Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.
Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
Erection begins with sexual stimulation. Sexual stimulation can be tactile (for example, by touching the penis) or mental (for example, by having sexual fantasies). Sexual stimulation or sexual arousal generates electrical impulses along the nerves going to the penis and causes the nerves to release nitric oxide, which in turn increases the production of cyclic GMP (cGMP) in the smooth muscle cells of the corpora cavernosa. The cGMP causes the smooth muscles of the corpora cavernosa to relax and allow rapid blood flow into the penis. The incoming blood fills the corpora cavernosa, making the penis expand.
How is erection sustained?
The pressure from the expanding penis compresses the veins (blood vessels that drain the blood out of the penis) in the tunica albuginea, helping to trap the blood in the corpora cavernosa, thereby sustaining erection. Erection is reversed when cGMP levels in the corpora cavernosa fall, causing the smooth muscles of the corpora cavernosa to contract, stopping the inflow of blood and opening veins that drain blood away from the penis. The levels of the cGMP in the corpora cavernosa fall because it is destroyed by an enzyme called phosphodiesterase type 5 (PDE5).
What are some of the risk factors for erectile dysfunction?
The common risk factors for ED include the following:
Recreational drug use
Depression or other psychiatric diseases
What are the causes of erectile dysfunction?
The ability to achieve and sustain erections requires
a healthy nervous system that conducts nerve impulses in the brain, spinal column, and penis,
healthy arteries in and near the corpora cavernosa,
healthy smooth muscles and fibrous tissues within the corpora cavernosa,
adequate levels of nitric oxide in the penis.
Erectile dysfunction can occur if one or more of these requirements are not met. The following are causes of erectile dysfunction:
Aging: There are two reasons why older men are more likely to experience erectile dysfunction than younger men. First, older men are more likely to develop diseases (such as heart attacks, angina, cardiovascular disease, strokes, diabetes mellitus, and high blood pressure) that are associated with erectile dysfunction. Second, the aging process alone can cause erectile dysfunction in some men, primarily by decreasing the compliance of the tissues in the corpora cavernosa, although it has been suggested, but not proven, that there is also decreased production of nitric oxide in the nerves that innervate the corporal smooth muscle within the penis.
Diabetes mellitus: Erectile dysfunction tends to develop 10-15 years earlier in diabetic men than among nondiabetic men. In a population study of men with type I diabetes for more than 10 years, erectile dysfunction was reported by 55% of men 50-60 years of age. The increased risk of erectile dysfunction among men with diabetes mellitus may be due to the earlier onset and greater severity of atherosclerosis that narrows the arteries and thereby reduces the delivery of blood to the penis. When insufficient blood is delivered to the penis, it is not possible to achieve an erection. Diabetes mellitus also causes erectile dysfunction by damaging both sensory and autonomic nerves, a condition called diabetic neuropathy. Smoking cigarettes, obesity, poor control of blood glucose levels, and having diabetes mellitus for a long time further increase the risk of erectile dysfunction in diabetes. In addition to atherosclerosis and/or neuropathy causing ED in diabetes, many men with diabetes also develop a myopathy (muscle disease) as their cause of ED in which the compliance of the muscles in the corpora cavernosa is decreased, and clinically this presents as an inability to maintain the erection.
Hypertension (high blood pressure): People with essential hypertension or arteriosclerosis have an increased risk of developing erectile dysfunction. Essential hypertension is the most common form of hypertension; it is called essential hypertension because it is not caused by another disease (for example, by kidney disease). It is not clearly known how essential hypertension causes erectile dysfunction; however, those with essential hypertension have been found to have low production of nitric oxide by the arteries of the body, including the arteries in the penis. High blood pressure also accelerates the progression of atherosclerosis, which in turn can contribute to erectile dysfunction. Scientists now suspect that the decreased levels of nitric oxide in patients with essential hypertension may contribute to erectile dysfunction.
Cardiovascular diseases: The most common cause of cardiovascular diseases in the United States is atherosclerosis, the narrowing and hardening of arteries that reduces blood flow. Atherosclerosis typically affects arteries throughout the body and is aggravated by hypertension, high blood cholesterol levels, cigarette smoking, and diabetes mellitus. When coronary arteries (arteries that supply blood to the heart muscle) are narrowed by atherosclerosis, heart attacks and angina occur. When cerebral arteries (arteries that supply blood to the brain) are narrowed by atherosclerosis, strokes occur. Similarly, when arteries to the penis and the pelvic organs are narrowed by atherosclerosis, insufficient blood is delivered to the penis to achieve an erection. There is a close correlation between the severity of atherosclerosis in the coronary arteries and erectile dysfunction. For example, men with more severe coronary artery atherosclerosis also tend to have more erectile dysfunction than men with mild or no coronary artery atherosclerosis. Some doctors suggest that men with new onset erectile dysfunction should be evaluated for silent coronary artery diseases (advanced coronary artery atherosclerosis that has not yet caused angina or heart attacks).
Cigarette smoking: Cigarette smoking aggravates atherosclerosis and thereby increases the risk for erectile dysfunction.
Nerve or spinal cord damage: Damage to the spinal cord and nerves in the pelvis can cause erectile dysfunction. Nerve damage can be due to disease, trauma, or surgical procedures. Examples include injury to the spinal cord from automobile accidents, injury to the pelvic nerves from prostate surgery for prostate cancer (prostatectomy), radiation to the prostate, surgery for benign prostatic enlargement, multiple sclerosis (a neurological disease with the potential to cause widespread damage to nerves), and long-term diabetes mellitus.
Substance abuse: Marijuana, heroin, cocaine, methamphetamines, crystal meth, and alcohol abuse contribute to erectile dysfunction. Alcoholism, in addition to causing nerve damage, can lead to atrophy (shrinking) of the testicles and lower testosterone levels.
Low testosterone levels: Testosterone (the primary sex hormone in men) is not only necessary for sex drive (libido) but also is necessary to maintain nitric oxide levels in the penis. Therefore, men with hypogonadism (diminished function of the testes resulting in low testosterone production) can have low sex drive and erectile dysfunction.
Medications: Many common medicines produce erectile dysfunction as a side effect. Medicines that can cause erectile dysfunction include many used to treat high blood pressure, antihistamines, antidepressants, tranquilizers, and appetite suppressants. Examples of common medicines that can cause erectile dysfunction include propranolol (Inderal) or other beta-blockers, hydrochlorothiazide, digoxin (Lanoxin), amitriptyline (Elavil), famotidine (Pepcid), cimetidine (Tagamet), metoclopramide (Reglan), indomethacin (Indocin), lithium (Eskalith, Lithobid), verapamil (Calan, Verelan, Isoptin), phenytoin (Dilantin), and gemfibrozil (Lopid).
Depression and anxiety: Psychological factors may be responsible for erectile dysfunction. These factors include stress, anxiety, guilt, depression, widower syndrome, low self-esteem, posttraumatic stress disorder, and fear of sexual failure (performance anxiety). It is also worth noting that many medications used for treatment of depression and other psychiatric disorders may cause erectile dysfunction or ejaculatory problems.