19 Microcirculation of the Diabetic Foot

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From: Contemporary Cardiology: Diabetes and Cardiovascular Disease, Second Edition Edited by: M. T. Johnstone and A. Veves © Humana Press Inc., Totowa, NJ

19 Microcirculation of the Diabetic Foot

Chantel Hile, ^MD and Aristidis Veves, ^MD , ^DS c

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INTRODUCTION

Diabetic foot problems are major contributors to health care costs and hospitalizations.

A complete understanding of how the disease process works is essential in learning how to best prevent and treat these complications. Abnormalities of the microcirculation are generally accepted as early changes in diabetes (1–7). Eventual manifestations of altered microcirculation, such as retinopathy, nephropathy, and neuropathy, are related to the duration and severity of diabetes (8–10). Intensive glycemic control was found in the Diabetes Control and Complications Trial to significantly delay the development and progression of these microvascular complications in type 1 diabetic patients, with similar results reported in type 2 diabetic patients (10–13). The capillary microcirculation to foot skin is no exception and has shown signs of significant impairment in diabetic patients, especially when metabolic control is poor (14). This chapter will focus on the changes that occur in the microcirculation of the diabetic foot and the different methods used for their evaluation.

The Concept of “Small Vessel Disease”

For the purpose of clarity in discussing microcirculation, the concept of “small vessel disease” must be eliminated. Early retrospective pathological studies in diabetic patients who underwent amputation led to the misconception that abnormalities in the microcir- culation are occlusive in nature, so-called “small vessel disease” (15). It was postulated that such occlusive small vessel disease occurs even in the absence of any macrovascular occlusive problem and causes ischemic lesions and impairment of wound healing (15).

This idea originated from the histological existence of periodic acid-Schiff-positive

material occluding the medium-sized or small arteries in amputated limb specimens (15).

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404 Hile and Veves

However, subsequent physiological studies (16) and other prospective staining and arterial casting studies (17,18) have demonstrated absence of such occlusive lesions.

Thus, it is clear that the term “small vessel disease” initially referred to medium or small size arteries, not to the microcirculation. As it stands, the phrase creates confusion and should no longer be used.

STRUCTURAL CHANGES OF THE FOOT MICROCIRCULATION Over the last two decades, it has become clear that metabolic alterations in diabetes cause both structural and functional changes in multiple areas within the arteriolar and capillary systems (19–21). The most characteristic structural changes of the capillary circulation in diabetic patients are a reduction in the capillary size and thickening of basement membranes (BM) (22,23). Skin capillary density in diabetics, on the other hand, does not differ from that of healthy subjects (24). These changes in capillary size and BM thickness are more pronounced in the legs. This phenomenon is most likely the result of the higher hydrostatic pressure in the lower extremities (25), especially in diabetic patients with poorly controlled blood sugar levels (26). It is currently believed that increased hydrostatic pressure and shear force in the microcirculation evokes an injury response in the microvascular endothelium. This injury may result in increased elaboration of extravascular matrix proteins leading to capillary BM thickening and arteriolar hyalinosis (27,28). Thickened membranes impair the migration of leukocytes and hamper the hyperemic response to injury, increasing the susceptibility of the diabetic foot to infection (29,30). These structural modifications also decrease the elastic prop- erties of the capillary vessel walls, limiting their capacity for vasodilatation, and may eventually result in a significant loss of the autoregulatory capacity (31). It is of interest that these changes do not result in narrowing or occlusion of the capillary lumen; on the contrary, the arteriolar blood flow may be normal or even increased (32).

Another factor that is most probably involved in the impaired vasodilatory capacity of diabetic patients is the increased stiffness of precapillary vessel walls as a result of increased glycosylation and formation of nonenzymatic advanced glycosylation end- products (AGE). Irreversible chemical processes occur slowly as these compounds accumulate over time (33). AGE receptors are found on both endothelial cells and monocytes (34). It has been postulated that AGE contributes to the development of diabetic microangiopathy (33), a hypothesis supported by the fact that diabetics have higher serum and arterial wall concentrations of AGE than healthy subjects. The differ- ence was even more striking between diabetics with nephropathy as compared to healthy subjects (35). A study of the effect of AGE on endothelial function in experimental animals showed that they inhibit endothelium-dependent vasodilatation, an effect that can be reversed by an AGE inhibitor (36).

FUNCTIONAL CHANGES OF THE MICROCIRCULATION

In addition to the structural changes wrought by diabetes on the microcirculation,

techniques that allow the measurement of skin blood flow have highlighted functional

disturbances as well. Using these techniques, researchers have observed that diabetic

patients have reduced maximal hyperemic response to heat, even in the early stages of the

disease (37). The idea that impaired capillary microcirculation could be a major contrib-

uting factor in the development of diabetic foot pathology has encouraged more in-depth

research in this direction (18,29,38). Further development of new techniques to evaluate

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the microcirculation to peripheral tissues has expanded the understanding of these func- tional changes and their role in altering the microvascular blood flow. Before discussing the changes in vascular reactivity, it would be of particular importance to review the different techniques currently used for evaluating the microcirculation.

Methods of Evaluating the Microcirculation of the Feet M

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Currently, this method is the most widely accepted technique for evaluating blood flow in the skin microcirculation. Basically, it measures the capillary flux, which is a combination of velocity and the number of moving red cells. This is achieved by employ- ing red laser light that is transmitted to the skin through a fiber-optic cable. The frequency shift of light back-scattered from the moving red cells beneath the probe tip is computed to give a measure of the superficial microvascular perfusion (39).

Either a single-point laser probe, which evaluates the microvascular blood flow at one point of the skin, or a real-time laser scanner, which evaluates the blood flow in an area of skin, can be used. The single-point laser probe is used mainly for evaluating the hyperemic response to a heat stimulus, or for evaluating the nerve-axon-related hyper- emic response. To assess heat-related hyperemic response, the baseline blood-flow measurements are made first. The skin is then heated to 44

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C for 20 minutes using a small brass heater, following which the maximum blood flow is measured to evaluate the magnitude of change from baseline. To measure nerve-axon-related hyperemic response, two single-point laser probes are applied (Fig. 1). One probe measures the blood flow to an area of skin, which is exposed directly to acetylcholine (Ach). The second probe, placed in close proximity (5 mm), measures the indirect effect of applied Ach. This indirect effect results from stimulation of C nociceptive nerve fibers in the area and reflects the integrity of the nerve-axon-related reactive hyperemia.

Fig. 1. Measurements of direct and indirect effect of vasoactive substance using single-point laser probes: One probe is used in direct contact with the iontophoresis solution chamber (colored ring) and measures the direct response. The center probe measures the indirect response (nerve-axon- related effect). A small quantity (<1 mL) of 1% acetylcholine chloride solution or 1% sodium nitroprusside solution is placed in the iontophoresis. A constant current of 200 mA is applied for 60 seconds achieving a dose of 6mC/cm^–2between the iontophoresis chamber and a second nonactive electrode placed 10 to 15 cm proximal to the chamber (black strap around the wrist).

This current causes a movement of solution to be iontophorized toward the skin.

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The laser scanning method is also used for evaluating the endothelium-dependent microvascular reactivity (the magnitude of change in blood flow in response to Ach admitted to the skin through the iontophoresis technique), and the endothelium-indepen- dent microvascular reactivity (the magnitude of change in blood flow in response to sodium nitroprusside [SNP]).

The iontophoresis technique is used to apply these vasoactive substances to a localized area of the skin. In this technique, a delivery vehicle device is attached firmly to the skin with double-sided adhesive tape. The device contains two chambers that accommodate two single-point laser probes. A small quantity of (<1 mL) of 1% Ach solution or 1% of SNP solution is placed in the iontophoresis chamber and a constant current of 200 mA is applied for 60 seconds, achieving a dose of 6 mC/cm

^–2

between the iontophoresis chamber and a second nonactive electrode placed 10–15 cm proximal to the chamber.

This current causes a movement of solution to be iontophoresed toward the skin, resulting in vasodilatation (Fig. 1).

After the adhesive device has been removed, the localized area exposed to either of the vasoactive substances is scanned. The laser Doppler perfusion imager employs a 1-mW helium-neon laser beam of 633-nm wavelength, which sequentially scans an area of skin (Fig. 2). The maximum number of measured spots is 4096, and the apparatus produces a color-coded image of skin erythrocyte flux on a computer monitor. The scanner is set up to scan up to 32 × 32 measurement points over an area approx 4 × 4 cm.

All laser measurements are expressed as volts and depend on the voltage difference created by the returned light to the computer. Higher blood flow at the skin level results in a higher amount of light picked by the single probe or the scanner and a higher voltage recorded by the computer. This technique is best suited for studying the relative changes in flow induced by variety of physiological maneuvers or pharmaceutical intervention procedures.

Fig. 2. Laser Doppler flowmetry: A helium-neon laser beam is emitted from the laser source to sequentially scan the circular hyperemic area (seen surrounding the laser beam) produced by the iontophorized vasoactive substance to a small area on the volar surface of the forearm.

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The technique has been validated against direct measurements of the capillary flow velocity (40). The day-to-day reproducibility of the technique was evaluated in healthy subjects who were repeatedly tested at their foot and arm for 10 consecutive working days in our lab. The coefficient of variation (CV) for the baseline blood-flow measurement obtained with the laser probe evaluating the response to heat was 44%, although that for the maximal response to heat was 27.9%. The indirect response to Ach, measured by a single-point laser probe, had a CV of 60.6% for the baseline measurements and 35.2%

for the maximal hyperemic response after the iontophoresis. The laser scanner had a significantly better reproducibility with the CV at the foot and forearm level being between 14% and 19%.

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The technique of measuring oxygen tension transcutaneously is based on the fact that oxygen is capable of diffusing through tissue and skin. Although the rate of diffusion is very low at normal surface body temperature, the application of heat to a localized area sufficiently enhances the flow of oxygen through the dermis to allow noninvasive mea- surement of capillary oxygen level. The measurements are affected by the affinity of blood for oxygen and the tissue properties, and the change in skin temperature. These factors may influence, to some extent, the accuracy of these measurements.

Changes in Vascular Reactivity

The classic description of the diabetic neuropathic foot as warm and red with palpable pulses and distended veins points to a possibility of increased blood flow in the affected limb. Studies to explore this presentation found that the blood flow in the nutritional skin microcirculation is stable or even reduced (41,42), indicating a functional ischemia of the skin microcirculation and maldistribution of blood flow to the foot (14). It was also suggested that both structural and functional changes in the skin microcirculation result in a significant shifting of the blood flow away from nutritional capillaries to subpapillary arteriovenous shunts of a much lower resistance (43). As these shunts are innervated by sympathetic nerves (44), co-existing autonomic neuropathy and sympathetic denerva- tion (such as occurs in diabetic patients with severe neuropathy) may lead to an opening of these shunts, augmentation of the maldistribution of blood between the nutritional capillaries and subpapillary vessels (45,46), and consequent aggravation of microvascu- lar ischemia. Studies using venous occlusion plethysmography, Doppler sonography, and venous oxygen tension measurements support this concept (46,47). These distur- bances in nutritive microcirculation may be of importance in the development of diabetic foot complications and may help explain why the diabetic foot is more susceptible to the effect of pressure and has an impaired ulcer-healing process.

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Using the new technique of measuring capillary blood flow by laser Doppler flowmetry has enabled researchers to evaluate endothelial function in diabetic limbs more precisely.

Early application of this technique showed a reduced hyperemic response to heat stimu-

lus and pointed to the role of endothelial dysfunction as the cause of the impaired vascular

reactivity at microcirculatory level (37). Such dysfunction was shown to occur early in

the course of diabetes and may even predict diabetic micro- and macrovascular compli-

cations (4,6,48,49). More recently, endothelial dysfunction was also reported in patients

with impaired glucose tolerance and in relatives of type 2 diabetic patients (50).

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To evaluate the relation between changes in microcirculation and neuropathy in the presence or absence of peripheral vascular disease, the skin microcirculation of foot was thoroughly investigated using both single-point laser imaging and laser scanning tech- niques in five groups (51). The first group included diabetic patients with neuropathy (diabetic neuropathic [DN]), the second group included diabetic patients with both neu- ropathy and peripheral vascular disease (diabetic ischemic [DI]), the third group included diabetic patients with Charcot arthropathy (diabetic arthropathy [DA]), the fourth group included diabetic patients without complications (DC) and the fifth group included healthy control subjects (C). As seen in Fig. 3A, the percentage of increase in blood flow over baseline in response to heating the skin to 44°C was reduced in the DN and DI

Fig. 3. (A) The maximal hyperemic response to heating of foot skin at 44°C for at least 20 min (expressed as the percentage of increase over baseline flow measured by a single-point laser probe) is reduced in the diabetic with neuropathy (DN) and in diabetic patients with neuropathy and peripheral vascular disease (DI) when compared with diabetic patients with Charcot arthropathy (DA), diabetic patients without complications (DC), and normal control subjects (C) (p < 0.001).

(B) The response to iontophoresis of acetylcholine and sodium nitroprusside (SNP) (expressed as the percentage of increase over baseline flow measured by laser scanner). The response to acetylcholine is equally reduced in the DN, DI, and DA groups when compared with the DC and C groups (p < 0.001). The response to SNP was more pronounced in the DI group and also reduced in the DN and DA groups compared with the DC and C groups (p < 0.001).

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patients, whereas no difference existed among the remaining three groups. On the other hand, the endothelium-dependent vasodilatation (response to iontophoresis of Ach) was reduced in DN, DI, and DA patients. The endothelium-independent vasodilatation (response to iontophoresis of SNP) was more severely reduced in the ischemic-neuro- pathic patients compared with other groups and was reduced in the neuropathic groups with or without Charcot disease compared to the controls (Fig. 3B). These findings pointed to the close association between diabetic neuropathy and microcirculatory impairment in the form of reduced endothelium-dependent and endothelium-indepen- dent vasodilation at the foot level even in the absence of large vessel peripheral vascular disease. They also implied that the presence of neuropathy may be an important contrib- uting factor as the coexistence of neuropathy and peripheral vascular disease did not result in a greater decrease in endothelium-dependent vasodilation than that as a result of neuropathy alone.

The Role of the Nerve-Axon Reflex in Vasodilation

In healthy subjects, the ability to increase blood flow depends on the existence of normal neurogenic vascular response. The normal neurovascular response is conducted through the C nociceptive nerve fibers. Stimulation of these nerve fibers leads to antidromic stimu- lation of adjacent C fibers, which secrete substance P, calcitonin gene-related peptide and histamine, causing vasodilatation and increased blood flow to the injured tissues, thereby promoting wound healing (Lewis’ triple-flare response) (Fig. 4). In cases of diabetic neu- ropathy, this neurovascular response is impaired, leading to a significant reduction of blood flow under conditions of stress (such as injury or infection) and increasing the vulnerability of the neuropathic limb to severe diabetic foot problems (52,53).

Evidence that diabetic neuropathy contributes to vasodilatory impairment is provided

by studies in our lab that used the previously described single-point laser probe technique

to evaluate the nerve-axon-related vasodilatory response. We found that the indirect

response to iontophoresis of Ach was significantly reduced in DN, DI, and DA patients,

when compared with healthy subjects or DC patients (54,55) (Fig. 5). Further evidence

is provided by a study designed to evaluate the role of the C-nociceptive nerve fibers in

nerve-axon reflex-related vasodilation. In this study, nerve-axon reflex-related vasodi-

Fig. 4. Stimulation of the C-nociceptive nerve fibers leads to antidromic stimulation of the adja- cent C fibers, which secrete substance P, calcitonin gene-related peptide (CGRP), and histamine that cause vasodilatation and increased blood flow.

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Fig. 5. The response of blood flow (expressed as the percentage of increase over baseline flow measured by a single-point laser probe) in a skin area adjacent to, but not in direct contact with, the iontophoresis solution. During the iontophoresis of deionized water, a mild response is observed in all groups. In contrast, during iontophoresis of acetylcholine (Ach), the response is reduced in diabetic patients with neuropathy (DN, first column), diabetic patients with neuropathy and peripheral vascular disease (DI, second column) and diabetic patients with Charcot arthropathy (DA, third column), when compared with diabetic patients without complications (DC, fourth column) and normal control subjects (C, fifth column) (p < 0.001). A similar response is observed during iontophoresis of sodium nitroprusside (SNP), but is less than half when compared with the response achieved with Ach.

lation was measured in three groups: DN, DC, and C. Measurements were first taken on the forearm and the foot of each subject. Then, after blocking the C-nociceptive nerve fibers with dermal anesthesia, measurements were repeated. A clear reduction in nerve- axon reflex-related vasodilation occurred in all three groups on the forearm but only in the two non-neuropathic groups on the foot, indicating that C-nociceptive fiber function is the main factor that influences nerve-axon reflex-related vasodilation (56) (Fig. 6).

The contribution of the nerve-axon reflex-related vasodilatation response to the total endothelium-dependent and endothelium-independent vasodilation was also studied in a group of diabetic patients vs a control group at both forearm and foot level (57). The nerve-axon-related response in healthy subjects was found to be 35% of the total response at the forearm level and 29% at the foot level (Fig. 7). In contrast, the response to SNP, a substance that does not specifically excite the C-nociceptive fibers, was 13% and 12%, respectively, indicating that the presence of a nonspecific galvanic response may also be implicated (Fig. 8). In the presence of neuropathy the response significantly reduced, at a level of only 8% of the total response. These findings indicate that although the neu- rovascular response is an important factor in skin microcirculation function, it is not the sole or dominant pathway through which vasodilation is achieved (52).

The abnormality in nerve-axon-related vascular reactivity is believed to further aggra-

vate the abnormalities in the microcirculation and contribute to a vicious cycle of injury

(51). It becomes apparent that involvement of C-nociceptive fibers in diabetes not only

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Fig. 6. Total and nerve-axon reflex-related vasodilatory response to acetylcholine before (black columns) and after (white columns) the application of local anesthesia in healthy subjects (A), nonneuropathic diabetic patients (B), and diabetic neuropathic patients (C).

leads to impaired pain perception but also to impaired vasodilation under condition of stress, such as injury or inflammation.

Differences Between Forearm and Foot Microcirculation

As mentioned previously, erect posture may lead to differences in the microcirculation

at the foot level when compared to other parts of the body that are closer to the heart and

therefore have a reduced hydrostatic pressure. In order to test this hypothesis, we have

examined the differences in the foot and forearm skin microcirculation in diabetic patients

with or without neuropathy and healthy subjects (54). No differences were found in the

maximal hyperemic response between forearm and foot level, although the response in

the DN group was significantly lower at both levels in comparison to the DC and the C

subjects (Fig. 9). The endothelium-dependent and endothelium-independent vasodilata-

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Fig. 7. The total and the nerve-axon-related vasodilatation in the upper extremities in response to acetylcholine (Ach), sodium nitroprusside (SNP) ,and deionized-water (H₂O) in a group of diabetic patients vs a control group of healthy subjects. The contribution of nerve-axon-related response to the total response to Ach is 35% in diabetic patients and 31% in control group (p > 0.05) and the contribution of nerve-axon-related response to the total response to SNP is 13% in diabetic patients and 11% in control group (p > 0.05).

tion was significantly lower at the foot level when compared to the forearm level in both Cs and in DNs and DCs (Fig. 10). Additionally, the DN group showed a significantly lower response at both forearm and foot levels when compared to the DC and C groups.

Evaluation of the nerve-axon-mediated vasodilatation response also revealed a signifi- cantly lower response at the foot level vs the forearm level in the three groups (Fig. 9).

These results indicate that the microcirculation at the foot level is compromised even in healthy subjects when compared to the forearm level. The presence of diabetes may further compromise the microcirculation to a level that creates a hypoxic environment and allows the development of neuropathic changes. These factors may also explain why neuropathy initially occurs in the lower extremities of diabetic patients (58,59).

Expression of Endothelial Nitric Oxide Synthetase

One possible mechanism for impairment of endothelial functions in diabetic neuropa-

thy is the reduction in the expression of the endothelial nitric oxide synthetase (eNOS)

activity (51). We have tested this hypothesis by evaluating the immunohistochemistry

staining for eNOS of foot skin biopsies taken from diabetic neuropathic patients with

or without peripheral vascular disease and healthy subjects (51). The results showed

reduced staining for the diabetic patients when compared to the healthy subjects. Similar

results were reported by other investigators using immunohistochemistry and Western

blotting techniques (60).

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Fig. 8. The total and the nerve-axon related vasodilatation in the lower extremities in response to acetylcholine (Ach), sodium nitroprusside (SNP), and deionized-water (H₂O) in a group of diabetic patients with (DN) or without (DM) neuropathy vs a control group of healthy subjects. The contribution of nerve-axon-related response to the total response to Ach is 8% in DN, 29% in DM and 36% in the control group (p < 0.001 between DN and DM and controls) and the contribution of nerve-axon-related response to the total response to SNP is 8% in DN, 12% in DM and 9% in the control group (p > 0.05 between DN and DM and controls).

Fig. 9. The hyperemic response to heat stimulus (expressed as the percentage of increase over baseline flow measured by single-point laser probe) at forearm vs foot level in diabetic patients with or without neuropathy and in healthy control subjects. No difference was observed between the forearm and foot in any of the three groups. The response in neuropathic group is significantly lower compared with the other two groups at both forearm and foot level (p < 0.001).

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Poly(ADP-Ribose) Polymerase Role in Impaired Vascular Reactivity Recently, data has emerged showing poly(ADP-ribose) polymerase (PARP) to be involved in endothelial dysfunction as well. PARP is a nuclear enzyme that responds to oxidative DNA damage by activating an inefficient cellular metabolic cycle, leading to cell necrosis. In a study done in our unit, nondiabetic controls were compared to three groups: those with type 2 diabetes, those with glucose intolerance, and those with a family history of type 2 diabetes but no intolerance themselves. PARP activation was higher in all three diabetes-associated groups than in the healthy controls (61). The activation of PARP was associated with changes in the vascular reactivity of the skin microcirculation in forearm biopsies taken from these subjects, supporting the hypothesis that PARP activation contributes to changes in microvascular reactivity. Further study is required to prove this association.

Fig. 10. The response to iontophoresis of acetylcholine (A) and sodium nitroprusside (B) (ex- pressed as the percentage of increase over baseline flow measured by laser scanner) at forearm vs foot level in diabetic patients with or without neuropathy and in healthy control subjects. The response at the foot level is significantly lower than that of the forearm in all groups (p < 0.01).

The response in neuropathic group is significantly lower compared with the other two groups at both forearm and foot level (p < 0.05).

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Microvascular Changes in Diabetic Foot With Charcot Arthropathy The diagnosis of Charcot neuroarthropathy is made when gross destruction of the joints of the mid-foot results in significant foot deformity. The skin temperature of Charcot feet is usually higher as a result of increased blood flow in arterio-venous shunts. Although the endothelial-dependent and endothelial-independent vasodilata- tion is impaired in Charcot patients, the maximal hyperemic response to heat is pre- served (Fig. 4). These findings indicate that the hyperemic response in Charcot disease is present but is probably unregulated and results in excessive bone resorption. The final results of these changes are complete joint destruction and gross deformity of the foot shape. These findings are consistent with clinical observations that the develop- ment of Charcot neuroarthropathy is extremely rare in the presence of peripheral vas- cular disease (62). Poor blood flow to the extremity would prevent much of a hyperemic response, protecting the foot from bone resorption and deformation, although certainly contributing to other microcirculatory derangement.

Fig. 11. The axon-reflex-mediated vasodilatation related to the C-nociceptive fibers (expressed as the percentage of increase over baseline flow measured by two single-point laser probe) at forearm vs foot level in diabetic patients with or without neuropathy and in healthy control subjects. The response to acetylcholine, which directly stimulates the C fibers, is lowest in the neuropathic group, but is also reduced in nonneuropathic group (p < 0.001), although no differ- ences are found at the forearm level. A much smaller response is observed during the iontophoresis of sodium nitroprusside (nonspecific stimulus) at both foot and forearm level and is smaller in all three groups.

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CONCLUSIONS

Microcirculation to the diabetic foot suffers multiple significant structural and func- tional changes. Nerve-axon-related microvascular reactivity is clearly impaired in the diabetic population. There is a growing belief that both the failure of the dysfunctional vessels to dilate and the impairment of the nerve-axon reflex are major causes for impaired wound healing in diabetic patients. Further studies are required to clarify the precise etiology of observed endothelial dysfunction in diabetic patients and to identify the possible potential therapeutic interventions to prevent it or to retard its progression.

Studies are also required to examine the vascular changes in the peripheral nerves, rather than in the skin.

Currently research is also ongoing in the connection of inflammatory states with both the development of vascular disease and diabetes. The hypothesis that inflammatory factors such as vascular cell adhesion molecule, interleukin-1, and tumor necrosis factor- F have a role in the development and progression of atherosclerosis is intriguing. These inflammatory factors are necessary in the cascade of wound healing, presenting a conflict of interest within the injured diabetic body. The factors required for healing a diabetic foot ulcer may actually worsen the atherosclerosis that is preventing adequate blood flow to heal the foot ulcer in the first place. An attempt to break the resulting cycle of wound, attempted healing, worsening circulation, worsening wound, is the next major focus in the field of diabetic microcirculation.

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19 Microcirculation of the Diabetic Foot