Transperineurial capillary abnormalities in the sural nerve of patients with diabetic neuropathy

The relationship between sudomotor function and skin microvascular reactivity in individuals with type 1 diabetes of long duration

AIM

The aim of this study was to assess the relationship between sudomotor function and microvascular perfusion in patients with type 1 diabetes (DM1).

METHODS

We evaluated 415 patients (206 women), with DM1, median age of 41 (IQR: 33-53) years, disease duration of 25 (IQR: 20-32) years. We assessed metabolic control of diabetes and the presence of peripheral and cardiac autonomic neuropathy. Sudomotor function was assessed using Sudoscan device by electrochemical skin conductance (ESC). Microvascular function was measured by laser-Doppler flowmetry with basal perfusion, the peak flow after occlusion (PORHpeak) and THmax which is the percentage change between basal perfusion and the peak flow during thermal hyperemia (TH). The accumulation of advanced glycation end products in the skin was assessed by skin autofluorescence (AF) measurement using AGE Reader. We subdivided patients based on the presence of diabetic peripheral neuropathy (DPN), cardiac autonomic neuropathy (CAN) and according to normal value of ESC.

RESULTS

Patients with abnormal ESC had higher skin AF [2.5 (2.1-2.9) vs 2.1 (1.9-2.5) AU, p < 0.001], lower eGFR [83 (72-96) vs 98 (86-108) ml/min/1.73 m2, p < 0.001], higher basal perfusion [25 (12-81) vs 14 (7-43) PU, p < 0.001], lower THmax [664 (137-1461) vs 1115 (346-1933) %, p = 0.002], higher PORHpeak [104 (59-167) vs 70 (48-135) PU, p < 0.001] as compared to subjects with normal ESC results. We found negative correlation between THmax and TG level (Rs = -0.14, p < 0.005), AF (Rs = -0.19, p = 0.001), vibration perception threshold - VPT (Rs = -0.24, p < 0.001) and positive correlation with HDL level (Rs = 0.14, p = 0.005), Feet ESC (Rs = 0.21, p < 0.001) and Hands ESC (Rs = 0.14, p = 0.004). We found positive correlation between PORHpeak and TG level (Rs = 0.14, p = 0.003), skin AF (Rs = 0.29, p < 0.001), VPT (0.27, p < 0.001) and negative correlation with eGFR (Rs = -0.2, p < 0.001), HDL (Rs = -0.12, p = 0.01), Feet ESC (Rs = -0.27, p < 0.001) and Hand ESC (Rs = -0.16, p = 0.002).

CONCLUSION

Impaired microvascular reactivity is associated with sudomotor dysfunction in patients with type 1 diabetes.

Mechanisms of diabetic neuropathy: Schwann cells

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Pathology of human diabetic neuropathy

Pathologic study of a disease provides insights into the precise mechanisms and targets of damage and may provide insights into new therapies. The main targets in diabetic neuropathy are myelinated and unmyelinated fibers as dysfunction and damage to them explains the symptoms of painful neuropathy and the major end points of foot ulceration and amputation as well as mortality. Demyelination and axonal degeneration are established hallmarks of the pathology of human diabetic neuropathy and were derived from pioneering light and electronmicroscopic studies of sural nerve biopsies in the late 1960s and early 1970s. Additional abnormalities, which are relevant to the pathogenesis of human diabetic neuropathy, include pathology of the microvessels and extracellular space. Intraepidermal and sudomotor nerve quantification in skin biopsies provides a minimally invasive means for the detection of early nerve damage. Studies of muscle biopsies are limited and show significant alterations in the expression of neurotrophins, but limited changes in muscle fiber size and capillary density.

Magnetic resonance imaging of the central nervous system in diabetic neuropathy

Diabetic 'peripheral' neuropathy (DPN) is one of the common sequelae to the development of both type-1 and type-2 diabetes mellitus. Neuropathy has a major negative impact on quality of life. Abnormalities in both peripheral vasculature and nerve function are well documented and, in addition, evidence is emerging regarding changes within the central nervous system (CNS) that are concomitant with the presence of DPN. The often-resistant nature of DPN to medical treatment highlights the need to understand the role of the CNS in neuropathic symptomatology and progression, as this may modulate therapeutic approaches. Advanced neuroimaging techniques, especially those that can provide quantitative measures of structure and function, can provide objective markers of CNS status. With that comes great potential for not only furthering our understanding of involvement of the CNS in neuropathic etiology but also most importantly aiding the development of new and more effective, targeted, analgesic interventions.

Pathogenesis of diabetic neuropathy: focus on neurovascular mechanisms

Neuropathies of the peripheral and autonomic nervous systems affect up to half of all people with diabetes, and are major risk factors for foot ulceration and amputation. The aetiology is multifactorial: metabolic changes in diabetes may directly affect neural tissue, but importantly, neurodegenerative changes are precipitated by compromised nerve vascular supply. Experiments in animal models of diabetic neuropathy suggest that similar metabolic sequelae affect neurons and vasa nervorum endothelium. These include elevated polyol pathway activity, oxidative stress, the formation of advanced glycation and lipoxidation end products, and various pro-inflammatory changes such as elevated protein kinase C, nuclear factor κB and p38 mitogen activated protein kinase signalling. These mechanisms do not work in isolation but strongly interact in a mutually facilitatory fashion. Nitrosative stress and the induction of the enzyme poly (ADP-ribose) polymerase form one important link between physiological stressors such as reactive oxygen species and the pro-inflammatory mechanisms. Recently, evidence points to endoplasmic stress and the unfolded protein response as forming another crucial link. This review focuses on the aetiopathogenesis of neurovascular changes in diabetic neuropathy, elucidated in animal studies, and on putative therapeutic targets the majority of which have yet to be tested for efficacy in clinical trials.

Emerging therapy for diabetic neuropathy: cell therapy targeting vessels and nerves

Diabetic neuropathy (DN), the most common complication of diabetes, frequently leads to foot ulcers and may progress to limb amputations. Despite continuous increase in incidence, there is no clinical therapy to effectively treat DN. Pathogenetically, DN is characterized by reduced vascularity in peripheral nerves and deficiency in angiogenic and neurotrophic factors. We will briefly review the pathogenetic mechanism of DN and address the effects and the mechanisms of cell therapies for DN. To reverse the changes of DN, studies have attempted to deliver neurotrophic or angiogenic factors for treatment in the form of protein or gene therapy; however, the effects turned out to be very modest if not ineffective. Recent studies have demonstrated that bone marrow (BM)-derived cells such as mononuclear cells or endothelial progenitor cells (EPCs) can effectively treat various cardiovascular diseases through their paracrine effects. As BM-derived cells include multiple angiogenic and neurotrophic cytokines, these cells were used for treating experimental DN and found to reverse manifestations of DN. Particularly, EPCs were shown to exert favorable therapeutic effects through enhanced neural neovascularization and neuro-protective effects. These findings clearly indicate that DN is a complex disorder with pathogenetic involvement of both vascular and neural components. Studies have shown that cell therapies targeting both vascular and neural elements are shown to be advantageous in treating DN.

Central nervous system involvement in diabetic neuropathy

Diabetic neuropathy is a chronic and often disabling condition that affects a significant number of individuals with diabetes. Long considered a disease of the peripheral nervous system, there is now increasing evidence of central nervous system involvement. Recent advances in neuroimaging methods detailed in this review have led to a better understanding and refinement of how diabetic neuropathy affects the central nervous system. Recognition that diabetic neuropathy is, in part, a disease that affects the whole nervous system is resulting in a critical rethinking of this disorder, opening a new direction for further research.

Endoneurial microvascular pathology in feline diabetic neuropathy

Endoneurial capillaries in nerve biopsies from 12 adult diabetic cats with varying degrees of neurological dysfunction were examined for evidence of microvascular pathology and compared to nerves obtained at necropsy from 7 adult non-diabetic cats without clinical evidence of neurological dysfunction. As reported previously [Mizisin, A.P., Nelson, R.W., Sturges, B.K., Vernau, K.M., LeCouteur, R.A., Williams, D.C., Burgers, M.L., Shelton, G.D., 2007. Comparable myelinated nerve pathology in feline and human diabetes mellitus. Acta Neuropathol. 113, 431-442.], the diabetic cats had elevated glycosylated hemoglobin and serum fructosamine levels, decreased motor nerve conduction velocity and compound muscle action potential (CMAP) amplitude, and markedly decreased myelinated nerve fiber densities. Compared to non-diabetic cats, there was a non-significant 26% increase in capillary density and a significant (P<0.009) 45% increase in capillary size in diabetic cats. Capillary luminal size was also significantly (P<0.001) increased, while an index of vasoconstriction was significantly decreased (P<0.001) in diabetic cats compared to non-diabetic controls. No differences in endothelial cell size, endothelial cell number or pericyte size were detected between non-diabetic and diabetic cats. In diabetic cats, basement membrane thickening, seen as a reduplication of the basal lamina, was significantly (P<0.0002) increased by 73% compared to non-diabetic controls. Regression analysis of either myelinated nerve fiber density or CMAP amplitude against basement membrane size demonstrated a negative correlation with significant slopes (P<0.03 and P<0.04, respectively). These data demonstrate that myelinated nerve fiber injury in feline diabetic neuropathy is associated with microvascular pathology and that some of these changes parallel those documented in experimental rodent and human diabetic neuropathy.

Early involvement of the spinal cord in diabetic peripheral neuropathy

OBJECTIVE

The pathogenesis of diabetic peripheral neuropathy (DPN) is poorly understood. We have recently reported a significant reduction in spinal cord cross-sectional area at the stage of clinically detectable DPN. In this study, we investigated whether spinal cord atrophy occurs in early (subclinical) DPN.

RESEARCH DESIGN AND METHODS

Eighty-one male type 1 diabetic subjects, 24 nondiabetic control subjects, and 8 subjects with hereditary sensory motor neuropathy (HSMN) type 1A underwent detailed clinical and neurophysiological assessments. Diabetic subjects were subsequently divided into three groups based on neuropathy severity (19 with no DPN, 23 with subclinical DPN, and 39 with clinically detectable DPN). All subjects underwent magnetic resonance imaging of the cervical spine and cord area measurements at disc level C2/C3.

RESULTS

Mean corrected spinal cord area index (SCAI) (corrected for age, height, and weight) was 67.5 mm [95% CI 64.1-70.9] in diabetic subjects without DPN. Those with subclinical (62.4 mm [59.5-65.3]) and clinically detectable DPN (57.2 mm [54.9-59.6]) had lower mean SCAIs compared with subjects with no DPN (P = 0.03 and P < 0.001, respectively). No significant difference was found between diabetic subjects without DPN and nondiabetic control subjects (69.2 mm [66.3-72.0], P = 0.47). Mean SCAIs in subjects with HSMN type 1A (71.07 mm [65.3-76.9]) were not significantly different from those for nondiabetic control subjects and diabetic subjects without DPN. Among diabetic subjects, SCAI was significantly related to sural sensory conduction velocities and the Neuropathy Composite and Symptom Scores.

CONCLUSIONS

Spinal cord involvement occurs early in DPN. There is also a significant relation between reduction in SCAI and neurophysiological assessments of DPN.

The role of PPARs in the microvascular dysfunction in diabetes

There is a major defect in skin blood flow (SkBF) in people with type 2 diabetes (T2DM). This defect is associated with relatively normal nitric oxide (NO) production in the skin. The abnormal blood flow cosegregates with hypertension, dyslipidemia, abnormal fatty acid composition, a proinflammatory state, and insulin resistance. Since these covariates are an integral part of the insulin resistance syndrome, we examined the effects of the thiazoledindiones (TZDs) as insulin sensitizers for their ability to correct the abnormal blood flow. The PPARgamma rosiglitazone improved NO production to normal levels, but had a small effect on SKBF. In contrast, pioglitazone had a small effect on skin neurovascular function but a dramatic effect on reducing nitrosative stress. These effects do not appear to be due to the insulin sensitizing properties of these compounds but are associated with a reduction in indices of inflammation, hemodilution, and are likely to be due to one of the many "vascular" effects of TZDs. The role of inflammation in the disordered neurovascular function in diabetes cannot be underplayed and the possible contribution of PPARalpha agonists to alter the inflammatory state needs to be explored further. Since blood flow regulation is mediated by mechanisms other than NO, such as prostaglandins and endothelial derived hyperpolarizing factor, which, in turn, are compromised by the inflammatory state, we anticipate that activation of both the PPARgamma as well as PPARalpha should ameliorate the disordered blood flow in type 2 diabetes. While it now appears that the PPARs may have a major role to play in protection from macrovascular disease, their contribution to amelioration of the microvascular defects in type 2 diabetes has fallen short of spectacular success. In this respect, the combinations of PPARalpha, PPARbeta and PPARgamma may better serve the unique requirements for improving the microvascular defect in diabetes.

Intraepidermal nerve fibers are indicators of small-fiber neuropathy in both diabetic and nondiabetic patients

OBJECTIVE

Small-fiber neuropathies may be symptomatic yet escape detection by standard tests. We hypothesized that morphologic changes in intraepidermal nerves would correlate with clinical measures of small-fiber neuropathy.

RESEARCH DESIGN AND METHODS

We studied 25 diabetic and 23 nondiabetic patients with neuropathy defined by signs, symptoms, and quantitative testing and 20 control subjects. Skin biopsies were obtained from forearm, thigh, proximal leg, and distal leg, and nerves identified using immunofluorescence with antibody to protein gene product (PGP) 9.5.

RESULTS

Mean dendritic length (MDL) (P < 0.01) and intraepidermal nerve fiber density (IENF) (P < 0.001) progressively decreased from proximal to distal sites only in patients with neuropathy. There was a significant reduction in IENF when comparing control subjects and patient groups in the distal leg (P < 0.001). MDL was significantly decreased in the thigh (P < 0.005) and in the proximal (P < 0.01) and distal (P < 0.002) leg in patients compared with control subjects. IENF was not significantly altered in diabetic patients of <5 years' duration, but significantly decreased in patients with >5 years' duration. MDL showed a linear decrease with increasing duration of diabetes. Distal leg IENF showed significant negative correlations with warm (P < 0.02) and cold (P < 0.05) thermal threshold, heat pain (P < 0.05), pressure sense (P < 0.05), and neurological disability score total sensory (P < 0.03) and total neuropathy (P < 0.03) values.

CONCLUSIONS

IENF was not significantly altered in these patients at <5 years' duration of diabetes, but fell significantly after 5 years of diabetes. MDL exhibited a linear loss with time, suggesting a different mechanism of change. MDL and IENF together may prove a useful end point in therapeutic trials for neuropathy.

Diabetic neuropathy: pathogenesis and therapy

Diabetic neuropathies are complex, heterogeneous disorders that encompass a wide range of abnormalities affecting both peripheral and autonomic nervous systems, causing considerable morbidity and mortality. Treatment should be based upon the underlying etiology and not symptoms alone, although symptomatic therapy is needed. Neuropathies may be focal or diffuse, proximal or distal, and involve somatic and autonomic nerves. Focal syndromes are classified as (1) entrapment syndromes or (2) mononeuropathies. Entrapment syndromes are treated by means of relieving compression within confined spaces. Mononeuropathies are due to a vascular insult and resolve spontaneously. They are best treated by supportive therapy. Proximal neuropathies are usually due to an inflammatory, vasculitic, or autoimmune condition and are best treated with specific therapies for the underlying disorder based on biopsy findings. Therapies for distal polyneuropathies include metabolic treatments (e.g., aldose reductase inhibitors, aminoguanidine, gamma-linolenic acid), autoimmune therapies, and nerve growth factors. No definitive treatment is available for painful diabetic neuropathy. Several medications have been used, among them tricyclic antidepressants, antiepileptic drugs, phenothiazines, calcitonin, local anesthetics, nonsteroidal anti-inflammatory drugs, and dextromethorphan. Nonpharmacologic therapies include surgical sympathectomy, spinal cord blockade, electrical spinal cord stimulation, and prostaglandin.

Diabetic neuropathy

Better clinical characteristics and a standardized approach to the definition of neuropathy has enabled us to define more precisely the natural history of diabetic neuropathy. Detailed studies on the pathology and pathogenesis have allowed dissection of important pathogenetic pathways. Effective treatment is currently limited, although a number of new and potentially important therapeutic interventions, including modification of the vascular supply and antioxidant status and growth factors, may prove to be of benefit in preventing damage and also promoting repair of peripheral nerves in human diabetic neuropathy.

Increased NADH oxidase activity in the retina of the BBZ/Wor diabetic rat

This morphological study demonstrates a role for endothelial cells in generating reactive oxygen species in early stages of retinopathy in the BBZ/Wor rat, an obese, noninsulin dependent model of diabetes. Hyperglycemia induced pseudohypoxia results in an imbalance in cytosolic NADH/NAD+. In the oxygen-rich environment of the retina, NADH oxidase generates superoxide radical which is dismutated to hydrogen peroxide. Localization of hydrogen peroxide by the cerium NADH oxidase enzyme activity cytochemical localization technique shows a statistically significant increase of peroxide localization in the central retina of diabetic rats as compared to age-matched, nondiabetic controls. Endothelial cell dysfunction, indicated by leakage of endogenous serum albumin, coincided with areas of NADH oxidase activity localization. In diabetic rats there are increased levels of fibronectin in areas of hydrogen peroxide localization. This in vivo, morphological study is the first demonstration of oxidative injury and endothelial cell dysfunction in the retina of a spontaneous, noninsulin dependent model of diabetes.

Diabetes and the nervous system

Hyperglycemia and its vascular complications affect the entire nervous system, contributing to increased morbidity and mortality. Chronic hyperglycemia is not only a known and major risk factor for cerebral vascular diseases but also the presence of hyperglycemia at the time of a cerebrovascular event may adversely influence the outcome. It also affects the treatment of some neurodegenerative disorders, and there are suggestions that diabetes may in fact suffer from a "chronic diabetic encephalopathy." Its varied effects on the peripheral nervous system result in several forms of diabetic neuropathies, the exact pathogenesis of which is still obscure. There is, however, some new information that may link metabolic and vascular hypotheses.

Arterio-venous shunting and proliferating new vessels in acute painful neuropathy of rapid glycaemic control (insulin neuritis)

Insulin neuritis, or painful neuropathy following rapid improvement in glycaemic control, is well recognised but its aetiology is unclear. An understanding of the processes involved in the genesis of acute painful neuropathy of rapid glycaemic control may give an insight into the early pathogenetic factors leading to diabetic nerve damage in general. We have identified five subjects with insulin neuritis including one who developed severe autonomic neuropathy following treatment with insulin. Subjects underwent: 1) assessment of neuropathic symptom and deficit scores; 2) quantitative sensory and electrophysiological studies and 3) sural nerve epineurial vessel photography and fluorescein angiography in vivo. The sural nerve photographs were independently graded by an ophthalmologist. All subjects with insulin neuritis presented with severe sensory symptoms but clinical examination and electrophysiological tests were normal except in the subject with the severe autonomic neuropathy in whom all the tests were abnormal. On nerve photography, there was an abundance of epineurial nutrient vessels although these showed severe abnormalities including arteriolar attenuation, tortuosity and arterio-venous shunting in all subjects. Proliferating neural 'new vessels' which bear striking similarities to those found in the retina and that were more leaky to fluorescein than normal vessels, were observed in three subjects. Venous distension and/or tortuosity was also observed in three subjects and this was most marked in the subject with severe autonomic neuropathy. This study shows that epineurial nutrient vessel anatomy is abnormal in subjects with acute painful neuropathy of rapid glycaemic control, a condition previously thought to be purely metabolic in origin. The presence of epineurial arterio-venous shunting and a fine network of vessels resembling the new vessels of the retina, may lead to a 'steal' effect rendering the endoneurium ischaemic. This process may be important in the genesis of neuropathic pain, and further supports the importance of vascular factors in the pathogenesis of diabetic neuropathy.

Vascular factors in diabetic neuropathy

Despite considerable research we still do not have a comprehensive explanation for the pathogenesis of diabetic neuropathy. Although chronic hyperglycaemia is almost certainly involved, it is not known whether the primary pathology is metabolic, microvascular, or an interaction between the two. Hyperglycaemia-induced polyol pathway hyperactivity associated with nerve sorbitol accumulation and myo-inositol depletion may play a part in the genesis of diabetic neuropathy. The case for microvascular disease in diabetic neuropathy is now strong. Fibre loss in human sural nerve is multifocal, suggesting ischaemia. The degree of vessel disease has been related to the severity of neuropathy. People with chronic obstructive pulmonary disease develop the so called "hypoxic neuropathy" in which similar microvascular changes occur as in diabetic neuropathy. In rats with experimental diabetic neuropathy nerve blood flow is reduced and oxygen supplementation or vasodilator treatment improved the deterioration in conduction velocity and nerve blood flow. Similarly, in human diabetic neuropathy, there is impaired nerve blood flow, epineurial arterio-venous shunting and a reduction in sural nerve oxygen tension. At what stage during the development of nerve damage these changes occur is yet to be determined.