One of the most commonly used tests to distinguish psychogenic from organic erectile dysfunction (ED) is to monitor nocturnal penile tumescence using the RigiScan device and its new software called RigiScan Plus. To give a true estimate of the predictive ability of the new RigiScan software parameters, tumescence activity units (TAUs) and rigidity activity units (RAUs), we conducted this study on 639 RigiScan night records of 416 ED patients. For study purposes, these records were transferred to a personal computer and classified as normal and abnormal. We recorded the TAU and RAU provided by the RigiScan software for each event separately and also for the total night. We then estimated the diagnostic performance of these two parameters using cutoff values with highest accuracy plotted against the previously reported normal and abnormal curves. We then made four new calculations to improve the diagnostic accuracy of TAU and RAU for the total night. On estimating the highest diagnostic accuracy of RAU and TAU, it ranged from 67.8 to 73.7% for the single best event and from 68.4 to 74.2% for the total night. When using the newly computed units, the highest diagnostic accuracy did not exceed 75.9%.
Erectile dysfunction (ED) is a worldwide problem that affects ∼10–25% of men.1 After the introduction of new oral agents for the treatment of ED, most researchers and clinicians adopted a goal-oriented approach to handle most cases of ED. However, in cases when these new oral agents fail to correct the ED, both the patient and physician may attempt to identify the underlying etiology of the patient's ED. The physician may refer to one or two reliable tests to fulfill this purpose. Monitoring of nocturnal penile tumescence (NPT) is a helpful test to diagnose ED and to distinguish psychogenic from organic impotence.2 The RigiScan device (Dacomed Corporation, Minneapolis, MN, USA) is one of the most reliable tests commonly used to monitor NPT. It was designed to monitor NPT and rigidity through two loops held around the base and tip of the penis. A patient is considered to have a normal NPT if he has three to five erectile episodes during 8 h of sleep fulfilling the following criteria: tip and base rigidity 70%, an increase in tumescence at the tip 2 cm and at the base 3 cm, and erectile episode 10 min.3, 4 However, most clinicians now agree that a single erectile event fulfilling the above criteria is adequate to consider an NPT normal.5, 6
In 1994 RigiScan monitors were modified and new software was developed to further improve the reliability of the test to differentiate between organic and psychogenic ED. This software provides summary statistics that include: the number of detected events, event duration, average tumescence and rigidity readings during each event, integrated time-intensity area measures of tumescence (tumescence activity unit, TAU), and integrated time-intensity area measures of rigidity (rigidity activity unit, RAU).7, 8
Despite the lapse of a decade since introducing these units, only one study with only 40 subjects has been conducted to determine their usefulness in the evaluation of ED.8 This study was conducted on 416 subjects monitored over 639 nights to give a true estimate of the predictive ability of these parameters and their associated cutoff values when diagnosing a case of ED.
Materials and methods
Retrieval of data
This retrospective study was carried out on 639 RigiScan night records of 416 patients, aged 19–89 years. These records dated from September 1997 to August 2005 and were transferred to a personal computer for study purposes. Each record provides curves for penile tumescence and rigidity together with summary statistics of both penile tip and base activity during the 8 h of sleep. Each curve was visually analyzed and then categorized as normal or abnormal. A curve was considered normal if the best event had 70% rigidity at both the tip and base with increase in tip tumescence 2 cm and in base tumescence 3 cm with the activity in the base and tip synchronous and maintained for at least 10 min.
Summary statistics provided by the RigiScan software are numerical presentations of the erectile events. The software recognizes an event if there is a 20% increase in base loop circumference maintained for at least 3 min. Summary statistics also provide two new parameters, tumescence and rigidity activity units, for each event separately and as a total for each night. RAUs represent the product of elapsed time during a detected erectile event multiplied by the associated rigidity, with rigidity expressed as a fraction between 0 and 1, and resulting values ranging from 0 to 120. Similarly, TAUs represent the product of elapsed event time multiplied by the increase in tumescence (measured as penile circumference) over baseline tumescence with resultant values ranging from 0 to 120.8
Total RAU and TAU for each night and of the best event, as well as the result of visual evaluation of the curves (normal vs abnormal) were transferred to statistical software (SPSS version 10, Chicago, IL, USA) for statistical analysis. The obtained data were explored by one sample Kolmogorov–Smirnov test to determine the distribution of data as Gaussian or non-Gaussian. For description of different groups, mean and s.d. were used. Two samples Kolmogorov–Smirnov test was used to test differences of sample means. Diagnostic performance (sensitivity, specificity and diagnostic accuracy) of TAU and RAU were calculated according to the cutoff values with highest accuracy.
Further computations on activity units
In an attempt to increase the diagnostic accuracy of the RAU and TAU, four additional computations were performed. The diagnostic performance of each new output was calculated as follows:
Normalized activity units (RAU/h and TAU/h)
As activity units are time dependent, they will vary with the duration of sessions. For the units to be comparable between different sessions regardless of duration, the activity units for the total night were normalized by dividing them by the number of hours in each session.
Standardized activity units (RAU/e and TAU/e)
As activity units of total nights are the summation of units in a particular night, they will vary according to the number of events per night. Thus, their values depend on the number of events whether these events are normal or abnormal. To be comparable between different sessions regardless of the number of events, the activity units were standardized by dividing the activity units of each session by the number of events in that session.
Summated activity units (sRAU and sTAU)
To overcome the problem of dissociation that frequently happens between the tip and base in RigiScan records, we summed tip and base RAU and tip and base TAU of the total night.
R/T ratio (tip R/T ratio and base R/T ratio)
To overcome the problem of uncoupling that can occur between rigidity and tumescence in RigiScan records, RAU was divided by TAU for the tip and the base.
Using the classic NPT criteria, 262 sessions of 639 session records were determined to be normal and 377 sessions to be abnormal. Our findings can be categorized as follows:
Single best events
The RAU and TAU values of the single best events were found to be statistically significant (P<0.001) with normal sessions generally higher compared with abnormal sessions. However, there was a wide range of values within each parameter in each group with a resultant overlap between normal and abnormal (Figure 1). This can be observed from the high s.d. for RAU and TAU in Table 1. As a result of this overlap, some parameters in many sessions considered normal were found to have lower values than that of some abnormal sessions (Figure 2). Another result of this overlap is that the highest diagnostic accuracy of the single best event RAU and TAU obtained with different cutoff values ranging from 67.8 to 73.7% (Table 2).
Total night events
Similarly, activity units of normal sessions were found to be nearly double that of abnormal sessions (P<0.001). In addition, there were great overlap between ranges as evident in the histogram (Figure 1) and the high s.d. (Table 3). The diagnostic accuracy of total night activity units (68.4–74.2%) were found to be marginally higher than that of the single best event (67.8–73.7%) (Tables 2 and 4).
Newly computed activity units of the total nights
Despite this modification of activity units, an overlap between normal and abnormal sessions was still present. The diagnostic accuracy of these units was slightly higher (67.1–75.4%) than that of total night events before normalization (66.7–75.4%) (Tables 4 and 5).
Despite controlling for the number of events per night, overlap between normal and abnormal sessions did not disappear. This modification resulted in a marginal decrease in diagnostic accuracy (66.7–73.9%) (Table 5).
Controlling for the disassociation between the tip and base activity units slightly increased the diagnostic accuracy (67.8–75.9%) (Table 5).
Correcting the problem of uncoupling between rigidity and tumescence resulted in lowered diagnostic accuracy (63.5–64.2%) (Table 5).
Since the introduction of the new RigiScan objective parameters, rigidity and tumescence activity units in 1994, only three studies have evaluated their potential to simplify the interpretation of the RigiScan results.7, 8, 9 Levine and Carroll7 studied these units in RigiScan recordings of 44 men with normal sexual function but not on patients with ED. They obtained only a cumulative distribution curve for these parameters but did not determine a cutoff value to differentiate between normal and abnormal readings. They suggested that the use of these cumulative distributions of tumescence and rigidity activity units provides a convenient way to rapidly compare the percentile ranking of other NPT and rigidity readings with the studied normal population. Furthermore, they recommended examining these newly developed measurements in a population of men with ED.
Benet et al.8 studied only 80 sessions of 40 patients and obtained cutoff values with good diagnostic accuracy (ranged from 87.5 to 92.5%). When using the average readings, they found the best overall predictor of diagnosis to be the tip TUA (accuracy 92.5%). They found that three additional parameters had high predictive ability: base and tip RUA and base TUA (accuracy 90%). However, owing to the relatively small number of patients studied, they recommended another study on a second independent group of subjects to obtain a true estimate of the predictive ability of these new parameters. When we used the cutoff values they proposed in their study on our larger group of sessions, the diagnostic accuracies for the best event ranged from 61.2 to 66.5% and for the total night from 50.2 to 57.3%. Hatzichristou et al.9 studied these parameters in only 12 young males with normal sexual function, and thus could not give cutoff values to differentiate between normal and abnormal sessions.
This study was conducted on a large number of sessions (639 sessions) in a total of 416 patients with ED. According to the classic interpretation, using the currently available evaluation criteria, we had 262 normal sessions and 377 abnormal sessions. The RAU and TAU values obtained from the summary provided by the new Rigiscan Plus software were subjected to further statistical analysis to obtain cutoffs for these new measurements and their predictive values. This large number of sessions may allow us to obtain a more accurate estimate of the predictive ability of these parameters that can be generalized to other groups of patients. We tested these new measurements (RAU and TAU for the base and tip) against the currently available diagnostic criteria (base and tip rigidity 70%, increase in the tip and base tumescence 2 and 3 cm, respectively, maintained for 10 min). For the single best event, we reached a diagnostic accuracy ranging from 67.8 to 73.7% (Table 2). When repeating the above test for the total night, the diagnostic accuracies were also low (66.7–75.4%).
Four new calculations were used in an attempt to improve the diagnostic accuracy obtained, that is, normalized units, standardized units, summated units and R/T ratio. However, none of these new calculations raised the diagnostic accuracy above 75.9% (Table 5). A high diagnostic accuracy could not be obtained in this study, neither by studying activity units of the single best events nor of the total night events, or after trying further calculations as described above. The relatively low values of diagnostic accuracies can be explained by the overlap of values of activity units between normal and abnormal sessions as shown in Figure 1. This overlap is attributed to the fact that activity units are the product of elapsed time during a detected erectile event multiplied by the associated rigidity or increase in tumescence. Therefore, events with abnormal rigidity (that is, <70%) but with long duration can have the same or even higher values of RAU than events with normal rigidity, but with relatively short duration (Figure 2).
In conclusion, on the basis of the above results and after using the most appropriate cutoff values, the diagnostic accuracy of RAU and TAU did not exceed 75.4% and reached a maximum of 75.9% after using the new calculations. Thus, one can conclude that the new software, RigiScan Plus, and its new output, RAU and TAU, are not sufficient to differentiate between normal and abnormal NPT sessions and cannot replace the currently used RigiScan criteria of normality. However, these activity units have high utility as a research tool to quantitatively compare potency of different etiological groups of ED and as a clinical follow-up tool to compare potency of an ED patient before and after ED treatments.
Conflict of interest
The authors declare no conflict of interest.