathy by preventing bone resorption. It remains unclear then whether the Charcot arthropathy is a direct result of a failure to decrease blood flow to bone, or is the manifestation of some other pathology.

Impairment of peripheral blood flow responses in diabetes resembles an enhanced aging effect.

OBJECTIVE: To test the hypothesis that skin blood flow responses in the fingertip of diabetic patients are impaired and to examine the role of aging in both healthy control subjects and diabetic patients. RESEARCH DESIGN AND METHODS: We measured cutaneous blood flow using laser Doppler techniques in 40 people with diabetes and in 20 age- and sex-matched healthy control subjects. To induce vasoconstriction, subjects were asked to perform three 1-min stressor tasks: mental arithmetic, contralateral hand grip, and immersion of the contralateral hand in ice water. To induce vasodilatation, a local heat stimulus of 45 degrees C was applied for 5 min. RESULTS: Basal blood flow did not differ between groups, but vasoconstrictive responses induced by arithmetic or immersion of the contralateral hand in ice-cold water and vasodilatation induced by local heating were severely impaired in diabetic subjects, compared with healthy control subjects (P < 0.01). These responses correlated with autonomic nerve function and deteriorated significantly with advancing age in control subjects, but not in diabetic subjects. Blood flow in younger diabetic subjects resembled that of older control subjects. CONCLUSIONS: These data demonstrate that diabetes has effects on precapillaries that may by direct or mediated via autonomic nerves, which result in a deficit that resembles premature aging.

Impaired peripheral vasomotion in diabetes.

OBJECTIVE: To test the hypothesis that vasomotion, the rhythmic contraction exhibited by small arteries and arterioles, is impaired in diabetic subjects compared with healthy control subjects. RESEARCH DESIGN AND METHODS: We mathematically modeled the oscillations in laser Doppler microvascular measurements taken from the pulpar surface of the index finger in 20 healthy control subjects and 20 age-matched diabetic subjects (8 with type I and 12 with type II diabetes). The mean duration of diabetes was 17.1 +/- 2.3 years, and mean HbA1c was 9.1 +/- 0.4%. Blood flow was measured for 5 min as subjects rested quietly in a closed room. Fast Fourier transformation was performed to provide the frequency power spectrum of each recording. Amplitude of vasomotion was correlated with six quantitative measurements of neuropathy. RESULTS: Diabetic subjects had impaired low-frequency oscillation vasomotion in 75% of age-matched patients (15 of 20 patients), with mean amplitudes of 24.9 +/- 6.4 vs. 129.0 +/- 33.2 (P < 0.0039). Of six somatic and autonomic neuropathy variables, only the warm thermal sensory threshold correlated significantly with the mean amplitude of vasomotion (r = -0.75, P < 0.0009). CONCLUSIONS: Patterns of peripheral vasomotion are clearly disordered in diabetes. The loss of low-frequency oscillations observed here suggests a peripheral vascular abnormality that extends past the capillary network to arterial vessels. It is uncertain whether the accompanying small unmyelinated nerve C-fiber dysfunction is a cause or consequence of the impaired microvascular function. Measurement of vasomotion may prove useful as a novel test for peripheral neurovascular function.

The natural progression of autonomic neuropathy and autonomic function tests in a cohort of people with IDDM.

OBJECTIVE: To test the natural progression of symptoms of autonomic neuropathy (AN) and function tests in subjects with IDDM. RESEARCH DESIGN AND METHODS: Seventy-six subjects with IDDM of < 10 years duration had cardiovascular autonomic reflex tests and were evaluated for signs and symptoms of AN. RESULTS: Fifty-seven subjects (66%) were available for restudy 9 years later (15 had died, 4 could not be located). Of the symptoms of AN, only gastroparesis increased in prevalence (P < 0.01). Of the five cardiovascular AN measures, only the R-R response to the Valsalva maneuver deteriorated (F[1,44] = 10.61, P < 0.01). CONCLUSIONS: The progression of AN in IDDM is monitored best longitudinally by the Valsalva maneuver because of the small variance ratio in repeated measures. Prevalence rates can be monitored by expiration-to-inspiration R-R or Valsalva ratios. Most clinical signs and symptoms of AN do not progress, underscoring the need for objective and quantitative autonomic function tests to identify people at risk for premature death.

Phospholipid and glutamic acid decarboxylase autoantibodies in diabetic neuropathy.

OBJECTIVE: To determine the prevalence and significance of phospholipid autoantibodies (PLAs) and glutamic acid decarboxylase (GAD) autoantibodies in the circulation of normal patients and diabetic patients with and without neuropathy. RESEARCH DESIGN AND METHODS: We measured PLAs in a total of 78 patients (a diabetic group with somatic or autonomic neuropathy [n = 40] another group without neuropathy [n = 38]), and GAD autoantibodies in a subset of 22 patients. RESULTS: PLAs are found in 2% of the general population. We found PLAs in 32% of the diabetic population without neuropathy, in 88% of those with neuropathy, in 55% of those with retinopathy, and in 25% of those with established nephropathy. The frequencies of immunoglobulins in the neuropathic group were: IgG = 78%, IgM = 33%, and IgA = 23%. There was no correlation between PLAs and microalbuminuria, macrovascular disease, fibrinogen, duration of diabetes, or neuropathy, but there was a strong correlation with total neuropathy score. Sera with high PLA IgG titers bound to the surface of neuroblastoma cells and inhibited cell growth. Antibodies to GAD65 were present in 32% and to GAD67 in 0% of patients. No titers of GAD65, GAD67, or the GAD65 ratio were associated with the degree of neuropathy of the presence of PLAs. CONCLUSIONS: PLAs occur frequently in the sera of patients with diabetes and correlate with the extent of neuropathy, suggesting a role for PLAs in the etiology thereof. The measurement of PLAs may constitute a marker for ongoing damage to nerves.

Quantitative measurement of cutaneous perception in diabetic neuropathy.

To determine the diagnostic value of various cutaneous sensory modalities in diabetic neuropathy, we studied cutaneous perception at the dominant hallux of 113 subjects (32 normal healthy controls and 81 diabetic subjects). The cutaneous sensory perception tests included warm and cold thermal perception, vibration, touch-pressure sensation, and current perception testing (CPT). The sensitivity of each modality when specificity is held greater than 90% was as follows: warm = 78%, cold = 77%, vibration = 88%, tactile-pressure = 77%, 5-Hz CPT = 52%, 250-Hz CPT = 48%, and 2000-Hz CPT = 56%. Combination thermal and vibratory gave optimum sensitivity (92-95%) and specificity (77-86%). We conclude that vibratory and thermal testing should be the primary screening tests for diabetic peripheral neuropathy. Other modalities may be of use only in specific situations.

Diabetic neuropathies.

Diabetic neuropathy is a common complication of diabetes that may be associated both with considerable morbidity (painful polyneuropathy, neuropathic ulceration) and mortality (autonomic neuropathy). The epidemiology and natural history of diabetic neuropathy is clouded with uncertainty, largely caused by confusion in the definition and measurement of this disorder. We have reviewed various clinical manifestations associated with somatic and autonomic neuropathy, and we herein discuss current views related to the management of the various abnormalities. Although unproven, the best evidence suggests that near-normal control of blood glucose in the early years after diabetes onset may help delay the development of clinically significant nerve impairment. Intensive therapy to achieve normalization of blood glucose also may lead to reversibility of early diabetic neuropathy, but again, this is unproven. Our ability to manage successfully the many different manifestations of diabetic neuropathy depends ultimately on our success in uncovering the pathogenic processes underlying this disorder. The recent resurgence of interest in the vascular hypothesis, for example, has opened up new avenues of investigation for therapeutic intervention. Paralleling our increased understanding of the pathogenesis of diabetic neuropathy, refinements must be made in our ability to measure quantitatively the different types of defects that occur in this disorder. These tests must be validated and standardized to allow comparability between studies and more meaningful interpretation of study results.