Infertility - a Great Challenge of the Past, Present, and Future.

For writing this review authors, as sources, used published relevant facts in the articles deposited in important scientific indexed databases Medline, PubMed Central, Scopus, Hinari, etc.

Causes of female infertility There are many factors that contribute to female infertility and their significance only increases with age. The most common are ovulation disorders, fallopian tube disorders, pelvic diseases, and abnormalities of the uterus ( Figure 1 ). Ovarian and ovulation disorders Ovulation disorder implies an absence of ovulation or its irregular timing. Today, an increasingly common cause of the hypothalamus-pituitary-ovary axis is polycystic ovary syndrome (PCOS). There are four phenotypes of PCOS (A-D), determined by key clinical features – oligo-/anovulation, hyperandrogenism, and polycystic morphology on ultrasound examination ( 3 ). Oligoovulation and anovulation are key parameters for diagnosing infertility, often manifesting as oligomenorrhea or amenorrhea. New findings have discovered correlations between PCOS and smoking, harmful environmental factors such as heavy metals, a high-calorie diet leading to obesity, and certain genetic factors ( 3 ). Obesity is a global issue causing significant problems in many aspects of health and the reproductive system is no exception. Changes in adipokine levels in obese patients, especially reduction in adiponectin and rise in leptin, are associated with pathogenic mechanisms leading to infertility. Multiple studies have researched the complex relationship between obesity and female infertility and have noted a linear decline in spontaneous pregnancy occurrence with the rise of body mass index (BMI). There are many more potential mechanisms that link obesity to infertility, some of which are disruptions in the hypothalamus-pituitary-ovary axis, endocrine effects of adipose tissue, insulin resistance, hyperinsulinemia, oxidative stress, dysregulation of lipid metabolism, mitochondrial dysfunction, etc ( 4 ). Fallopian tube disorders and pelvic disease Fallopian tube disorders include obstruction and epithelial irregularities which interfere with the motility of the zygote. These can manifest as a consequence of inflammatory pelvic diseases. Less frequently, fallopian tube disorders can be a consequence of congenital anomalies of the tube, which include agenesis, hypoplasia, cysts, additional ampules, multiple lumens, unilateral agenesis, partial or total duplication of the tube, dislocation, and segmental agenesis of the tube ( 5 ). Functional abnormalities of the fallopian tube can be caused by pelvic inflammatory disease, ruptured appendicitis, or ectopic pregnancy. A relatively common pelvic disease in infertile women is endometriosis, which implies ectopic endometrial tissue in the peritoneal cavity. This is a finding present in up to 70% of infertile women. Pelvic adhesions can also have an effect by impacting pelvic anatomy and prohibiting the physiological release of the oocyte and its transport via the fallopian tube. Chronic inflammation in the pelvis can be the cause of such complications. Cytokines, growth factors, prostaglandins, and oxygen radicals, which are present in higher levels at sites of endometriotic lesions, can interfere with ovulation, sperm function, fertilization, and implantation. In patients with endometriosis, the peritoneal cavity can be filled with macrophages which leads to an increase in the production of pro-inflammatory factors, thus stimulating inflammation ( 6 ). Additionally, new meta-analyses have identified up to 14 genomic regions related to endometriosis, such as the region pertaining to estrogen receptor 1 (ESR1). Polymorphisms in estrogen and progesterone receptor genes are also an important contributing factor to infertility ( 7 ). Another frequent cause of pelvic inflammatory disease is Chlamydia which, while often asymptomatic, leads to the formation of adhesions and a decrease in fertility. In fallopian tube-related infertility, ultrasound hysterosalpingography and laparoscopy are used for diagnostic, but also therapeutic purposes. Uterine disorders Disorders of the uterus that can lead to infertility include congenital anomalies, polyps, myomas, and adenomyosis. Myomas are benign tumors of the muscular layer of the uterus that can grow inside, outside, or in the wall of the uterus. The estimated prevalence of myomas is 4.5% to 68.6%. Generally, myomas have little effect on female fertility. However, it has been established that they are more frequent among infertile women. They are diagnosed in about 5-10% of infertile women but are responsible for only 2-3% of infertility cases. Myomas that affect the anatomical structure of the uterine cavity, most often submucosal myomas, can affect implantation mechanically. On the other hand, intramural myomas have a greater effect on the functionality of the myometrium. Both of these groups can negatively affect the receptivity of the uterine wall ( 8 ). Endometrial polyps are found in 3-10% of infertile women and are mostly benign, with malignant alteration occurring in up to 13% of cases. Since hyperplasia is the base pathogenic mechanism, hyperestrogenism poses a higher risk of development. The utility of removing endometrial polyps in treating infertility has not been completely cleared up. Currently, an assessment is done on the individual level, taking into account the duration of infertility, polyp size, and presence of additional symptoms. A rare acquired uterine disorder is Asherman syndrome which is present in 1% of infertile women and is defined by the presence of intrauterine adhesions, most commonly caused by medical procedures or endometriosis. Congenital anomalies that lead to infertility are septate uterus, bicornuate uterus, arcuate uterus, uterine agenesis, and uterus didelphys ( 9 ). Adenomyosis is characterized by the presence of endometriotic tissue inside the myometrium and can increase the risk of miscarriage, preeclampsia, or implantation disorders ( 6 ). Genetic causes Chromosomal aberrations which cause female infertility are translocations (Robertsonian and reciprocal), gonadal dysgenesis with short stature, inversions, X chromosome structural abnormalities, mixed gonadal dysgenesis, Swyer syndrome, mosaicism with Y chromosome, X-related autosomal translocations, X-related aneuploidy (X trisomy and Turner syndrome). An estimated 30-40% of patients with primary amenorrhea have Turner syndrome. Genetic disorders leading to female infertility also include fragile X syndrome, Kallman syndrome, syndrome of androgenic insensitivity, mutations in FSH/LH beta-component genes, and mutations in FSH/LH receptor genes. Genetic causes of female infertility are still being thoroughly researched and will hopefully be more illuminated in the near future. Causes of male infertility Causes of female infertility can be divided into pre-testicular, testicular, and post-testicular causes. Pre-testicular causes include endocrine disorders such as gonadotropin deficiency, systemic disorders such as liver cirrhosis, and immune system disorders. It is important to stress that an unhealthy lifestyle, often involving insufficient physical activity, a sedentary lifestyle, and an unhealthy diet, significantly impacts reduction in fertility. This trend has been observed especially in the last two decades ( 9 ). Testicular causes refer to spermatogenesis disorders, such as anorchia, testicular dysgenesis syndrome, or cryptorchism. Congenital anomalies, such as the absence of the vas deference, also have an effect. Conditions such as varicocele or orchitis can lead to structural and functional impairments of the testis, also contributing to infertility. Post-testicular causes of infertility include obstruction, ejaculation disorders, or abnormalities in semen fluid composition. Low semen quality can be caused by inflammatory conditions such as epididymitis, prostatitis, and urethritis. Additionally, sperm function can be altered in conditions such as primary ciliary dyskinesia, which makes fertilization more difficult. Erectile dysfunction is another contributing disorder, which can be organic or psychological in origin ( 2 ). Novel studies have demonstrated the effect of the microbiome on male infertility, which is moderated via the immune system ( 10 ). Some studies investigating the role of the human papillomavirus (HPV) in infertility have found that infection can negatively affect sperm DNA integrity, motility, count, and morphology. It can also stimulate the development of antibodies against sperm ( 11 ). Additionally, genetic factors are being researched in the context of male infertility. The most common are chromosomal aberrations, such as Y chromosome microdeletions, translocations, aneuploidy, and certain monogenic disorders. Cystic fibrosis, caused by CFTR variants, is often mentioned as a monogenic disorder that causes male infertility.

The aim of this article was to discrabe the observing of various causes of infertility. When observing them, both male and female causes must be observed in details.

Treatment of infertility is very individual as it primarily focuses on the specific causes behind it. If treatment based on etiology, either conservative or surgical, fails, assisted reproduction methods are put in place. Some conditions benefit from both conservative and surgical approaches, such as cryptorchism and varicocele. Cryptorchism can be treated with human chorionic gonadotropin (hCG) and gonadotropin-releasing hormone (GnRH), but surgical intervention is also necessary to preserve testicular function and prevent malignant alterations ( 15 ). In complex cases of male infertility, intracytoplasmic sperm injections (ICSI) can be used. Studies have demonstrated the importance of treating obesity as an initial step in obese women with infertility issues, before employing specific procedures ( 4 ). Where female infertility is concerned, the first line of therapy is pharmacological stimulation of ovulation using letrozole or clomiphene citrate. Clomiphene is a selective estrogen receptor modulator (SERM) with agonist and antagonist effects, resulting in an increased release of gonadotropins from the pituitary gland. Letrozole is an aromatase inhibitor that prevents estrogen production by blocking the conversion of testosterone. Another option is gonadotropin therapy, which is often more intense. Alongside pharmacological treatment, surgical treatment of intrauterine adhesions, myomas, congenital anomalies, and endometrial polyps is also an option. Hysteroscopy, while primarily a diagnostic method, can also be used for therapeutic purposes. Laparoscopy is used for both diagnosis and treatment and allows for the removal of endometriotic lesions or pelvic adhesions. It can also be used to assess fallopian tube status. Advancements in minimally invasive procedures in the treatment of infertility have resulted in a lesser need for “classical” surgical procedures which carry a higher risk of complications ( 16 ). In the context of infertility treatment, assisted reproductive technology (ART) is greatly meaningful. These include intrauterine insemination (IUI), in vitro fertilization (IVF), and intracytoplasmic sperm injection (ICSI) ( Figure 2 ). IUI implies the insertion of processed ejaculate directly into the uterus at the moment of ovulation. A variation on this procedure is intratubal insemination, which implies the insertion of ejaculate into the fallopian tube. These methods can be used with or without ovarian stimulation. However, a necessary precondition for these techniques is a normal fallopian tube structure, which can be assessed using hysterosalpingography or ultrasound ( 9 ). In vitro fertilization is often used in treating infertility. Oocytes are acquired through ultrasound-guided aspiration and fertilized in vitro using the partner’s sperm. If fertilization is successful, usually one to two embryos are returned to the uterus. According to current recommendations, the return of one embryo is optimal, five days after fertilization in the blastocyst phase (if possible). This method is also known as a single embryo transfer (SET) ( 9 ). High-quality embryos can be stored in specialized containers with liquid nitrogen at a temperature of -196⁰C. In cases of complex male infertility or unsuccessful fertilization via IVF, the ICSI method can be used, in which a healthy sperm is injected into the cytoplasm of an oocyte ( 9 ). Assisted reproductive methods can be used in a woman’s natural menstrual cycle, carrying a success rate of 10-15% per month, with mild ovarian stimulation, which raises the success rate to 25-30% per cycle and can be done 4-5 times a year, or with full ovarian stimulation, which raises the success rate to 40-60% and can be done 2-3 times a year ( 9 ). The latter does carry a higher risk of certain complications during pregnancy, such as hypertension, gestational diabetes, placenta previa, placental abruption, bleeding, polyhydramnios, and oligohydramnios ( 16 ). C-sections are also more common in ART-achieved pregnancies. In terms of pregnancy outcomes, premature labor, low birth weight, perinatal mortality, congenital malformations, and intrauterine growth disorders are more common ( 17 ). Some studies show that children conceived using ART have an increased risk of malignancies, but this is a topic still being researched ( 18 ).

The condition of infertility is defined as the lack of natural conception in a heterosexual couple after 12 months of regular unprotected sexual relations. For women over 35 years of age, evaluation is recommended after 6 months of conception failure. Zegers Hochschild F et al. defined fertility as the ability to achieve clinical pregnancy, fecundity as the probability of achieving pregnancy in a menstrual cycle, and subfertility as a term, partly interchangeable with infertility, which denotes an ability of achieving pregnancy after a longer period than is expected. The definition of infertility implies a specific timeframe, while sterility is a permanent state of infertility ( 1 ). Infertility can be categorized as primary or secondary. Primary infertility pertains to individuals who’ve never achieved pregnancy, while secondary infertility pertains to those who fail to achieve pregnancy after at least one successful one. The rate of infertility correlates with age, especially in women. An estimated 8-12% of couples worldwide are affected by this condition ( 2 ). In 25% of those cases, the etiology cannot be determined, which is called idiopathic infertility. In the remaining cases, contributing factors are equally divided among both genders. The most prevalent is the age of the female partner, followed by history of sexually transmitted diseases, history of pelvic inflammatory disease, metabolic syndromes including obesity and polycystic ovary syndrome, thyroid disorders, diabetes, etc. Certain genetic disorders, such as fragile X syndrome, Turner syndrome, cystic fibrosis and Klinefelter disease, also contribute to this condition. However, novel genetic studies have demonstrated a much wider range of contributors, which were previously unknown, allowing for more precise diagnostics. When observing various causes of infertility, both male and female causes must be observed in detail ( Figure 1 ). Often, it is the joint effect of these two groups of factors that leads to the infertility of a couple.

Infertility is a complex condition that requires a broad diagnostic and therapeutic approach in order to achieve optimal conditions for pregnancy. The definition of infertility, both primary and secondary, highlights the importance of timing in making the diagnosis. Different causes of infertility, including ovarian disorders, fallopian tube disorders, pelvic diseases, and uterine abnormalities, require an individualized approach to diagnostics and treatment. Causes of male infertility, divided into pre-testicular, testicular, and post-testicular, also require an individualized approach. Accurate and timely discovery of etiological factors in an individual is key for effective interventions and treatment. With modern advances in medical genetics, testing for chromosomal and genetic abnormalities is another useful tool in the diagnostic arsenal. While etiological treatment, either conservative or surgical, can lead to resolution in a certain number of cases, ART is the method of choice in most cases. This is an umbrella term consisting of multiple effective methods and as an approach, it can be used with a wide variety of etiological factors. With advancements further advancements in genetics, especially next-generation sequencing, it is expected that new causes of infertility will be discovered, continually improving our approaches to combat this frequent condition.

Diagnostics of female infertility The diagnostics of female infertility is performed by a gynecologist and implies taking a detailed patient history, ultrasound examination, gynecological examination, cervical swabs and Pap test. Hormone level assessment is also required and involves FSH, LH, estrogen, progesterone, prolactin, and TSH levels. A complete hormone level test is done between the second and fifth day of the menstrual cycle, with progesterone being reevaluated around the 21st day. Additionally, anti-Muller hormone (AMH), vitamin D, complete blood count, and iron levels are determined, irrespective of menstrual cycle phase. If necessary, additional endocrine tests can be conducted. Laboratory test results can significantly aid in discriminating patients with regular ovulation, patients with reduced ovarian reserve, and patients with PCOS ( 9 ). The diagnosis is often made by imaging and endoscopic examination methods, such as transvaginal ultrasound, hysteroscopy, laparoscopy, fallopian tube openness assessment, and pelvic magnetic resonance imaging. The most commonly used method for ovulation timing assessment is folliculometry, which entails keeping track of follicular growth. Aside from folliculometry, orientational methods such as menstrual cycle tracking, basal temperature measurements, or LH strips can be used. Hysterosalpingography is done after menstruation is complete and is used to evaluate the uterine cavity and fallopian tubes. A more exact evaluation is provided by hysteroscopy, which involves observing the uterine cavity via a transvaginal device. Aside from diagnostics, this method provides therapeutic possibilities in certain conditions, such as septate uterus, polyps, and submucosal myomas. A dangerous complication of hysteroscopy is uterine wall rupture followed by potential damage of other intraabdominal organs ( 9 ). Laparoscopy is a minimally invasive procedure that can also be both diagnostic and therapeutic and allows for the visualization of the entire abdominal cavity. It allows for the discovery of certain fallopian tube-related causes of infertility, as well as endometriotic lesions. Specialized laboratory diagnostics in the case of infertility may include thrombophilia testing, immune system tests, genetic testing, or endometrial biopsy. These are more frequently used in patients with primary ovarian insufficiency. In terms of genetic disorders, the main structural abnormalities are Robertsonian and reciprocal translocations. Women with reciprocal translocations are exposed to a higher risk of infertility. Syndromes such as Fragile X can lead to menstrual abnormalities, decreased ovarian reserve, and premature ovarian insufficiency ( 12 ). Diagnostics of male infertility The diagnosis of male infertility is primarily based on sperm evaluation or spermiogram. Optic microscopy is used to visualize and determine the concentration of the sperm. World Health Organization guidelines include criteria such as a minimum volume of 1.4 mL, minimum sperm count of 15 x106/mL, minimum total motility of 42%, minimum progressive motility of 30%, and minimum normal morphology of 4% ( 13 ). One of the novel methods for early and precise infertility diagnosis is nanotechnology, which utilizes biosensors to detect new biomarkers. These biosensors can be calibrated for proteins, DNA, and hormones. Currently being researched are nanoparticles with ligand-binding and anti-oxidative capabilities and their potential to improve male infertility ( 13 ). A key diagnostic method in male infertility diagnostics, especially where oligozoospermia and azoospermia are concerned, is genetic testing. Contributing genetic factors have been established in all categories of male infertility. According to OMIM (Online Mendelian Inheritance in Man), there are over 200 genetic conditions associated with male infertility. These include chromosomal aberrations such as Klinefelter syndrome, characterized by a 47 XXY genotype. Microdeletions in the Yq region, also known as the azoospermia factor region, are present in 8-12% of men with azoospermia and 3-7% of men with oligozoospermia, making it the most common genetic cause of male infertility. Regarding genetic mutations associated with infertility, CFTR mutations are a relatively frequent occurrence, causing cystic fibrosis and vas deferens abnormalities ( 12 ). Male infertility is also associated with other genetic factors, such as ADGRG2 mutations and AR mutations, which can affect both anatomical and functional aspects of the male reproductive system ( 14 ).