A. Nocturnal Penile Tumescence (NPT) Test
Nocturnal erections occur in healthy males of all ages. Of the nocturnal erections, about 80% occur during REM sleep. The average man has 3-5 episodes of NPT each night, with each episode yielding 30-60 min of erection time. Total NPT time declines with increasing age. NPT has been measured by several methods: stamp test, Snap Gauges, strain gauges, NPTR (_{Rigiscan, Timm Medical Technologies, [Levine and Lenting, 1995]}) sleep lab NPTR and NPT electrobioimpedance (_{NEVA, American Medical Systems, [Knoll and Abrams, 1999]}).

In its classic form, NPT monitoring devices measure the number of erectile episodes, maximal penile rigidity, tumescence (circumference), and duration of the nocturnal erections. These data are recorded in conjunction with electroencephalographic, electro-oculographic, and electromyographic, nasal air flow, and oxygen saturation data, to document REM sleep, the presence or absence of hypoxia, and abnormal limb movement (_{Karacan, 1988}). Because of its expense, sleep lab NPT has been replaced by simpler outpatient devices such as Rigiscan and NEVA. These newer devices have the advantage of electronically recording the number, duration, rigidity, and circumference of penile erections. NPT was originally designed to differentiate psychogenic from organic ED. In the United States, it is now mostly used in medicolegal or complicated cases.

B. Psychological Evaluation
The contemporary definition of psychogenic ED is, “persistent inability to achieve or maintain erection satisfactory for sexual performance which is due to predominantly or exclusively to psychological or interpersonal factors” (_{Lizza and Rosen, 1999}). Therefore, a skillful diagnostic interview is the mainstay of a good psychological evaluation. The diagnostic interview should focus more on the current sexual problem and its immediate causation, partner relationship, and any psychiatric symptoms. Because psychogenic impotence is the result of changes in affect and mood, it usually occurs in a specific pattern. A suggestive history includes sudden onset, selective dysfunction (eg, rigid erection with one partner and poor erection with others, or normal erection during masturbation or fantasy but not during intercourse), and normal pattern of nocturnal erections but abnormal pattern during waking hours. This is often associated with anxiety, guilt, fear, emotional stress, and religious or parental inhibition.

Although psychological consultation is not indicated for most patients, it is very useful in evaluating and treating men with deep-seated psychological problems. Three groups of psychometric instruments are available: (1) personality questionnaires; (2) the depression inventory; and (3) questionnaires for sexual dysfunction and relationship factors. The Minnesota Multiphasic Personality Inventory (MMPI)-2 is a valuable tool for evaluating the patient’s personality and its relevance to sexual dysfunction. The Beck Depression Inventory is a self-reported test for which a score exceeding 18 is considered indicative of significant clinical depression. For relationship assessment, the Short Marital Adjustment Test (for married couples) and the Dyadic Adjustment Inventory (for unmarried people) can be used to determine overall relationship quality.

C. Neurologic Tests
There are 3 types of erections: nocturnal, psychogenic, and reflexogenic. In a broader sense, neurologic testing should assess peripheral, spinal, and supraspinal centers as well as both somatic and autonomic pathways associated with all 3 types of erection and sexual arousal. However, the effect of neurologic deficit on penile erection is a complicated phenomenon and, with a few exceptions, neurologic testing will rarely change management. Therefore the aim of neuro-urologic testing is to (1) uncover reversible neurologic disease such as dorsal nerve neuropathy secondary to long-distance bicycling; (2) assess the extent of neurologic deficit from a known neurologic disease such as diabetes mellitus or pelvic injury; and (3) determine whether a referral to a neurologist is necessary (eg, workup for possible spinal cord tumor).

1. Biothesiometry - This test is designed to measure the sensory perception threshold to various amplitudes of vibratory stimulation produced by a hand-held electromagnetic device (biothesiometer) placed on the pulp of the index fingers, both sides of the penile shaft, and the glans penis (_{Bemelmans et al, 1995}).

2. Bulbocavernosus reflex latency - This test is performed by placing 2 stimulating ring electrodes around the penis, one near the corona and the other 3 cm proximal. Concentric needle electrodes are placed in the right and left bulbocavernous muscles to record the response. Square-wave impulses are delivered via a direct current stimulator. The latency period for each stimulus response is measured from the beginning of the stimulus to the beginning of the response. An abnormal bulbocavernosus reflex (BCR) latency time, defined as a value greater than 3 standard deviations above the mean (30-40 msec), carries a high probability of neuropathology.

3. Genitocerebral evoked potential - This test involves electrical stimulation of the dorsal nerve of the penis as described for the BCR latency test. Instead of recording EMG responses, this study records the evoked potential waveforms overlying the sacral spinal cord and cerebral cortex.

4. Smooth muscle EMG - This involves direct recording of cavernous electrical activity with a needle electrode. The normal resting flaccid electrical activity from the corpora cavernosa was a rhythmic slow wave with an intermittent burst of activity. These bursts virtually ceased during visual sexual stimulation or after intracavernous injection of a smooth-muscle relaxant. Patients with suspected autonomic neuropathy demonstrated a discoordination pattern with continuing electrical activity during visual sexual stimulation or after intracavernous injection of a smooth-muscle relaxant (_{Junemann, Buhrle, and Stief, 1993}).

5. Tests for penile vascular function -

a. CIS (Combined Intracavernous Injection and Stimulation) test - Office pharmacotesting consists of an intracavernous injections, visual or manual sexual stimulation, and a rating of the subsequent erection. This test is the most commonly performed diagnostic procedure for ED. This screening test allows the clinician to bypass neurogenic and hormonal influences and to evaluate the vascular status of the penis directly and objectively. The most commonly used vasodilator is 10 ug of alprostadil or 0.3 mL of a mixture of papaverine and phentolamine. A rigid erection lasting for more than 20 min is indicative of normal venous function (_{Virag et al, 1984; Donatucci & Lue, 1992}). However, the same conclusion cannot be made for arterial function because some men with mild arterial insufficiency can also have the same response.

b. Duplex ultrasonography (gray-scale and color-coded) - When further vascular diagnostic testing is indicated, the color duplex ultrasound study - which consists of an intracavernous pharmacotest and measurement of blood flow by duplex Doppler ultrasound - is the most reliable and least invasive evidence-based assessment of ED (_{Lue et al, 1985;}

Figure 37-4). Normal arterial response is a peak flow velocity measured at the base of penis of more than 30 cm/s, a sharp upstroke of the waveform, and absence of diastolic flow after sexual stimulation. Ultrasound can also be used to detect penile abnormalities such as Peyronie’s plaque, calcification, thickened vessel wall, and intracavernous fibrosis. The parameters useful in diagnosing venous leakage include a diastolic venous flow velocity of > 5 cm/s, a resistive index (RI) of < 0.75, or both (_{Naroda et al, 1994}). RI = peak flow velocity (PSV) - end diastolic flow velocity (EDV)/PSV.

The Pulsed Doppler study test is performed with a portable Midus pulsed Doppler unit. It records the Doppler waveform of the cavernous arteries without providing a real-time image. Cavernous artery blood flow is studied in a similar fashion to duplex Doppler.

c. Cavernous arterial occlusion pressure - This test involves infusing saline solution into the corpora after intracavernous injection of vasodilators to raise the intracavernous pressure above the systolic blood pressure. A pencil Doppler transducer is then applied to the side of the penile base. The saline infusion is stopped, and the intracavernous pressure is allowed to fall. The pressure at which the cavernous arterial flow becomes detectable is defined as the cavernous artery systolic occlusion pressure (CASOP). A gradient between the cavernous and brachial artery pressures of less than 35 mm Hg and equal pressure between the right and left cavernous arteries have been defined as normal.

d. Cavernosometry and cavernosography - Pharmacologic cavernosometry involves intracavernous injection of a vasodilator combination (papaverine + phentolamine + alprostadil) followed by saline infusion and simultaneous monitoring of intracavernous pressure. In men with normal venous function, the maintenance flow should be less than 10 mL/min at 100 mm Hg intracavernous pressure, and the intracavernous pressure drop rate after the infusion is stopped should be less than 50 mm Hg in 30 seconds (_{Padma-Nathan, 1989}).

Cavernosography involves the infusion of diluted radiocontrast solution into the corpora cavernosa during an artificial erection to visualize the site of venous leakage. It should always be performed after activation of the veno-occlusive mechanism by intracavernous injection of vasodilators. Minimal or no contrast is seen outside the corpora cavernosa in men with normal veno-occlusive function. In patients with venous leakage of congenital or traumatic origin, the leakage is seen most often in the crura or at the site of injury, respectively (

Figure 37-5). The typical finding in men with intrinsic disease of the corpus cavernosum or the tunica albuginea is a diffuse leakage through all penile venous channels.

e. Arteriography - Arteriography is reserved for the evaluation of the complex patient when revascularization surgery is contemplated. The study is performed by intracavernous injection of a vasodilating agent (papaverine, papaverine + phentolamine, or alprostadil) followed by selective cannulation of the internal pudendal artery and injection of a diluted contrast solution of low osmolarity. The anatomy and radiographic appearance of the cavernous arteries are then evaluated (

Figure 37-6). The evaluation should include the size and length of the inferior epigastric arteries as well as the cavernous and dorsal arteries of the penis.