Influence of Neuropathy on the Microvascular Response to Local Heating in the Human Diabetic Foot

Abstract

1. The diabetic neuropathic foot exhibits excess arteriovenous anastomotic shunt flow due to a reduced sympathetic vasoconstrictor tone. Local axon reflexes (mediating postural vasoconstriction, for example) are preserved even in severe diabetic neuropathy. This excess shunt flow and its local neurogenic control may be important in the development of complications of the neuropathic limb. 2. The response of arteriovenous anastomoses to local heating was assessed in 13 diabetic patients with neuropathy (12 insulin-dependent), 10 diabetic control patients (seven insulin-dependent) and 10 normal control subjects. The aim was to study the local reflex control of arteriovenous flow when central sympathetic tone had been largely removed. 3. The change in skin blood flow on local heating to 44°C was measured by using a laser Doppler flowmeter in standard environmental conditions with the foot at heart level. Two sites were assessed: (i) the plantar surface of the great toe (a site in which skin blood flow is dominated by arteriovenous shunt flow) and (ii) the dorsum of the foot (a site without anastomotic flow). 4. It was found that when heat was applied to the plantar surface of the great toe in the diabetic patients with neuropathy a paradoxical decrease in flow through arteriovenous anastomoses occurred, flow declining to 65% (P < 0.05) of its resting value. This could be compared with an increase in flow over the same time period of 262% and 228% (P < 0.01) in diabetic control patients and normal subjects, respectively. When the foot dorsum was heated flow increased in a similar fashion in all patient groups, and a decline in flow was not observed. 5. Peak flow at the great toe was significantly lower in the diabetic patients with neuropathy compared with both the diabetic control patients and the normal subjects [23.68 (44.64) flow units versus 57.0 (43.92) flow units and 76.32 (40.88) flow units, respectively, medians and interquartile ranges, P < 0.02]. 6. Thus we have demonstrated that in the presence of neuropathy local heating of the toe pulp may produce (i) a paradoxical fall in skin blood flow and (ii) a greatly reduced peak skin blood flow. The importance of this abnormal vascular control in the pathogenesis of complications of the neuropathic limb has now to be determined.