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Erectile Dysfunction*

Managing Impotence - A Patient Guide

Neurogenic Impotence**

Successful Self Penile Injection

*Deeplink to New England Journal of Medicine, 342(24):1802-1813, Lue TF. Copyright (2000), with permission from Massachusetts Medical Society, Publishing Division.

**Reprinted from Journal of Urology, June 165:2103-2109, Bakircioglu ME; Lin CS; Fan P; Sievert KD; Kan YW; Lue TF. Copyright (2001), with permission from Lippincott Williams & Wilkins.

Blood Flow and Penile Erection

Molecular Mechanisms of Penile Erection

During sexual excitement, smooth muscle surrounding the cavernosal artery (red circle in the center of diagram) relaxes, causing the artery to widen. This causes an increase blood flow. Blood collects in the spongy erectile tissues of the penis, causing an increase in penile width and hardness. As the spongy tissue swells it presses up against veins that are arranged around the inside of the erectile bodies. This pressure causes them to be squeezed shut, trapping blood inside the penis and maintaining rigid penile erection.

During sexual excitement, nitric oxide is released from the cavernous nerves (which control erections). Nitric oxide activates an enzyme in smooth muscle called guanylyl cyclase. This activated enzyme in turn transforms GTP, an important energy source inside cells, into cGMP. cGMP is a molecule that through a complex process causes smooth muscle relaxation, leading to dilation of arteries and the rapid filling of the spongy erectile tissues described in figure 1.

cGMP is normally broken down by an enzyme called phosphodiasterase type 5 (PDE5). This enzyme is important in reversing penile erection. Because this enzyme causes loss of penile erection, medications that block PDE5's activity (known as PDE5 inhibitors, examples include drugs like Viagra®, Levitra®, and Cialis®) will lead to increased levels of cGMP inside the penile erectile tissues and thereby enhance penile erection.

Proposed pathophysiologic mechanism of Peyronie's disease

Mild trauma to the penis may cause bleeding and scar formation inside the tunica of the penis or around one of the internal struts of the erectile tissues. Uninjured portions of the penis expand with penile erection while scar tissue (plaque) will not. This is thought to be a leading cause of the penile curvature often found in Peyronie's disease.

Low flow pripaism

During penile erection, relaxation of the cavernous artery permits increased blood flow into the erectile tissue of the penis. This causes penile enlargement, which in turn squeezes veins around the inside of the erectile bodies and prevents blood from exiting. For various reasons, sometimes the process of erection persists abnormally. After several hours the oxygen content of the blood in the penis declines, causing the blood to turn dark red.

High flow pripaism

A small branch of the main cavernous artery may be damaged after a trauma to the penis, groin, or scrotum. While there may not be any immediate problems, if a penile erection occurs this damaged artery may break, leading to uncontrolled bleeding into the erectile tissue. Usually this flow is less than what normally occurs with penile erection, so that the veins around the outside of the penis are able to drain normally and full erection does not happen. There is however usually some degree of penile enlargement in these cases.

Treatment of low flow priapism with penile aspiration

In this procedure, a small needle is used to drain blood out of the erectile tissues. This causes the erectile tissues to shrink, which relieves pressure on the penile veins and may help restore the normal process of blood drainage and help reverse penile erection.

Treatment of low flow priapism with penile injection

In this procedure, a medication which causes contraction of the cavernous artery is injected into the penis. This causes a reduction of penile blood flow, shrinking of the erectile tissue, and opening of the veins draining the penis. This is a very effective way to help reverse ischemic priapism.