Introduction

Varicocele, from Latin varix (dilated vein) and Greek kele (tumor), consists of an abnormal dilatation of the veins of pampiniform plexus. It is commonly seen in the general male population, affecting 15% of individuals at reproductive age, 35% of those with primary infertility, and up to 80% of men with secondary infertility [1,2,3].

The varicocele diagnosis is primarily based on physical examination alone or combined with imaging studies [4,5,6]. The Dubin’s grading system is the most commonly used criteria to determine its presence and severity. The system categorizes varicoceles on a 1–3 scale, in which a grade 3 (large) varicocele is detected by visual inspection of the scrotum, whereas a grade 2 (moderate) varicocele is readily palpable. In contrast, grade 1 (small) varicoceles are those palpable with the aid of a Valsalva’s maneuver [7]. Treatment is usually recommended for infertile men with varicoceles detected during physical examination (any grade) and abnormal semen [5]. The reason stems from the fact that semen parameters and chances of conception, both natural and assisted, are overall increased after varicocele repair in men with palpable (clinical) varicoceles, but not in those with subclinical varicoceles (i.e., solely detected by imaging studies) [7, 8].

The controversy concerning varicocele mainly stems from its unclear pathophysiology that would lead to infertility [5,6,7,8,9]. Furthermore, the reasons why most men with varicocele have no apparent fertility issues remain unclear [10]. Recent studies, however, have shed light on possible pathways by showing that reactive oxygen species (ROS) and apoptosis markers are elevated in the semen of infertile men with varicocele [9,10,11,12,13].

An imbalance between ROS production and antioxidant protection leads to oxidative stress (OS), which causes damage to lipids, proteins, and nucleic acids in living sperm [14]. As a consequence, sperm motility and sperm–oocyte fusion are impaired. Moreover, OS can disrupt sperm chromatin structure by inducing breaks in the DNA strands [9, 15, 16], which has been shown to have a negative impact on embryo development and implantation [17,18,19,20,21].

In fact, the role of OS as a central element of varicocele-induced infertility and its association with sperm DNA breaks (so-called sperm DNA fragmentation [SDF]) have gained increased attention [9, 22, 23]. Urologists should be familiar with the evidence data linking varicocele-related infertility to OS and SDF as it has obvious implications for practice. In this review, we briefly discuss the current literature concerning the association between varicocele, oxidative stress, and SDF, and the possible mechanisms involved in infertility. Then, we examine in detail the role of varicocele repair as a means of alleviating SDF and improving fertility. Lastly, we critically appraise the evidence-based algorithm recently issued by the Society for Translational Medicine aimed at guiding urologists on the use of SDF testing in men with varicocele seeking fertility.

Varicocele and oxidative stress

Despite the current debate about varicocele pathophysiology, evidence concerning the role of OS and DNA fragmentation on varicocele-related infertility is increasing steadily. Small quantities of ROS play essential roles in sperm function as ROS are involved in sperm capacitation, acrosome reaction, hyperactivation, and the sperm–oocyte fusion [24]. In contrast, a disproportionate increase in ROS usually leads to OS [9]. The imbalance between ROS production and antioxidant protection causes alterations in nuclear and mitochondrial sperm DNA, including base modification, strand breaks, and chromatin cross-links, and is associated with apoptosis-like processes that affect sperm maturation and nuclear protamination [13,14,15,16,17,18,19,20,21,22,23].

Studies comparing the seminal levels of OS markers among fertile men with and without varicocele have shown increased OS in varicocele men [16, 25, 26]. Likewise, infertile men with varicocele exhibit elevated OS markers. Among these, ROS, nitric oxide, and lipid peroxidation products are common findings [27,28,29], thus indicating that the presence of a varicocele exacerbates the generation of OS [10]. Along the same lines, infertile men with varicocele have diminished seminal antioxidant capacity when compared to their fertile counterparts [12, 25, 29,30,31]. Notably, an association between varicocele grade and OS seems to exist, as larger varicoceles are associated with higher levels of seminal OS than smaller ones [31,32,33,34,35,36,37,38].

In varicocele, ROS and nitrogen species are released in the endothelial cells of the dilated pampiniform plexus, testicular cells (germ cells, Leydig cells, macrophages, and peritubular cells), and principal cells of the epididymis [9, 39, 40]. In such condition, excessive ROS negatively affect the sperm membrane and chromatin by causing lipid peroxidation and inducing DNA breaks, respectively [13, 16, 41].

Despite the fact that the mechanisms by which varicocele increases ROS and/or decreases antioxidant capacity are not fully elucidated, the central theory is that ROS generation is related to scrotal hyperthermia, testicular hypoxia, reflux of adrenal/renal metabolites, cadmium accumulation, and epididymal response, as discussed below. Yet, it is still unknown by which mechanisms infertility is prevented in fertile varicocele men. It has been speculated that intrinsic factors either protecting an individual from the deleterious effect of varicocele or exacerbating the harmful effects of oxidation on germ cells modulate the fertility status of men with varicocele [9]. For instance, antioxidant enzymes, such as catalase, superoxide dismutase, vitamin C, and glutathione peroxidase, counteract ROS [9]. In the fertile varicocele population, the equilibrium between oxidants and antioxidants might be more efficient in counteracting the increased ROS levels. Furthermore, other protective mechanisms might exist, including a slowed rate of germ cell apoptosis, enhanced turnover machinery for the oxidized proteins to prevent their aggregation, and reduced cellular signal-transducing effects of ROS [10]. While the disruption of these protective antioxidants can result in OS, it is still unknown which mechanisms exert major protective roles.

Heat stress and SDF

The reflux of abdominal blood through incompetent valves of the internal spermatic and cremasteric veins into the pampiniform plexus leads to scrotal hyperthermia. This change in testicular thermostasis goes against the optimal temperature for spermatogenesis, which is 2.5 °C lower than the body’s temperature. Scrotal hyperthermia is the most widely accepted hypothesis to explain OS in varicocele [9, 42, 43]. Heat stress is associated with increased ROS production by cell mitochondria, plasma membrane, cytoplasm, and peroxisomes. Cell damage resulting from hyperthermia occurs in different grades in the various cell compartments [13]. In the testes, spermatogonia B and the developing spermatozoa are highly vulnerable to heat stress. On the contrary, spermatogonia A, as well as Leydig and Sertoli cells, are thermo-resistant [9].

Testicular hypoxia

Infertile men with varicocele can present with signs of ischemia due to the stagnation of blood on the microcirculatory vessels [44]. Arteriolar occlusion by microthrombi, germ cell degeneration, Leydig cell atrophy, and fibrotic thickening of the basement membranes of seminiferous tubules have been observed in testicular biopsy specimens [45]. It seems that ischemia occurs in varicocele patients when the venous hydrostatic pressure of internal testicular vein exceeds the testicular arteriolar pressure [45, 46]. ROS are produced by various sources during this hypoxic state, including activation of hypoxia-inducible factor 1 (HIF-1), mitochondrial dysfunction, xanthine dehydrogenase/oxidase, membrane-associated NAPDH oxidase 5 (NOX5), and phospholipase A2 [9]. Moreover, hypoxia can lead to increases in the expression of leptin and cytokines in testicular tissue, including interleukin (IL)-1 and IL-6, which can induce ROS generation [34, 47,48,49].

Reflux of adrenal/renal metabolites and cadmium accumulation

The retrograde blood flow through the left testicular vein with adrenal prostaglandins and renal and adrenal metabolites can induce cellular OS [50]. Norepinephrine also contributes to vasospasm and aggravate hypoxia, thus generating more ROS [9]. Cadmium is a natural metal that has been identified in elevated levels in the wall of the internal spermatic veins, testicular tissue biopsy specimens, and the seminal fluid of patients with varicocele [51,52,53]. It is hypothesized that increased hydrostatic pressure and hypoxia might result in a porous blood–testis barrier that enables cadmium to build up [53]; however, it is still unclear how cadmium affects fertility.

Epididymis dysfunction

Experimental varicoceles have been used to study the epididymal structural and functional changes [9]. In the epididymis, there are three important sources of ROS, namely the luminal fluid from the testis, the endothelial cells layering the rich capillary network around the caput, and the metabolically active principal cells [9]. The initial epididymal segment seems to be the primary site of ROS accumulation. However, cells capable of generating enzymatic and nonenzymatic antioxidants seem to exist in all epididymal sections. Hypoxia and heat stress are the likely triggers underlying the imbalance between ROS and antioxidant defenses in the epididymis. Under these stressful conditions, the principal cells can generate excessive ROS that combined with the impaired production of antioxidants result in oxidative damage to the maturing sperm and epididymal cells [13].

Varicocele and sperm DNA fragmentation

Sperm DNA integrity is critical to the development of a healthy embryo [54, 55]. Damage to sperm DNA is a complex process involving multiple, non-mutually exclusive, causative mechanisms that generate a variety of insults to DNA [39]. Among DNA lesions, two main types are of utmost clinical importance: single-strand DNA breaks (SS-DB) and double-strand DNA breaks (DS-DB) [55]. SDF usually refers to either SS-DB or DS-DB, or both, and is more common in infertile men than in fertile counterparts. Several etiological factors have been implicated in the impairment of sperm DNA content, including varicocele [17, 43, 56, 57].

A variety of assays have been developed to measure the proportion of sperm with SDF [42, 57]. Probes or dyes are used to identify the existence of DNA breaks in specimens examined by fluorescence and optical microscopy or flow cytometry [12, 17, 56, 58, 59]. The sperm chromatin structure assay (SCSA), terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), sperm chromatin dispersion test (SCD), and single gel electrophoresis (Comet) are the most commonly used methods to measure SDF [60, 61].

Men with high levels of SDF in semen have difficulties to impregnate their partners, both naturally and assisted [62]. Among those establishing a pregnancy unassisted, the time-to-pregnancy is longer in couples whose male partners have high SDF [63]. SDF has also been associated with poor intrauterine insemination and assisted reproductive technology (ART) outcomes [64]. Although sperm with fragmented DNA may fertilize an egg with apparently similar efficiency as sperm without DNA fragmentation [65, 66], the negative impact of a damaged paternal chromatin to the integrity of embryonic genome is usually observed after implantation [67] and is often manifested by early pregnancy loss [19, 20]. However, massive SDF can also promote embryonic arrest [17, 18]. It has also been speculated that SDF might lead to a higher risk of congenital disabilities in the offspring [20, 68].

In men with varicocele, SDF is probably one of the critical consequences of OS via ROS as depicted in Fig. 1. This fact is supported by the usual observation of a concomitant impairment in sperm DNA integrity and altered oxidative stress markers in such men [13]. In the sections below, we discuss the clinical evidence of the OS-induced SDF in men with varicocele and the effect of interventions.

Fig. 1
figure 1

Pathophysiology of varicocele and its association with reactive oxygen species and sperm DNA fragmentation (solid lines and dotted lines indicate direct and indirect effects, respectively). Reprint from Cho et al. Novel insights into the pathophysiology of varicocele and its association with reactive oxygen species and sperm DNA fragmentation. Asian J Androl. 2016 Mar–Apr; 18(2):186–193, under the terms of the Creative Commons Attribution-Non-Commercial-ShareAlike License, which permits non-commercial use, distribution and reproduction in any medium, provided the original work is properly cited

Full size image

Clinical evidence of the association between varicocele and SDF

Following the confirmation of a consistent association between conventional semen parameters and varicocele, the majority of recent studies addressing varicocele and sperm quality have focused on sperm function markers and genetic defects. These range from markers of oxidative stress, mitochondrial activity, chromatin compaction, DNA methylation, and DNA fragmentation (reviewed by Agarwal et al. [69]).

As for SDF, infertile men with varicocele often present with elevated DNA damage in semen. In an early study involving 55 infertile men with clinical varicocele and 25 normozoospermic donors, elevated SDF (defined as the mean of the control group plus 2 SD) was seen in 49% varicocele patients with normal semen profile and 58% of those with abnormal semen parameters [70]. In another study involving 593 men with various etiologies attending infertility clinics, including a control group of semen donors, SDF rates (by SCD) were highest in both varicocele patients (35.7 ± 18.3%) and those with subclinical genital infection (41.7 ± 17.6%) [22]. Notably, two distinct sperm populations with fragmented DNA were identified, namely standard DNA fragmentation and degraded DNA fragmentation (DDS). Spermatozoa with standard fragmented DNA exhibited either the absence or presence of a small halo of chromatin dispersion around a compact nucleoid, whereas spermatozoa with degraded DNA showed a ghost-like morphology owing to massive SS-DB and DS-DB, as well as nuclear protein damage. In the study mentioned above, the proportion of sperm with degraded DNA was eightfold higher in varicocele patients than donors. Moreover, although the presence of sperm with degraded DNA was not pathognomonic of varicocele, it was possible to identify varicocele patients by computing the index of sperm with degraded DNA with 94% accuracy [22].

The observations mentioned above were corroborated by two systematic reviews. In one report, Zini and Dohle assessed 16 case–control studies evaluating SDF in fertile and infertile men with and without varicocele [12]. The authors found that in four out of nine studies, SDF rates were overall higher in infertile men with varicocele than infertile counterparts without varicocele. Moreover, the group of patients with varicocele had poorer seminal parameters than the group of infertile patients without varicocele. The remaining seven studies specifically included fertile men with varicocele. In six studies, SDF rates were higher in men with varicocele (and no history of infertility) than in fertile men or sperm donors without varicocele [12]. Another systematic review followed by meta-analysis compiled the data from seven studies including 240 patients with varicocele and 176 normal healthy controls without varicocele [71]. In this study, SDF was higher in varicocele men than controls without varicocele (mean difference: 9.84%; 95% CI 9.19–10.49, P < 0.00001).

In fact, confirmatory data concerning the association between varicocele and elevated SDF have increased steadily [72, 73]. Furthermore, it has been shown that other essential markers of sperm function, including epididymal neutral α-glucosidase and sperm PLCζ levels, are also reduced in men with high SDF and varicocele [72]. Despite that, the impact of varicocele grade on SDF levels remains poorly studied as does the effect of subclinical varicocele on sperm DNA integrity.

Collectively, objective evidence indicates that SDF is overall increased in men with palpable varicocele, particularly in those with abnormal semen parameters, and that such increase is usually accompanied by alterations in markers of oxidative stress and sperm function.

Effect of varicocele treatment on SDF

Surgical repair has been used as the treatment for infertile men with varicocele for over a century [74]. Indeed, such intervention has been associated with significant improvements to various biomarkers of male infertility, such as semen parameters and pregnancy rates [8, 75,76,77,78]. Recently, varicocele repair has been used as an attempt to alleviate oxidatively induced SDF and protect against the progressive nature of varicocele and its consequent upregulation of systemic OS [10].

In fact, over 20 studies accounting for more than 1200 treated subjects were published in the last 12 years addressing the effect of varicocelectomy on SDF (Tables 1, 2, 3) [12, 29, 79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97]. The overwhelming majority of studies included men with clinical varicocele and abnormal semen parameters according to the WHO criteria. Despite using different SDF assays, heterogeneous design, and variable sample size, all studies reported a significant decrease in SDF rates after varicocele repair in a follow-up period ranging from 3 to 12 months (Tables 1, 2, 3). Yet, the exact percentage of men who benefit from surgery concerning SDF remains poorly reported. In a retrospective small cohort study including 37 men, Moskovtsev et al. reported improvements in SDF rates in 78% of the treated patients [81]. In another report, Werthman et al. studied 11 men with clinical varicocele and observed that 90% of the patients showed a significant decrease in the rates of SDF 3–6 months after varicocelectomy [80] (Table 1).

Table 1 Studies evaluating the effect of varicocelectomy on sperm DNA fragmentation, not including controls or concomitant assessment of oxidative stress markers
Full size table
Table 2 Studies evaluating the effect of varicocelectomy on sperm DNA fragmentation, including controls or concomitant assessment of oxidative stress/sperm function markers
Full size table
Table 3 Studies evaluating the effect of varicocelectomy on sperm DNA fragmentation, including both controls and concomitant assessment of oxidative stress/sperm function markers
Full size table

Of the few studies providing pregnancy outcomes, postoperative SDF rates were overall lower in men from couples who achieved pregnancy success than those who did not. In one report, Smit et al. prospectively evaluated 49 men with clinical varicocele, oligozoospermia, and at least 1-year infertility duration subjected to varicocelectomy. These authors observed improvements in SDF rates 3 months after varicocelectomy (preoperative 35.2 ± 13.1%; postoperative 30.2 ± 14.7%, P = 0.019; SCSA). In their study, couples that conceived naturally or with ART exhibited lower postoperative SDF levels (26.6 ± 13.7%) than those who did not (37.3 ± 13.9%, P = 0.013) [82]. In another study, Ni et al. [93] evaluated 42 subfertile patients with clinical varicocele grades 2 and 3 and altered seminal parameters subjected to microsurgical varicocelectomy. SDF was measured by SCSA, and the preoperative results were compared to a control group of semen donors. The SDF levels were significantly higher preoperatively in the patient group than in the control group. After 3–6 months postoperatively, SDF decreased overall (preoperative: 28.4%; postoperative: 22.4%; P = 0.018), despite remaining higher than controls. Notably, SDF levels in patients who achieved pregnancy naturally after varicocele repair (20.6 ± 3.5%) were not significantly different than controls (11.5 ± 3.9%), but were lower than both preoperative values (27.4 ± 6.3%; P < 0.01) and non-pregnant patients (24.7 ± 6.5%; P < 0.010) [93]. Recently, Mohammed et al. prospectively evaluated 75 infertile men with clinical varicocele and abnormal semen parameters and found that couples with positive pregnancy outcome at 1-year follow-up had had significantly lower DFI (16.4 ± 6.4%) than those who did not (24.2 ± 4.1%, P = 0.04) [94]. Notwithstanding, contrary results were reported by Baker et al. who retrospectively evaluated data from a small group of 24 infertile men with clinical varicocele who underwent microsurgical varicocele repair and had pre- and postoperative SDF results [89]. The authors observed that despite a significant decrease in SDF rates from a preoperative mean of 40.8% to a postoperative mean of 24.5% (P = 0.001), DFI results in pregnant and non-pregnant couples did not differ (22.2 ± 14.4 vs. 25.7 ± 14.5%, respectively).

Several studies evaluating the impact of varicocelectomy on SDF also assessed oxidative stress markers, sperm chromatin compaction, or other advanced sperm function characteristics. Decreases in such markers were noticeable in most studies, thus underscoring the association among varicocele, OS, and SDF (Tables 2, 3) [87,88,89,90,91,92,93,94,95,96,97]. Yet, although these studies unequivocally reported significant reductions in SDF after varicocelectomy, some studies have failed to demonstrate reduction in OS markers after surgery [87, 89], rendering it unclear as to why not all men with signs of OS improve after varicocele repair.

The published literature on varicocelectomy and SDF contains a few controlled studies, comprised of either healthy fertile men with normal semen parameters (WHO criteria) and without varicocele, infertile men with clinical or subclinical varicocele and normal semen parameters, or infertile men without varicocele. Despite limitations concerning confounding factors and design, SDF is shown to be significantly higher in varicocele patients than controls [88, 92,93,94,95,96,97] (Tables 2 and 3). Of the controlled studies assessing OS markers and other sperm functional characteristics, the overwhelming majority report higher levels of oxidative stress, DNA decondensation, and SDF in infertile men with clinical varicocele than healthy fertile counterparts without varicocele and men with subclinical varicocele (Table 3). Notably, such markers seem to be elevated in both patients with normal and abnormal semen parameters according to the WHO criteria.

In contrast, repair of subclinical varicoceles concerning SDF does not seem beneficial, but the evidence is based on a single study [60]. In this report, Garcia-Peiró et al. evaluated 60 infertile patients with varicocele using several SDF methods (TUNEL, SCD, and SCSA). While SDF rates decreased after repairing clinical varicoceles, there were no improvements in SDF rates in infertile patients with subclinical varicocele subjected to surgery [98].

As for the role of other treatment modalities for decreasing SDF in varicocele-related infertility, the published literature is very scarce. In a small cohort non-controlled study, 20 patients with grade 1 varicocele were treated with oral antioxidants (1500 mg L-Carnitine, 60 mg vitamin C, 20 mg coenzyme Q10, 10 mg vitamin E, 200 μg vitamin B9, 1 μg vitamin B12, 10 mg zinc, 50 μg selenium) daily for 3 months [99]. The relative reductions in SDF and the percentage of highly degraded sperm cells—assessed by the SCD assay—were 22.1% (P = 0.02) and 31.3% (P = 0.07), suggesting a possible role for oral antioxidants in men with clinical varicocele and SDF. In a recent prospective trial involving 80 infertile men with clinical varicocele (grades 2 or 3) and DFI > 30%, the patients were randomized to (1) microsurgical subinguinal varicocelectomy, (2) varicocelectomy followed with 1 mg ketotifen (mast cell stabilizer) twice daily for 3 months, and (3) oral ketotifen 1 g twice daily for 3 months [86]. The percent improvement in sperm DFI after treatment was significant (P < 0.05) but not different between varicocelectomy alone versus oral therapy, whereas the highest percent improvement was seen with the combination of surgery and medication (Table 1). Despite these results, the evidence is too limited to draw any definite conclusions about the potential role of antioxidants as a treatment for SDF-infertility in men with clinical varicocele.

In conclusion, the existing evidence is reassuring as to the effectiveness of varicocele surgical repair as a means of alleviating oxidatively induced sperm DNA damage. Given the current observations, urologists should advise male partners of infertile couples presenting with palpable varicoceles of the connection with SDF and oxidative stress, and discuss varicocele repair as a way of both decreasing SDF and potentially improving fertility.

Clinical practice guidelines on SDF testing in varicocele patients

While the essential role of sperm DNA integrity in human reproduction has been extensively studied, the clinical indication of SDF testing is less clear. In the context of varicocele, current guidelines issued by major professional societies recommend varicocelectomy to be considered in infertile men with clinical varicocele and abnormal semen analysis [100, 101]. These guidelines also recommend that varicocele treatment should not be offered to patients with normal semen quality. However, it is well-established that conventional semen analysis alone is not enough to assess semen quality [102,103,104]. Reference ranges and interpretation vary according to the World Health Organization (WHO) edition utilized for the examination of human semen [14]. Moreover, a routine semen analysis cannot identify abnormalities affecting the sperm chromatin. Assessment of sperm DNA integrity in the context of varicocele has been proposed as complementary to conventional semen analysis. Indeed, SDF can be present even in men with semen parameters within normal ranges as per the WHO criteria [56].

A 2017 clinical practice guidelines (CPG) issued by the Society for Translational Medicine (STM) provides evidence-based recommendations for SDF testing in male infertility scenarios, including varicocele [105]. The guidelines recommend testing to patients with varicocele grades 2 or 3 with normal conventional semen parameters as per the WHO criteria. SDF testing was also prescribed to patients with grade 1 varicocele with borderline/abnormal traditional semen parameter results (Table 4). The reasoning of these recommendations relies on the previously discussed association between the presence of palpable varicoceles and increased SDF, and the overall positive effect of varicocelectomy on sperm DNA damage. Notably, the CPG mentioned above propose the utilization of SDF testing results for clinical decision-making in varicocele clinical scenarios in which treatment is not warranted by itself (Fig. 2). It has been postulated that identification of the affected individuals might allow urologists to better select varicocele candidates for early surgical interventions and  potentially halt further deterioration of semen and fertility [57]. Moreover, SDF test results can be used to monitor the effectiveness of varicocelectomy [10].

Table 4 Excerpt of the evidence-based clinical practice guidelines issued by the Society for Translational Medicine on indications for sperm DNA testing in varicocele.
Full size table
Fig. 2
figure 2

Algorithm for sperm DNA fragmentation testing in patients with clinical varicocele. Reprinted with permission from Esteves et al., A Strengths–Weaknesses–Opportunities–Threats (SWOT) analysis on the clinical utility of sperm DNA fragmentation testing in specific male infertility scenarios. Transl Androl Urol. 2017 Sep; 6(Suppl 4):S734–S760

Full size image

Interestingly, results of a 2017 cross-sectional questionnaire-based survey involving 65 participants with expertise in male infertility, mostly urologists, indicated that while SDF testing is commonly utilized (61.2%) in infertile men with high-grade varicocele and “normal” semen parameters (by WHO criteria), it was least ordered for the evaluation of low-grade varicocele in patients with subnormal semen analysis results (46.9%) [60]. In this study, 1/3 participants responded that SDF testing is not currently offered in any of the clinical scenarios listed above in their practices, whereas 1/6 revealed uncertainty about its clinical utility in such situations. The CPG issued by the STM clarify these issues and provide useful guidance to urologists and other healthcare practitioners as to enhance the quality of healthcare deliverable to varicocele patients as well as to discourage potentially harmful or ineffective interventions [105].

While it is important to contemplate that the CPG on SDF testing synthesized their recommendations based on the available evidence, these were based overwhelmingly on non-randomized clinical trials and retrospective studies. Therefore, most recommendations were graded B and C, like those issued by most male infertility guidelines [43, 101, 106, 107]. Concerning varicocelectomy in men with clinical varicocele/normal semen analysis and low-grade varicocele/borderline semen analysis, the evidence is still limited, thereby warranting further research. However, as with all male infertility CPG, the guidelines on SDF testing concerning varicocele is not aimed at dictating an exclusive course of treatment. Other management and treatment strategies might be appropriate, taking into account the available resources, patient needs, and specific practice conditions. The essence of any CPG should be to translate the best evidence into practice and serve as a framework for standardized care while maintaining clinical autonomy and physician judgment [108, 109].

Future research

Despite convincing evidence of a positive effect of varicocele repair on SDF, there exist gaps in knowledge as to the exact prevalence of elevated SDF among varicocele patients and the association between SDF and varicocele grade (reviewed by Esteves et al.) [39]. Although reduction in SDF levels after surgery is shown to be more common in men who have a concomitant improvement in conventional semen parameters [70], further research is needed to clarify whether improvements in SDF alone after varicocele repair in men with clinical varicocele and semen analysis within normal ranges can increase pregnancy success. Additionally, investigations are warranted to ascertain the proportion of patients with high SDF levels that resolve to normal levels after varicocelectomy. Lastly, there is a need for further evidence that SDF is reduced in patients with low-grade varicocele and borderline routine semen analysis and that such decline in SDF levels translates into better pregnancy outcomes.

Conclusions

Current evidence supports oxidative stress as a primary factor in the pathophysiology of varicocele-related infertility. The mechanisms by which varicocele increases oxidative stress are not fully elucidated, but reactive oxygen species generation in response to scrotal hyperthermia, testicular hypoxia, reflux of adrenal/renal metabolites, and cadmium accumulation is the leading theory. The testis and epididymis react to oxidative stress via several mechanisms—including the generation of antioxidants that may maintain fertility potential in men with varicocele. Failure of these mechanisms might explain testicular/epididymal dysfunction and infertility observed in a subset of men with varicocele. In this scenario, increased sperm DNA fragmentation—as often seen in men with clinical varicocele—is likely the final result of this oxidative-induced damage. Many assays are available to identify abnormal sperm DNA fragmentation levels in semen of men with varicocele. Surgical varicocele repair seems beneficial not only for decreasing sperm DNA fragmentation but also for increasing the likelihood of pregnancy, both natural and assisted, in men with palpable varicocele and damaged sperm chromatin. While gaps in knowledge exist, particularly concerning the understanding of varicocele grade on sperm DNA fragmentation and the utility of varicocelectomy in men with palpable varicocele and normal/borderline semen analysis, recent guidelines have provided evidence-based indications for SDF testing and guidance for management of the infertile man with varicocele.

Review criteria

An extensive search of studies examining the relationship between varicocele and sperm DNA fragmentation was performed using PubMed and MEDLINE. The start date for the search was not specified, and the end date was December 2017. The overall strategy for study identification and data extraction was based on the following keywords: “varicocele,” “male infertility,” “sperm DNA fragmentation,” “sperm DNA damage,” “varicocele repair,” “varicocelectomy,” “varicocele treatment,” “varicocele embolization,” and “antioxidants,” with the filters “humans” and “English language.” Data that were solely published in conference or meeting proceedings, websites or books were not included. Citations dated outside the search dates were only included if provided conceptual content.