Trabeculectomy has statistically superior IOP-lowering effects compared to PMS [33] and is indicated for cases that require a lower target IOP. However, in addition to the need for postoperative management, including laser suture lysis, the risk of vision loss due to postoperative hypotony makes it unsuitable for some glaucoma patients. By contrast, postoperative management of PMS—apart from the need for needling—is simpler than trabeculectomy because laser suture lysis is unnecessary. PMS is also less prone to intraoperative and postoperative hypotony, which may help preserve visual acuity after surgery.

This study evaluated the efficacy and safety of PMS in Japanese patients with XFG, a population considered at high risk for IOP management due to both their non-Caucasian ethnicity and the presence of XFG. PMS led to a significant reduction in IOP over 6 months, without severe vision-threatening adverse effects. Propensity score matching was used to compare the efficacy of PMS between XFG cases and a control group of POAG patients with similar baseline characteristics. Both groups showed significant reductions in IOP and medication scores; however, at 6 months, the IOP and medication scores were slightly higher in the XFG group than in the POAG group. The survival rate for IOP control at 6 months was also significantly lower in the XFG group than in the POAG group.

Several reports have examined the efficacy and safety of PMS for eyes with XFG. Antonio et al. conducted a multicenter study in which PMS with MMC concentrations of 0.02–0.05% was performed on 81 eyes with POAG and 23 with XFG [17]. At 12 months postoperatively, IOP had decreased from 25.0 ± 6.7 to 14.3 ± 3.6 mmHg in POAG eyes, and the number of glaucoma medications had decreased from 2.9 ± 1.0 to 0.8 ± 0.9. In comparison, in XFG eyes, IOP had decreased from 25.0 ± 5.9 to 13.5 ± 2.4 mmHg and the number of medications had decreased from 3.0 ± 1.0 to 0.8 ± 1.0. The IOP reduction (survival rate) at 12 months did not differ significantly between the two groups (p = 0.52), and no difference was observed between phakic and pseudophakic eyes. In another report using a 0.02% MMC concentration, Nobl et al. inserted the PMS in 20 XFG and 26 POAG eyes. At 12 months postoperatively, no significant difference in IOP or medication use was found between the two groups [28]. The survival rate (IOP reduction of > 20% from baseline and within 5–16 mmHg) also did not differ significantly between the groups. Regarding complications, XFG patients had higher incidences of anterior chamber hemorrhage, hypotony, and choroidal detachment than POAG patients. A report from Japan by Wakuda et al. describes PMS with 0.04% MMC in 29 eyes of Japanese patients with XFG [29]. At 46 weeks postoperatively, IOP had decreased from 32.6 ± 9.1 to 16.9 ± 10.5 mmHg, and the number of glaucoma medications had decreased from 3.4 ± 1.0 to 1.0 ± 1.3. In their study, which evaluated IOP management under relatively similar conditions (success defined as a ≥20% reduction in IOP, with 5–15 mmHg), the complete success rate was 49%, and the conditional success rate was 53%, comparable to our findings. Table 6 summarizes a comparison between the results of previous studies and those of the present study.

Our study and the aforementioned reports from abroad differ in terms of the target population, making direct comparisons challenging, particularly given that the results are being compared at 6 months postoperatively. The mean IOP and number of glaucoma medications after PMS surgery for XFG in our study were slightly higher than those reported internationally. This discrepancy may be attributed to the Asian race, a potential risk factor for intraocular pressure management [34]. In contrast, a study by Wakuda et al. on Japanese XFG patients [29]—similar to those in our study—reports a higher preoperative mean IOP (32.5 mmHg). Additionally, their study included cases in which the tube was inserted in the lower quadrant. Despite these differences, the success rate was comparable to our results, with a success rate of 49–67% at 24 weeks. Further investigation is needed to better understand postoperative IOP dynamics in Japanese XFG patients.

The rate of additional surgery was 10% in the study by Nobl et al., 31% in Wakuda et al., and 16% in our study. Since the criteria for additional surgery vary across institutions, it is difficult to determine the significance of these differences. In our propensity score-matched comparison with POAG, no significant difference was observed in the rate of additional surgery.

The novelty of this study lies in the use of propensity score matching to establish a control group (POAG group) with baseline characteristics that matched those of the XFG group, enabling a direct comparison of postoperative outcomes between the two glaucoma types. At 6 months postoperatively, IOP and glaucoma medication scores were significantly higher in the XFG group than in the POAG group. The qualified success rate was also higher in the POAG group. So far, studies that have compared XFG and POAG in PMS are limited; although the follow-up period was short (one year), one study reports that micro-shunts demonstrated similar efficacy in XFG and POAG, despite a high incidence of transient hypotony and choroidal detachment [28].

On the other hand, previous reports indicate that the success rate of trabeculectomy for XFG is lower than for POAG [8, 9]. This difference is thought to arise from the distinct pathophysiological characteristics of XFG. After trabeculectomy in XFG, disruption of the blood-aqueous barrier, inflammatory responses, and fibrin exudation are thought to accelerate filtering bleb scarring and reduce its filtering capacity [10, 35]. The higher risk of anterior chamber inflammation and the greater tendency for scar tissue formation in XFG further contribute to its lower success rate. On the contrary, the PMS is thought to cause less tissue invasion and weaker inflammatory responses than trabeculectomy; these factors may still lead to more pronounced filtering bleb scarring in XFG than in POAG. Sugimoto et al. report that in eyes with XFG undergoing Ex-PRESS surgery, the volume of the fluid-filled space within the filtering bleb was significantly smaller at 3 and 6 months postoperatively compared to eyes with POAG [36]. This finding suggests a greater propensity for scar tissue formation in the bleb during the 3–6-month postoperative period in XFG eyes, which may account for the higher intraocular pressure observed at 6 months in XFG eyes compared to those with POAG in the present study. As the follow-up period extends, the impact of this scarring would become more evident, making long-term IOP control increasingly challenging.

Postoperative IOP values in XFG patients may be slightly higher than in POAG patients; however, PMS surgery remains a viable option due to its potential to suppress postoperative IOP fluctuations. Studies indicate that IOP fluctuation is greater in XFG than in POAG [4] and this is thought to affect glaucoma progression negatively. In cases where progression occurs despite normal IOP management, both short- and long-term IOP fluctuations are thought to contribute to disease progression [37,38,39]. If the PMS can effectively stabilize IOP fluctuations, it may play a significant role in slowing disease progression. One study suggests that the PMS can reduce IOP fluctuations [40], but further investigations in larger patient cohorts are necessary to confirm this.

Marta et al. report a 5.1% decrease in CECD at 6 months following PMS implantation [41]. In their study, 43 of 46 eyes were diagnosed with POAG, and 2 eyes with XFG. In the present study, no significant decrease in CECD was observed at 6 months postoperatively; however, a reduction exceeding 5% was noted in 11 of 31 eyes (35.5%). Consistent with previous reports, which predominantly involved eyes with POAG, this study also found no significant CECD decrease at 6 months postoperatively in eyes with XFG. Nevertheless, long-term follow-up is necessary to ascertain whether this trend persists and to evaluate the potential cumulative effects of the procedure on corneal endothelial cell density.

This study has several limitations that should be considered when interpreting the results. First, as a retrospective cohort study, it was impossible to eliminate all known and unknown confounding factors or biases. To mitigate this, relatively strict inclusion and exclusion criteria were applied; however, complete control of confounders was not achievable. Additionally, patients who were transferred to other institutions during the follow-up period were excluded from the analysis, potentially limiting the generalizability of the findings. Furthermore, the criteria for repeat surgery, needling, and resumption of glaucoma medications were not standardized, and their potential influence on the outcomes must be considered. Postoperative steroid tapering protocols may also have varied between XFG and POAG patients because of the experience that XFG is more likely to cause conjunctival scarring, potentially affecting IOP management. However, it is unclear at which postoperative stage the difference arises, due to the small number of measurement points, it cannot be clearly stated. Thus, extended observation is necessary to fully understand the postoperative management required for XFG. A comparison of the efficacy and safety of PMS and trabeculectomy in XFG eyes remains an issue to investigate. In a cumulative case series performed by a single surgeon at a single institution, the six-month outcomes of trabeculectomy in XFG eyes (mean age: 74.4 years, baseline IOP: 23.1 mmHg) showed a mean IOP of 12.9 mmHg and a cumulative survival rate (IOP < 15 mmHg) of 78.3% [42]. Although direct comparison is difficult due to differences in patient backgrounds, our PMS results show higher IOP and lower survival rates at six months postoperatively compared to their report. The limited sample size and involvement of multiple surgeons may have contributed to this outcome, but PMS may have inferior results compared to trabeculectomy. Another limitation is the lack of a clear consensus on the optimal MMC concentration for glaucoma surgery. In this study, a concentration of 0.04–0.05% was used in both the XFG and POAG groups, which is slightly higher than the concentrations commonly used in other countries. This concentration represents the standard protocol adopted at our institution for both POAG and XFG cases [26]. Given these limitations, caution is needed when generalizing the findings of this study. Finally, when evaluating CECD, gonioscopic evaluation was not performed in all cases, and detailed anterior segment imaging using OCT was also not performed, making it difficult to interpret the cause of the decrease in CECD. To overcome these limitations, future multicenter collaborative studies and prospective research are warranted to address these limitations and strengthen the evidence base.

In conclusion, this study demonstrates that the PMS significantly reduced IOP and decreased glaucoma medication scores in the eyes of Japanese patients with XFG at 6 months postoperatively. Notably, the improvement in IOP control and reduction in the burden of glaucoma medications implies that this treatment enhances patients’ quality of life. Regarding safety, although some transient adverse events were observed, no severe long-term complications, such as vision loss, were reported, supporting the overall safety of this procedure. However, compared to POAG, treatment outcomes appeared less favorable in XFG eyes. These findings provide preliminary evidence supporting the usefulness of the PMS as a treatment option for Japanese XFG patients. However, further research is warranted to confirm its long-term efficacy and safety.