Role of Vascular Disease in Erectile Dysfunction.
An erection is essentially a vascular event requiring an intact penile circulation. Given alterations in both the macro- and the microcirculation of diabetic men, abnormalities in the penile circulation have frequently been implicated in the pathophysiology of diabetic ED. The penile blood supply may be impaired by either diffuse atherosclerosis or isolated external iliac disease resulting in “pelvic steal” syndrome causing blood to be diverted away from the pelvis. Additionally, penile veno-occlusive dysfunction and resulting cavernosal fibrosis are probably secondary to arterial disease.
Diabetes is associated with accelerated atherosclerosis resulting in a two- to fourfold increase in cardiovascular mortality and with similar deleterious changes being observed in the peripheral circulation. Diabetic microangiopathy is characterized by the thickening of the capillary basement membrane, which is pathognomonic of diabetic retinopathy. Diabetic retinopathy has itself been linked with the development of ED. Moreover, vascular endothelial dysfunction, a reputed precursor of atherosclerosis, has been reported in both type 1 and type 2 diabetes, while abnormalities in the penile endothelial function of impotent men have also been reported.
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Pathophysiology of Diabetic Vasculopathy
Type 2 diabetes is inexorably linked with the metabolic syndrome of insulin resistance, hypertension, and dyslipidemia, all independent vascular risk factors predisposing to atherosclerosis. Untreated hypertension per se is associated with a threefold increase of ED and may often precede the development of diabetes. The pathophysiology of vascular disease in type 1 diabetes is less clear, although hyperemic changes in response to the metabolic abnormalities and other consequences of prolonged hyperglycemia—including free radicals, advanced glycosylation end-products (AGEs), and the polyol pathway—have all been implicated. The etiology of vascular endothelial dysfunction in the diabetic circulation is analogous to the alterations in the availability of nitric oxide (NO) and other vasoactive substances within the corporal smooth muscle as discussed earlier.
Contribution of Vascular Impairment to Diabetic ED
Wide-ranging evidence supports a role for vascular factors in the etiology of diabetic ED, but the extent of this contribution is unclear. Doppler studies have shown up to 95% of diabetic impotent men may have abnormalities in penile blood flow compared to nondiabetic healthy controls; however, other studies have found that penile blood flow and penile-brachial index were reduced in diabetic males irrespective of potency. Angiography has shown that stenosis of the internal pudendal artery predicts ED in the diabetic population, but a similar pattern was also demonstrated in the nondiabetic population. This suggests the excess of generalized arterial disease seen with diabetes is responsible for the increased ED rather than penile vascular abnormalities specific to diabetes. Thus, vascular disease per se is not as strong a predictor of ED as neuropathy in the diabetic population.