The currently accepted definition of chronic endometritis (CE) was given in 2018 by I. Moreno, E. Cicinelli, et al.: a chronic inflammation of the endometrium caused by its abnormal microbiome [1]. Recent years have seen an increasing interest in this pathology, primarily due to its presumed role in infertility, recurrent miscarriage, and recurrent implantation failure (RIF) during in vitro fertilization (IVF) procedures [2–10]. It is noted that the prevalence of CE exceeds 30% in these conditions [11–14].

A variety of theories are available to explain the CE-related reduction in the implantation capacity of the endometrium [15–20] — activation of local inflammatory processes with altered secretion of cytokines and chemokines [19][21–25]; abnormal leukocytic infiltration, decidualization, and vascularization in the uterine cavity mucosa [13][19][23][24][26][27][29][30]; altered uterine contractility [16][28]. Although these theories clearly reflect some features of CE pathogenesis, the current evidence for a correlation between this pathology and implantation defects is often based on data from studies that are not without their limitations, such as heterogeneous sampling and questionable diagnostic criteria for CE [31][32]. Therefore, the scientific community remains divided between those who are for and against recognizing CE as the real cause of female infertility.

One of the most frequently asked questions regarding CE is the methodology used to diagnose it. Hysteroscopy has sufficient sensitivity; however, it is highly dependent on the technician [33–35]. Therefore, the current gold standard for CE diagnosis is the detection of plasma cells (PCs) in endometrial biopsy material [36–38]. The number of PCs required to diagnose CE remains a matter of discussion [38][39].

In this systematic review involving a meta-analysis, we examined scientific and clinical studies of infertile women with repeated IVF failures; specifically, we studied the role of CE in reducing IVF effectiveness, evaluated the impact of CE treatment on IVF outcomes, and analyzed the propensity to implantation impairment depending on CE severity (determined by plasma cell infiltration).

The article aims to analyze the effect of chronic endometritis of varying severity and its treatment on the outcomes of in vitro fertilization.

The systematic review with meta-analysis was reported in line with the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines [40].

The evaluation of studies for compliance with the inclusion criteria was conducted in three stages: evaluation of the title, abstract, and full text of the article.

Inclusion criteria

All studies (experimental and observational) evaluating the impact of chronic endometritis on the outcome of in vitro fertilization.

Exclusion criteria

Studies that include patients with recurrent miscarriage; evaluate spontaneous conception rate in women with CE; evaluate other types of endometrial inflammation (acute, subacute, or tuberculous endometritis).

Diagnostic criteria

Chronic endometritis was defined as the presence of at least one endometrial stromal plasma cell in the entire section; immunohistochemical analysis was used as an evaluation method for CD138+ cells (syndecan-1 staining). Severe CE was defined as the presence of ≥5 PCs in the endometrial tissue per HPF, while the mild form was defined as the presence of 1–4 PCs per HPF.

The following comparisons were made: 1) CE patients vs non-CE patients (with normal endometrial histology); 2) patients with CE (untreated or persistent after antibiotic therapy) vs treated CE, i.e., following antibiotic therapy (endometrial biopsy showed no CE); 3) treated CE patients (following antibiotic therapy) vs non-CE patients (with normal endometrial histology).

Also, a comparison was conducted between CE patients, which was defined by the number of PCs in endometrial tissue: ≥5 PCs per HPF (severe CE) vs 1–4 PCs (mild CE). Also, a subgroup of patients with 1–4 PCs was compared with non-CE patients.

Two authors conducted an independent search for publications in all languages across electronic databases (PubMed, Medline, Scopus, Embase, ELibrary, Cochrane Central Register of Controlled Trials (CENTRAL), WHO International Clinical Trials Registry, and RSCI) that were published between January 2010 and January 2022.

The search query included the following words: chronic endometritis; infertility; endometrial plasma cells (EPC); endometrial CD-138; hysteroscopy; in vitro fertilization (IVF); assisted reproductive technology (ART); embryo transfer; recurrent implantation failure (RIF). For domestic databases, Russian equivalents for these terms were used.

Initially, the title and abstract of potential studies were examined, and in case of insufficient information, the full text of the article was studied. Reviews, case histories, comments, and letters were excluded. In order to increase the transparency of the method, we selected only randomized controlled trials (RCTs) in human beings. The references of all found studies were examined to identify additional, previously undiscovered publications. Duplicate results obtained in the search across different databases were also excluded.

Two researchers independently examined databases and found potentially relevant studies. Then, another reviewer assessed whether the full text of the articles met the inclusion criteria. Points at issue were resolved through discussion. For the sake of transparency and complete reporting of the meta-analysis, the search for studies was presented as a PRISMA block diagram.

With the aim to establish the relationship between chronic endometritis and in vitro fertilization outcomes, the meta-analysis used the following indicators as primary endpoints: ongoing pregnancy or live birth rate (per patient — OPR/LBR), clinical pregnancy rate (per patient — CPR), and miscarriage rate (per clinical pregnancy — MR). OPR/LBR (“ongoing pregnancy”) — after 12 weeks of gestation; “live birth” — birth of one or more viable children; CPR — presence of an intrauterine gestational sac as indicated by transvaginal ultrasound or other reliable clinical signs; MR — fetal loss before 20 weeks of gestation expressed as mean values (M) ± standard deviations (SD).

RCT data meeting inclusion and exclusion criteria were obtained by the reviewers according to the predefined criteria. Special attention was paid to data concerning severity, diagnostic criteria for chronic endometritis, its therapy methods, and treatment outcomes (biochemical pregnancy, clinical pregnancy, live birth rate, and miscarriage rate). All authors independently assessed the risk of systematic error in each study using guidelines provided in the Cochrane Handbook for Systematic Reviews of Interventions, version 5.1.0 (www.handbook.cochran.org). In the course of the study, reviewers also evaluated the randomization method; the degree of blinding of patients, medical staff, and researchers, as well as the availability and completeness of data on outcomes presented by the authors.

The meta-analysis was performed using Stata 11.0 and Review Manager (version 5.4, The Cochrane Collaboration, 2020, Oxford, UK). The study results were expressed using an odds ratio (OR) with a 95% confidence interval (95% CI); values of p < 0.05 were considered statistically significant.

The heterogeneity was considered low at I2 <30%, moderate at 30–50%, and high at >50%. The relation I2>50% indicated significant heterogeneity between studies and prevented reliance on a combination of study results, so a random effects model was used to summarize the effects. The random effects model (DerSimonian & Laird method) was applied in the meta-analysis [41]. The analysis of subgroups and sensitivity was used to examine the sources of heterogeneity in the studies. The condition was to follow the Cochrane Handbook recommendations for assessing publication bias [42].

The recommendations provided in the Cochrane Handbook for Systematic Reviews were used to assess the risk of bias in RCTs. The following criteria were assessed: presence of randomization; allocation concealment; blinding of participants and staff; blinding of the researcher evaluating outcomes; presence of incomplete data on outcomes; selective reporting of data; and other risks of bias. The systematic error risk assessment was denoted as follows: low risk — “+”; questionable risk of bias, as well as incompletely reported data — “?”; high risk of bias — “−.”

No additional analyses were planned for this study.

We conducted a search for papers published between January 2010 and January 2022 that met the inclusion criteria. The search scheme is presented in Fig. 1.

After removing duplicate publications, the search yielded 1905 relevant studies; of these, 19 full-text articles that met the eligibility criteria for this meta-analysis were selected. After evaluating the full text, a total of ten studies in the English language were included in this meta-analysis [6][9][11][12][14][17][18][44][45][46]. The most important data from the ten RCTs included in the analysis are summarized in Table 1.

The studies included a total of 4145 patients. All studies were observational: five prospective [9][18][43][44][46], five retrospective [6][12][14][17][45], and one crossover study [11].

Two studies compared non-CE patients with treated CE and recurrent CE patients [18][46]; one study compared women with treated and recurrent CE [6]; and two studies compared non-CE, treated CE, and infertile patients who were not tested for CE [9][17]. Three studies compared CE and non-CE patients [43][44][45], and one compared CE, non-CE, and treated CE patients [11]. Y. Li, S. Xu et al. (2021) divided patients into groups according to the number of plasma cells (0, 1, 3, 4, and ≥5 per HPF) and compared pregnancy outcomes in women with <5 and ≥5 CD138+ cells [12]. Also on the basis of PC count, Y. Xiong et al. (2021) compared pregnancy outcomes in patients with different CD138+ counts and recurrent CE following antibiotic therapy [14]. As the main criteria of CE recovery, T.M. Motovilova et al. chose sonographic markers—increase in endometrial thickness and blood flow change (resistance index, RI) in radial, spiral, and basal uterine arteries [43].

Four studies included patients with recurrent implantation failures (defined as failure of at least two or three previous IVF attempts (fresh or frozen/thawed embryos), including at least one high-quality cleavage-stage embryo or blastocyst transferred per cycle) [6][9][17][18][44]. One study included patients with only one previous embryo transfer failure [45], while four studies analyzed infertile patients with previous non-elective embryo transfers [11][12][14][44].

All of the women underwent IVF. Information on IVF protocols was missing in four studies [11][17][43][44], while the remaining seven provided adequate information on the protocols of assisted reproductive technologies (ART). Ovarian stimulation was conducted by administering recombinant FSH alone or in combination with human menopausal gonadotropin using GnRH. The administration of hCG was performed following visualization (transvaginal ultrasound) of at least two preovulatory (17 mm) follicles; oocyte retrieval was conducted 34–36 h following ovulation induction (transfer of no more than three embryos or two blastocysts): two studies transferred only cleavage-stage embryos (up to three) [6][46]; two studies transferred only blastocysts [11][44]; and two studies transferred both cleavage-stage embryos and blastocysts [14][17].

Two studies provided no data on the embryo stage [9][43]. Gestagen luteal phase support was carried out in all studies providing data on IVF protocols.

All RCTs used randomization without allocation concealment—blinding of study participants and staff was not conducted due to patient specificity (infertility, in vitro fertilization protocol). Three studies had adequate sample representativeness [6][17][45], while the rest had a high risk of systematic error. Three studies reported an adequate (consistent) sampling strategy [11][17][18], whereas most studies provided no accurate data. In the diagnosis of chronic endometritis, all studies had a low risk of bias (≥3 points) (Table 2).

Two studies failed to clearly describe the study population or did not fully report descriptive statistics [18][46], while the others had a low risk of bias for this domain. Three studies provided incomplete data on outcomes [17][18][45]. According to the assigned points, all studies have a low risk of bias.

Results of the meta-analysis of primary endpoints

Diagnosis of chronic endometritis

Plasma cell identification was achieved via hematoxylin and eosin (H&E) staining alone or in combination with immunohistochemical analysis of CD138+ cells, with the exception of the study that only provided an immunohistochemical analysis of PCs [43]. Six studies collected endometrial samples during the follicular phase in [6][14][18][44][45][46]; one study performed endometrial biopsies either in the follicular or mid-luteal phase (cycle days 21–23) [17]; two studies collected samples in the mid-luteal phase [11][12].

CE therapy

The first-line antibiotic therapy for CE was microbe-specific, with a microbiological culture assessment of the endometrium [6][11][46] or empiric (doxycycline [9][14][1][43] or ciprofloxacin and metronidazole for 14 days [18]).

CE patients vs non-CE patients

Data from eight studies [9][11][12][14][17][18][44][46] showed significantly lower OPR/LBR (OR 1.97; 95% CI 1.11–3.48; I2=64%; p < 0.02) and CPR (OR 2.28; 95% CI 1.34–3.86; I2=70%; p < 0.002) in CE patients as compared to non-CE patients, with no difference in the MR (p < ns) (Tables 3–5; Figs. 2–4). The sequential exclusion of each study from the meta-analysis provided no significant changes in the pooled results in terms of OPR/LBR, CPR, and MR.

CE patients vs treated CE patients

Four studies revealed higher OPR/LBR (OR 5.33; 95% CI 2.41–11.79; I2=0%; p < 0.0001) and CPR (OR 3.64; 95% CI 1.89–7.04; I2=0; p < 0.0001) in patients with treated CE as compared to patients with untreated/recurrent CE [6][11][18][46], with a borderline significance in terms of MR (p < 0.05) (Tables 6–8; Figs. 5–7). The sequential exclusion of individual studies from the meta-analysis provided no significant changes in the pooled results for OPR/LBR and CPR (sensitivity analysis was impossible for MR).

Treated CE patients vs non-CE patients

An analysis of 609 patients from three studies [11][18][46] showed no difference between the groups in terms of OPR/LBR, CPR, and MR (p < ns) (Tables 9–11; Figs. 8–10).

A sensitivity analysis was not possible due to the small number of studies included in the meta-analysis (n = 3).

CE patients vs patients who were not tested for this pathology

A pooled analysis of data on 1,556 patients from two studies [9][17] showed lower OPR/LBR (OR 0.55; 95% CI 0.37–0.82; I2=0%; p < 0.003) and CPR (OR 0.59; 95% CI 0.41–0.85; I2=0%; p < 0.005) in untreated CE patients as compared to those who was not tested for CE. No differences in miscarriage rates (p < ns) were found (Tables 12–14; Figs. 11–13).

A sensitivity analysis was not possible due to the small number of studies included in the meta-analysis (n = 2).

Severe CE patients vs mild CE patients

Two studies [12][14] showed that severe CE (≥5 PCs per HPF) was associated with significantly lower OPR/LBR (OR 0.43; 95% CI 0.25–0.74; I2=0%; p < 0.003) and CPR (OR 0.40; 95% CI 0.24–0.68; I2=0%; p < 0.0007) as compared to mild CE (1–4 PCs per HPF), with no difference in the miscarriage rate (Table 15–17; Figs. 14–16). A sensitivity analysis was not possible due to the small number of studies included in the meta-analysis (n = 2).

Mild CE patients vs patients untested for CE

No differences were found between the groups [12][14] in terms of OPR/LBR, CPR, and MR (p < ns) (Tables 18–20; Figs. 17–19).

A sensitivity analysis was not possible due to the small number of studies included in the meta-analysis (n = 2).

Due to the limited number of included studies, publication bias could not be effectively represented using a funnel chart.

No additional analyses were planned for this study.

The present systematic review summarizes the available data on the impact of chronic endometritis, its therapy, and its severity on the outcome of assisted reproductive technologies. The analysis included a total of 4,145 infertile patients from 11 studies [6][9][11][12][14][17][18][43][44][45][46]; of these, 1,716 were women with recurrent implantation failures. The overall quality of the included studies was considered satisfactory (no study had a high risk of bias).

Noteworthy is that non-CE women showed a significantly higher live birth rate (OPR/LBR) (OR 1.97; 95% CI 1.11–3.48) and clinical pregnancy rate (CPR) (OR 2.28; 95% CI 1.34–3.86) as compared to patients with diagnosed CE. This fact is consistent with the number of failed embryo transfers (p < 0.05) and supports the concept that CE has a negative effect on implantation [39], which is confirmed by a sufficiently large number of studies included in this meta-analysis, having equivalent approaches to CE diagnosis (i.e., CD138+ immunohistochemistry) [6][9][11][12][14][17][18][44][45][46].

Several major factors are currently considered responsible for reproductive dysfunction in CE [20]: this is primarily the abnormal uterine microbiome, as demonstrated by a sufficient number of microbiological studies [1][47–50], the disruption of cytobiochemical and immune processes in the endometrium [28][36][51–54], and also confirmed by the improved fertility following antibacterial therapy in CE patients [34][55][56]. The abnormal uterine microbiome leads to immune deviations primarily affecting the leukocyte pool of immunocompetent cells with a change in the ratio of pro- and anti-inflammatory cytokines, which results in the reduced implantation capacity of the endometrium and prevents embryo invasion [19][20][22][57]. In addition, prolonged activation of proliferation gene regulation with the downregulation of apoptotic genes (necessary for maintaining adequate endometrial responses) may contribute to the development of proliferative lesions such as polyps [25][34][55][58][61], whereas changes in the vascularization and decidualization of secretory endometrium in the presence of CE may additionally contribute to its receptivity impairment [9][59][60][62].

On the basis of data from two studies (which somewhat limits the value of their interpretation), patients with diagnosed CE were found to have worse IVF outcomes as compared to a group of infertile women who were not tested for this pathology (OPR/LBR: OR 0.55; 95% CI 0.37–0.82; CPR: OR 0.59; 95% CI 0.41–0.85; p < 0.05) [9][17]. If this result is confirmed by further studies, it could theoretically justify CE testing prior to IVF to identify (and treat) women with an expected poor reproductive prognosis. This is supported by the significant improvement in IVF outcomes following CE treatment that are presented in the review. It is true that the OPR/LBR and CPR were significantly higher following CE treatment as compared to those of patients with untreated or recurrent CE (OR 5.33; 95% CI 2.41–11.79; OR 3.64, 95% CI 1.89–7.04; p < 0.05). In addition, treated CE patients had similar IVF outcomes as women without this pathology (p>0.05), potentially indicating endometrial receptivity recovery after therapy.

Our conclusions for the MR seem to deviate from those for the other outcomes (OPR/LBR and CPR). The meta-analysis found neither a significant effect of CE on the miscarriage rate nor improvements in pregnancy outcomes in treated patients (p < 0.05). It can be assumed that spontaneous abortion involves too many different etiopathogenetic factors related to both the mother and the embryo which in this case reduces the role of endometrial inflammation [63–66]. In particular, aneuploidy is considered to be the main factor behind miscarriage, while maternal age (≥35 years) constitutes a major risk factor [61][67–70]. Most studies included in this meta-analysis had specific age limits: under 44 [12], 40 [44], 39 [6][14][17], and 38 years [45], while others (where age limits were not specified) reported mean patient ages close to [9] or above 35 years [11][18]. None of the studies conducted preimplantation genetic testing for aneuploidies. The only study conducted in women aged under 35 years showed a trend toward a higher miscarriage rate in CE patients as compared to healthy women or treated CE patients [46]. Also, we should note statistical problems associated with the sample size of patients taking into account the realization of their reproductive function and a high risk of error, as any comparative studies of miscarriage rate (MR) are inadequate as compared to the population-based estimation of CPR or OPR/LBR.

Unambiguous results were obtained when comparing the effect of severe (≥5 PCs per HPF) and mild (1–4 PCs per HPF) CE on IVF outcomes. Two studies showed that severe CE is associated with lower OPR/LBR (OR 0.43; 95% CI 0.25–0.74) and CPR (OR 0.40; 95% CI 0.24–0.68; p < 0.05) [12][14]. Of note is that mild CE patients exhibited similar results in terms of clinical pregnancy and live birth rates as women without this pathology (p>0.05). These data are consistent with the conclusions that a higher number of CD138+ cells determines a worse IVF outcome [12][45]. Although the possibility of categorizing CE by severity is attractive from a practical point of view, the available evidence is insufficient to consider “mild CE” as a condition that does not pose a significant threat to the implantation capacity of the endometrium [37]. CE classification that is based solely on plasma cell counts, although practical, is potentially prone to clinical errors due to the ambiguity of sampling methods—blind endometrial biopsy (pipelle biopsy or curettage), where the diagnosis may depend on the amount of captured endometrial tissue, especially if the distribution of PCs is heterogeneous. In addition, the severity of the process may be underestimated in the case of focal CE due to the random nature of morphological material collection from the uterine cavity. An equally important issue is that if PC counts are based solely on CD138+ staining, there is a possibility of overdiagnosis due to the background reaction of adjacent cells [34].

One of the most promising additional methods for diagnosing CE is hysteroscopy [6][16][32][34][65][71], especially in cases of diagnostic uncertainties [70][72][73]. Through visual evaluation of the entire endometrial surface, hysteroscopy can enable the identification of specific endometrial changes that are consistent with severe forms (e.g., micropolyps) [73][74]. In this connection, some studies indicate inconsistency between the CE diagnosis made on the basis of plasma cell counts and those made via hysteroscopy [6][22][32]. Therefore, we cannot exclude that the combination of the two methods may provide a higher diagnostic and prognostic value than immunohistochemistry alone. For example, in a study by Yang R. et al. (2014), patients whose control hysteroscopy showed no CE signs exhibited better IVF outcomes as compared to women whose recovery was assessed via immunohistochemistry (no PCs) [75]. Also, hysteroscopy is a technique that enables endometrial material sampling under visual control [76–78]. However, despite the fact that the use of hysteroscopy with biopsy is mandatory in focal endometrial lesions, its effectiveness is yet to be evaluated in CE diagnosis [15][79–83].

This meta-analysis has several limitations. The first is that the analysis is based on only ten RCTs and, therefore, a small number of women; thus, it is necessary to conduct more RCTs using a larger sample size. The second is the heterogeneity of patient characteristics (including IVF cycles and days of embryo transfer) included in the study—the variability of CE therapeutic regimens across studies and the inclusion of patients aged over 35 years without correction for the possibility of aneuploidy.

The evaluation of methods for diagnosing and treating chronic endometritis as one of the widespread factors in female infertility and, especially, implantation failures in assisted reproductive technologies, will help to promptly identify this pathology, conduct adequate therapy, and increase the effectiveness of in vitro fertilization programs.

Chronic endometritis can significantly reduce the effectiveness of in vitro fertilization in infertile women. Of note is that a rational antibacterial therapy of this pathology seems to improve the reproductive outcome, which becomes similar to that of women with no inflammatory changes in the endometrium.

The quality and number of studies on the limited negative impact of only severe CE (≥5 PCs per HPF) on IVF outcomes can currently be considered highly controversial, so further evidence is required.

Future RCTs are needed to check the effectiveness of the required CE tests in infertile patients prior to IVF in order to improve reproductive outcomes and increase live birth rates. In addition, further studies are recommended to evaluate the impact of chronic endometritis of varying severity on the IVF outcome and the feasibility of hysteroscopy in this pathology.