Penile shear-wave elastography predicts the outcome of botulinum neurotoxin (Botox) in the management of non-responders to phosphodiesterase-5-inhibitors: A pilot study

ABSTRACT

Objective

Botulinum neurotoxin (BoNT) was introduced as a minimally invasive treatment option for erectile dysfunction (ED). However, there is no proven method that predicts improvement after BoNT injection. Shear wave elastography is an emerging imaging modality that evaluates tissue stiffness. This study aims to assess the reliability of SWE in predicting BoNT response in patients with ED and phosphodiesterase 5 inhibitors (PDE-5i) non-responders.

Methods

This study comprised 20 men presenting with ED and PDE-5i non-responders. Mean tissue stiffness values (TSVs) were measured by SWE, international index of erectile function (IIEF-5), erection hardness score (EHS), sexual encounter profile questions 2 and 3 (SEP-2 and SEP-3), and global assessment questionnaire (GAQ) were evaluated before and after 100 IU of BoNT injection by 6 and 12 weeks.

Results

In comparison with baseline, there was a significant improvement in IIEF-5 and EHS scores after BoNT injection at 6- and 12 weeks (p < 0.001). Similar improvements were observed in SEP-2&3 (p < 0001 & p = 0.001, respectively) and GAQ-1&2 (p = 0.008 & p = 0.006, respectively). We found that mean SWE in a penile flaccid state can predict failure of clinically significant improvement after BoNT injection using a cutoff point of 12.7 kPa (sensitivity = 100%, specificity = 54%, AUC = 0.86, p = 0.014).

Conclusion

This finding could be applied to avoid unnecessary BoNT injections in men with ED and PDE-5i non-responders.

Introduction

Erectile dysfunction (ED) is defined as failure to attain or maintain an erection for satisfactory sexual performance [1]. Lifestyle modification and Phosphodiesterase-5 inhibitors (PDE-5i) are considered first-line treatment options for the management of ED. Patients who fail to respond to the maximum tolerable dose of PDE-5i on four different occasions under the optimal conditions of timing and hormonal levels are considered PDE-5i non-responders [2]. In PDE-5i non-responders, different treatment modalities are available, including prostaglandin E1 (PGE1) intra-cavernosal injections (ICI), low-intensity shockwave therapy, and penile prosthesis implantation [3].

Botulinum neurotoxin (BoNT) has been introduced as a minimally invasive treatment option for erectile dysfunction (ED) [4]. BoNT is produced by anaerobic bacteria termed Clostridium botulinum, with seven different serotypes (BoNT A-G) [5]. BoNT serotype A is the most common form used clinically, with successful application in the management of overactive bladder syndrome, neurogenic detrusor over-activity, blepharospasm, achalasia, and hyperhidrosis [6]. Prolonged cavernosal smooth muscle tone reduction has been suggested to underlie the observed clinical improvement after BoNT injection in ED patients [4]. BoNT facilitates smooth muscle relaxation by inhibiting the presynaptic release of pro-contractile norepinephrine from adrenergic neurons [7,8]. Furthermore, BoNT inhibits acetylcholine release from cholinergic neurons and nitric oxide (NO) synthase, leading to increased NO production and promoting cavernosal smooth muscle relaxation [8]. After a human pilot trial for the management of ED by BoNT [9], the efficacy and safety of this treatment modality were further confirmed by randomized controlled trials and a meta-analysis [9–11]. However, no proven method could predict clinical improvement after BoNT injection.

Elastography is an emerging class of medical imaging that evaluates tissue stiffness. The two main types of elastography are strain elastography and shear wave elastography (SWE). The results of strain elastography are semi-quantitative and operator-dependent methods. In contrast, SWE is a quantitative measure of tissue stiffness. SWE has been applied to several hepatic, testicular, prostatic, and thyroid pathologies with promising results [12]. Tissue stiffness value (TSV) measured by SWE was found to be significantly higher in ED patients in comparison with controls, reflecting different numbers and tone of cavernosal smooth muscles and content of collagen fibers in corpora cavernosa [12–14].

This study aims to assess SWE predictability for clinical improvement after BoNT in managing men with ED and PDE-5i non-responders.

Patients and methods

This prospective study comprised 20 men presenting with ED and PDE-5i non-responders. It was registered under protocol record NCT06044883 and approved by the local research ethical committee under code MS-191-2023. A written informed consent was obtained from all participants.

All included patients have previously failed to achieve or maintain a rigid erection for at least four consecutive occasions using the maximum tolerable doses of PDE-5i. Men with a previous history of priapism, penile surgery, Peyronie’s disease, hormonal abnormalities, psychogenic ED, and neurogenic ED were excluded.

Each patient received 100 U of Onabotulinumtoxin-A (Botox™; Allergan Pharmaceuticals, NJ, USA) diluted in 2 ml of normal saline. Doses were distributed to four quarters along the proximal and distal right and left sides of the shaft of the penis using two 1 ml syringes equipped with a 31-gauge needle after the application of a basal tourniquet. Patients were instructed to resume sexual activity after 1 week using the maximum tolerable dose of PDE5-i. International Index of Erectile Function (IIEF-5), Sexual Encounter Profile questions 2 and 3 (SEP-2 and SEP-3), and Erection Hardness Score (EHS), penile hemodynamic parameters using duplex ultrasound and mean tissue stiffness value (TSV) using SWE were measured before and after BoNT injection by 6 and 12 weeks.

Ultrasonographic, duplex, and elastography measurements were done using Aplio 500 (Toshiba Medical Systems, Japan) with a linear probe (7.5 − 13 MHz). Penile duplex ultrasound was done to evaluate hemodynamic parameters: peak systolic velocity (PSV), end diastolic velocity (EDV) and resistive index (RI). The arithmetic means of PSV, EDV, and RI for both the right and left sides were calculated and recorded before and after ICI at 5–20-min intervals. Induced erection during penile duplex and SWE was done using 20 µg PGE-1 (Alfaprostin, Rotabiogen Pharmaceuticals, Egypt). Penile cavernosal elastography measurements were obtained before PGE-1 injection and at 20 min post-PGE-1 injection. SWE Measurements were taken in the longitudinal plane at the mid-third of the penis. Three regions of interest (ROI) circle area elastography measurements were taken for each corpora cavernosa, and the arithmetic mean of the six readings was calculated. Mean TSV was measured in kilo Pascal (kPa), with a higher value referring to a greater stiffness as described by Cheng et al. [15].

Statistical analysis

In this pilot study, statistical analysis was conducted using the 27th edition of SPSS (SPSS Inc., Chicago, IL, USA); categorical variables were presented in count and percent, paired comparison of categorical variables at baseline, 6 weeks, and 12 weeks after Botox injection was conducted using Fisherman’s test. Quantitative variables were presented in mean, standard deviation, minimum, and maximum, and paired comparisons of baseline, 6-week, and 12-week findings were conducted using the Cochrane Q test. Any p-value of <0.05 was considered significant.

Results

This study comprised 20 men with acquired ED refractory to PDE-5i, with a mean age of 57.2 ± 8.1 years. The mean duration of the complaint was 7.4 ± 5.3 years. Around a third of included patients had diabetes mellitus, 7/20 (35%). The peak response after treatment in terms of improvement of IIEF-5, EHS, SEP2–3, and GAQ2–3 was observed in 6 weeks of follow-up, followed by a decline in the same parameters after 12 weeks, as described in Table 1. No considerable side effects have been observed throughout the study duration.

Table 1.

Comparison of IIEF-5, EHS, SEP2, SEP3, GAQ-1, and GAQ-2 score across study duration.

		Baseline	6 weeks follow-up	12 weeks follow-up	p value
IIEF-5 (Mean ± SD)		8.6 ± 3.2	13.2 ± 3.1	10.4 ± 3.1	<0.001
EHS (Mean ± SD)		1.3 ± 0.7	2.5 ± 0.9	1.5 ± 0.8	<0.001
SEP2 positive responders [No. (%)]		6 (30%)	17 (85%)	9 (45%)	<0.001
SEP3 positive responders [No. (%)]		1 (5%)	10 (50%)	2 (10%)	0.001
GAQ-1 positive responders [No. (%)]		0	17 (85%)	9 (45%)	0.008
GAQ-2 positive responders [No. (%)]		0	10 (50%)	2 (10%)	0.006

IIEF-5, International Index of Erectile Function; EHS, Erection Hardness Score.

SEP, Sexual Encounter Profile questionnaire; GAQ, Global Assessment Questionnaire.

Follow-up using conventional penile duplex parameters illustrated significant improvement in PSV and RI after 5 and 20 min of ICI by 20 µg PGE1 but not in non-PGE1-induced PSV (i.e. basal PSV), as observed in Table 2.

Table 2.

Comparison of penile duplex findings at baseline versus 6- and 12-weeks after BoNT injection.

	Baseline	6 weeks follow-up		12 weeks follow-up			p value
	mean ±SD	min-max	mean ±SD	min-max	mean ±SD	min-max
Pre PGE1 PSV (cm/s)	9.4 ± 3.3	4.8–17	9.8 ± 2	6.9–13	9.6 ± 1.9	6.9–12.7	0.227
5 min. post PGE1 PSV (cm/s)	20.8 ± 2.6	16.3–24.8	37.2 ± 4.7	30.5–48.1	27.7 ± 3.8	21.5–34.7	<0.001
5 min. post PGE1 EDV (cm/s)	1.7 ± 3.1	−4.9–5.7	1.9 ± 3.5	−4.9–11.1	1.6 ± 3.7	−7.5–9.5	0.284
5 min. post PGE1 RI	0.91 ± 0.14	0.69–1.2	0.94 ± 0.09	0.7–1.12	0.93 ± 0.13	0.65–1.21	0.008
20 min. post PGE1 PSV (cm/s)	25.1 ± 3.5	19.3–34.1	47.8 ± 7.9	36.7–64.2	38.8 ± 4.9	31.4–47.6	<0.001
20 min. post PGE1 EDV (cm/s)	2.3 ± 3.7	−5.1–8.4	2.3 ± 4.1	−5.2–12.9	2.4 ± 4	−5.1–11.6	0.369
20 min. post PGE1 RI	0.89 ± 0.15	0.68–1.19	0.95 ± 0.09	0.72–1.1	0.93 ± 0.11	0.67–1.12	0.01

PGE1, Prostaglandin E1; PSV, Peak Systolic Velocity; EDV, End Diastolic Velocity; RI, Resistive Index.

In the flaccid state, mean tissue stiffness values (TSVs) as measured by SWE showed significant reductions in the 6- and 12-week follow-up after BoNT injection. Similar improvements were observed during PGE1-induced erection, as illustrated in Table 3.

Table 3.

Comparison of tissue stiffness values measured by shear wave elastography at baseline versus 6- and 12-weeks after BoNT injection.

	Baseline		6 weeks post Botox		12 weeks post Botox		p value
	mean ± SD	min-Max	mean ± SD	min-max	mean ± SD	min-max
Mean tissue stiffness values (kPa) pre-PGE1 ICI	22.57 ± 12.1	8.3–46.5	19.66 ± 8.2	10.32–38.3	20.9 ± 9.8	9.6–42.8	0.041
Mean tissue stiffness values (kPa) 20 min. post-PGE1 ICI	9.5 ± 2.5	5.1–14.3	8.3 ± 1.8	5.2–11.2	8.7 ± 1.7	5–11.7	0.026

PGE1, Prostaglandin E1; kPa, Kilopascal; ICI, Intra-Cavernosal Injection.

After the BoNT injection, 7/20 (35%) regained an erection sufficient for vaginal penetration by using maximum tolerable PDE-5i doses. A mean TSV value in a flaccid state of >12.7 kPa was found predictive of failure of regaining erection after BoNT injection with the aid of a maximum tolerable dose of PDE-5i (sensitivity = 100%, specificity = 54%, AUC = 0.86, p = 0.014). In contrast, mean TSV in PGE1-induced erection was not a significant predictor of regaining PDE-5i-induced erection after BoNT injection (AUC = 0.53, p = 0.8), as described in Figure 1.

Figure 1.

ROC curve showing predictors for outcome of BoNT injection.

Discussion

This study demonstrates that in men with ED and PDE-5i non-responders, clinical improvement after BoNT injection could be predicted by SWE. The higher the value of mean tissue stiffness measured by penile SWE, the less likely to regain an erection sufficient for vaginal penetration by using maximum tolerable PDE-5i doses after treatment with BoNT injection. This finding could be clinically applied to patient selection for treatment by intra-cavernosal BoNT injection.

BoNT was introduced as a treatment modality for ED by Ghanem et al. in 2016, followed by an animal study and a pilot human study [16]. The current study showed significant improvements in IIEF-5 score from 8.6 ± 3.2 to 13.2 ± 3.1 after 6 weeks and 10.4 ± 3.1 after 12 weeks of ICI by BoNT. Similar improvement was observed in EHS from 1.3 ± 0.7 to 2.5 ± 0.9 after 6 weeks and 1.5 ± 0.8 after 12 weeks. A meta-analysis published in 2022 has further confirmed the beneficial effects of BoNT injection over placebo in terms of clinical improvement measured by EHS with a mean difference of 0.7 from the baseline [10]. However, this meta-meta-analysis highlighted an insignificant improvement in terms of IIEF-5, which could be explained by the heterogeneity and small number of cases in the included studies. In a randomized placebo-controlled study, significant improvements in IIEF-5 from a mean of 5.4 ± 1.7 at baseline to 6.7 ± 2.2 and 9.97 ± 5.9 after 2 and 6 weeks, respectively, of 100 units of BoNT injection. EHS also improved significantly from 2.3 ± 0.6 to 2.89 ± 0.8 two weeks after treatment [9]. In a three-arm randomized placebo-controlled study comparing the effect of BoNT 50 and 100 units vs. placebo by El-Shaer et al., significant improvements in both IIEF-5 and EHS in a follow-up period of 2 weeks 3-and 6 months after treatment in compared with the placebo group [11]. A later sizeable retrospective study demonstrated an IIEF-5 categorial improvement of ED severity using repeated BoNT intra-cavernosal injections. This improvement ranged between 64% and 95% of the included cases [17].

Penile rigidity is affected by the tone and number of cavernosal smooth muscles and the percentage of collagenous fibers in corpora cavernosa [18]. This composition varies between men with and without ED [19]. A significant negative correlation was confirmed by animal model studies between tissue stiffness measured by SWE, smooth muscle content of corpora cavernosa [20] and also with the collagen content of corpora cavernosa [21,22]. A single human study failed to confirm a significant correlation between cavernosal smooth muscle cells, elastic fibers, and SWE findings, but this study was limited by a small sample size (n = 12) [23]. SWE was investigated to compare the tissue stiffness between ED patients and controls [12–15]. Turkey et al. observed a significant difference in SWE measurements between men with ED (defined by IIEF-5 score <17) and healthy volunteers. A cut-off value of 17.1 kPa was found to differentiate both groups by SWE [12]. In another study, a lower cut-off value of 8.05 kPa in an erect state was found to differentiate vascular and non-vascular causes of ED [14]. This difference could be explained by the decline of SWE measurements in erectile state, as confirmed by our study. Illiano et al. concluded in a prospective bicentric study, that elasticity of the corpora cavernosa decreases with worsening IIEF-5 and EHS scores [18]. In contrast, Cheng et al. found that EHS was not correlated with SWE measurements of corpora cavernosa but with tunica albuginea of the penis in a controlled study [15].

During the follow-up period of this study, there was a significant increase in both PSV and RI after PGE-1 injection with penile duplex evaluation. This observed improvement could be explained by a synergistic effect between cavernosal smooth muscle relaxation by BoNT and the vasodilator effect of PGE-1 on cavernosal arteries. These findings are in coordination with Abdelrahman et al. observation of improvement in post-PGE-1 PSV after 2 weeks of BoNT treatment [9].

Our study applied SWE as a diagnostic tool for predicting erectile function improvement after BoNT injection in ED and PDE-5i non-responders cases. A mean tissue stiffness value (TSV) in a flaccid state of >12.7 kPa was found to be predictive of poor outcomes after BoNT treatment and failure to regain PDE-5i-induced erection after BoNT injection. Men with higher TSV are less likely to benefit from BoNT ICI due to the advanced deterioration of smooth muscles and collagenous content of corpora cavernosa. In our study, seven patients comprising 35% of the study population regained an erection enough for vaginal penetration by using maximum tolerable PDE-5i doses. In a randomized controlled study by Abdelrahman et al. [9]18/35 (53%) patients in the BoNT treatment arm achieved a quality of erection sufficient for successful intercourse. This difference in outcome could be explained by the fact that our study population had a lower EHS at baseline (1.3 ± 0.7) compared with 2.3 ± 0.6 in the Abdelrahman et al. study.

A small sample size limits this study. However, the observed significant findings may encourage further studies with larger study populations and different BoNT doses. Moreover, we used a fixed dose and BoNT brand, which may limit the generalizability of the concluded outcome. However, one study found no significant difference between 50- and 100 units of BoNT in terms of efficacy after 3 months of injections, but 100 units were found to have a more prolonged effect than 50 units [11].

In conclusion, penile SWE could be used to evaluate cavernosal tissue integrity and patient selection before BoNT injection in men with ED and PDE-5i non-responders. This finding could be applied to avoid unnecessary BoNT injections in men with terminal forms of ED who are less likely to benefit from this treatment modality. Integrating SWE into Andrological clinical practice may enhance treatment monitoring and optimize outcomes for individuals with ED as the field advances.

Funding Statement

This study was fully funded by Kasr Al-Ainy Teaching Hospital, Faculty of Medicine, Cairo University.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Author contributions

M W Ragab and A M Albohy designed this study. M W Ragab, A Z Bendary, and K Shawky wrote the manuscript, which A M Albohy edited. M W Ragab was involved in patient acquisition and BoNT injection. K Shawky performed ultrasonography under the observation of A Z Bendary and A M Albohy.