Primary Research on Correlation between Erectile Function Assessment through International Index of Erectile Function Score and Nocturnal Penile Tumescence and Rigidity Measurements in Men with Erectile Dysfunction

Nocturnal Penile Tumescence and Rigidity (NPTR)

NPTR was evaluated using the Rigiscan^® device with Rigiscan Plus software (Timm Medical Technologies Inc., USA). The data collected from the NPTR examinations included the number of erections recorded, number of erections with normal tip duration, number of erections with normal tip tumescence, number of erections with adequate tip criteria, tip tumescence increment, average tip rigidity, duration of tip with > 60% rigidity, number of erections with normal base duration, number of erections with normal base tumescence, number of erections with adequate base criteria, base tumescence increment, average base rigidity, and duration of base with > 60% rigidity. Blood samples were collected before the NPTR examination and analyzed for total cholesterol, HbA1c, vitamin D-25(OH), and testosterone.

Software availability

The Rigiscan device operates with the proprietary Rigiscan Plus software, which is included with the purchase of the device from Timm Medical Technologies Inc., USA. This software is essential for collecting and analyzing Nocturnal Penile Tumescence and Rigidity (NPTR) data. As this software is proprietary and bundled with the device, no free or open-source alternatives are available for equivalent functionality.

The Rigiscan device and its software are widely used in clinical practice as standard tools for erectile dysfunction assessment, and their use in this study complies with institutional clinical protocols.

Statistical analysis

All statistical analyses were performed using SPSS 27.0 software (IBM, Armonk, NY, USA). All continuous data are presented as meaminus SD for normal distributions and medians (min-max) for data, not within the normal distribution. The normal distribution of variables was analyzed using the Kolmogorov–Smirnoff sample test. Table 1 summarizes the patients’ characteristics. The correlation between the IIEF-5 score and NPTR measurements was performed using Pearson bivariate analysis for normally distributed data or Spearman bivariate analysis for skewed distribution. A p<0.05 indicated statistical significance. Further bivariate analysis was performed to analyze the correlation between blood parameters and NPTR measurement.

Correlation between IIEF-5 and NPTR parameters

To our knowledge, this study was the first to explore the association between the IIEF-5 score and specific components of nocturnal erection using NPTR or NPT measurements. Although previous studies have examined the correlation between nocturnal erections and IIEF-5 questionnaire scores, serving as a clinical measure of ED, these studies have primarily categorized nocturnal erections as either “adequate” or “inadequate” and compared them with the IIEF-5 questionnaire scores. The components of nocturnal erections encompass various aspects, including the number of erections, the quality of erections, and aspects related to both the tip and the base of the erection. The present study aimed to compare and correlate these individual aspects with IIEF-5 questionnaire scores.

Overview of NPTR physiology and assessment

Spontaneous nighttime penile erections, initially observed by Halverson et al. in infants during sleep in 1940 and confirmed by subsequent studies, are part of the normal NPTR phenomenon.¹⁴ They involve 3–6 episodes of tumescence during sleep, including at least one instance with a tip rigidity exceeding 60%, lasting 1–15 min over 8 h.¹⁵ These regular NPTR occurrences indicate a healthy vascular and neural supply to the penis and the structural integrity of penile components. NPTR assessment employs seven methods, with the Rigiscan device introduced in 1985.¹⁶ Rigiscan device provides continuous and quantitative measurements of penile circumference and rigidity, helping distinguish between physiological ED and organic cases of ED. This differentiation relies on the assumption that psychological factors do not substantially impact nighttime erectile activity.⁸

Strengths and limitations of NPTR testing

The application of NPT relies on the premise that to generate a nocturnal erection, both the corticospinal nerve signals to the penis and the vascular responsiveness of penile tissue to these signals must remain functional. Nevertheless, achieving an erection during sexual activity also necessitates proper responsiveness to sensory cues. Notably, the inability to assess any deficits in afferent signals originating from the penis is a potential limitation of NPT.¹⁷

Consequently, NPTR has been subjected to validation procedures recently. The evolution of the Rigiscan Plus software has led to a standardized approach to NPTR examinations, thereby enhancing sensitivity and specificity and allowing for the precise identification of erections possessing the requisite rigidity and duration for engaging in sexual intercourse. Empirical studies have reported sensitivities ranging from 42% to 85% and specificities ranging from 93% to 100%.^17–19 In another investigation, the study examined the sensitivity, specificity, positive predictive value, and accuracy rate of Rigiscan device monitoring in differentiating organic and pED.²⁰ These assessments were made in the context of a comprehensive diagnostic approach incorporating advanced techniques, and the results indicated values of 81%, 82%, 89%, and 81%, respectively, for NPTR measurements using the Rigiscan device.²⁰

However, the utilization of NPTR has several limitations, as highlighted in previous studies. One study that employed NPT with the Minnesota Multiphasic Personality Inventory for diagnosing primary organic or primary psychogenic ED reported a misclassification rate of 63%.⁹ Certain specific populations further complicate the accuracy of NPT assessment, with a decreased diagnostic accuracy observed in men experiencing depression.¹⁰ Additionally, the absence of universally accepted normative data for interpreting NPTR readings makes it challenging to reproduce the assessments.¹⁵

The utilization of NPTR assessment presents a substantial drawback owing to its cost, complexity, and relatively low reproducibility. Moreover, it is worth considering that endocrine, vascular, and neurological diagnostic techniques offer cost-effective alternatives and can identify organic causes of ED to resolve ED issues.²¹ These limitations prompt us to question whether standardized questionnaires, such as the IIEF-5, may suffice as more cost-effective diagnostic measures in conjunction with laboratory investigations. Consequently, our primary aim was to explore the correlation between the IIEF-5 questionnaires and specific parameters within NPTR measurements.

Diagnostic value and limitations of IIEF-5

The IIEF questionnaire was initially created not for diagnostic purposes but as a tool to track treatment outcomes over time.¹⁷ Moreover, IIEF and similar self-assessment questionnaires cannot discriminate among different etiologies of ED.^22,23 Nevertheless, examining its association with objective assessments like NPTR is intriguing. Kassouf et al. examined the diagnostic utility of the IIEF-5 in identifying the vascular causes and severity of ED, comparing it with pharmacological testing and duplex Doppler ultrasonography.²⁴ Their findings showed no significant differences in IIEF scores among patients with normal vascular responses, arterial insufficiency, or venous leakage.

Correlation between IIEF-5 and NPTR findings

The median number of erections recorded in our study was three, which is consistent with previous studies that reported a similar number of erection episodes in patients without diabetes.²⁵ Our findings revealed a significant correlation between the IIEF-5 questionnaire and the number of recorded nocturnal erections. These findings bolster the hypothesis that a reduced number of nocturnal erections may manifest as subjective erectile symptoms.

Our findings indicate that the base aspects of penile erections are the most strongly correlated with the IIEF-5 score, with no significant correlation observed for tip erections. In contrast, Lee et al. (2021) analyzed the potential link between lower urinary tract symptoms (LUTS) associated with benign prostatic hyperplasia (BPH) and erectile function in eugonadal men and demonstrated that the rigidity activity unit in the penile tip emerged as the most statistically significant parameter in its association with the IIEF score.²⁶

Both tip and base erections are essential in assessing erectile function. However, based on a cross-sectional study on the effect of prolactin on penile erection, the penile base may be more meaningful than the penile tip in assessing the effect of prolactin on erectile function.²⁷ Another cross-sectional study on the effect of estradiol on penile erection found no apparent relationship between erection time at the penile tip and estradiol levels. However, a negative correlation was observed between base erection time and estradiol levels.²⁸

Association with metabolic and hormonal parameters

In the context of blood parameters, this study determined that only HbA1c levels displayed a statistically significant correlation with several NPTR parameters. Several studies have investigated the relationship between diabetes and nocturnal penile tumescence (NPT); NPT test results were abnormal in sexually functional men with diabetes.²⁹ 10 men with diabetes who reported normal daytime sexual function with four nights of polysomnography, including NPT assessment, had significantly diminished NPT profiles compared to an age-matched, nondiabetic, healthy control group.³⁰ Anwar et al. revealed that in ED and diabetes, men with diabetes experiencing severe ED tended to be older and experience more challenges maintaining proper glycemic control.³¹

No significant correlation was observed between NPTR variables and other blood parameters (total cholesterol, vitamin D-25(OH), and testosterone). The two main types of cholesterol are high-density lipoprotein (HDL) and low-density lipoprotein (LDL). Elevated levels of LDL can contribute to the development of ED, whereas HDL cholesterol improves penile erectile function.³² Therefore, the relationship between cholesterol and nocturnal erections may depend on the measured cholesterol type. Furthermore, a study has found that statin drugs improve erectile function in men with hypercholesterolemia.³³ Therefore, statin drugs may mask any potential correlation between cholesterol and nocturnal erections.

Low serum 25-hydroxyvitamin D (25(OH)D) levels were associated with an increased risk of ED in older adults with moderate to severe lower urinary tract symptoms.³⁴ Furthermore, a study by Dumbraveanu et al. investigated the correlations of clinical and biochemical indices of vitamin D with ED.³⁵ The study found that vitamin D level reduction, concomitantly with decreased testosterone and increased cholesterol, contributed to the development and maintenance of ED. Further research is needed to validate this association.

A study investigating the relationship between sleep-related erections and testosterone levels in men found that the serum testosterone threshold for sleep-related erections was lower than the low end of the standard laboratory male range at approximately 200 ng/dL.³⁶ The study also found that participants with higher testosterone serum levels showed higher values for some erectile parameters than those with serum testosterone between 100 and 199 ng/dL, without any significant difference among the groups with testosterone serum levels in the normal range.³⁶ Although some studies suggest a correlation between NPT and testosterone, other studies demonstrated no significant correlation. A review article on the relationship between testosterone and sleep-related erections reported that men with androgen deficiency may have normal NPT, and sleep-related erections increased in response to testosterone administration.³⁷ A prospective cross-sectional pilot study found that testosterone levels were weakly associated with penile rigidity and disappeared when associated with metabolic syndrome.³⁸

Study limitations

The present study had several limitations. First, the study was not population-based. The patients included in the study were individuals seeking treatment at a urologist’s office who reported erectile ED and underwent NPTR testing. Therefore, the current study was prone to selection bias. Second, not all patients were assessed for essential laboratory exams such as HbA1c, cholesterol, testosterone, and vitamin D-25(OH). Another aspect to consider is the use of single NPTR testing in our study, which may not have accurately evaluated nocturnal erectile capacity. Consecutive nocturnal measurements are widely accepted to differentiate pED from organic ED. However, only a single NPTR test was performed in this study to prioritize patient comfort. Moreover, the correlation study design might have introduced biases, and the sample size, constrained by specific criteria and exclusions (e.g., depression, anxiety, poor sleep quality), may have impacted the generalizability of the results. Additionally, reliance on secondary data and potential variations in recording practices might have affected the reliability of the results. We acknowledge that psychosocial and interpersonal aspects were not thoroughly evaluated in this study and should be considered in future research.