Since in healthy men erections of 20 to 50 min duration occur physiologically during the REM phases of sleep, it was believed that by measuring nocturnal penile tumescences it would be possible to distinguish between organic and psychogenically induced impotence.
This concept was based on the notion that lack of nocturnal tumescence indicates an organic cause of erectile dysfunction. In the case of organic disorders, characteristic curves were also presented (Fig. 3.10). For measurement, 2 mercury strain gauges were placed around the base and fore-shaft of the penis and circumferential changes were recorded during sleep using a monitoring unit (Fig. 3.11). As a simplified version, the Eska erectiometer is available for the outpatient setting, in which the changes are given in centimeter increments. Since it was soon recognized that an organic cause may be present even in the case of inconspicuous nocturnal tumescences, devices were developed which determine both the nocturnal penile tumescences and the associated rigidities (Rigidiscan, Dacomed, USA). A correlation of noninvasive measurement of penile rigidity to intracavernosal pressure was confirmed by Virag . Normal findings for the Rigidiscan are at least 3 erections per night with a duration of about 15 min, a rigidity of at least 70,070 and a circumferential increase of more than 3/2 cm at the base/tip of the penis.
Fig. 3.10. Normal tumescent during REM phases (upper curve). Premature drop in case of venous occlusion disorder (missing plateau phase).
Fig.3.11. Device for tumescence measurement using mercury strain gauges.
The measurement of nocturnal tumescence as well as rigidity  with the aid of a monitoring unit is a very time-consuming procedure that has to be performed on a stationary basis and is therefore expensive. The measurements should be performed in at least 2–3 nights to make a meaningful assessment, . The assessability is also limited by the fact that after the age of 50, the duration and severity of nocturnal erections decrease, but it is precisely this clientele that constitutes the majority of patients. Normal NPT curves are also found at rest in pelvic steal syndrome (redistribution of blood in extremities in arterial occlusive disease). On the other hand, purely psychological causes, e.g., depression, can lead to complete suppression of nocturnal erections and thus to an erroneous diagnosis of an organic disorder . Abnormal NPT measurements are found in approximately 20% of patients without verifiable organic pathological findings . What remains is the realization that if nocturnal erections of full tumescence and rigidity are detected, an organic disorder is unlikely—or somewhat low—given the high technical and cost involved. However, documentation of normal erections may be of interest in the context of an expert opinion. This shows that NPT measurements cannot replace any of the other clinical examinations.
Furthermore, the combination of rigidity measurement through visual sexual stimulation under intracavernosal application of pharmaceuticals  cannot be classified regarding its diagnostic significance at present. Visual sexual stimulation does not appear to be unproblematic for etiological clarification, since it insufficiently considers individual, cultural and religious aspects and thus could hardly be standardized.
NPT measurements represent time-consuming, expensive examinations with a not inconsiderable error rate, which are not absolutely necessary for the clarification of erectile dysfunction.