Recent advances in the understanding of the physiology of erections and improvements in technology have greatly increased our ability to evaluate and offer therapeutic choices to the impotent patient. Both noninvasive and invasive imaging modalities have helped define the evaluation of many types of impotence. Although these advances have greatly aided in detecting the cause of impotence in many patients, it is important to realize the limitations and technical pitfalls of each study.
In general, all laboratory evaluations of impotence are performed in an artificial, nonsexual setting with little privacy. This sometimes leads to a high anxiety level and an increased sympathetic response on the part of the patient, which can significantly influence the results of the test. In addition, due to variations in study technique and the unavailability of a large population of normal study subjects, the normative values for many of these studies have not been well established. Therefore, some patients undergoing evaluation may require more than 1 evaluation technique in order to arrive at a diagnosis.
Evaluation of nocturnal penile tumescence (NPT) is one of the earliest tests devised to study erectile dysfunction. The association between sleep and erections was documented as early as 1940 by Halverson. Further studies revealed the association between REM sleep and penile erections. In 1970, Karacan suggested that NPT could be used to evaluate erectile dysfunction. The mechanism of NPT is presumed to rely on neurovascular response mechanisms similar to those seen in erotically induced erections. Therefore, patients who are documented to have normal NPT are presumed to have a normal capacity for spontaneous, erotically induced erections.
The primary goal of NPT studies is to distinguish psychogenic causes from organic (vascular, neurogenic, hormonal) causes of impotence. In its sophisticated form, this study involves the use of nocturnal monitoring devices that measure the number of erectile episodes, maximal penile rigidity, tumescence (circumference), and duration of nocturnal erections. Since nocturnal erections occur during REM phase sleep, tumescence monitoring may have to be repeated over 2 to 3 nights to overcome the "first night effect" (ie, suboptimal study results due to poor sleep pattern related to a foreign environment).
Nocturnal penile erections have been associated with rapid eye movement (REM) sleep. Nocturnal penile tumescence (NPT) helps maintain erections by providing oxygenation to the penis. During REM sleep, men normally have several erections each night, each one lasting up to an hour. Thus, during the erection, the corporal bodies are exposed to the same oxygen level that the rest of the body experiences for up to four hours per day. In men who have poor penile blood flow, and therefore poor erections, this does not occur, hastening the development of scar tissue and loss of corporal smooth muscle. Therefore, nocturnal erections are extremely important for the maintenance of good erectile functioning. As men age, these episodes become fewer and shorter. Nocturnal penile tumescence monitoring is useful in patients who report a complete absence of erections but in whom a psychological component is suspected.
Before the advent of the newer techniques using the rigiscan, physicians used the postage stamp test. Basically, a series of stamps was placed around the base of the penis. If the patient awoke the next morning with the stamps unbroken, this indicated an organic problem. However, if the stamps were broken, this was felt to be a psychological problem. Following the postage stamp test was the development of the snap gauge, a Velcro band placed around the base of the penis that had three colored plastic film elements. Each film ruptured at a specific known force. It took 10 ounces of radial force to rupture the blue tab, 15 ounces to rupture the red tab, and 20 ounces to rupture the clear tab. These snap gauge results were reasonable. However, there was no way to measure rigidity. When the criteria for the snap gauge was carefully examined, it was found that half the men who broke two to three films actually had no rigidity by visual inspection. Because of this, it has lost some favor and it has been mostly supplanted by the rigiscan.
The rigiscan allows us to measure continuous tumescence monitoring, but it also provides rigidity information during the times when the patient is achieving an erection. In addition, it gives detailed information about how often the erections occur, for what period of time, and the rigidity and change in diameter. Rigidity is measured by placing a loop around the base of the penis, which is tightened every thirty seconds with a force of 2.8 Newtons. It records three sessions, and then it is downloaded into a computer. The newest version of the rigiscan uses software that can calculate an entire evening of tumescence and rigidity data into rigidity activity units and tumescence activity units. Several different types of patterns are measured by the rigiscan, including dissociation which is seen with Peyronie's disease. This pattern shows how the base of the penis gets hard and the tip past the areas of the Peyronie's plaque has poor rigidity and tumescence.
Another pattern, termed uncoupling, occurs when there is good tumescence on both the base and tip, but poor rigidity. While rigiscan testing doesn't give an exact diagnosis, it can be extremely useful.
NPT studies are not perfect. Some problems, including sleep disorders and depression, can cause abnormal readings. This should be recognized when performing the study. Testing prior to surgery demonstrates that the patient does indeed have erectile dysfunction. If the patient has complications or needs surgery, such as repairing the implant, testing protects the patient against cases in which an insurance company attempts to avoid payment for a service and in medical malpractice. An NPT study supports the urologist's diagnosis of erectile dysfunction.
Neither of these methods is helpful in determining a physical cause for erectile dysfunction if erection does not occur during sleep. A normal finding of an NPT study in patients in whom a psychogenic etiology is suspected may help reduce costs by preventing unnecessary endocrine and vascular evaluations.
Although NPT study is noninvasive, its validity and usefulness in the evaluation of impotence have been questioned. First, normative values as well as a standardized technique for determining parameters such as the number of episodes, degree of tumescence, and rigidity have not been well established. What constitutes a normal or abnormal NPT study still has not been determined. The interpretation of nocturnal monitoring studies may therefore differ among investigators. In addition, the parameters measured. In an NPT study may not correlate well with the clinical findings. For example, early NPT studies relied on measurement of penile tumescence and not rigidity. However, significant penile tumescence can occur in the absence of penile rigidity, and measurement of penile tumescence may not always correlate with penile rigidity, which is necessary for vaginal penetration. Therefore, overreliance on these parameters may lead the physician to misdiagnose the condition as having a psychological basis.
The validity of NPT studies relies on the assumption that nocturnal erections are prompted by neurovascular responses similar to those of erotically induced erection. The truth of this assumption, however, has not been proven. Indeed, some investigators have reported that NPT studies correlate poorly with patient-reported sexual performance. Therefore, the findings of NPT tests often must be confirmed independently by other studies.
One useful way to evaluate the nerves that carry sensation away from the penis is by the use of a technique called penile biothesiometry. This is a quantitative measure of the vibratory sense of the penis. A biothesiometer consists of a device that vibrates at a known frequency, and it is compared to other parts of the body with known vibration thresholds. This tends to become less reliable in older patients because as men age they tend to lose sensation. Still, it is a reasonable, cost-effective test.
The device is placed on the tip of the finger, and slowly the frequency is increased until the vibration is felt. This is then used as a baseline to compare the vibration sense of the penis as well. It is a useful way to detect early neuropathic disease in younger men, particularly in men with diabetes. It is also used in men who have had circumcisions and complain that the head of the penis has lost sensation.
Differentiation of psychogenic, neurogenic, and vascular causes of impotence is often difficult even with a complete history, physical exam, and endocrine evaluation. Following the completion of a thorough history and physical exam and basic endocrine evaluation, additional information regarding the vascular status of the penis is often helpful in directing further evaluation.
Intracorporal injection of papaverine, first introduced by Virag and associates, was found to be useful as a diagnostic tool in patients with suspected vasculogenic impotence. The pharmacologic screening test allows the clinician to bypass neurogenic and hormonal influences on the penis and objectively evaluate the vascular status of the penis directly.
The pharmacologic test is an extremely useful, inexpensive, and minimally invasive screening exam for patients with suspected vasculogenic impotence.
Pharmacologic evaluation of the impotent patient consists of testing the erectile response of the penis to an intracavernosally administered vasodilating agent, which causes relaxation of the penile vascular sinusoids and increased blood flow in the cavernous arteries. The resultant engorgement of the corpora results in compression of the subtunical venules and decreases penile venous outflow. Therefore, in order to produce a normal erection, arterial vasodilation, sinusoidal relaxation, and decrease in venous outflow must all occur in response to the vasodilating agent.
In the past, a number of agents have been used for this study, including papaverine (a direct-acting, smooth-muscle relaxant) and phentolamine (an alpha-adrenergic blocking agent). Later, prostaglandin E1 (PGE1), a potent vasodilating agent metabolized locally in the penis was used in many centers.
The technique involves injecting 10ug PGE1 through a 28-gauge needle into the corpus cavernosum. The needle site is compressed manually for at least 5 minutes to prevent hematoma formation. The erectile response is periodically evaluated for both rigidity and duration. In a normal study, a full erection is achieved within 10 to 15 minutes and is defined as one that is firm to palpation and positioned at above 90deg.. The erectile response to the injection should be sustained for at least 15 minutes to complete a normal test.
The pharmacologic test yields important information regarding the vascular status of the penis. The finding of a normal erectile response to the injection rules out the possibility of venous leakage, although some patients (about 20%) with arterial insufficiency may also achieve rigid erection due to an intact veno-occlusive mechanism.
The finding of a normal response to intracavernosal injection in an impotent patient suggests that neurogenic, psychogenic, or hormonal factors may be primarily responsible, but it is difficult to distinguish among these causes. However, a normal pharmacologic screening test makes further evaluation for veno-occlusive dysfunction unnecessary.
An abnormal finding on the pharmacologic test suggests that penile vascular disease (arterial, venous, or sinusoidal) is responsible for impotence.
In the appropriately selected patient, abnormal findings on pharmacologic screening should lead the clinician to perform further evaluation for a vasculogenic etiology of impotence. The most important role of the test lies possibly on the selection of those patients who are candidates for the intracavernosal self-injection therapy.
Although many tests have been suggested and are carried out in very specialized centers, testing for nocturnal penile tumescence (NPT), penile biothesiometry and intracavernosal injections, and their correct interpretation, are sufficient in most cases to point towards the etiological factor(s) leading to erectile dysfunction. And this is especially so for patients with diabetes.
In recent times, one other test has gained prominence in giving a more quantifiable result for vascular anomalies leading to erectile dysfunction.
In 1984, Lue et al introduced the duplex ultrasound for evaluation of male impotence. Notwithstanding the fact that duplex exams evaluate the flow speed and not the arterial flow, they have proved a reliable and reproducible method for the evaluation of penile arterial insufficiency. Duplex ultrasound combined with an intercavernosal injection has pretty much replaced all other tests that are currently available. This single test can evaluate both the early and late stages of an erection, as well as venous leakage.
After much controversy regarding which parameters should be evaluated (flow speed, artery caliber or form of the flow waves), a consensus has been currently reached: the only important parameter is the speed peak of cavernosal artery systolic flow, which must be greater than 30cm/s after the intracavernosal injection. Clinically, a veno-occlusive dysfunction can be suspected when patients with systolic speed peak greater than 30cm/s eventually show high speed of diastolic flow. However, this last concept is not widely accepted.
In recent times, with machine improvement and their lower costs, the duplex ultrasound has turned out to be the simplest and most reliable means for the penile artery tree evaluation.