Vascular disease is probably the most common cause of ED, and of all the vascular causes, the commonest is atherosclerosis. However, not only is atherosclerosis associated with ED, but its risk factors, namely smoking, hypertension, hyperlipidaemia and diabetes, are also associated with the development of ED. The Massachusetts Male Aging study described an association between ED and hypertension, diabetes and hypercholesterolaemia. The Cologne study of men with ED also confirmed an association between ED and diabetes and hypertension, while several studies have demonstrated an association between smoking and ED. At a cellular level, it has been suggested that a reduced arterial inflow leads to relative hypoxia within the penis with subsequent cellular effects. The crucial cellular mediator appears to be Transforming Growth Factor Beta 1 (TGF1), which is increased in hypoxia and induces trophic changes in the cavernosal smooth muscle.

In addition to atherosclerosis, other disorders of both the arterial supply and the venous drainage of the penis can result in ED, and examples of both.

When there is failure of the veno-occlusive mechanism, the phenomenon of venous leakage occurs. This is a purely radiological phenomenon seen during specialised radiological imaging of the penis (cavernosography). When originally described, it was thought that the abnormal veins represented the primary pathological pathological abnormality, and for some years surgical ligation of these veins was undertaken as a means of treating ED. However, with the understanding that the venous leak was almost always secondary to abnormalities of the tunica albuginea or to disease of the cavernosal smooth muscle (and with the realisation that the results of venous surgery were poor), surgical treatment has almost completely disappeared.

Cellular causes of erectile dysfunction

Two types of cavernosal cells are central to penile erection; namely smooth muscle cells and endothelial cells. The vascular endothelial cells line the trabecular spaces of the cavernosal sinusoids and release a variety of vasoactive chemicals which control smooth muscle tone within the penis. The most important of these is NO. Diseases which damage the endothelium impair the vascular response of the penis to neural stimuli. A number of diseases damage the endothelium, including hypercholesterolaemia, but the most important is diabetes mellitus.

The structural changes in the endothelium that are produced by diabetes are accompanied by functional changes that result in impaired smooth muscle relaxation.

The diseases of smooth muscle that can result in erectile dysfunction have already been alluded to above. Aging results in reduced penile smooth muscle, as does atherosclerosis, while renal failure results in smooth muscle dysfunction. When the smooth muscle malfunctions, arterial dilatation is incomplete, cavernosal relaxation fails to occur and the veno-occlusive mechanism fails.