Erectile dysfunction (ED), difficulty in obtaining or maintaining an erection to complete sexual intercourse, is found in approximately 40% of men. ¹ Because of the existing cultural factors, reporting bias, and patient considerations, such as shame and embarrassment, the true prevalence of ED is yet to be determined. ED may occur due to various etiologies and is often multifactorial. ² Establishing ED diagnosis and differentiating between psychogenic and organic ED is imperative for physicians. ^{3, 4}

Diagnostic approaches in ED remain challenging, as information obtained from patient history and physical examination may not suffice. Supplemental examinations may be needed, and non-invasive tests are generally preferred. Therefore, the scoring system and questionnaire are vital for assessing ED severity and treatment progress. ⁵

The International Index of Erectile Function (IIEF) is a widely accepted patient-reported outcome measure (PROM) to evaluate ED. The IIEF aims to assess several domains of male sexual function, consisting of erectile function, orgasmic function, sexual desire, intercourse satisfaction, and overall satisfaction. ⁶ A shorter version of the five questions, known as the IIEF-5, was developed to assess patients’ erectile function and sexual intercourse satisfaction. The lower the IIEF-5 scores, the higher the severity of ED. ⁷

Nocturnal penile tumescence and rigidity (NPTR) monitoring with RigiScan device has been considered one of the most reliable methods to differentiate psychogenic erectile dysfunction (pED) from organic ED and to diagnose ED in general. ⁸ Nonetheless, NPTR monitoring has shortcomings, as previously reported. ^{9, 10} Furthermore, the main disadvantages of NPTR assessment include its high cost, intricate nature, and limited ability to yield consistent results.

Other means of ED diagnostics incorporate laboratory tests such as hypothalamic-pituitary-gonadal axis evaluation, including testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH), lipid profile, blood glucose, and thyroid function tests. ¹¹ However, as non-invasive and cost-efficient diagnostic evaluations are preferred, the IIEF could be an alternative diagnostic tool to assess ED. Therefore, this study aimed to compare the parameters of NPTR testing with the domain score of the IIEF to determine the sufficiency of the IIEF in diagnosing ED.

This was a retrospective cohort study to determine the correlation between the IIEF-5 questionnaire and NPTR parameters. The data were collected retrospectively through a database.

A total of 138 men presenting with ED between August 2017 and March 2023 who had undergone NPTR assessment were included in the study. The inclusion criteria were complaints of ED for at least 3 months without medical treatment. Men with depression, anxiety, and psychological problems, or those on antidepressant, antipsychotic, or anxiolytic medications, were excluded. Men with poor quality of sleep were also excluded from the study. All the patients were evaluated for complete medical histories, sexual histories, and physical examinations. The IIEF-5 questionnaire was used to evaluate the presence and severity of ED. The IIEF-5 contains five questions on erectile function and intercourse satisfaction. ⁷ This questionnaire is a non-proprietary tool that is widely used in clinical practice, including in our hospital, as a reliable and routine method for assessing erectile dysfunction severity.

Ethical approval was obtained retrospectively from the Health Research Ethics Committee, Faculty of Medicine, Universitas Indonesia, and Cipto Mangunkusumo National General Hospital (No: KET-1624/UN2.F1/ETIK/PPM.00.02/2023), covering the use of patient data recorded in routine clinical care prior to study initiation. Study was conducted per the Declaration of Helsinki. As this study involved retrospective analysis of de-identified medical records, the ethics committee waived the requirement for individual informed consent. All patient data used in this study were collected as part of routine standard care, and no identifying information was included in the analysis to ensure confidentiality.

A total of 139 men with ED were included in the study. The median patient age was 42 years. The IIEF-5 scores ranged between 0 and 24, with a median of 11. Comorbidities included dyslipidemia in 27 patients (20%), prediabetes in 65 patients (47%), and diabetes in 16 patients (12%).

The Kolmogorov–Smirnov normality test showed that all numeric variables of NPTR measurements did not have a normal data distribution ( Table 2). Pearson’s correlation for the IIEF scores vs. NPTR variables showed significant correlations with the number of erections recorded r=0.188 (95% confidence interval (CI), 0.017–0.348; P=0.026), base tumescence increment r=0.198 (95% CI, 0.027–0.357; P=0.020), average base tumescence r=0.185 (95% CI, 0.014–0.346; P=0.029), and duration of base erection with more than 60% rigidity r=0.198 (95% CI, 0.027–0.357; P=0.020). No correlations were observed between the IIEF scores and other NPTR variables ( Table 3).

Pearson’s correlation for HbA1c vs. NPTR variables showed significant correlations with the number of erections recorded r=-0.280 (95% CI; P<0.01), number of erections with normal tip duration r=-0.238 (95% CI; P<0.05), tip tumescence increment r=-0.269 (95% CI; P<0.05), average tip rigidity r=-0.336 (95% CI; P<0.01), duration of tip erection with > 60% rigidity r=-0.277 (95% CI; P<0.01), number of erections with normal base duration r=-0.249 (95% CI; P<0.05), and duration of base erection with >60% rigidity r=-0.262 (95% CI; P<0.05). No significant correlations were observed between NPTR variables and other blood parameters (total cholesterol, vitamin D 25(OH), and testosterone) ( Table 4).

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.

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. ¹² 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. ¹³ 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. ¹⁴ 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

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. ¹⁵

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%. ^{15– 17} In another investigation, the study examined the sensitivity, specificity, positive predictive value, and accuracy rate of Rigiscan device monitoring in differentiating organic and pED. ¹⁸ 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. ¹⁸

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. ¹³

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. ¹⁹ 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.

The IIEF questionnaire was initially created not for diagnostic purposes but as a tool to track treatment outcomes over time. ¹⁵ Moreover, IIEF and similar self-assessment questionnaires cannot discriminate among different etiologies of ED. ^{20, 21} 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. ²² Their findings showed no significant differences in IIEF scores among patients with normal vascular responses, arterial insufficiency, or venous leakage.

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. ²³ 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. ²⁴

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. ²⁵ 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. ²⁶

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. ²⁷ 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. ²⁸ 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. ²⁹

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. ³⁰ 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. ³¹ 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. ³² Furthermore, a study by Dumbraveanu et al. investigated the correlations of clinical and biochemical indices of vitamin D with ED. ³³ 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. ³⁴ 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. ³⁴ 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. ³⁵ A prospective cross-sectional pilot study found that testosterone levels were weakly associated with penile rigidity and disappeared when associated with metabolic syndrome. ³⁶

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 retrospective 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 retrospective data and potential variations in recording practices might have affected the reliability of the results.

Despite its limitations, this study provides insights into IIEF-5 and NPTR correlations within a specific cohort. Current research suggests that subjective questionnaires, including the IIEF-5, can be utilized in the diagnosis of ED, particularly in settings where advanced diagnostic tests, such as the NPTR assessment, are not available. We observed a correlation between IIEF questionnaire scores and the frequency of nocturnal erections, as well as the tip aspects of these nocturnal erections. Additionally, we identified a significant correlation between HbA1c levels and nocturnal erections. The method used to diagnose ED should be personalized and multidisciplinary. The primary objective should be to conduct a thorough assessment of erectile function, and NPTR assessment, which was used to monitor NPT, remains a valuable diagnostic tool for ED. Although the IIEF can facilitate cost-effective diagnoses, it should not be considered as a substitute for objective testing.

Ethical approval for this study was obtained from the Health Research Ethics Committee, Faculty of Medicine, Universitas Indonesia, and Cipto Mangunkusumo National General Hospital. The approval number is KET-1624/UN2.F1/ETIK/PPM.00.02/2023, and the approval was granted on November 17, 2023.

The study was conducted in accordance with the ethical principles outlined in the Declaration of Helsinki. As the study involved retrospective analysis of anonymized medical records collected during routine care, the ethics committee waived the requirement for individual informed consent. To ensure confidentiality, all patient data were de-identified prior to analysis.

This study utilized retrospective data from patient medical records collected during routine clinical care. Ethical approval was obtained from the Health Research Ethics Committee, Faculty of Medicine, Universitas Indonesia, and Cipto Mangunkusumo National General Hospital (Approval No: KET-1624/UN2.F1/ETIK/PPM.00.02/2023). The committee waived the requirement for individual informed consent as the data were anonymized and de-identified prior to analysis to ensure patient confidentiality. No additional data collection or interventions were performed as part of this study, and all analyses were conducted in compliance with the Declaration of Helsinki.