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Overview
Erectile dysfunction (ED) is the inability of a man to achieve or maintain an erection sufficient for his sexual needs or the needs of his partner. Most men experience this at some point in their lives, usually by age 40, and are not psychologically affected by it.  Some men, however, experience chronic, complete erectile dysfunction (impotence), and others, partial or brief erections. Frequent erectile dysfunction can cause emotional and relationship problems, and often leads to diminished self-esteem. Erectile dysfunction has many causes, most of which are treatable, and is not an inevitable consequence of aging.

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Incidence & Prevalence
According to the National Institutes of Health in 2002, an estimated 15 million to 30 million men in the United States experience chronic erectile dysfunction.  According to the National Ambulatory Medical Care Survey (NAMCS), approximately 22 out of every 1000 men in the United States sought medical attention for ED in 1999.

Incidence of the disorder increases with age. Chronic ED affects about 5% of men in their 40s and 15-25% of men by the age of 65. Transient ED and inadequate erection affect as many as 50% of men between the ages of 40 and 70.

Diseases (e.g., diabetes, kidney disease, alcoholism, and atherosclerosis) account for as many as 70% of chronic ED cases and psychological factors (e.g., stress, anxiety, depression) may account for 10-20% of cases. Between 35 and 50% of men with diabetes experience ED.

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Anatomy of the Penis
The internal structure of the penis consists of two cylinder-shaped vascular tissue bodies (corpora cavernosa) that run throughout the penis; the urethra (tube for expelling urine and ejaculate); erectile tissue surrounding the urethra; two main arteries; and several veins and nerves. The longest part of the penis is the shaft, at the end of which is the head, or glans penis. The opening at the tip of the glans, which allows for urination and ejaculation, is the meatus.

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Physiology of Erection
The physiological process of erection begins in the brain and involves the nervous and vascular systems. Neurotransmitters in the brain (e.g., epinephrine, acetylcholine, nitric oxide) are some of the chemicals that initiate it. Physical or psychological stimulation (arousal) causes nerves to send messages to the vascular system, which results in significant blood flow to the penis. Two arteries in the penis supply blood to erectile tissue and the corpora cavernosa, which become engorged and expand as a result of increased blood flow and pressure. Because blood must stay in the penis to maintain rigidity, erectile tissue is enclosed by fibrous elastic sheathes (tunicae) that cinch to prevent blood from leaving the penis during erection. When stimulation ends, or following ejaculation, pressure in the penis decreases, blood is released, and the penis resumes its normal shape.

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Causes
There are many underlying physical and psychological causes of erectile dysfunction. Reduced blood flow to the penis and nerve damage are the most common physical causes. Underlying conditions associated with erectile dysfunction include the following:  Vascular Disease, Diabetes, Drugs, Hormone Disorders, Neurological Disorders, Pelvic Trauma, Surgery, Radiation Therapy, Peyronie’s disease, Venous Leak, and Psychological Conditions.

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Diagnosis
A medical examination may indicate neurological, vascular, or hormonal disease, or Peyronie’s disease. History of illness, smoking, drug use, and hypertension can be ascertained with a thorough examination of health history. Laboratory tests are performed to identify the underlying cause.
Duplex ultrasound-Duplex ultrasound is used to evaluate blood flow, venous leak, signs of atherosclerosis, and scarring or calcification of erectile tissue. Erection is induced by injecting prostaglandin, a hormone-like stimulator produced in the body. Ultrasound is then used to see vascular dilation and measure penile blood pressure (which may also be measured with a special cuff). Measurements are compared to those taken when the penis is flaccid. Vasoactive injection-When injected into the penis, certain solutions cause erection by dilating blood vessels in erectile tissue. Normally, these injections produce an erection lasting about 20 minutes.

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Medical Treatment
Oral Medication Oral medications used to treat erectile dysfunction include selective enzyme inhibitors (e.g., sildenafil (Viagra), vardenafil HCl (Levitra),and tadalafil (Cialis)). Selective enzyme inhibitors are available by prescription and may be taken up to once a day to treat ED. They improve partial erections by inhibiting the enzyme that facilitates their reduction and increase levels of cyclic guanosine monophosphate (cGMP, a chemical factor in metabolism), which causes the smooth muscles of the penis to relax, enabling blood to flow into the corpora cavernosa.

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Patients taking nitrate drugs (used to treat chest pain) cannot take Viagra, Levitra, or Cialis as the drug interaction can be fatal. Viagra should not be taken within four hours of taking alpha-blockers (used to treat high blood pressure and benign prostatic hyperplasia) Cialis can be used with flomax (0.4 mg). Levitra should not be mixed with any alpha blockers (Hytrin, Cardura, and Flomax). Men who have had a heart attack or stroke within the past 6 months and those with certain medical conditions (e.g., uncontrolled high blood pressure), severe low blood pressure or liver disease, unstable angina) that make sexual activity inadvisable should not take Cialis, Viagra, or Levitra. Dosages of the drug should be limited in patients with kidney or liver disorders.

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Viagra® is absorbed and processed rapidly by the body and is usually taken on an empty stomach 30 minutes to 1 hour before intercourse. Results vary depending on the cause of erectile dysfunction, but studies have shown that Viagra is effective in 75% of cases. It helps men with erectile dysfunction associated with diabetes mellitus (57%), spinal cord injuries (83%), and radical prostatectomy (43%).   In clinical studies, Levitra® has been shown to work quickly, provide consistent results, and improve sexual function in most men the first time they take the drug. It also has shown to be effective in men of all ages, in patients with diabetes mellitus, and in men who have undergone radical prostatectomy.  Cialis® has been shown in clinical trials to stay in the body longer than the other selective enzyme inhibitors. It promotes erection within 30 minutes and enhances the ability to achieve erection for up to 36 hours.

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Common side effects of selective enzyme inhibitors include headache, reddening of the face and neck (flushing), indigestion, and nasal congestion. Cialis® may cause muscle aches and back pain, which usually resolve on their own within 48 hours.

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Self-Injection
Self-injection involves using a short needle to inject medication through the side of the penis directly into the corpus cavernosum, which produces an erection that lasts from 30 minutes to several hours. Prostaglandin (alprostadil, Caverject®, Edex®), and phentolamine (Regitine®) produce results similar to Viagra but are localized in the penis after injection. They cause vascular dilation and a relaxation of smooth muscle. Prostaglandin is the only substance currently approved for erectile dysfunction treatment. Phentolamine is a heart medication with similar effects used by some physicians to treat impotence.

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These drugs have been shown to produce erections in 80% of men who inject them. Some men claim that they produce erections that feel natural and improve sex. The injections are relatively painless and create an erection that begins about 5 to 15 minutes after the injection. It is recommended that self-injection be performed no more than once every 4 to 7 days. Side effects include infection, bleeding, and bruising at the injection site, dizziness, heart palpitations, and flushing. There is a small risk for priapism (an erection that lasts for more than 6 hours and requires medical relief). Repeated injection may cause scarring of erectile tissue, which can further impair erection.

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Urethral suppositories containing prostaglandin (aprostadil), like Muse® (Medicated Urethral System for Erections), may be an alternative to injection. Using a hand-held delivery device, a man inserts a prostaglandin pellet through the meatus (penis opening) into the urethra. Prostaglandin is absorbed through the urethral mucosa and into the surrounding erectile tissue. It is available with a prescription, is well tolerated, and may improve erections in 60% of men who use it.

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In addition to the side effects associated with injecting aprostadil, pain in the penis and perineum (area between scrotum and rectum) may occur with suppository use.

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Vacuum Devices
Vacuum devices work by manually creating an erection. The penis is inserted into a plastic tube, which is pressed against the body to form a seal. A hand pump attached to the tube is used to create a vacuum that draws blood into the penis, causing the penis to become engorged. After 1 to 3 minutes in the vacuum, an adequate erection is created. The penis is removed from the tube and a soft rubber O-ring is placed around the base of the penis to trap blood and maintain the erection until removed. The ring can be left in place for 25 to 30 minutes.

Vacuum devices work best in men who are able to achieve partial erections on their own. They are easy to use at home, require no other procedure, and typically improve erections regardless of the cause of impotence. Some men experience a numbing feeling after placing the O-ring. Since the penis is flaccid between the ring and the body, the erection may be somewhat floppy.

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Surgical Treatments
Penile implants involve surgical insertion of malleable or inflatable rods or tubes into the penis. A semi-rigid prosthesis is a silicon-covered flexible metal rod. Once inserted, it provides the rigidity necessary for intercourse and can be curved slightly for concealment. It requires the simplest surgical procedure of all the prostheses. Its main disadvantage is that concealment can be difficult with certain types of clothing.

An inflatable penile prosthesis consists of two soft silicone or bioflex (plastic) tubes inserted in the penis, a small reservoir implanted in the abdomen, and a small pump implanted in the scrotum. To produce an erection, a man pumps sterile liquid from the reservoir into the tubes by squeezing the pump in the scrotum. The tubes act as erectile tissue and expand to form an erection. When the erection is no longer desired, a valve allows the fluid to return to the reservoir. Inflatable prostheses are the most natural feeling of the penile implants and they allow for control of rigidity and size. The surgical procedure to implant the inflatable prosthesis is slightly more complicated than for a semi-rigid implant. Also, because there are more mechanical parts, there is a higher risk for mechanical failure requiring repair or adjustment.

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A self-contained inflatable prosthesis is similar but has fewer parts. It consists of a pair of inflatable tubes in the penis with a pump attached directly to the end of the implant. The reservoir is also located in the shaft of the penis. Its compact design allows for simpler implantation, but because it takes up more space in the penis, there is less room for expansion.