INTRODUCTION

Approximately 10–15% of all couples will seek assessment for infertility, which is the inability to conceive after 12 months of frequent unprotected vaginal intercourse.¹ Men and women seeking evaluation tend to be married, older and more educated than those not seeking care; and therefore, are not representative of the underlying population. Additionally, only 50% of such couples that reside in nations with well-resourced healthcare systems will undergo treatment after evaluation.^2,3 This treatment is usually female-directed, requiring assisted reproductive techniques (ART), even though male factors explaining subfertility (defined as reduced fertility that might require therapy for successful conception) are present in 20–25% of all cases and are a contributing factor in a further 30%.^4,5,6 Research to broaden therapies for male subfertility is required given this imbalance between the prevalence of male factors and the female-directed nature of the therapy – particularly if rates of male factor subfertility are truly increasing, as suggested by meta-analyses showing temporal declines in sperm concentrations.⁷ Although alarming, the phenomenon has not been universally observed and may be overstated due to publication bias.⁸ Irrespective of these presumptive temporal changes in sperm concentrations, a specific cause for male infertility is often not established, even after a complete workup, leaving a large undiagnosed heterogeneous group of men with idiopathic oligo-astheno-teratozoospermia (OAT).

ASSESSMENT OF MALE SUBFERTILITY

The assessment of male subfertility begins with evaluation for sexual disorders such as low libido and erectile dysfunction; and ejaculatory disorders such as anorgasmia. Symptoms of low libido, particularly in conjunction with decreased spontaneous morning erections or delayed puberty, should prompt assessment for male hypogonadism. Male hypogonadism is a clinical syndrome that results from dysfunction of the hypothalamic-pituitary-testicular (HPT) axis from organic or functional causes^9,10 that primarily affect the testes (termed primary hypogonadism) versus those that affect the hypothalamic-pituitary unit (termed secondary hypogonadism). The diagnosis hinges upon identifying a clinical syndrome for which multiple societal guidelines exist to help steer the clinical assessment. However, it ultimately requires biochemical confirmation by measuring testosterone in blood collected in the morning from men who have fasted, are not acutely ill, and after a good night's sleep.^10,11,12 The distinction between primary and secondary hypogonadism is important because induction of spermatogenesis with gonadotropin [or with pulsatile gonadotropin-releasing hormone (GnRH) in those with preserved pituitary function] therapy has only been reliably achieved in men with secondary hypogonadism.

Infrequent sexual intercourse (less than 2–3 times each week) due to low libido, relationship or situational causes or due to mental health problems (such as depression) can contribute to subfertility.¹³ Past paternity, history of undescended testes or recent genitourinary infection is relevant, as is painful or abnormal curvature during penile erection causing erectile dysfunction and postcoital micturition that is cloudy, which may be due to retrograde ejaculation. It is important to obtain a history of testicular causes of subfertility such as prior scrotal or pelvic irradiation; surgery, trauma or torsion, as well as systemic chemotherapy and features of Klinefelter syndrome (the most common genetic cause of testicular failure), particularly behavioral or educational problems. In fact, ascertaining the presence of any systemic disease is important because severe systemic disease of any cause can temporarily decrease sperm output. Diseases of the hypothalamo-pituitary unit, including tumors and infiltrative diseases, hemochromatosis, hyperprolactinemia and hypogonadotropic hypogonadism, including Kallmann syndrome, are important to uncover because male-directed therapy to increase sperm output (see later) is effective. Endocrine disorders are likely to cause subfertility and should be carefully screened. These disorders include obesity; diabetes mellitus, particularly in those with neuropathy and dysautonomia which can cause ejaculatory dysfunction; hypothyroidism, hyperthyroidism and hypercortisolemia from any cause, which impair spermatogenesis; and adrenal rest tumors from severe untreated congenital adrenal hyperplasia. The use of medications that may impair spermatogenesis should also be elicited, including prescribed drugs such as corticosteroids, 5-α reductase inhibitors and sulfasalazine; abused drugs such as opiates and androgens; and recreational drugs such as tobacco, alcohol and marijuana; and other drugs that affect ejaculation, such as serotonin reuptake inhibitors (anorgasmia) or α-adrenergic blockers (retrograde ejaculation).

The physical examination should include a general examination for any severe systemic disease, particularly the endocrine disorders listed above. Upon completion, it should then focus on evaluating secondary sexual characteristics and features of Klinefelter syndrome (such as tall stature, gynecomastia) and sellar masses (e.g., visual field confrontation) if indicated. The genital examination is crucial and includes an examination of the penis for hypospadias (indicating prenatal androgen deficiency) and fibrosis, unilateral or bilateral absence of the vas deferens, normal epididymal size and structure, varicoceles (examined in the standing position), and the testes to ensure that both are present and to measure size and masses. Clinically determining testicular size requires the use of a Prader orchidometer.

Initial diagnostic testing includes semen analysis, which remains the gold standard for assessing male factor fertility, and should be performed according to contemporaneous WHO standards.¹⁴ A low ejaculate volume should prompt query regarding spillage or inadequate abstinence, which in the presence of azoospermia (absent sperm) and low seminal fluid pH (<7.4) should suggest the need to determine seminal fructose concentrations. If seminal fructose is low, this suggests congenital absence of one or both vas deferens, which is a disorder that occurs in men with mutations of the cystic fibrosis transmembrane conductance regulator (CFTR), even in the absence of clinically apparent lung disease. Blood should be collected for measurement of luteinizing hormone (LH), follicle-stimulating hormone (FSH), sex hormone-binding protein (SHBG), estradiol (E₂), testosterone and prolactin, and for panhypopituitarism if indicated by history, or the initial testing (e.g., hypogonadotropic hypogonadism). A testicular ultrasound may be needed to confirm the absence of testes or inguinal testes since men with undescended testes are at increased risk of germ cell tumors or to work up a varicocele. Genetic testing for karyotype and Y chromosome microdeletions should be considered in all men with non-obstructive azoospermia or severe oligozoospermia and CFTR mutations with obstructive azoospermia. In select cases, the pituitary sella needs to be imaged. Testicular biopsy is now almost always performed in the context of surgical sperm retrieval.

MEDICAL THERAPY

Lifestyle modifications

Many lifestyle behaviors in men (e.g., smoking, increased consumption of food resulting in overweight/obesity, excessive alcohol intake, and lack of physical exercise) have been associated with impairment in seminal parameters and fertility. This association constitutes the rationale for eliminating these behaviors in men seeking fertility.

Cigarette smoking in men has been associated with impaired seminal parameters¹⁵ and adverse ART outcomes.¹⁶ There is limited evidence of a trans-generational effect, as compromised seminal parameters have been reported in the offspring of smoking fathers.¹⁷

The association between overweight and impaired seminal parameters is conflicting, with some studies indicating a positive dose-response association,¹⁸ whereas others do not.¹⁹ However, it should be recognized that the studies present important risk of bias due to the heterogeneity of the sperm parameters considered. Accordingly, a recent meta-analysis using only WHO 2010 sperm criteria confirmed a strong relationship between overweight/obesity and impaired sperm parameters.²⁰ In line with the latter observation, a single non-randomized study suggested that weight reduction improved seminal parameters in severely obese men (BMI >33 kg/m²).²¹ In addition, weight loss independently of the applied method (e.g., hypocaloric diet, bariatric operation) results in increased serum total testosterone concentrations.²²

Similarly, the association between moderate alcohol intake and impaired seminal parameters is inconclusive. Most studies show a negative effect;^23,24,25 nevertheless, they are not randomized. The evidence is most robust regarding severe alcohol consumption and its negative effect on erectile function.^26,27

Although the association between lack of physical activity and impaired seminal parameters has been documented in observational studies,^28,29 there is a lack of evidence from interventional studies regarding the effect of physical exercise on male fertility. Obviously, physical activity and body weight are interrelated.

There is also no evidence to recommend behaviors, such as scrotal cooling or changing clothing and working conditions for decreasing scrotal heating to increase fertility.

As the lifestyle choices under investigation (i.e., smoking, obesity, alcohol) are potentially hazardous, the evidence is exclusively gathered by observational (as opposed to interventional) studies, a fact that will not change in the future. The avoidance of unfavorable lifestyle behaviors, which coexist in most cases, will have a positive impact not only on reproductive issues (e.g., semen quality, fertility, testosterone production), but also on general male health. Additionally, eradicating these behaviors in one partner may facilitate similar actions in the other, affecting the cumulative fertility of the couple.

Hormonal approach

Gonadotropins

Male-directed therapy should be considered in causes of subfertility, where the specific underlying disorder is reversible. This includes erectile or ejaculatory dysfunction, treatable systemic illnesses, and cessation of drugs (such as opiates, androgens, antidepressants, 5-α reductase inhibitors, serotonin reuptake inhibitors or α-adrenergic blockers) that impair spermatogenesis, erectile or ejaculatory function. Male-directed therapy is also possible for subfertility due to hypothalamo-pituitary disease, which is characterized by low or low-normal serum gonadotropins, low testosterone (almost universally, although there are rare examples of fertile eunuch syndrome), and low sperm concentrations. Some causes of secondary hypogonadism, such as hemochromatosis, prolactinoma and pituitary infection, may be directly treated by venesection, dopamine agonists, and antibiotics, respectively, which may reverse infertility. However, treating other causes of secondary hypogonadism, such as a pituitary tumor or craniopharyngioma by surgery or external beam radiation, may induce or worsen infertility. For all other causes of secondary hypogonadism (including genetic causes such as Kallmann syndrome and idiopathic hypogonadotropic hypogonadism), referral to a specialist andrologist/endocrinologist should be considered. Gonadotropin replacement therapy with human chorionic gonadotropin (hCG) for LH replacement is administered at 1000–2000 IU sc 2–3 times weekly. Therapy generally continues for 6 months, then FSH is added if sperm concentrations remain below 10 million/ml. The initial starting dose is 75 IU sc every other day and maybe doubled after 6 months if sperm concentrations remain suboptimal (<15 million/ml) and pregnancy has not occurred. A large meta-analysis has clarified that the combination of FSH and hCG results in better outcomes compared to single isolated therapy.³⁰ Hence, combined therapy with hCG and FSH initially can be considered.³¹ Retrospective studies show that conception occurs on average after 2–3 years of gonadotropin therapy when sperm concentrations are between 5 and 15 million/ml, but can occur with sperm concentrations less than 5 million/ml.^30,32 Because gonadotropin therapy is inconvenient, and pregnancies occur at low sperm concentrations, gonadotropin therapy is not often continued beyond the first trimester of pregnancy, and, therefore, higher sperm concentrations are not often attained but can be with prolonged therapy.³³ Therapy should be continued if sequential pregnancies are desired. Men with small testicular volumes (each less than 4 ml) are more likely to require prolonged therapy.^32,33 There is controversy about whether prior androgen therapy may decrease the likelihood of response to gonadotropin therapy.^30,33 However, data from meta-analyses seem to suggest the neutral role of prior androgen therapy on sperm parameters in subjects with secondary hypogonadism.³¹

SERMs and aromatase inhibitor

Selective estrogen receptor modulators (SERMs) and aromatase inhibitors act to reduce feedback inhibition on GnRH and gonadotropin secretion, thereby increasing pulsatile gonadotropin release by the pituitary and testicular testosterone secretion by the testes.^9,34 Their ability to stimulate spermatogenesis is controversial, and aromatase inhibitors, in particular, are likely to negatively impact skeletal health, sexual function, and body composition.^35,36 Accordingly, the duration of therapy should be limited to no more than 1 year. Nevertheless, two metanalyses of SERMs show improvements in sperm concentration, motility and/or pregnancy rates,^37,38 and one small randomized controlled trial (RCT) comparing the aromatase inhibitor letrozole with placebo claimed that letrozole increased sperm concentration.³⁹ However, the latter study did not report group differences between treatment and placebo, only that there was a significant increase in sperm concentrations in those receiving letrozole but not among those receiving placebo. Furthermore, the second larger meta-analysis included only 16 controlled and uncontrolled trials, meaning that the paucity of data did not allow for firm conclusions. Hormonal therapies also do not improve sperm retrieval rates.⁴⁰

FSH for idiopathic male infertility

The use of follicle-stimulating hormone (FSH) has been proposed as an empirical treatment in subjects with idiopathic male infertility due to its action in regulating spermatogenesis.^41,42 Accordingly, although data derived from pre-clinical⁴¹ and clinical^30,31 studies showed that either luteinizing hormone (LH) or FSH is crucial to support normal spermatogenesis, some evidence has clarified that FSH alone can sustain it even in the absence or impaired LH action.⁴³ Recently, Santi et al.¹⁶ reported one of the largest retrospective real-world studies, including 194 men with infertility treated with FSH (mean therapy duration 9.1 ± 7.1 months). Overall, they reported 43 pregnancies (27.6%), of which 22 occurred naturally and 21 after assisted reproduction. In addition, a significant improvement in sperm parameters was observed, particularly in men of couples achieving pregnancy.¹⁶ Despite this observation, only a few placebo-controlled RCTs have investigated the use of FSH treatment in subjects with idiopathic infertility. Specifically, 21 clinical trials and four meta-analyses have addressed this therapeutic option. The first meta-analysis that included four RCTs with 278 participants concluded that FSH resulted in a better pregnancy rate (PR) compared with placebo or no treatment.^44,45 Some years later, the Cochrane study group published an updated revision of their first analysis that included six RCTs enrolling 456 participants; they showed that FSH resulted in better PR and live birth rates (LBR) compared with placebo or no treatment.⁴⁵ However, the data at that time were too limited to be conclusive.^44,45 More recently, Santi et al.,⁴⁶ by collecting 15 RCTs that included 614 men treated with FSH and 661 treated with placebo or no treatment, concluded that FSH improved spontaneous and ART-derived PR and sperm concentration but not sperm with progressive motility. In addition, the same authors clarified that no differences were observed when different FSH preparations (purified, recombinant) were considered.⁴⁶ Finally, in the most recent meta-analysis on this topic, Cannarella et al.³⁸ suggested, for the first time, that the use of a higher FSH dose (>450 IU per week) might have resulted in better outcomes.

Despite the positive results derived from the meta-analyses mentioned above, there are several concerns related to the use of FSH in men with idiopathic infertility. First, all the meta-analyses are characterized by a high heterogeneity among studies and a high risk of bias due to the lack of precise criteria to guide FSH administration. The high cost of this treatment represents a further limitation. Italy is the only country that allows and reimburses the empirical FSH administration in cases of male idiopathic infertility, considering the latter as a partial, functional form of hypogonadotropic hypogonadism with a serum FSH threshold of 8 UI/l. Finally, several polymorphisms on FSH receptors with different activities represent a further source of bias in evaluating FSH outcomes.⁴⁷

In conclusion, FSH treatment represents an intriguing therapeutic option for men with idiopathic infertility. However, more placebo-controlled RCTs are advisable before firm conclusions are made.^48,49

Antioxidants and Nutraceuticals

Oxidative stress is a common pathology affecting approximately 50% of men with infertility.⁵⁰ It occurs when producing potentially destructive reactive oxygen species (ROS) exceeds the antioxidant capacities. ROS, free radicals and peroxides are generated by immature sperm and seminal leukocytes and induce infertility by damaging the sperm membrane and altering the sperm DNA. Advancements in the understanding of oxidative procedures have been made through their quantification through direct (e.g., oxidative stress products) and indirect (e.g., sperm DNA integrity) methods. The rationale behind oral antioxidants is that these compounds reduce oxidative stress thereby restoring the oxidation-reduction balance.

Many primary and meta-research studies have been conducted, including four Cochrane meta-analyses. The Showell et al.⁵¹ meta-analysis included 48 RCTs in 4179 men with subfertility. It concluded that the quality of the evidence from only four small RCTs was low, but there was a suggestion that antioxidant supplementation could improve live birth rates for couples attending fertility clinics. The most recent meta-analysis^52,53,54 updated the evidence to 246 live births from 1283 couples in 12 small or medium-sized RCTs. The authors concluded that if the baseline probability of live birth following placebo or no treatment is 16%, the use of antioxidants increases it to 17–27%. However, when studies at high risk of bias were removed from the analysis, there was no evidence of increased live birth. Regarding adverse effects, there was no evidence of an increased risk of miscarriage, although antioxidants may result in mild gastrointestinal discomfort.⁵⁴

Obstacles to the interpretation of the evidence include their large number and diversity (indicatively zinc, folic acid, N-acetylcysteine, Coenzyme Q10, vitamins E and C, selenium, carnitines, pentoxifylline), their use as mixture of substances (obscuring the effect of each of them), the use of extracts with no defined substance nature and dose, the diversity in the treatment duration and the lack of the indications for their use (e.g., the cause of male infertility). Additional methodological barriers are the non-randomized nature of most studies and the use of different outcomes (e.g., pregnancy rates, seminal parameters) that prohibit data synthesis and direct comparison among the substances.

Based on the current evidence, no recommendations can be made for or against using antioxidants to treat male infertility. Further large RCTs are needed to clarify the results.

Etymologically, the term “nutraceutical” derives from the words "nutrition" and "pharmaceutical". A nutraceutical is a pharmaceutical compound, which claims physiological benefits. In the US, nutraceuticals are classified by the FDA in the same category as dietary supplements and food additives, being under the authority of the Federal Food, Drug, and Cosmetic Act.

Most of the study design and data interpretation obstacles mentioned for antioxidants apply to nutraceuticals. The current evidence does not allow for recommendations regarding using nutraceuticals to treat male infertility.

SURGICAL APPROACH

Varicocele

Varicocele may compromise seminal parameters and male fertility. The correlation between unilateral or bilateral varicocele and disturbed seminal parameters is controversial, as the former occurs in 15% of the healthy general male population, 35% of men with primary infertility, and up to 80% of men with secondary infertility.^55,56 The rationale behind the surgical repair of varicocele is that it may improve seminal parameters temporarily or permanently, thus resulting in a live pregnancy.

A few primary studies and some meta-analyses have been conducted on the effect of the surgical repair of varicocele on male subfertility. Meta-research demonstrates improvement in seminal parameters for palpable varicocele^57,58,59 and, possibly, higher pregnancy rates and lower sperm DNA damage.⁶³ Additionally, varicocelectomy may increase serum total testosterone concentrations in men with hypogonadism and subfertility.⁶⁴ No such effect has been demonstrated in men with subclinical varicocele.⁶⁵  The more recent meta-analyses^60,61,62 show that varicocele repairs improve the pregnancy rate in subgroups of selected men. Favorable pre-treatment parameters include oligo-astheno-teratozoospermia (as opposed to azoospermia), secondary infertility (as opposed to primary), palpable varicocele (as opposed to subclinical) and young age with progressive testicular failure and/or seminal deterioration (as opposed to men of advanced age with seminal parameters within the reference range).

The selection of varicocele repair seems to constitute an etiologic approach for men with infertility compared with the empiric approach of intra-cytoplasmic sperm injection (ICSI), which is more expensive and incorporates risks for the female partner. Nevertheless, the aim of treating varicocele differs with age: in adolescents it is to prevent testicular damage and maintain future fertility, whereas in adults it is to improve current fertility potential. Many other parameters must be considered concerning the decision to treat varicocele. Indicatively, these factors would favor varicocele intervention: younger female age (<35 years old), higher degree of sperm DNA fragmentation, bilateral varicocele, and planning to use varicocele in conjunction with assisted reproductive techniques (ART) since there is some evidence that varicocelectomy can temporarily improve sperm parameters (after an initial post-surgical decline) and the sperm used may result in higher pregnancy rates with ICSI. The cost-effectiveness of the surgical procedure and the possible adverse effects should still be weighed against the expected efficacy.

Some obstacles hinder the interpretation of the evidence. These obstacles include the type of procedure [e.g., surgical repair (microsurgery, laparoscopic surgery)⁶⁶ or radiological techniques (sclero-embolization)^67,68] and the lack of uniform indications for its use (e.g., men with seminal parameters within or outside the reference range, co-existence of other causes of male infertility). Additional methodological barriers are the non-randomized type of several studies (e.g., “before-after” design), the small sample size (as it is difficult to enroll men in the no-treatment group) and the use of different outcomes (e.g., pregnancy rates, seminal parameters) that prohibit data synthesis and direct comparison among the substances.

In conclusion, varicocele treatment could improve seminal parameters in selected men. Favorable pre-treatment parameters include oligo-astheno-teratozoospermia, palpable varicocele and young age with progressive testicular failure and/or seminal deterioration.

Testicular sperm extraction (TESE)/microdissection (mTESE)

Non-obstructive azoospermia (NOA) is considered the most severe clinical condition related to male factor infertility, accounting for about 5% of infertile couples and resulting in the absence of sperm in the ejaculate.^48,69 Histological features of NOA are variable, ranging from hypospermatogenesis and maturation arrest to Sertoli cell-only syndrome. Hence, a surgical approach has been proposed for more than 30 years with the combination of conventional (non-magnified) testicular sperm extraction (cTESE) and intracytoplasmic sperm injection (ICSI).^48,69,70 In 1999, Schlegel et al.⁷¹ introduced a modified technique allowing magnification of the testis parenchyma under an operating microscope: microdissection TESE (mTESE). The latter can potentially permit the selection of the whitish, larger and more opaque tubules, which are more likely to contain sperm.⁷¹ Despite the potential theoretical advantages of mTESE over cTESE, real-life available data are confusing and cannot completely support the use of mTESE. The largest meta-analysis, including 117 studies and 21,404 men, actually showed similar sperm retrieval rates (SRR) when cTESE was compared to mTESE [46%, 95% confidence interval (CI) 43–49% for cTESE vs. 46%, 95% CI 42–49% for mTESE].⁶⁹ In addition an live birth rates (LBR), per ICSI cycle, of 24%, 95% CI 20–28% was also reported.⁶⁹ Interestingly, meta-regression analysis showed that reduced testis volume (<12.5 ml) and a higher prevalence of Klinefelter’s syndrome were the main factors negatively predicting a lower SRR.⁶⁹ However, the lack of superiority of mTESE over cTESE, as determined by this meta-analysis, should be considered with caution due to several potential biases. In particular, the retrospective nature of most studies and the lack of direct RCTs constitute major limitations. Therefore, current guidelines suggest choosing between mTESE and cTESE on a case-by-case basis, according to the ART center set-up and the surgeon’s expertise.^48,49 In addition, it should be recognized that the detection of complete or partial AZFa or complete deletion of AZFb region on Y chromosome are contraindications for sperm surgical retrieval due to the impossibility of finding sperm.^48,49

The use of hormonal stimulation, including administration of SERMs or gonadotropins, has been hypothesized to improve SRR in men with NOA. However, this hypothesis has not been extensively studied. A recent meta-analysis, including 22 studies and 1706 men, showed a higher SSR rate in subjects pre-treated with hormonal therapy [odds ratio (OR) 1.96, 95% CI 1.08–3.56]. However, when the data were analyzed according to the pre-surgical hormonal status, the latter observation was confirmed only in subjects with normal (OR 2.13, 95% CI 1.10–4.14) but not increased FSH concentrations (OR 1.73, 95% CI 0.44–6.77).⁴⁰ Hence, hormonal stimulation can be considered only in men with normal gonadotropin concentrations before surgery.

CONCLUSIONS

Despite the high prevalence of the male factor in couples seeking medical care for infertility, the appropriate evaluation and treatment of male factor infertility is still negligible or even neglected in many cases. Analysis of the semen remains a crucial step to assess male fertility, but, unfortunately and quite often, analysis of the semen is the only male factor considered, despite semen parameters showing low predictive value for pregnancy outcomes.⁷² However, the choice of the correct method to address male factors, among couples with infertility, should be based on an appropriate clinical and laboratory evaluation of the man. Research, including precision medicine approaches, to better evaluate and characterize male subfertility and to directly treat male subfertility are needed.

Promotion of healthy life-style behaviors should be encouraged in all men who are overweight or obese. The combined used of gonadotropins (FSH and hCG) represents the most effective choice in men with secondary hypogonadism. In the latter cases a possible alternative could be the use of SERMs only if the pituitary axis is preserved, although the evidence is still limited and the quality of the available studies poor. FSH therapy can be considered in subjects with idiopathic infertility and normal FSH levels (<8 mU/l). The use of antioxidants can also be considered, particularly in men with high sperm DNA fragmentation, but the evidence is still conflicting. Varicocele repairs can be offered in symptomatic men, and in those couples where the female partner is younger (<35 years), whereas surgical sperm retrieval is the gold standard in subjects with NOA (Table 1).

PRACTICE RECOMMENDATIONS