Penile extracorporeal low-intensity shock wave therapy (LIST) to the penis has recently emerged as a novel and promising modality in the treatment of erectile dysfunction (ED). LIST has angiogenic properties and stimulates neovascularization. If applied to the corpora cavernosa, LIST can improve penile blood flow and endothelial function. In a series of clinical trials, including randomized double-blind sham-controlled stud-ies, LIST has been shown to have a substantial effect on penile hemodynamics and erec-tile function in patients with vasculogenic ED. LIST is effective in patients who are responsive to phosphodiesterase 5 inhibitors (PDE5i) and can also convert PDE5i non-responders to responders. The response to LIST wanes gradually over time, and after 2 years, about half of the patients maintain their function. Extensive research is needed to understand the effect of LIST on erectile tissue, to modify the treatment protocol to maximize its outcomes, and to identify the patients who will benefit the most from this treatment.
Keywords: Erectile dysfunction; Penile; Shockwave treatment; Therapy
Great advances in the understanding and treatment of erectile dysfunction (ED) have been made in the past two decades. Most patients suffering from ED respond to the available options for treatment, including oral phospho-diesterase type 5 inhibitors (PDE5i) and intracavernosal injections of vasodilating agents. However, these medi-cations are unable to alter the underlying pathophysiology of the erectile mechanism. They are usually taken on de-mand, prior to the sexual act, and do not improve sponta-neous erections . Therefore, a need exists for an effective new treatment concept that would have a durable effect on spontaneous erectile function. Recently, a promising mo-dality was introduced: the application of low-intensity ex-tracorporeal shock wave therapy (LIST) on the penis . Shock waves are acoustic waves that generate a pressure impulse and that carry energy when propagating through a medium. The degree of focus can be modulated non-invasively, resulting in variable concentration of energy at a desired location . When shock waves are applied to an organ, the focused waves interact with the targeted deep tissues and act as transient micromechanical forces that initiate several biological changes . Extracorporeal shock wave therapy has been clinically examined and ap-plied in various medical fields. High-intensity shock wave therapy is used for lithotripsy because of its focused me-chanical destructive nature. Medium-intensity shock waves have been shown to have anti-inflammatory proper-ties and are used for treating a wide range of orthopedic con-ditions, such as nonunion fractures, tendonitis, and bursitis. Low-intensity shock waves probably have angio-genic properties and are used in the management of chronic wounds, peripheral neuropathy, and in cardiac ischemic tissue [1,2,5,6].
When LIST is applied to an organ, the relatively weak yet focused shock waves interact with the targeted deep tis-sues where they cause mechanical stress and micro-trauma, also known as shear stress . This shear force then triggers a chain of events that cause the release of an-giogenic factors inducing neovascularization of the af-fected tissues and enhancing the blood flow. Extrapolation of these findings to ED has led to the assumption that if ap-plied to the corpora cavernosa, LIST could improve penile blood flow and endothelial function by stimulating angio-genesis in the penis . This review summarizes the effect of LIST on erectile tissue as well as the clinical evidence of its effect in patients with vasculogenic ED.
POTENTIAL MECHANISM OF SHOCK WAVE THERAPY
Research on the biological changes that result from LIST has mainly been focused on angiogenesis and local neovascularization. More than a decade ago, Wang et al.  revealed that LIST stimulates the expression of angio-genesis-related growth factors, such as endothelial nitric oxide synthase (NOS), vascular endothelial growth factor (VEGF), and endothelial cell proliferation factors, e.g., pro-liferating cell nuclear antigen (PCNA). This group also re-ported that LIST induces neovascularization and cell proliferation. This effect is evident 4 weeks after treatment and persists for more than 12 weeks . Moreover, previous studies on transgenic mice revealed an up-regulation of VEGF receptor 2, which is considered a primary mediator of angiogenic signaling. Quantitative immuno-histo-logical studies evaluating ischemic tissue vessel formation showed higher vessel densities in the shock wave-treated groups [6,9-12].
In recent years, LIST has also been studied in regard to its effect on stem cell recruitment in tissue repair. Stem cells and progenitor cells have the ability to divide and dif-ferentiate into specialized cell types. Their key role in the neovascularization of ischemic tissues was widely studied in recent years . One example is the circulating progeni-tor cells (CPCs). During acute ischemia, the release of che-moattractant factors (i.e., VEGF) acts as a homing factor for CPCs that may contribute to the ongoing endothelial re-pair . Aicher et al.  investigated the effect of LIST on the homing of infused human CPCs in rats with chronic hind limb ischemia. In their study, LIST-treated ischemic muscles attracted significantly more labeled CPCs than did the untreated muscles. The treatment significantly in-creased the blood flow in the ischemic muscles. Nishida et al.  studied LIST in a porcine model of ischemia-induced myocardial dysfunction and found that LIST significantly up-regulated VEGF expression in ischemic myocardium and improved regional myocardial blood flow and left ven-tricular ejection fraction. Further studies have also demon-strated beneficial effects of LIST on left ventricle remodel-ing in pigs with experimentally induced myocardial in-farction  and myocardial ischemia-reperfusion injury .
CLINICAL APPLICATIONS OF LIST
Encouraging results in animal model have set the ground for future studies in humans. Several studies have shown positive effects of LIST in human patients with severe is-chemic heart disease. Kikuchi et al.  reported that LIST improved the ejection fraction and stroke volume and sig-nificantly improved chest pain after a 6-minute walk test. Similar results have also been reported by other groups [18-20] in patients with severe coronary artery disease and refractory angina to whom LIST was applied. Following the promising results in patients with ischemic heart dis-ease, other reports on the capability of LIST were published with positive results on a wide range of ischemic wounds and ulcers [21-24].
LIST FOR ED
Because one of the underlying functional causes of ED is poor cavernosal arterial blood flow, it has been suggested that induction of neovascularization by LIST could im-prove cavernosal arterial flow, which in turn would im-prove erectile function . The first study of the efficacy of LIST for ED was conducted by Vardi et al.  in 2010. In this pilot study, Vardi et al.  applied LIST to 20 mid-dle-aged men with mild to moderate vasculogenic ED who responded well to the use of PDE5i. The treatment protocol was based on the described methodology used in patients with cardiovascular disease  with modifications ac-cording to anatomical differences. The main challenge pre-sented by the authors was to apply shock waves to the whole area of the corpora cavernosa, including the crus. Another challenge was to adapt the shock wave delivery probe to the anatomy of the penis because the corpora cavernosa lies im-mediately under the penile skin. The protocol consisted of two treatment sessions per week for 3 weeks, a 3-week no-treatment interval, and a second 3-week treatment pe-riod of two treatment sessions per week. Each session com-prised the application of 300 shock waves (energy intensity of 0.09 mJ/mm2) to each of five different sites: three along the penile shaft and two at the crura (Fig. 1). The results showed that 1 month after treatment, erectile function im-proved in 15 men (75%). An increase by more than five points in the International Index of Erectile Function Erectile Function (IIEF-EF) domain score was noted in 14 men and an increase of more than 10 points was noted in 7 men. The average increase in the IIEF-EF domain score was 7.4 points. Furthermore, erectile function and penile blood flow were measured by using nocturnal penile tu-mescence (NPT) and flow-mediated dilation methodology (FMD) of the penis. In the 15 men who responded to LIST, all NPT parameters improved as recorded by significant in-creases in the duration of the erections and penile rigidity. Penile blood flow also improved significantly at the 1-month follow-up examination. At the 6-month follow-up visit, 10 men reported that they still had spontaneous erec-tions and did not require PDE5i support.
Following these impressive preliminary clinical results, the effect of LIST on erectile function was examined in an animal model. Several groups investigated the effects of LIST on erectile function in diabetes mellitus rats, using the changes in the intracavernosal pressure following elec-trostimulation of the cavernous nerve to assess erectile function. Qiu et al.  found that erectile function was sig-nificantly decreased in all diabetic rats, but this effect was less evident in the LIST group. Additionally, histological analysis revealed an abundant amount of nNOS-contain-ing nerves, endothelial and smooth muscle cells, and mes-enchymal stem cells (MSCs) in the LIST group compared with untreated diabetic rats. Such findings support the idea that the underlying mechanism of the therapeutic ac-tion of LIST is the recruitment of MSCs, which was first postulated by Chen et al.  almost a decade before. Two more groups have recently examined the effect of ex-tracorporeal shock waves on nerve stimulation-induced erection in rats. Chen and Chang  and Liu et al.  demonstrated that shock wave energy improved erectile function as well as pathological changes in the erectile tis-sue of diabetic rats.
Vardi and colleagues  further extended their clinical study in vasculogenic ED patients and examined LIST as a possible treatment for patients with severe ED who re-sponded poorly to PDE5i therapy. The aim of this second study was to investigate the ability of LIST to convert non-responders to PDE5i therapy to PDE5i responders so that they were able to achieve satisfactory erections for vaginal penetration. This study included 29 men who had multiple cardiovascular disease risk factors, cardiovascular dis-ease, and diabetes mellitus and who did not respond to oral PDE5i therapy. The study results showed that 3 months after the completion of the LIST protocol, the IIEF-EF do-main scores improved by at least five points in 76% of men, and the mean IIEF-EF domain score increased by 10 points. Overall, 72% were converted to PDE5i responders. This increase in the IIEF-EF domain scores was also ac-companied by improvements in cavernosal blood flow and penile endothelial function as measured by FMD. One of the key issues addressed in these studies was the partic-ipants’ subjective opinions regarding their sexual function. At screening, all patients were thoroughly interviewed about their medical and sexual history. The total IIEF questionnaire, the Erection Hardness Scale (EHS), the Quality of Erection, the Self-Esteem and Relationship questionnaires, and the Clinical Global Impression of Change rating scale were used to subjectively determine the sexual function of each patient. In these studies, a sig-nificant correlation between the subjective assessment of sexual function by use of validated sexual function ques-tionnaires and the objective results of penile blood flow and endothelial function was found. Most importantly, none of the men in either study reported treatment-associated pain or any adverse events during or after the treatment.
A prospective, randomized, double-blind, sham-con-trolled study was designed to confirm these positive outcomes. Sixty ED patients were treated in a similar fash-ion . The probe used for the sham treatment looked identical to the active probe and made the same noise yet did not produce any shock wave energy. The patients in the active treatment group had significantly higher IIEF-EF domain scores as well as improved penile endothelial func-tion compared with the sham-treated men. The overall suc-cess rate of LIST was later analyzed by Gruenwald et al. . In this cohort of 184 patients from various studies, the mean IIEF-EF domain scores increased after LIST by 7 points with the greatest improvement occurring in men with severe ED. Moreover, further analysis revealed that the increases in the IIEF-EF domain scores in the men who were PDE5i nonresponders were higher than in the men who were PDE5i responders (7.52 points vs. 5.7 points). FURTHER CLINICAL STUDIES
Another study recently introduced by Vardi et al.  eval-uated the effect of LIST after 2 years of follow-up and re-ported that the beneficial response that was achieved im-mediately after therapy had not faded in about 50% of the patients. The success rate after 2 years was lower mainly in patients with severe ED and diabetes. The same group also reported their preliminary results regarding a second round of penile LIST. They aimed to discover whether an additional LIST protocol would be therapeutically benefi-cial in patients whose first round of treatment was unsuccessful. Thirty patients were included in this study, 22 of whom (73.3%) had severe ED. Their median baseline IIEF-EF score before treatment was 8 and improved to a median of 10 points after the first treatment protocol. In the “second round” protocol, LIST was effective in approx-imately half the patients, and the median IIEF-EF score increased from 8 to 13.5 points, demonstrating that there are patients who probably need additional exposure to shock wave energy in order to respond .
Several other groups have recently studied penile LIST with different shock wave machines. Preliminary results with a machine that produced linear shock waves at 4 ana-tomical areas were presented by Reisman et al. . In this study, 82% out of 57 patients had a successful treatment, with the average IIEF-EF domain score increase of 8 points. Similar results were presented by Lacono . The preliminary results of a third shock wave machine were presented by Lund . In a prospective, randomized, blinded, placebo-controlled study, they demonstrated that LIST significantly improved the EHS in more than 50% of patients, allowing full sexual intercourse without medication.
LIST is a promising novel therapeutic modality for restor-ing erectile function in men with vasculogenic ED. It is a safe and nonpainful treatment with a long-lasting effect on erectile function. Additional well-designed, randomized, large-scale studies are necessary to confirm these findings and to outline the optimal treatment protocol and the suit-able candidates.