Do I need any specialized tests?
In most cases, the history, physical examination, and blood testing allow the physician to identify possible causes of your ED, but in some cases, a more advanced evaluation may be required. In men who seem to have a psychogenic basis for their problem, the doctor might order nocturnal penile tumescence (NPT) studies to confirm this theory. (Tumescence is the state of being swollen, in this case referring to having an erection.) This test involves wearing a specialized device around the penis at night on several occasions when sleeping. The device records whether you have erections during your sleep, which is both normal and common; if you do, it would suggest that the ED could be psychogenic in origin, which can help determine the correct treatment strategy. However, NPT studies have limitations, and it has been suggested that sleep-related erections may not be the same as sexually induced erections.
If your doctor wishes to evaluate your penile vasculature and obtain a preliminary assessment of the arterial and venous function therein, the best initial test is Doppler ultrasonography in conjunction with injection therapy. In ultrasound studies, internal organs are visualized by measurement of reflected sound waves. In this particular study, you are injected with a chemical that causes smooth muscle relaxation and increases penile blood flow, usually 10 |ig of alprostadil (Caverject, Edex), but other agents, such as trimix/triple P (prostaglandin E1, papaverine, and phentolamine), may be used. After injection, sequential blood flow studies are performed. The rate at which blood is flowing through the cavernosal artery on each side of the penis (the peak systolic velocity) can be determined. A peak systolic velocity over 25 to 30 mL/sec is considered normal. The Doppler ultrasound also allows the diameter of the cavernosal arteries to be measured. In addition, the function of the penile veins can be assessed by measuring the end-diastolic velocity (venous resistance). The Doppler ultrasound study may be affected by patient anxiety and may require stimulation to achieve the maximum response. As with injection therapy, there is a small risk of priapism and penile pain related to the injection or to the medication used. If the Doppler ultrasound study demonstrates an abnormality of the arteries or veins and the patient wishes to be evaluated for possible surgical correction, then further studies would be required. Arteriography, or cavernosography are performed to better assess possible arterial or venous abnormalities. These more invasive tests are not required in all individuals and should be performed in select institutions where penile artery bypass surgery or venous ligation surgery is performed.
What are nocturnal penile tumescence (NPT) studies?
Formal nocturnal penile tumescence (NPT) testing requires spending two to three consecutive nights in the sleep laboratory. Men undergoing NPT tests must abstain from alcohol, caffeine, and medications that may affect sleep or erections for 8 hours before the test. Because sleep erections occur primarily during rapid eye movement (REM) sleep, one must assess the quality of REM sleep, which is done through the use of an electroen-cephalographic (EEG) test, an electro-oculographic test, and a submental electromyographic test. To assess penile tumescence, two mercury-in-rubber strain gauges (expandable rubber loops) are placed on the penis, one at the base and the other just behind the glans penis. When the loop circumferences increase, the mercury column is thinned, this increases the electrical resistance through the mercury. Each time there is an increase in the electrical resistance, it is recorded on a tracing. The high point of the deflection on the tracing corresponds to the change in penile circumference. The study also allows one to detect discrepancies between the changes in both loops, meaning that if one loop expands substantially more than the other, the test can detect this. One of the biggest drawbacks of NPTs is that they are measuring nocturnal erections, not erections related to sexual arousal, and each may occur via different mechanisms. Secondly, there can be false-positive NPT results; these more commonly occur in patients with neurogenic ED and pelvic steal syndrome, and false-negative NPT results occur in patients with depression, alcohol use, medication ingestion, and sleep apnea. In addition, the measurements of penile rigidity with NPT studies may be insufficient to differentiate between organic and psychogenic erectile dysfunction. Formal nocturnal penile tumescence testing is also laborious and expensive.
What is the RigiScan?
The RigiScan (Timm Medical Technologies, Eden Prairie, MN) was developed as a portable home device to evaluate the quality and the quantity of nocturnal penile erections. It is used to provide continuous measurements of penile rigidity and tumescence, and it represents an improvement over previous techniques of assessing nocturnal penile tumescence. The RigiScan consists of a portable battery-powered unit that is strapped around the thigh and has two loops that are connected to a direct-current torque motor. One loop is placed around the base of the penis and the other is placed just below the corona (the area of the penis just before the glans penis). To measure tumescence every 30 seconds, the loops tighten and the penile circumference (loop length) is recorded. Fifteen seconds later, a second measurement is taken without active tightening of the loops. If tumescence (measured by the length of the loop) increases by 10 mm or more from the initial measurements, rigidity measurements are taken. Rigidity is
Axial rigidity is the most important measurement in predicting vaginal penetration because it is used to assess the ability of the penis to stay straight despite pressure against the tip.
measured every 30 seconds by a slight tightening of the loops. Rigidity is expressed as a percentage, with 100% equaling the rigidity of a noncompressible rubber shaft. Rigidity measurements are discontinued when tumescence decreases to within 10 mm of baseline. The RigiScan can record three uninterrupted 10-hour sessions. The information stored in the device can then be downloaded into a microprocessor, and the nocturnal penile tumescence and rigidity information can be analyzed, displayed, and printed. The RigiScan measures only radial rigidity, that is, rigidity across the width or circumference; it does not measure axial rigidity, or rigidity across the length of the penis. Axial rigidity is the most important measurement in predicting vaginal penetration because it is used to assess the ability of the penis to stay straight despite pressure against the tip.
In most patients, the RigiScan device can distinguish functional from inadequate erections and is helpful in distinguishing psychogenic from organic ED.
-  Use of a Doppler probe during ultrasound to look at flow through vessels.
-  A test for identifying and locating arterial disease in the penis, using injection of contrast into the arteries supplying the penis to look for areas of "blockage."
-  A technique used to visualize areas of venous leak. It involves the injection of a cavernous smooth-muscle dilator (e.g., prostaglandin E1 or trimix), followed by placement of a butterfly needle into the corpora, instillation of a contrast agent into the corpora, and X-ray photographs to visualize the sites of venous leak.
-  A condition in which a person stops breathing for a short period of time during sleep (anywhere from a few seconds to a minute or two), causing him to wake repeatedly and get insufficient sleep.