Erectile dysfunction is a significant and common medical problem. Recent epidemiologic studies suggest that approximately 10% of men aged 40-70 have severe or complete erectile dysfunction, defined as the total inability to achieve or maintain erections sufficient for sexual performance. An additional 25% of men in this age category have moderate or intermittent erectile difficulties. The disorder is highly age-dependent, as the combined prevalence of moderate to complete erectile dysfunction rises from approximately 22% at age 40 to 49% by age 70. Although less common in younger men, erectile dysfunction still affects 5%-10% of men below the age of 40. Findings from these studies show that erectile dysfunction impacts significantly on mood state, interpersonal functioning, and overall quality of life.

Erectile dysfunction is strongly related to both physical and psychological health. Among the major risk factors are diabetes mellitus, heart disease, hypertension and decreased HDL levels. Medications for diabetes, hypertension, cardiovascular disease and depression may also cause erectile difficulties. In addition, there is a higher prevalence of erectile dysfunction among men who have undergone radiation or surgery for prostate cancer, or who have a lower spinal cord injury or other neurological diseases (e.g. Parkinson’s disease, multiple sclerosis). Life style factors, including smoking, alcohol consumption and sedentary behavior are additional risk factors. The psychological correlates of erectile dysfunction include anxiety, depression and anger. Despite its increasing prevalence among older men, erectile dysfunction is not considered a normal or inevitable part of the aging process. It is rarely (in fewer than 5% of cases) due to aging-related hypogonadism, although the relationship between erectile dysfunction and age-related declines in androgen remains controversial.

Erectile impairment is a condition with profound psychologic consequences and may interfere with a man's overall well-being, self-esteem and interpersonal relationships. Conservative estimates of its incidence have been made at between 10-20 million men. Furthermore, it has been shown that erectile problems account for 400,000 out-patient physician visits, 30,000 hospital admissions and an annual financial outlay by our health industry of 146 million dollars.

Kinsey's report in 1948 was the first study to address the occurrence of sexual dysfunction in the general population. Results from this study, based on the detailed interview of 12,000 males, stratified for age, education and occupation, indicated an increasing rate of impotence with age. Its prevalence was cited as less than 1% in men under 19 years of age, 3% of men under 45 years, 7% less than 55 years and 25% by the age of 75 years. In 1979, Gebhard reanalyzed the Kinsey data and in a chort of over five thousand men, 42% admitted to erectile difficulties.

Other studies performed on subjects derived from general populations have suffered from two major problems, the use of non-representative samples due to the method of sampling and the unknown value of the instrument used in the study. Ard, in 1977, reported on the sexual behavior of 161 couples married for greater than 20 years and noted a 3% incidence of erectile problems. In 1978, Frank studied 100 volunteer couples, reportedly normal, who were married and sexually active, with a mean age of 37 years. Forty percent of men reported difficulty with either erection of ejaculation. A year later, Nettelbladt found that 40% of randomly selected, sexually active males (mean age of 31 years) noted some degree of erectile problems. Other studies have reported a variable incidence of erectile impairment, from 3-40%. The Baltimore Longitudinal Study of Aging cited erectile impairment as being present in 8% of men 55 years or less, 25% of 65 year olds, 55% of 75 year olds and 75% of 80 year olds. The Charleston Heart Study Cohort reported on sexual activity rather than erectile dysfunction. It reported a 30% incidence of inactivity between he ages of 66-69 years. In subjects over 80 years of age this figure rose to 60%.

Subjects obtained from medical health statistics have also been analyzed for the occurrence of erectile difficulties. In an analysis of family practice patients, Schein noted a prevalence of erectile difficulties of 27% in 212 patients with a mean age of 35 years. Mulligan cited a 6-fold increase in erectile problems in middle-aged men with self-reported poor health, and a 40-fold increase in similar patients over 70 years of age. In a cohort of 50 year old men undergoing a nutritional and general health screening, Morley found a 27% incidence of impotence. This finding is in keeping with other data from Masters and Johns and Slag, inferring that men with medical conditions have is a higher incidence of erectile dysfunction.

The Massachusetts Male Aging Study (MMAS) was a cross-sectional, community-based, random-sample, multidisciplinary epidemiologic survey of aging and health in men aged 40-70 years. The study was conducted between 1987-1989, in and around Boston. The responses of 1290 subjects were evaluated following the administration of a detailed, comprehensive questionnaire-based instrument. This work represents the largest work since the Kinsey report in 1948. The MMAS study differed from prior studies in both size and content. It included four groups of intervening variables (confounders) that could be related to sexual function: health status and medical care use, sociodemographic data, psychosocial and lifestyle characteristics.

All data were collected at the subject's home by trained interviewers. The multidisciplinary approach included gerontologists, behavioral scientists, endocrinologists and sexual dysfunction clinicians. The study design allowed precise estimation of key parameters while controlling for potentially important confounders and permitted identification of statistically predictive risk factors. The sample group was as close to the general population as could be achieved. The population studied were a free-living, non-institutionalized group, only a fraction of which was sick and interacting with the health system.

The MMAS instrument contained 23 questions, 9 of which related to erectile capability. a subjective assessment of erectile potency was made as opposed to a more concretely defined erectile dysfunction state. A calibration study was conducted to discriminate different potency profiles. Potency was divided into 4 grades: not impotent, minimally impotent, moderately impotent and completely impotent.

The overall rate of any degree of impotence the MMAS was 52%, including 17% minimally impotent, 25% moderately impotent and 10% completely impotent. The overall probability of impotence, of any degree, at 40 years was 39% and at 70 years 67%. Extrapolating these data, there would be 30 million men in the United States with some form of erectile impairment. The conditions that were associated with impotence in this study included, diabetes, hypertension, heart disease, untreated ulcer disease, arthritis, cardiac medications (including vasodilators and antihypertensive agents) in cigarette smokers, hypoglycemic agents and depression.

The association between vascular disease and erectile dysfunction has been recognized and well documented. Indeed, alterations in the vascular hemodynamics (whether, arterial insufficiency or corporovenocclusive dysfunction) are believed to be the most common cause of organic erectile dysfunction. Such vascular disease as myocardial infarction, coronary artery bypass surgery, cerebral vascular accidents, peripheral vascular disease and hypertension have all been shown to have a higher incidence of impotence compared to the general population without documented vasculopathies. Myocardial infarction (MI) and corornary artery bypass surgery have been associated with erectile difficulties in 64% and 57% respectively. Furthermore, in a group of 130 impotent men, the incidence of MI was 8 times higher in men with abnormal penile-brachial indices (PBI) than those with normal PBI (12% vs 1.5%). In men with peripheral vascular disease (PVD), the incidence of erectile dysfunction has been estimated at 80%. This figure is 10% in untreated hypertensive males.

Diabetes with its related vasculopathy is associated with a higher incidence of impotence at all ages compared to the general population. The prevalence of impotence in all-comer diabetics has been variably estimated at between 35 and 75%. Erectile difficulties may be the harbinger of diabetes, this phenomenon occurring in 12% of newly diagnosed diabetics. The incidence of impotence in diabetics is age-dependent and is higher in men with juvenile-onset diabetes compared to to adult-onset diabetics. Of those diabetic men who will develop impotence 505 will do so within 5-10 years of the diagnosis of their diabetes. When combined with hypertensive disease impotence in diabetics is even more prevalent.

As the number of vascular risk factors (such as, cigarette smoking, hypertension, cardiac disease, hyperlipidemia, and diabetes) increases so too does the likelihood of erectile dysfunction. This finding was confirmed in Virag's analysis of 400 impotent men, demonstrating that 80% of these men had physiologic abnormalities and that vascular risk factors were more common in this group compared to the general population

While androgens are essential to the growth and differentiation of the male genital tract, the development of secondary sexual characteristics and the presence of libido their role in the erectile process remains unclear. At this time, the nature of an appropriate hormonal investigation, whether a complete hormone panel is required for every patient or whether a single testosterone determination constitutes effective screening remains debated. Indeed, disagreement exists on whether free or total testosterone levels are more important in he evaluation of the impotent male. Nevertheless, endocrinopathies probably account for up between 3-6% of all organic erectile dysfunction and those endocrinopathies that may lead to impotence include hypogonadism, hypothyroidism, hyperthyroidism, hyperprolactinemia, diabetes mellitus, adrenal disorders, chronic liver disease, chronic renal failure and AIDS.

Drug associated erectile dysfunction is common and the list of medications that can induce erectile dysfunction is significant. Medication-induced impotence has been estimated occuring in up to 25% of patients in a medical outpatient clinic. Antihypertensive agents are associated with erectile difficulties, depending upon the specific agents in 4-40% of patients. They induce impotence either by actions at the central level (clonidine), by direct actions at the corporal level (calcium channel blockers) or by purely dropping systemic blood pressure upon which the patient has relied to maintain an intracorporal pressure sufficient for the development of penile rigidity.

Several medications cause impotence based on their anti-androgen actions, for example estrogens, LHRH agonists, H2 antagonists, and spironolactone. Digoxin induces erectile difficulties via blockade of the NA-K-ATPase pump resulting in a net increase in intracellular Ca and subsequent increased tone in the corporal smooth muscle. The psychotropic medications alter CNS mechanisms. Chronic use of recreational drugs has been associated with erectile dysfunction. Other agents affect erection through, as of yet, unknown mechanisms. Ultimately, it is essential to define a mechanism for each medication suspected of causing impotence. Furthermore, the diagnosis of drug-induced erectile dysfunction must be predicated upon reproducibility of the problem with medication administration and cessation of the problem upon its discontinuation.

Pelvic trauma, in particular injuries to the perineum and pelvic fractures, are associated with erectile dysfunction. In an analysis of patients presenting to a university-based practice, Goldstein reported that 35 of the patients seen had erectile dysfunction resulting from trauma. Furthermore, the pathophysiologic mechanisms for the development of such impotence has been previously postulated. In recent years it has been recognized that a disproportionate number of young men with erectile difficulties have a history of bicycling accidents. Disruption of the prostatomembranous urethra, as seen in severe pelvic fractures has been reported to be associated with a up to a 50% incidence of impotence.

Urologic surgery of a variety of types has been implicated in erectile dysfunction. The operations that have been reported to cause erectile dysfunction include, radical prostatectomy, retropubic and perineal, whether nerve-sparing or not, TURP, internal urethrotomy, perineal urethroplasty and pelvic exenterative procedures.

Until 15 years ago impotence was believed to be the result of psychological issues in the majority of men. Various workers have demonstrated the association between depression and erectile dysfunction. The presence of erectile dysfunction correlated with marital discord in 25% of couples. In the MMAS, psychologic factors associated with erectile problems included depression, anger and low levels of dominance.

Apart from the factors already outlined (vascular risk factors, endocrinopthies and psychologic problems) that may lead to impotence the following conditions may induce erectile problems:

Renal Failure
Up to 40% of men suffering from chronic renal failure have some form of erectile dysfunction. The mechanism by which impotence results in this disorder is probably multifactorial, involving endocrinologic (hypogonadism, hyperprolactinemia), neuropathic (diabetes-induced nephropathy) and vascular factors. Hatzichristou investigated the vascular etiologies in a cohort of men with chronic renal failure who had undergone hemodynamic evaluation and found an inordinately high incidence of corporovenocclusive dysfunction. The role of renal transplantation in the development of erectile dysfunction in these patients is variable. In some, transplant improves the renal function to the point where the patients erectile function also improves and in others, particularly those men who had received 2 transplants, the erectile function may deteriorate further.

Neurologic disorders
Neurogenic erectile dysfunction may be caused by disorders such as, stroke, brain and spinal tumors, cerebral infection, Alzheimer's disease, temporal lobe epilepsy and multiple sclerosis (MS). Agarwal cited a 85% incidence of impotence in a group of men following stroke, while Goldstein noted 71% of men with MS were affected by erectile difficulties. More recently, it has beenn recognized that AIDS has associated with an autonomic neuropathy which may cause neurogenic erectile dysfunction.