Study selection results and basic characteristics

We initially identified a total of 727 records related to our topic, including 507 records from PubMed, 137 records from Cochrane Library, 77 records from Web of Science. Six studies were identified by manually searching the reference lists of individual studies. First, we excluded 240 duplicated records by comparing title, authors, and publication year. Subsequently, a total of 464 records were excluded due to irrelevance, based on the content of their titles and abstracts. Then, 23 records remained for assessing the full-text eligibility. Upon reviewing the full texts of the remaining records, only four studies [2, 24,25,26] were included in our meta-analysis. Additionally, 19 articles were excluded for various reasons: eight were not related to semen (N = 8), one lacked a comparative group (N = 1), six were animal test articles (N = 6), two were review articles (N = 2), and two were letters to the editors (N = 2). The detailed study selection process is shown in Fig. 1. The publication year of the included studies ranged from 2012 to 2023, including a total of 279 varicocele patients. All these studies were prospective studies, and semen parameters were recorded pre-surgery, 3 months, 6 months, or 12 months after surgery. Table 1 detailed individual data regarding the name of the first author, year of publication, and location of recruitment, design of the study, size of the sample, age of the population, follow-up duration, and sperm parameters. The quality assessment results are shown in Table 2. All studies were regarded as high-quality studies based on NOS scores.

Fig. 1

Flow diagram of the study selection process

Table 1 Characteristics of studies included in the meta-analysisTable 2 Quality assessment of studies included in the meta-analysis by using the Newcastle–Ottawa ScaleSperm parameters in varicocele patients before surgery and 3 months after surgery

All the semen parameters were compared between pre-surgery and 3 months after surgery. The results were pooled to calculate the WMD and 95% CI. Random effects models or fixed effects models were used according to the heterogeneity among the included studies. The results were showed in Fig. 2.

Fig. 2

Forest plot and meta-analysis of semen parameters between pre-surgery and 3 months after varicocelectomy. (A Semen volume; B sperm concentration; C: sperm motility; D sperm morphology; E total motile sperm count)

Semen volume

The semen volumes were reported in all included studies. The fixed-effect model was used to pool the data, based on the heterogeneity detection (I2 = 0.0%, P = 0.818). After varicocelectomy, no significant difference in semen volume was found between pre-surgery and 3 months after surgery (WMD: − 0.05 (− 0.26, 0.16)) (Fig. 2A).

Sperm concentration

Similarly, all eligible studies reported the sperm concentration. We still compared the sperm concentration between pre-surgery and 3 months after surgery. It increased by 7.23 million per ML when 3 months after varicocelectomy (WMD: − 7.23 (− 10.89, − 3.57)). Moderate heterogeneity was detected among studies (I2 = 60.7%, P = 0.057) (Fig. 2B).

Sperm motility

Sperm motility was compared between pre-surgery and 3 months after varicocelectomy. The results showed that sperm motility increased by 6.31% when 3 months after varicocelectomy (WMD: − 6.31 (− 9.13, − 3.50)), with medium heterogeneity (I2 = 58.2%, P = 0.066) (Fig. 2C).

Sperm morphology

Only 2 out of 4 studies reported sperm morphology between pre-surgery and 3 months after varicocelectomy. However, no significant improvement in sperm morphology was revealed by meta-analysis (WMD: 2.39 (− 0.13, 4.91)). And no significant heterogeneity was detected among the 2 studies (I2 = 52.4%, P = 0.147) (Fig. 2D).

Sperm total motile count

Three studies reported the total motile sperm count (TMSC). The results showed a significant increase of TMSC when detecting in 3 months after surgery (WMD: − 19.08 (− 35.68, − 2.48)), but the degree of heterogeneity was high (I2 = 68.6%, P = 0.042) (Fig. 2E).

Sperm parameters in varicocele patients before surgery and 6 or 12 months after surgery

Similar semen parameters were compared between pre-surgery and 6 or 12 months after surgery. The pooled results are shown in Fig. 3. All the results showed that semen parameters improvements of 6 or 12 months after surgery were lower than those of 3 months after surgery.

Fig. 3

Forest plot and meta-analysis of semen parameters between pre-surgery and longer time (6 or 12 months) after varicocelectomy. (A Semen volume; B sperm concentration; C sperm motility; D sperm morphology; E total motile sperm count)

Semen volume

Four studies reported the semen volume in varicocele patients between pre-surgery and 6 or 12 months after surgery. The pooled results showed no significant improvement in semen volume with a WMD of − 0.11 (− 0.34, 0.12), and the degree of heterogeneity was acceptable (I2 = 0.0%, P = 0.755). (Fig. 3A).

Sperm concentration

Four studies provided data about sperm concentration. The meta-analysis result of the random-effect model indicated that the sperm concentration of 6 or 12 months after surgery in varicocele patients was higher than that before surgery (WMD: − 6.43 (− 12.41, − 0.46). The heterogeneity was significant based on I2 value (I2 = 68.8%, P = 0.022) (Fig. 3B).

Sperm motility

Four studies reported sperm motility before surgery and 6 or 12 months after surgery. The results showed that sperm motility increased by 4.00% when 6 or 12 months after varicocelectomy (WMD: − 4.00 (− 6.85, − 1.15)), with medium heterogeneity (I2 = 59.2%, P = 0.062) (Fig. 3C).

Sperm morphology

Only 2 out of 4 studies reported the sperm morphology between pre-surgery and 6 or 12 months after varicocelectomy. However, the sperm morphology of 6 or 12 months after surgery showed a significant decrease when compared to sperm morphology before surgery (WMD: 3.81 (1.50, 6.12)), and the heterogeneity was acceptable (I2 = 47.5%, P = 0.168) (Fig. 3D).

Sperm total motile count

Three studies reported the total motile sperm count. The results showed no significant increase of TMSC when detecting in 6 or 12 months after surgery (WMD: − 12.20(− 28.49, 4.09)), but the degree of heterogeneity was high (I2 = 68.6%, P = 0.042) (Fig. 3E).

Sperm parameters in varicocele patients 3 months after surgery and 6 or 12 months after surgery

All eligible studies reported the semen parameters of varicocele patients between 3 months after surgery and 6 or 12 months after surgery. The meta-analysis results of these comparisons are shown in Fig. 4.

Fig. 4

Forest plot and meta-analysis of semen parameters between 3 months after varicocelectomy and longer time (6 or 12 months) after varicocelectomy. (A Semen volume; B sperm concentration; C sperm motility; D sperm morphology; E total motile sperm count)

Semen volume

Four studies reported the semen volume in varicocele patients between 3 months after surgery and 6 or 12 months after surgery. The pooled results showed no significant difference in semen volume with a WMD of − 0.07 (− 0.29, 0.16), and the degree of heterogeneity was acceptable (I2 = 0.0%, P = 0.888) (Fig. 4A).

Sperm concentration

A total of four studies involved data on sperm concentration. The meta-analysis result of the fixed-effect model indicated that the sperm concentration of 6 or 12 months after surgery in varicocele patients was similar to those 3 months after surgery (WMD: − 1.33 (− 2.33, − 4.99). The heterogeneity was significantly based on the I2 value (I2 = 0.0%, P = 0.881) (Fig. 4B).

Sperm motility

Four studies reported sperm motility between 3 months after surgery and 6 or 12 months after surgery. The results showed that sperm motility had no significant difference between the two times (WMD: 2.31 (− 0.55, 5.18)), with negative heterogeneity (I2 = 0.0%, P = 0.562) (Fig. 4C).

Sperm morphology

Two studies reported sperm morphology between 3 months after surgery and 6 or 12 months after varicocelectomy. However, the sperm morphology of 6 or 12 months after surgery showed no significant difference when compared to sperm morphology 3 months after surgery (WMD: 1.29 (− 0.66, 3.24)), and the heterogeneity was acceptable (I2 = 0.0%, P = 0.899) (Fig. 4D).

Sperm total motile count

Three studies reported the total motile sperm count. The results showed no significant difference in TMSC when comparing TMSC between 3 months after surgery and 6 or 12 months after surgery (WMD: 3.95 (− 6.28, 14.19)), and the degree of heterogeneity was acceptable (I2 = 26.8%, P = 0.255) (Fig. 4E).

Subgroup analysis of sperm parameters in varicocele patients 3 months after surgery and 6 or 12 months after varicocelectomy

The subgroup analyses were performed only on the semen parameters between 3 months after surgery and longer time after surgery, which we were most concerned about. Notably, the degree of heterogeneity was acceptable. Time was the major factor that would influence the semen improvement. So, we conducted the subgroup analysis based on different months after surgery. Limited to the data on sperm morphology, no subgroup analysis was conducted to it. As shown in Fig. 5, all the semen parameters remained unchanged after subgroup analysis based on different times after surgery. Notably, the subgroup analysis revealed that there were no significant changes in semen parameters (semen volume, sperm concentration, sperm motility, and total motile sperm count) when comparing results 3 months after surgery to those at 6 months or 12 months after surgery.

Fig. 5

Subgroup analysis of semen parameters between 3 months after varicocelectomy and longer time (6 or 12 months) after varicocelectomy based on duration after surgery. (A Semen volume; B sperm concentration; C sperm motility; D total motile sperm count)

Sensitivity analysis and publication bias

Similar to subgroup analysis, the sensitivity analysis and publication bias were conducted on the semen parameters comparison between 3 months after surgery and 6 or 12 months after surgery. As for sensitivity analysis, it was conducted by omitting one study in turn to detect the stability of pooled results and assess the influence of each study on the pooled results. The results were shown in Fig. 6, and indicated stable results of our meta-analysis. In addition, we found no significant publication bias by the Begg’s test, and the visual results are presented in Fig. 7.

Fig. 6

Sensitivity analysis of semen parameters comparison between 3 months after varicocelectomy and longer time (6 or 12 months) after varicocelectomy. (A Semen volume; B sperm concentration; C sperm motility; D sperm morphology; E total motile sperm count)

Fig. 7

Publication bias of parameters comparison between 3 months after varicocelectomy and longer time (6 or 12 months) after varicocelectomy. (A Semen volume; B sperm concentration; C sperm motility; D sperm morphology; E total motile sperm count)