This article is derived from Mayo Clin Proc. 2002;77:276-282. It should not be regarded as authoritative medical advice, policy or treatment strategy.
Diabetic men have proportionately much more erectile dysfunction than other men. About three times as much, in fact. Around 50% of diabetic men aged 50 or over will have erectile difficulties. Now, as you may have read on another page on this site, erections are primarily caused by changes in the amount of blood flowing into the penis, though the regulatory factors are biochemical.
While any assessment of a diabetic man with erectile dysfunction would include possible vasculogenic, neurogenic, and hormonal causes, to rule out any hormonal causes such as hypogonadism or thyroid abnormalities, the usual treatment is with agents like Viagra, which act by inhibiting phosphodiesterase in the penile vasculature.
To recap, the usual mechanism of erections is as follows: increased blood flow into the corpora cavernosae and the bulbus spongiosum causes an increase in volume through engorgement of the cavernosal tissues. This leads to an increase in pressure which traps the veins leading out of the penis (the emissary veins), which in turn makes the penis even more engorged.
The penis has a high level of nitric oxide (or “NO”), which usually causes relaxation of the intracavernosal trabeculae, thereby maximizing blood flow and penile erection. Nitric oxide works by promoting the generation of cyclic guanosine monophosphate (cGMP), which encourages the smooth muscles of the penis to relax, and thereby increasing inflow of blood.
Detumescence (the loss of erection) is set in motion when nitric-oxide-induced vasodilation subsides because of metabolism of cGMP, which is a process regulated by intracavernosal type 5 cGMP phosphodiesterase. Viagra and similar chemicals inhibit the action of 5 cGMP phosphodiesterase.
In diabetic men, changes in the levels of potassium ion movement across the smooth muscles of the corpora can lead to a lack of erectile capacity.
Nervous & endocrine system influences on erectile dysfunction
The autonomic nervous system is responsible for starting the erectile process. Sexual stimuli, whether, they are real or in fantasy, will stimulate nerve impulses to travel along the nerves of the spinal thoracolumbar erection center to the pelvis, while physical stimulation of the penis activates a reflex arc in the sacral erection center.
Testosterone is implicit in maintaining NO synthase levels within the tissues of the penis. NO synthase impacts on the local production of Nitric Oxide. Clearly, therefore, testosterone replacement therapy is essential for hypogonadic men, whether diabetic or not, as it restores erectile function and sexual drive.
What causes erectile dysfunction in diabetic men?
The primary cause is that the primary arteries and other blood vessels of the penis narrow, so that they are unable to carry enough blood to promote engorgement. This is thought to be caused by microvascular degenerative changes in the smooth muscle. In addition, there is disturbance of both somatic and autonomic nerve systems in men with diabetes whose pudendal nerves and bulbocavernous and urethroanal reflexes have longer latencies of somatosensory-evoked potentials.
There appears to be resistance to the action of NO at the receptor level or an increased rate of breakdown of NO, due to high levels and activity of NO synthase. In addition, there are abnormalities in local levels of vasoactive intestinal peptide, prostaglandins, endothelins, and other mediators.
The psychological factors in Erectile Dysfunction
Since so much of the sexual arousal process is in the mind, it’s always important to assess whether psychogenic factors are playing a part in Erectile Dysfunction. Any stressful relationship or life event has the potential to cause Erectile Dysfunction, but nocturnal erections and erections during masturbation may remain unaffected. These conditions are discussed at length in this book about erectile dysfunction.
By contrast, organic Erectile Dysfunction is due to the causes mentioned above. When damage to the nerves is a factor, there will be clues such as lack of ability to masturbate, absent cremasteric reflex and bladder dysfunction.
Analysis of the blood flow may reveal a vasculogenic origin, which may be confirmed by the man’s smoking, low blood pressure in the penis or poor blood flow, no nocturnal or morning erections, and a gradual onset of erectile failure. Hormonal factors include loss of libido, gynecomastia, lack of maculinization, and decreased facial hair growth.
There are of course other factors, such as obesity, drug use – both prescription and non-prescription – including, ironically, drugs used to treat diabetes, prolaspe of the intervertebral discs, and vascular occlusion due to high cholesterol levels. Even the plaques of Peyronie’s disease can cause erectile dysfunction.
Test for all these physical factors can be carried out when treatment is being considered for diabetic men with Erectile Dysfunction. In addition, a hormonal profile including measurement of at least free and total testosterone and thyroid-stimulating hormone levels is considered good practice.
The measurement of testosterone needs to be taken carefully since there is considerable diurnal variation in the level of this hormone. Other tests which are available to the physician include nocturnal penile tumescence studies, intracavernosal injection, arteriography, and duplex penile ultrasonography, though this battery of test tends to be reserved for the more seriously affected men.
The most effective treatment is Viagra and other Phosphodiesterase Type 5 Inhibitors
Viagra is a powerful inhibitor of cGMP-specific PDE5. It works well in many men, including diabetics and those whose Erectile Dysfunction is due to vascular occlusion or drug use. The initial dose is usually of the order of 50 mg, though it may be increased gradually up to 100 mg depending on need.
The older and still effective therapies for Erectile Dysfunction involved a constricting ring placed around the base of the penile shaft, perhaps in conjunction with a vacuum suction device to induce engorgement of the corpora. This works well for some men, though others find it painful or have unsatisfactory orgasms and ejaculations.