Isolated asthenozoospermia: What should we know? A narrative review of literature

Abstract

Male infertility contributes to 20%–70% of infertile couple cases worldwide. One of the key challenges in this area is asthenozoospermia or reduced sperm motility. A particular subset, known as isolated asthenozoospermia (iASZ), in which low motility occurs without abnormalities in sperm count or shape, often goes unnoticed and is not well understood. This narrative review attempts to compile the state of knowledge regarding the etiology, diagnosis, and treatment of iASZ and to identify knowledge gaps that need further investigation with an emphasis on physiological, genetic, and molecular mechanisms. We conducted a focused literature search using PubMed and Web of Science, covering studies published between 2000 and 2024. We included articles that explored the structure, function, diagnosis, and treatment of iASZ. We find sperm motility is influenced by multiple factors, from structural components like mitochondria and the flagellum, to biochemical signals and genetic mutations. In men with iASZ, issues such as mitochondrial dysfunction, oxidative stress, or specific gene defects may impair motility. While routine semen analysis helps flag low motility, it often fails to explain why it occurs—highlighting the need for advanced tests like DNA fragmentation tests, genetic screening, and high-resolution imaging. As regards treatment, lifestyle changes, varicocele surgery, and antioxidants can offer some benefit. Assisted reproduction techniques, especially intracytoplasmic sperm injection (ICSI), remain central for achieving pregnancy when other options fail. iASZ deserves more attention as a distinct and treatable cause of male infertility. By improving how we diagnose and manage this condition, supported by further research, we can offer better outcomes for affected couples hoping to conceive.

Keywords

asthenozoospermia / genetics / isolated asthenozoospermia / male infertility / semen analysis / sperm motility

1 INTRODUCTION

Infertility is a common social and medical problem affecting approximately 8%–12% of reproductive-aged couples globally[1]. According to an epidemiological review, the male factor contributes to approximately 20%–70% of infertility conditions[2]. The initial evaluation of male infertility includes the assessment of medical and sexual history, physical examination, hormonal evaluation, and semen analysis. Notably, the semen analysis is the mainstay in the evaluation of male fertility as it can guide further investigations and management[3]. According to the sixth edition of the World Health Organization (WHO) manual of human semen analysis, the basic semen analysis should cover the evaluation of sperm concentration, motility, viability, and morphology.

Sperm motility defines the pattern of motility and velocity of spermatozoa, which can be classified into progressively motile, non-progressive motility (which describes only lateral head displacement without actual motility), and immotile spermatozoa[4]. Asthenozoospermia elucidates the reduction or absence of sperm motility in fresh ejaculate (< 40% totally motile and/or < 32% progressive motility)[5,6]. There is a substantial correlation between sperms motility and fertilization rates since the spermatozoa is deposited in the female vagina during sexual intercourse and should travel an average distance of 19 cm before meeting the oocyte at the ampullary site of the Fallopian tubes, where the fertilization process takes place[4,7].

Asthenozoospermia can be the sole cause of male infertility in approximately 16%–24% of cases, where this condition is known as isolated asthenozoospermia (iASZ). Furthermore, it can be a contributing factor with other defects in sperm characteristics, such as the reduction in the number of sperm per milliliter (oligozoospermia) or the abnormalities in the sperm morphology (teratozoospermia) in approximately 55%–61% of cases[8,9].

In this setting, iASZ is an important yet often under-recognized etiological factor of male infertility, where clinical attention focuses more frequently on overt abnormalities of semen parameters such as oligozoospermia, teratozoospermia, or combined abnormalities (oligo-astheno-teratospermia). This deficient clinical awareness about iASZ may lead to suboptimal management and delayed intervention. Furthermore, this finding is reflected in the presence of the scarce body of evidence focusing mainly on iASZ despite of the significant number of articles discussing different aspects of asthenozoospermia and sperm motility in general[10–20]. Additionally, the poor understanding of the etiological spectrum of iASZ in the clinical practice including mitochondrial dysfunction, oxidative stress, and genetic factors, highlights the gap in the literature requiring further studies to enhance its clinical awareness and understanding[11,16,17,21]. This is also reflected on the diagnostic algorithm of iASZ, where conventional tests like semen analysis is more of a descriptive rather than diagnostic as it provides no explanation for the reduced motility and lacks evaluation of critical aspects of sperm functions such as DNA integrity, underlying genetics, or reactive oxygen species level (ROS) rendering genetic and molecular testing essential in this kind of patients[21–24].

In this setting, the current narrative review aims to provide a comprehensive guide for iASZ starting from the physiological and anatomical factors of sperm motility to the diagnostic evaluation and treatment options.

2 METHODS

This narrative review was performed by the members of the andrology unit in the Urology Department of the Faculty of Medicine, Tanta University, Egypt. A comprehensive search of the literature was performed from August to December 2024 to identify the articles discussing iASZ. Two authors (AE & AZ) searched the PubMed database using a combination of the following medical subject headings (MeSH) terms: “sperm tail,” “sperm motility,” “diagnosis,” “therapeutics,” “epidemiology,” “etiology,” and “asthenozoospermia.” During the MeSH search, the subheading was limited to include searches related to drug effects, metabolism, pathology, physiology, ultrastructure, genetics, classification, complications, congenital, diagnosis, diagnostic imaging, diet therapy, drug therapy, epidemiology, etiology, history, physiopathology, prevention and control, surgery, and therapy. Furthermore, some other keywords were combined with the MeSH search including “isolated,” and “asthenospermia.” The search was limited to articles on human subjects published in English between January 1, 2000 and December 31, 2024, during the revision of this manuscript, to give an update about the etiology, diagnosis, and management options of iASZ. Furthermore, the Web of Science (WOS) core collection database was searched using a combination of the same keywords and the same search restrictions to identify all the relevant articles.

3 RESULTS

Overall, 255 and 64 articles were identified through the PubMed and WOS searches, respectively, of which 44 duplicate articles were excluded. Subsequently, 156 articles were excluded through the initial screening of the articles’ title and abstract, leaving 119 studies for the full text review. Finally, 50 manuscripts were excluded for different reasons including studies performed on animal subjects (n = 23), studies about other forms of sperm abnormalities (n = 6), non-English articles (n = 1), manuscripts outside the aim of the current study (n = 19), and articles that we were not able to gain access to (n = 1). Therefore, 69 articles were included in the current review. The included articles covered morphology and structure of the sperm[25], the role of calcium[26–29], capacitation[30–32], sperm maturation and motility[33–36], oxidative stress[16,37–39], genetic defects[21,40–50], proteomic and transcriptomic analysis[16,24,51–72], infection and inflammatory responses[73–76], diagnosis[14,23,77–79], and treatment[10,13,20,80–83].

4 DISCUSSION

4.1 Morphological structure of the sperm

The mature spermatozoon is a unique specialized single cell that is composed of head, midpiece, and tail (flagellum), all of which are surrounded by the plasmalemma, which is formed of a single continuous plasma membrane[84]. The head is composed of two parts: the acrosome (anteriorly located and produces important enzymes for fertilization) and the post-acrosomal region (posteriorly located and contains the nucleus). The midpiece (also known as the neck) contains an average of 13 gyres with two mitochondria (each containing a single copy of mitochondrial DNA) in each gyre. These mitochondria are responsible for the generation of energy required for the sperm tail. The reduction of the number or function of these mitochondria is correlated with the development of asthenozoospermia[16]. Finally, the sperm tail or the flagellum is arranged in a peculiar cylindrical structure known as axoneme. The axoneme constitutes a microtubule-based molecular engine that drives the sperm motility[85]. The axoneme is surrounded by outer dense fibers (ODF) forming the 9 + 2 structure[50]. The ODF are further surrounded by a fibrous sheath in the principal piece of the flagellum, which affects both the plane of flagellar beating and the tail's flexibility[84]. On the other hand, the midpiece of the flagellum is densely packed with mitochondria that produce energy for sperm motility[86]. The structure of the axoneme is controlled by hundreds of microtubule-associated proteins and dynein motor proteins, which have been linked to the control of traveling direction of the spermatozoon and play a paramount role in sperm motility.

4.2 Physiological aspects of sperm motility

4.2.1 Sperm maturation in relation to its motility

Normally, spermatozoa start to form in the epithelium of the testicular seminiferous tubules; however, the testicular spermatozoa are morphologically incomplete, immotile, and lack the ability to fertilize an oocyte[87]. Sperm morphological maturation, motility, fertilizing ability are acquired as the sperm transit through the epididymis from its proximal to distal end, where the sperm is exposed to a continuously modified epididymal milieu[88]. Theoretically, epididymal sperm maturation is controlled by extracellular vesicles mainly secreted by epididymis known as epididymosomes. These epididymosomes contain proteins and enzymes that are transported to maturing spermatozoa inside the epididymis and play an important role in sperm maturation, elimination of defective sperm, motility, and sperm-egg interaction[35,36]. Furthermore, the Hermes body; previously known as cytoplasmic droplet; is a specialized structure attached to the flagellum during its maturation in the epididymis that complement the functions of the epididymosomes (mainly energy production, membrane permeability, protein turnover, and protection against oxidative stress) through its abundant content of glycolytic enzymes, Golgi- and ER-derived membranes, stress-response proteins, and cytoskeletal elements. It plays a significant role in the initiation of sperm motility[33]. In this setting, the macrophage migration inhibiting factor (MIF); which is one of the constituents of the epididymosomes; become integrated with the sperm ODF to reduce the zinc content by 60%, which is a critical step in facilitating sperm motility[89]. On the same hand, extracellular vesicles secreted by the prostate are known as prostasomes and play an important role in the development of sperm motility through its content of Ca²⁺ signaling proteins[90].

4.2.2 Capacitation

Capacitation defines the morphological and functional transformations that the spermatozoa experience during their passage through the female genital tract on their way to fertilize the egg. It involves three main events: (1) lipid changes (mainly cholesterol), (2) change in the sperm membrane potential through ions movement, and (3) tyrosine phosphorylation of proteins[30–32]. Capacitation is a prerequisite for the development of sperm hyperactivated motility, which is an utmost pattern of sperm motility characterized by a relatively progressive movement with high amplitude dissymmetric flagellar bending[30]. This pattern of motility is essential for the spermatozoa to move through luminal structures filled with viscous fluid like the utero-tubal junction.

4.2.3 Functional aspects of sperm motility

Mechanism of sperm motility

Sperm motility is a process that is highly dependent on the consumption of energy produced by the sperm mitochondria, where the flagellum moves in a plane perpendicular to the central microtubules with an increasing amplitude during flagellar wave propagation[91]. By the end of the axoneme, the ODF layer tapers to reduce tail bending resistance, thereby facilitating sperm motility[92].

Motility patterns

The pattern of sperm motility is not fixed, and it varies as it progresses through its journey to fertilize an oocyte in the female genital tract. As mentioned previously, testicular sperm are immotile, and the movement ability is acquired with the sperm maturation within the epididymis[88,93]. Considering the motility patterns, sperms from efferent ducts showed non-progressive movement, while those from the proximal corpus epididymis showed forward progressive motility[93]. On the same hand, Calvin and Bedford, reported that the rigidity provided by the disulfide bonds in the ODF and the fibrous sheath explains the rapid and progressive motility of sperms obtained from the cauda epididymis and the ejaculate[94]. Finally, capacitated sperm acquire hyperactivated motility inside the female genital tract to be able to penetrate the zona pellucida[95].

4.2.4 Biochemical regulators of sperm motility

Calcium

Sperm intracellular Ca²⁺ concentration, which is regulated by the sperm-specific Ca²⁺ channel CatSper, plays a significant role in sperm hyperactivation, where the increase of intracellular calcium levels changes the flagellar beating of the spermatozoa from symmetrical to asymmetrical[26,27]. This ion channel is considered among the most complex channels in humans as it consists of 4 pore-forming subunits (CatSper 1-4) and multiple auxiliary subunits[26]. These channels are pH sensitive and require an alkaline pH to permit Ca²⁺ entry and subsequently allow sperm hyperactivation. They may be activated by some physiological stimulants such as progesterone, cyclic nucleotides, and zona pellucida glycoproteins[28]. Furthermore, low physiological concentrations of prostaglandin E2 and E2-alpha may significantly contribute to activation of sperm motility and binding to zona pellucida likely through the activation of CatSper channels.

This regulatory effect of Ca²⁺ is probably related to its role in the regulation of intracellular cyclic adenosine monophosphate (cAMP), which is essential for both sperm capacitation and acrosome reaction. Moreover, the activation of Ca²⁺/calmodulin (CALM) signaling pathway is related to the regulation of intracellular cAMP levels and sperm hyperactivated motility.

Kinases and phosphatases

Phosphorylation is the cornerstone for reversible control of protein function in almost all human cells. This process defines the addition of phosphate group to a molecule or an ion[96]. In this setting, protein kinases are essential for initiating different signaling pathways and regulating different cellular functions such as transcription, cell-cycle progression, apoptosis, intercellular communication, and immunological functions[97]. These protein kinases play a significant role in the process of phosphorylation of sperm amino acids as they enable the transfer of a phosphate group from the ATP to become attached to a free hydroxyl group of specific amino acids (mostly to serine and threonine amino acids)[98].

As mentioned before, this phosphorylation process is indispensable for sperm capacitation, hyperactivated motility, and fertilization process[99]. Leclerc et al.[100] supported the role of phosphorylation process in the controlling of sperm motility as the authors showed a significant increase in phosphotyrosine content of two main proteins of the human spermatozoa (105 and 81 kilo Dalton) during the sperm capacitation process[100]. Similarly, Martin-Hidalgo et al.[34] demonstrated that phosphorylation of glycogen synthase kinase 3 alpha (GSK3A) was significantly higher in high-motility sperm, supporting the role of phosphorylation in the regulation of sperm motility.

Cell volume and osmolarity

One of the important functions acquired by the spermatozoa during their maturation in the epididymis is the capacity to control their cell volume[101]. This function is essential for the fertilization process and the proper development of sperm motility. The ability to control the cellular volume is probably obtained through the uptake of organic osmolytes like L-carnitine and amino acids to resist any decrease in the cellular volume during the epididymal transit[102].

On the other hand, as the spermatozoa reaches the female genital tract (hypo-osmotic environment) it is exposed to a significant change in the osmotic pressure[103]. Accordingly, the sperm counteract this change to avoid swelling through losing the osmolytes and water that were uptaken in the epididymis. Any flaw in this precise process of cell volume control will subsequently affect sperm motility and fertility, and results in deformed sperms with large head and angulated tail.

Reactive oxygen species (ROS)

ROS particularly hydrogen peroxide (H₂O₂) has a dual effect on sperms’ metabolism and functions as it has a physiological effect by inducing cAMP in spermatozoa with subsequent inhibition of tyrosine phosphatase resulting in tyrosine phosphorylation, which is an important step for sperm capacitation[104]. On the other hand, the excessive oxidative stress results in an inhibitory effect of tyrosine phosphorylation process and increases the liability to DNA damage affecting the sperms’ motility and functions[105]. In this setting, proper sperm function requires a balance between ROS and antioxidants to buffer the effect of oxidative stress.

Oxidative stress can occur as a result of intrinsic factors such as spermatic leucocytes, infection, and hyperglycemia (as impaired glucose metabolism in patients with uncontrolled diabetes may contribute to asthenozoospermia[78]), or extrinsic factors such as smoking, excessive alcohol intake, or exposure to heavy metals[106]. Furthermore, coenzyme Q10 (CoQ10); a lipid molecule that plays an important role in the protection of cells from ROS and free radicals, and improving sperm motility[37]; demonstrated significantly higher levels in seminal plasma and lower cellular levels in varicocele patients reflecting reduced cellular utilization that potentially contributes to increased oxidative stress and reduced sperm motility.

Proteomic and transcriptomics analysis

Proteomics is a term that is used to describe the comprehensive analysis of the structure, function, and interaction of different proteins using tools such as two-dimensional (2D) electrophoresis, fluorescence 2D difference gel electrophoresis, isotope coded affinity tag[18], O stable isotope labeling, large-scale western blotting proteome analysis, protein microarray technology, and mass spectrometry[107]. Proteomic analysis of the proteins in the sperms and seminal fluid helps to enhance the understanding of different proteins integrated in sperm functions, such as motility and fertilization, and provides insights into the molecular mechanisms underlying asthenozoospermia.

Proteins integrated in sperm function can be divided into five categories including (i) sperm motility and morphological organization proteins (i.e., tubulin beta 2B (TUBB2B), tubulin beta 2C (TUBB2C), outer dense fibers protein 2 (ODF2), A-kinase anchor protein 4 (AKAP4), keratin type II cytoskeletal 1 (KRT1), tektin 1 (TEKT1), transition protein 1 and 2 (TNP1 and TNP2), spermatid-specific linker protein (SSLP), histone h1-like protein (HILS1), and Clusterin (CLU)); (ii) energy and metabolism proteins (i.e., triose phosphate isomerase (TPIS), testis-specific glycerol kinase 2 (GKP2), cytochrome c oxidase subunit 6B (COX6B), glyceraldehyde-3-phosphate dehydrogenase testis-specific (GAPDS), succinyl-CoA:3 ketoacid co-enzyme A transferase mitochondrial precursor (OXCT1), and phospholipids hydroperoxide glutathione peroxidase-mitochondrial (PHGPx)); (iii) stress response and turnover proteins (i.e., heat shock-related 70 kDa protein 2 (HSPA2), proteasome alpha 3 subunit (PSMA3), and stress 70 protein mitochondrial (HSPA9)); (iv) signaling and transport proteins (i.e., voltage-dependent anion-selective channel protein 2 (VDAC2), Aurora kinase B (AURKB), and glycodelin); and (v) proteins possessing antioxidant activities (i.e., glutathione S-transferase Mu3 (GST Mu3)). On the other hand, some proteins do not belong to any of the above-mentioned categories as their functions are not yet known (i.e., sperm protein associated with the nucleus on the X chromosome B (SPANXB), and isoaspartyl peptidase/L-asparaginase (ASRGL1)).

Several authors reported that sperm proteome in asthenozoospermic men revealed a statistically significant reduction of AKAP4, ODF2, ODF3, ADAM32, CCDC183 TUBB2B, COX6B, GST Mu3, PHGPx, GAPD-S, VDAC2, HSPA2, HSPA9, HABP1, and SPANX, while the expression of CLU, ASRGL1, lactotransferrin, AURKB, and KRT1 was significantly higher compared to normozoospermic men[16,24,51,57,59,61,69,70]. Similarly, HILS1, TNP1, and TNP2 are key proteins for chromatin condensation during spermatogenesis, which were reduced in samples obtained from patients with asthenozoospermia[53], with reduction of H4 acetylation, and alteration of H3K9 and H4K20 methylation[66]. On the other hand, seminal exosomes of men with sever asthenozoospermia showed higher levels of glycodelin protein, which negatively impact sperm function and motility as it inhibits albumin-induced cholesterol efflux from the spermatozoa, thus inhibit the capacitation process[52]. Other proteins and peptides such as angiotensin II type 2 receptors and pituitary adenylate cyclase-activating polypeptide (PACAP) were detected in seminal fluid and may play a role in the regulation of sperm motility[55,62]. Cannabinoid receptors (CB1 and CB2) are also expressed on human spermatozoa sharing in the regulation of sperm motility, where the activation of CB1 reduces the sperm motility and the activation of CB2 shift the motility pattern from rapid progressive to sluggish motility.

Transcriptomics refers to the study of the mRNA levels to identify proteins transcript levels[108]. Transcriptomic analysis of high- and low-motility sperms from normozoospermic men revealed that the transcripts related to nuclear condensation (protamine 1 and 2) and capacitation (endothelial nitric oxide synthase and neuronal nitric oxide synthase) were higher in low-motility sperms suggesting a potential link with reduced sperm motility[54,63,64,67,71]. Similarly, sperms obtained from asthenozoospermic men showed significantly lower levels of ribosomal proteins transcripts and key fertility related genes such as RPS24 (ribosomal protein S24), HNRNPC (heterogeneous nuclear ribonucleoprotein C), and RPS27A (ribosomal protein S27A) implying that alteration of sperm RNA profiles could be used as a potential biomarker for male infertility[60]. On the same hand, pro-enkephalin (PENK) transcript was identified in the postacrosomal region of the sperm head using reverse transcription-polymerase chain reaction (RT-PCR). PENK is the precursor of met-enkephalin, which is an opioid peptide also identified in the human sperm head and plays an important role in the autocrine regulation of sperm motility[56]. Endogenous met-enkephalin boosted sperm motility, while the exogenous met-enkephalin was associated with biphasic effect on sperm motility with initial increase of motility (2 min) followed by reduced motility (30 min)[56,65]. Other surface membrane enzymes such as aminopeptidase N (APN/CD13) and neutral endopeptidase (NEP/CD10) are found in human sperm cells and share in the regulation of sperm motility, where their inhibition improves either hyperactivated or progressive motility, respectively.

Immunological factors

There is a great discrepancy and inconsistency between studies and cohorts, rendering it impossible to define a conclusive role of cytokines in male infertility and to define reference ranges for different cytokines in fertile men[109]. Focusing on sperm motility, the concentration of several cytokines such as interleukin 1 beta, soluble interleukin 1 receptor agonist, interleukin 2, soluble interleukin 2 receptors, interleukin 6, cyclooxygenase-2, hypoxia-inducible factor 1-alpha, and tumor necrosis factors have been correlated with asthenozoospermia.

Gut microbiota significantly affects individuals immune and causative agent resistance, where dysbiosis may be associated with inflammatory response and autoimmune disease. In this setting, gut microbiota dysbiosis may alter the integrity of the blood-testis barrier affecting sperm functions. Pan et al.[74] reported in their study that the gut microbiota is significantly different in asthenozoospermic patients compared to normozoospermic individuals, highlighting the immunological role of gut microbiota in asthenozoospermia.

4.3 Genetic bases of asthenozoospermia

Sperm motility depends on several parameters, including a normally and fully functional flagellum, energy consumption, and multiple signaling pathways. Thus, any mutation in genes involved with these parameters can result in reduced or absent motility [21]. Moreover, it is widely accepted that genetic mutations (i.e. ADCY10 (adenylate cyclase 10), AKAP4 (A-kinase anchoring protein 4), CATSPER1 (cation channel sperm associated 1), CATSPER 2 (cation channel sperm associated 2), CATSPER3 (cation channel sperm associated 3), CATSPER4 (cation channel sperm assocaited 4), PLA2G6 (phospholipase A2 group VI), and A229V variant) and/or DNA methylation defects (CREM (cAMP responsive element modulator), HDAC1 (histone deacetylase 1), and DNMT3A (DNA methyltransferase 3 alpha)) are among the principal reason underlying a large population of men with iASZ[41,43,45,47]. In this setting, genetic factors may affect sperm motility through the alteration of; (1) sperm mitochondrial DNA and proteins, (2) ion transport and channels, and (3) flagellar proteins also known as multiple morphological alterations of the flagella (MMAF).

4.3.1 Sperms mitochondrial DNA and proteins

The sperm mitochondria are responsible for the production of energy required for sperm motility. Therefore, any genetic alteration of the mitochondrial DNA (mtDNA) can affect sperm motility such as MT-ND1 (mitochondrially encoded NADH; ubiquinone oxidoreductase core subunit 1), MT-ND4 (mitochondrially encoded NADH; ubiquinone oxidoreductase core subunit 4), and MT-COX1 (mitochondrially encoded cytochrome c oxidase 1).

4.3.2 Ion channels

Since ions transport is essential for the maturation and functions of sperms, any defect among the genes controlling ions, channels will in turn affect the sperm functions including motility. For example, mutations of CATSPER genes will cause disruption of the progesterone-sensitive calcium current reducing the intracellular calcium concentration, which will negatively affect sperm motility[27,41]. Similarly, sperm motility is affected by the mutation of SLC26 family genes, including SLC26A3 (solute carrier family 26 member 3), SLC9C1 (solute carrier family 9 member C1), and SLC26A8 (solute carrier family 26 member 8) genes (a group of transporters for small anions), and voltage-dependent anion-selective channels (VDACs) genes that play a role in regulating intracellular calcium levels.

4.3.3 Flagellar proteins

Mutations of genes that are responsible for the development of flagellar proteins result in abnormal architecture of the sperm tail, which subsequently affects its motility such as MMAF, primary ciliary dyskinesia (PCD), or dysplasia of fibrous sheath (DFS)[110].

MMAF

MMAF defines a condition in which sperms are immotile in the presence of morphological defects in the flagella such as absent, short, or angulated tail. It is easily diagnosed using during routine microscopic examination of semen. MMAF can occur as a result of mutation in several genes including DNAH1 (dynein axonemal heavy chain 1) (it encodes a dynein heavy chain that is critical for axonemal movement and it is considered the most important gene mutation as it is responsible for approximately 25% of MMAF cases), CFAP251 (cilia and flagella associated protein 251), DNAH2 (dynein axonemal heavy chain 2), DNAH6 (dynein axonemal heavy chain 6), DNAH8 (dynein axonemal heavy chain 8), DNAH9 (dynein axonemal heavy chain 9), and DNAH17 (dynein axonemal heavy chain 17) that results in defective outer and/or inner dynein arms[46,49,50]. Other gene mutations have also been correlated to MMAF, including AKAP3 (A-kinase anchoring protein 3), AKAP4 (A-kinase anchoring protein 4), and AK7 (adenylate kinase 7) (AK7 is a gene responsible for the production of some flagellar proteins whose mutation impairs energy metabolism with reduction of sperm motility)[44,111].

Primary ciliary dyskinesia (PCD)

PCD is an autosomal recessive disorder characterized by the presence of absolute asthenozoospermia (100% immotile sperms) in the presence of normal semen morphology and viability. Its diagnosis requires semen examination using transmission electron microscope to confirm the absence or reduction of outer and/or inner dynein arms in cilia and sperm flagella[112]. The most important genes involved in the development of PCD are DNAI1 (dynein axonemal intermediate chain 1) and DNAH5 (dynein axonemal heavy chain 5) (responsible for approximately 30% of cases); however, mutations of other genes have also been correlated with the development of PCD including but not limited to CCDC39 (coiled-coil domain containing 39), DNAH11 (dynein axonemal heavy chain 11), CCDC151 (coiled-coil domain containing 151), DNAI2 (dynein axonemal intermediate chain 2), SPAG1 (sperm associated antigen 1), SPAG16 (sperm associated antigen 16), and DNAAF4 (dynein axonemal assembly factor 4)[42,50,113]. Noteworthy, pregnancy have been reported in patients with PCD and was uneventful with the delivery of healthy children[114].

Dysplasia of fibrous sheath (DFS)

DFS results morphologically abnormal sperms in the form of spermatozoa with short tail or stump. This condition significantly affects sperm motility resulting in male infertility and most often causes MMAF[115]. There are scarce data in the literature about the genetic mutations involved in the development of DFS, but it may be related to mutations of AKAP3, AKAP4, DNAH1, and GAPDS (glyceraldehyde-3-phosphate dehydrogenase, spermatogenic) genes.

Others (rare forms)

Collodel et al.[40] reported a rare case of infertility in a 45-year-old man with severe asthenospermia related to the presence of extremely elongated and coiled tail rendering them prone to rupture at different levels and dynein deficiency. The authors reported that this defect potentially results from genetic abnormality because this patient was the son of first-degree cousins.

4.4 Absolute asthenozoospermia

Absolute asthenozoospermia describes a condition, where 100% of the sperms are immotile. It may occur in 1 of 5000 men. Absolute asthenozoospermia occurs as a result of either genetic abnormalities causing ultrastructure anomalies of the sperm flagellum (as discussed before) or the production of unviable sperms (necrozoospermia). Thus, it is important to differentiate between infertile men with viable immotile sperms (probably due to genetic anomalies) and men with non-viable spermatozoa (necrozoospermia) as a result of oxidative stress, antisperm antibodies, and environmental pollutants exposure [116].

4.5 Diagnosis of asthenozoospermia

Diagnosis should start with history taking and physical examination to define any associated comorbidities. Furthermore, semen analysis is the cornerstone evaluation test for any infertile man including those suffering from iASZ.

4.5.1 Laboratory investigation

Endocrinal assessment

Generally, infertile men with abnormal semen parameters should undergoes assessment of serum testosterone (and serum LH in patients with abnormal testosterone) and serum FSH[117]. Generally, patients with absolute asthenozoospermia may show a higher level of serum FSH compared to individuals with normally motile sperms reflecting some degree of testicular dysfunction.

Testing for genitourinary infections

If the history and physical examination are suggestive for the presence of urethritis, prostatitis, epididymitis, or orchitis, it is recommended to test the patient for genital tract infection[117]. For instance, infections like Candida albicans and Escherichia coli have been correlated with reduced sperm motility, membrane mitochondrial potential, membrane phosphatidylserine externalization, and increase of apoptosis and DNA fragmentation[73,75]. Similarly, leukocytozoospermia (the presence of more than one million white blood cells per milliliter in semen) is correlated with increased production of ROS resulting in oxidative stress, which may be underlying cause of asthenozoospermia.

Anti-sperm antibody (ASA) testing

ASA should be indicated in the following conditions: patients with iASZ, history of testicular trauma, previous history of male reproductive tract surgery, and past history of testicular torsion. Furthermore, sperm agglutination or the attachment of motile sperms to each other under optical microscopy should raise the concern about the presence of ASA[118].

Sperm DNA integrity testing

The clinical value of sperm DNA integrity testing is still controversial; however, it may be used for patients with idiopathic infertility. Yet, sperm DNA fragmentation (SDF) have been negatively correlated with fertility[119]. In this setting, Gill et al.[23] showed that men with asthenozoospermia exhibited significantly higher levels of SDF (24% versus 12%), along with a greater prevalence and increased odds ratio (OR) for SDF levels exceeding 20%, compared to the control group. In particular, the occurrence of > 20% SDF in studied subjects was over 4.5 times more common in men with reduced sperm motility (61.97% vs. 13.26%), and a 10-fold higher OR of > 20% SDF compared to the control group[23]. Similarly, other studies have confirmed that asthenozoospermic patients have a higher percentage of spermatozoa with fragmented DNA when compared to men with a normal semen profile or normokinetic sperm cells[120–122]. On the contrary, some authors reported that there is no significant difference in the SDF levels between asthenozoospermic and normozoospermic patients[123,124]. Interestingly, men with high DNA fragmentation and teratozoospermia were more likely to have erectile dysfunction; however, this finding was not consistent among men with iASZ[79]. On the contrary, another study demonstrated that DNA fragmentation was higher among men with iASZ (31%) compared to isolated oligozoospermia and isolated teratozoospermia.

Genetic screening

Genetic screening in patients with iASZ is of paramount importance as it helps in the identification of the underlying molecular defects that is not normally detectable by the conventional semen analysis. Furthermore, the accurate diagnosis of the underlying cause of iASZ can guide the treatment (i.e., early adoption of assisted reproductive technologies or mitochondrial-targeted therapies in case of energy metabolism defects) and allow genetic counseling about the fertility outcomes and inheritance risk.

4.5.2 Electron microscopy

Electron microscopy is essential for the evaluation of ultrastructure and functions of different sperm organelles, particularly in patients with severe asthenozoospermia (< 5% motile sperm) or absolute asthenozoospermia. It may be helpful in the assessment of the molecular structure of the sperm, such as axonemal defects, sperm mitochondrial abnormalities, and defects of the fibrous sheath or the flagellum[125].

4.5.3 Recent high-resolution methods

Advancement in the technology of microscopic examination has also been proposed for the evaluation of human spermatozoa abnormalities, such as confocal laser microscopy, that was used for the assessment of acrosomal and nuclear abnormalities, and to identify the location of androgen and estrogen receptors in the midpiece of human spermatozoa[126,127]. Furthermore, three-dimensional laser scanning microscope was applied for the examination of different laboratory animals' sperms providing images that are comparable to electron microscopy while using lower magnification power[128].

4.5.4 Radiological investigation

Scrotal Doppler ultrasound

Scrotal Doppler is mainly indicated in patients with simultaneous iASZ and varicocele[117]. Interestingly, a recent study by Elshibany et al., demonstrated that patients with iASZ showed significantly more heterogenous echotexture of the epididymal parts compared to fertile men.

Transrectal ultrasound

Transrectal ultrasound can be used for the visualization of seminal vesical and vas deference in case of suspected obstruction.

4.6 Treatment of asthenozoospermia

Addressing management strategies for iASZ is relatively deficient in the literature. There are no absolute recommendations available for treatment of iASZ. However, given the clear evidence linking asthenozoospermia to oxidative stress, smoking, varicocele and genital infection, there may be a place for treatment to improve fertility potential in such patients. Furthermore, it should be noted that men with iASZ should be informed that all reproductive options including spontaneous pregnancy (24%), intrauterine insemination (13%), and in vitro fertilization (13%) have similar pregnancy rates and depends on the initial total motility rates.

4.6.1 Lifestyle counseling

Lifestyle modification can be an additive treatment option to any patient with asthenozoospermia, and it should include avoidance of chronic alcohol consumption, increased intake of ultra-processed foods, and excessive testicular exposure to heat, as in saunas, for their negative impact on sperm parameters[129–131]. Cigarette smoking may also be associated with isolated asthenozoospermia (light smoking) and teratozoospermia (heavy smoking), thus infertile men should be counseled that giving up smoking may improve their semen parameters[81,82]. In the same setting, Eslamian et al. reported that persons whose diet are rich in vitamin E, vitamin D, vitamin C, zinc, folate, total fiber, selenium, and polyunsaturated fatty acids have 51% lower risk of asthenozoospermia.

4.6.2 The relation between varicocele and iASZ

Boman et al. looked at the effect of varicocele ligation on fertility potential in patients with iASZ[20]. They retrospectively analyzed 2 groups of patients (varicocele ligation group/69 patients; and a control group who opted not to undergo surgery/49 patients). They concluded a beneficial effect of varicocele ligation on iASZ patients as the mean total motile sperm count significantly improved from 29.6 million preoperatively to 39 million postoperatively. Moreover, the varicocele ligation group exhibited significantly higher natural pregnancy rate as compared to the control group (65% vs. 32%). The combined natural pregnancy and intrauterine insemination (IUI) pregnancy rate was also significantly improved in the varicocelectomy group as compared to the control group (74% vs. 36%). It was also to be noted that the decision to move for other assisted reproductive technologies (ART) techniques (in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI)) was significantly higher in the control group as compared to the surgery group (32% vs. 11%)[20]. Furthermore, varicocelectomy results in reduction of the serum and seminal levels of leptin, whose elevated level may result in reduced sperm motility. This supports the finding that varicocelectomy may improve the motility of sperms in varicocele patients with asthenozoospermia.

4.6.3 Human chorionic gonadotropin (HCG) for iASZ

Pusch et al.[132] in 1986 suggested a potential benefit of HCG in treating such patients. 16 out of 30 participants responded to therapy. The total motility significantly increased from 34% to 40% and six natural pregnancies occurred. This supports the concept that sperm motility is the most important factor for fertilization and natural pregnancy. However, this single study, conducted four decades ago, can not support a recommendation for empirical HCG stimulation in patients with absolute asthenozoospermia.

4.6.4 The effect of iASZ on the outcomes of ICSI

There is no study in the literature addressing the outcome of ICSI in cases of iASZ. However, the study of Chen et al.[133], unintentionally included a cohort of patients which is very close to the one we are trying to investigate. The study population exhibited iASZ except in En-group, which also showed mild oligozoospermia. They included 97 couples with severe or absolute asthenozoospermia who underwent ICSI between 2014 and 2018. They were categorized according to the type of sperm used for ICSI into four groups: patients with motile sperm obtained through ejaculation (Ep-group), non-progressive motile sperm obtained through ejaculation (En-group), immotile sperm obtained through ejaculation (Ei-group), and finally testicular sperm (TESE-group). The Ep-group (65.4%, p = 0.019) and TESE-group (63.6%, p = 0.035) showed significantly higher clinical pregnancy rate as compared to the Ei-group (23.1%). Considering the ongoing pregnancy rate, the Ei-group showed significantly lower rates compared to the Ep-group (23.1% vs. 61.5%, p = 0.041). In addition, the biochemical pregnancy rate, ongoing pregnancy rate, and live birth rate were significantly lower in the Ei-group than in the TESE-group (30.8% vs. 63.6%, 23.1% vs. 40.4% and 23.1% vs. 40.4%, respectively). They came to the conclusion that in complete asthenozoospermia, testicular sperm should be preferred to ejaculated one for better ICSI outcomes. The extent of severe or complete asthenozoospermia may not affect the ICSI outcomes if the testicular sperm was meticulously selected during the ICSI procedure[133]. Another study suggested that using the swim-up technique is useful for selecting sperm negative for phosphatidylserine membrane translocation, a marker of cell apoptosis; which enriches the sample with high motility sperm that may improve the outcomes of assisted reproduction.

4.6.5 Mitochondria-targeted treatment

As mitochondrial dysfunction is a major player in the development of asthenozoospermia, treatments that aimed at improving the mitochondrial functions emerged as a promising option to improve treatment. These treatments include antioxidants to guard against the effect of ROS, such as coenzyme Q10 and l-carnitine[38,39,77]. Coenzyme Q10 supplementation in patients with idiopathic asthenozoospermia was associated with significant improvement of the forward motility (from 9.13% ± 2.50% to 16.34% ± 3.43%, p < 0.05) at 6 months[134]. The same group of authors confirmed these results in another randomized controlled trial[135]. Similarly, l-carnitine was associated with improvement of sperm motility function and pregnancy rates among patients with asthenozoospermia[136].

Finally, this review article is not devoid of limitations. First, most of the evidence is derived from in vitro studies and small cohorts, which may limit the generalizability of these results. Second, the functional relevance of the many identified molecular markers still requires validation in larger studies in order to confirm their applicability. Third, the limited number of articles focusing mainly on iASZ, particularly in the field of treatment, warranted the need for larger randomized controlled trials to provide a more comprehensive guide for patients’ management. Finally, the lack of information about environmental exposures, epigenetic modifications, and personalized therapeutic strategies rendered these points scarcely covered in the current review.

5 CONCLUSION

In conclusion, absolute asthenozoospermia is a difficult case to treat. It can be attributed to antisperm antibodies, structural defects of spermatozoa, or oxidative stress in the male reproductive tract. Management should focus on lifestyle modifications and the treatment of potentially reversible causes, such as clinical varicocele. If this fails, ICSI remains the treatment of choice, and testicular sperm may show better outcomes as compared to ejaculated one. However, it should be made clear that the evidence supporting the above-mentioned treatment strategy is still weak and further well-designed studies are needed to reach stronger evidence-based recommendations.

Areas for future research include studies on sperm DNA fragmentation in patients with iASZ, and studies on the clinical efficacy and risk-benefit analysis of targeted therapies and assisted reproductive technology in the context of iASZ.

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