Factors Associated with Favorable One-Year Outcomes After PreserFlo MicroShunt Implantation: Insights from Anterior Segment Optical Coherence Tomography

Abstract

Introduction

This study aimed to identify factors associated with favorable 1-year surgical outcomes after PreserFlo MicroShunt (PMS) implantation, with a particular focus on baseline ocular anatomy and postoperative filtration patterns assessed by anterior segment optical coherence tomography (AS-OCT).

Methods

This cohort study included individuals with primary open-angle or exfoliation glaucoma who underwent PMS implantation. AS-OCT was performed 1 month postoperatively. Surgical success was defined by hypertensive thresholds of 12 mmHg (A), 15 mmHg (B), and 18 mmHg (C), with an intraocular pressure (IOP) reduction of ≥ 20% from baseline. Each criterion had a hypotensive limit of 6 mmHg. Multivariable Cox proportional hazards models were used to evaluate factors associated with favorable outcomes, considering both hypertensive and hypotensive failure limits.

Results

Eighty-eight eyes were analyzed. Median IOP decreased from 19 mmHg (interquartile range: 16–23.8) to 10 mmHg (8–13) at 12 months (p < 0.001). Qualified success rates at 12 months were 56.8% for criterion A, 70.5% for criterion B, and 73.9% for criterion C. Filtration toward the corneal limbus on AS-OCT was associated with a lower risk of hypertensive failure across all criteria (HR, 0.28–0.57; p = 0.008–0.031). Longer axial length was significant for criteria B and C (HR, 0.70 and 0.77; p = 0.002 and 0.008). Older age and exfoliation glaucoma were risk factors for hypotensive failure (HR, 1.05 and 3.75; p = 0.01 and 0.006).

Conclusion

Both baseline ocular anatomy, represented by axial length, and filtration toward the corneal limbus on AS-OCT were independently associated with favorable 1-year outcomes following PMS implantation. Integrating preoperative anatomical assessment with early postoperative imaging findings may help clinicians better understand individual surgical responses.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s12325-026-03727-9.

Key Summary Points

Why carry out this study?

Clinical outcomes after PreserFlo MicroShunt (PMS) implantation vary among patients, and factors associated with favorable surgical outcomes are incompletely understood.

This study aimed to identify both baseline anatomical factors, including axial length, and postoperative filtration patterns assessed by anterior segment optical coherence tomography (AS-OCT) that are associated with favorable one-year outcomes.

What was learned from the study?

Longer axial length and filtration toward the corneal limbus on AS-OCT were independently associated with favorable 1-year outcomes after PMS implantation.

Combining baseline anatomical assessment with early postoperative AS-OCT imaging may help clinicians better understand individual surgical outcomes.

Introduction

Glaucoma is a leading cause of irreversible blindness worldwide, and its progression can be effectively slowed by lowering intraocular pressure (IOP) [1–3]. Trabeculectomy has long been regarded as the gold standard surgical procedure for glaucoma because of its proven efficacy in reducing IOP [4, 5]. However, it often requires intensive and prolonged postoperative care, which may delay visual recovery and may carry a risk of serious, potentially sight-threatening complications [5, 6].

In recent years, less invasive surgical approaches have been developed to reduce postoperative complications. The PreserFlo MicroShunt (PMS; Santen, Inc., Osaka, Japan) is a novel device classified as a minimally invasive bleb-forming procedure. It has shown effective IOP reduction and a more favorable safety profile compared with trabeculectomy [7–9], although trabeculectomy remains an indispensable surgical option for achieving low-teen IOP levels, particularly in patients with advanced glaucoma, and cannot be replaced by minimally invasive bleb-forming procedures, as the two procedures have different indications [10]. The PMS is implanted via an ab externo approach, with the distal tip placed beneath the conjunctiva and Tenon’s capsule, approximately 6 mm posterior to the limbus. The filtering bleb that forms after PMS implantation is typically more posterior than that seen after trabeculectomy [11]. Nevertheless, blebs occasionally extend diffusely toward the corneal limbus after PMS implantation.

Recent studies have examined the association between bleb morphology and surgical outcomes after PMS implantation, mainly evaluating parameters such as bleb height, area, and internal reflectivity using anterior segment optical coherence tomography (AS-OCT). To date, however, the clinical impact of filtration extending toward the corneal limbus on surgical outcomes has not been clearly established.

The purpose of this study was to identify factors associated with favorable 1-year surgical outcomes following PMS implantation, focusing on both baseline ocular anatomy, including axial length, and postoperative filtration patterns assessed by AS-OCT.

Methods

Study Design

This was a single-center, prospective observational cohort study. The study was approved by the Medical Ethics Committee of Kanazawa University Hospital (2020-008) and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all participants. We evaluated bleb characteristics after PMS implantation and identified factors associated with 1-year surgical outcomes in individuals with primary open-angle glaucoma (POAG) or exfoliation glaucoma (XFG).

Patient Selection

Participants with POAG or XFG inadequately controlled with maximally tolerated medical therapy, or in whom glaucoma progression warranted surgery, were eligible. If both eyes met the inclusion criteria, only the first eye to be operated on was included. Exclusion criteria were angle-closure glaucoma, neovascular glaucoma, uveitic glaucoma, other secondary glaucoma types, eyes that underwent intraluminal stent placement during PMS implantation, and eyes that underwent combined PMS implantation and cataract extraction. Consecutive eligible patients who underwent PMS implantation between March 2023 and October 2024 were prospectively enrolled.

Clinical Examinations

Diagnosis of glaucoma was based on characteristic glaucomatous optic nerve head changes, including enlargement of optic disc cupping, neuroretinal rim thinning, and/or retinal nerve fiber layer defects, together with corresponding visual field defects. Glaucomatous visual field defects were defined as the presence of at least one of the following findings on automated perimetry [Humphrey Field Analyzer (HFA) 24-2 Swedish interactive threshold algorithm; Carl Zeiss Meditec, Inc., Dublin, CA, USA]: an abnormal glaucoma hemifield test, pattern standard deviation with a probability of < 5%, and/or a cluster of three or more contiguous points with probabilities of < 5% on the pattern deviation plot in at least one hemifield, including at least one point with a probability of < 1%. Glaucoma stage was defined according to baseline visual field mean deviation (MD) as follows: early, MD ≥ − 6 dB; moderate, − 12 dB ≤ MD < − 6 dB; and advanced, MD < − 12 dB. Eyes evaluated using the HFA 10-2 program or Goldmann perimetry instead of HFA 24-2 were classified as having advanced glaucoma because HFA 24-2 testing was considered clinically inappropriate owing to advanced visual field loss.

POAG was diagnosed in eyes with an open anterior chamber angle on gonioscopy and no ocular or systemic findings suggestive of secondary glaucoma. XFG was diagnosed based on the presence of pseudoexfoliative material on the pupillary margin and/or anterior lens capsule in eyes with glaucomatous optic neuropathy and corresponding visual field loss.

Axial length was measured preoperatively using optical biometry (OA-2000; Tomey, Japan). Postoperative examinations, including IOP measurement with Goldmann applanation tonometry, slit-lamp biomicroscopy, and funduscopy, were performed at 1, 3, 6, and 12 months. AS-OCT was performed 1 month postoperatively.

Surgical Procedure

All PMS implantations were performed by three experienced glaucoma surgeons. After sub-Tenon’s anesthesia, a fixation suture (7-0 Vicryl, Johnson & Johnson Ethicon, Tokyo, Japan) was placed in the cornea at the 12 o’clock position to expose the surgical field. A fornix-based conjunctival flap was created in the superonasal quadrant by peritomy between two rectus muscles, followed by posterior sub-Tenon’s dissection. When a prior surgical scar was present in the superonasal quadrant, the conjunctival flap was created in the superotemporal area. Sponges soaked in 0.04% mitomycin C (MMC) were applied to the sub-Tenon’s space for 4 min. After sponge removal, the area was irrigated with balanced salt solution. A scleral tunnel was created with a double-step knife, beginning 3 mm posterior to the corneal limbus and extending into the anterior chamber. The PMS tube was then inserted through the scleral tunnel. No eyes underwent placement of an intraluminal 10-0 nylon stent. Tenon's capsule and conjunctiva were sutured at the corneal limbus following the technique originally described for trabeculectomy [12]. Surgery was completed with a subconjunctival injection of dexamethasone sodium phosphate. No eyes underwent combined PMS implantation and cataract extraction.

AS-OCT

Blebs were evaluated using the AS-OCT imaging system (Casia 2; Tomey, Japan). Multiple serial radial sections perpendicular to the limbus and tangential sections along the limbus were obtained across the entire bleb area. Filtration toward the corneal limbus was determined by the presence or absence of fluid between the corneal limbus and the scleral entry site of the PMS. Evaluations were independently performed by two observers (T.M. and D.T.) in a masked manner using both radial and tangential AS-OCT images. Tangential images were acquired approximately 2 mm posterior to the corneal limbus. When judgments differed between the two observers, the final decision was made by a third grader (T.H.). The presence of filtration was defined as an episcleral fluid space, subconjunctival microcysts, and/or multiple internal layers (Fig. 1), with reference to the description by Nakano et al. regarding AS-OCT characteristics of trabeculectomy blebs [13]. Multiple internal layers were characterized as hyporeflective spaces in the deep and superficial conjunctival layers, with fluid channels running parallel to the scleral surface [13].

Fig. 1.

AS-OCT images of blebs 1 month after PMS implantation. A Presence of an episcleral fluid space near the corneal limbus. B Fluid with subconjunctival microcysts and multiple internal layers. C Absence of episcleral fluid near the corneal limbus. AS-OCT anterior segment optical coherence tomography, PMS PreserFlo MicroShunt

Surgical Success

We followed the outcome criteria recommended by the World Glaucoma Association guidelines [14]. Surgical success was defined using three criteria with hypertensive thresholds of 12 mmHg (A), 15 mmHg (B), and 18 mmHg (C), together with an IOP reduction of ≥ 20% from baseline. Each criterion included a hypotensive limit of 6 mmHg. Qualified success (QS) and complete success (CS) were defined as surgical success with or without antiglaucoma medications, respectively. Eyes requiring needling or intracameral injection of an ophthalmic viscosurgical device (OVD) were categorized as QS.

Failure was defined as any of the following: (1) failure to meet the IOP criteria on two consecutive visits; (2) hypotony maculopathy persisting for ≥ 3 months; (3) reoperation, including trabeculectomy, drainage device implantation, or bleb revision; or (4) loss of light perception. Failures corresponding to conditions (2) and (3) were considered to exceed the hypotensive and hypertensive limits, respectively. The time of failure was defined as the first visit at which IOP was outside the specified range.

Statistical Analysis

Data analysis was performed using STATA software (version 18.0, StataCorp, College Station, TX, USA). Normality was assessed with the Kolmogorov-Smirnov test. Changes in IOP over time were analyzed using Friedman’s test. Kaplan-Meier survival analysis was applied to estimate the cumulative probability of surgical success. Cox proportional hazards models were used to identify factors associated with surgical failure, analyzed separately for the hypertensive and hypotensive limits. The following variables were included as covariates in the multivariable Cox model: age, glaucoma type, axial length, preoperative IOP, number of preoperative medications (acetazolamide counted as one), and filtration toward the corneal limbus on AS-OCT. To account for intra-surgeon correlations arising from multiple cases performed by the same surgeon, all models were fitted with robust standard errors clustered by surgeon ID. A p-value < 0.05 was considered statistically significant for all analyses.

Results

Baseline Characteristics

In total, 88 eyes from 88 patients underwent PMS implantation and were followed for 12 months. Baseline characteristics of the study population are presented in Table 1. Among glaucoma subtypes, POAG accounted for 87.5% (n = 77) and XFG for 12.5% (n = 11). Visual field mean deviation values were available for 78 of 88 eyes; the remaining eyes lacked reliable MD data because of central visual field loss or inability to perform automated perimetry. Ten eyes (11.4%) were classified as early glaucoma, 22 eyes (25.0%) as moderate glaucoma, and 56 eyes (63.6%) as advanced glaucoma. At baseline, 98.9% (n = 87) of eyes were pseudophakic, and one eye (1.1%) was phakic. The phakic eye underwent PMS implantation alone, without concurrent cataract extraction. Among all cases, 1 eye had a prior trabeculectomy, while the remaining 87 eyes underwent first-time filtration surgery.

Table 1.

Demographics and baseline characteristics

	Overall population (N = 88)
Age, years, median [IQR]	73 [68–80]
Sex, male/female	43/45
Eye, right/left	44/44
Glaucoma type
Primary open-angle glaucoma, n (%)	77 (87.5)
Exfoliation glaucoma, n (%)	11 (12.5)
Lens status, n (%)
Phakic	1 (1.1)
Pseudophakic	87 (98.9)
Baseline IOP, mmHg, median [IQR]	19 [16–23.8]
Axial length, mm, median [IQR]	25.7 [24.4–27.3]
Corneal endothelial cell density, cells/mm2, median [IQR]	2430 [2144–2669]
Visual field MD, dB, median [IQR]	−14.94 [−9.44–−21.00]a
Glaucoma stagesb
Early	10 (11.4%)
Moderate	22 (25.0%)
Advanced	56 (63.6%)
BCVA, logMAR, median [IQR]	0.15 [0.00–0.40]
Previous laser or incisional surgeries, n (%)
Selective laser trabeculoplasty	4 (4.5)
Trabeculotomy	27 (30.7)
Trabeculectomy	1 (1.1)
Micropulse cyclophotocoagulation	3 (3.4)
Pars plana vitrectomy	11 (12.5)

IQR interquartile range, IOP intraocular pressure, MD mean deviation (Humphrey Field Analyzer 24-2), BCVA best-corrected visual acuity

^aVisual field MD values were available for 78 of 88 eyes

^bGlaucoma stage was defined according to baseline visual field MD as follows: early, MD ≥ − 6 dB; moderate, − 12 dB ≤ MD < − 6 dB; advanced, MD < − 12 dB. Ten eyes without 24-2 MD values were classified as having advanced glaucoma because 24-2 testing was not appropriate owing to severe visual field loss; these eyes were evaluated using the 10-2 program or Goldmann perimetry

Intraocular Pressure, IOP-Lowering Medications, and Bleb Features by AS-OCT

Median IOP (interquartile range) decreased from 19 [16–23.8] mmHg at baseline to 10 [8–13] mmHg at 12 months after PMS implantation (Fig. 2A). The Friedman test showed a significant IOP reduction over time (p < 0.001). The median number of glaucoma medications also declined significantly, from 4 [3.25–5] at baseline to 0 [0–0] at 12 months (p < 0.001, Friedman test) (Fig. 2B). At 1 month postoperatively, 67 eyes (76.1%) exhibited a fluid space extending toward the corneal limbus on AS-OCT, while 21 eyes (23.9%) showed no detectable fluid space. AS-OCT grading showed excellent interobserver agreement between the two graders (κ = 0.88).

Fig. 2.

Postoperative IOP and mean number of glaucoma medications over 12 months. A IOP. Boxes represent interquartile ranges, whiskers denote maximum and minimum values, and the horizontal line indicates the median. The Friedman test showed a significant change from baseline to 12 months (p < 0.001). B Mean number of glaucoma medications. Error bars represent standard deviations. IOP intraocular pressure

Surgical Success

Kaplan-Meier survival curves for overall surgical outcomes are shown in Fig. 3. The QS rates for criteria A, B, and C were 56.8%, 70.5%, and 73.9% at 12 months, respectively (Fig. 3A). The CS rates for the same criteria were 46.6%, 54.5%, and 55.7% at 12 months, respectively (Fig. 3B). In the multivariable Cox proportional hazards model for hypertensive failure, filtration toward the corneal limbus on AS-OCT was independently associated with a lower risk of failure across all three criteria (criterion A: HR = 0.57, 95% CI = 0.36–0.91, p = 0.018; criterion B: HR = 0.35, 95% CI = 0.16–0.76, p = 0.008; criterion C: HR = 0.28, 95% CI = 0.09–0.89, p = 0.031). Longer axial length (criterion B: HR = 0.70 per mm, 95% CI = 0.56–0.87, p = 0.002; criterion C: HR = 0.77 per mm, 95% CI = 0.63–0.93, p = 0.008) and older age (criterion B: HR = 0.95 per year, 95% CI = 0.90–1.00, p = 0.033; criterion C: HR = 0.94 per year, 95% CI = 0.92–0.96, p < 0.001) were also independently associated with a lower risk of failure. Higher preoperative IOP showed criterion-dependent associations, increasing the risk of failure under criterion A while reducing the risk under criterion C. A greater number of preoperative medications was associated with a lower risk of failure under criterion A (Table 2). Additional multivariable Cox analyses using advanced glaucoma as a covariate showed that the main findings, including the associations of axial length and filtration toward the corneal limbus on AS-OCT with hypertensive failure, remained materially unchanged (Supplementary Table 1). In the multivariable Cox model for hypotensive failure, older age (HR = 1.05 per year, 95% CI = 1.01–1.09, p = 0.01) and exfoliation glaucoma (XFG vs. POAG: HR = 3.75, 95% CI = 1.47–9.53, p = 0.006) were identified as independent risk factors for surgical failure (Table 3).

Fig. 3.

Kaplan-Meier curve showing the cumulative probability of success over 12 months. A Qualified success. B Complete success

Table 2.

Multivariable Cox proportional hazards model for factors associated with surgical failure by the hypertensive limits of three criteria

Variables	Criterion A			Criterion B		Criterion C
	HR (95% CI)	P value		HR (95% CI)	P value	HR (95% CI)	P value
Age (per year)	0.99 (0.95–1.05)		0.84	0.95 (0.90–1.00)	0.033	0.94 (0.92–0.96)	< 0.001
Axial length (per mm)	0.88 (0.72–1.08)		0.22	0.70 (0.56–0.87)	0.002	0.77 (0.63–0.93)	0.008
Preoperative IOP (per mmHg)	1.05 (1.03–1.07)		< 0.001	1.00 (0.95–1.06)	0.92	0.93 (0.90–0.97)	0.001
Presence of filtration toward the corneal limbus	0.57 (0.36–0.91)		0.018	0.35 (0.16–0.76)	0.008	0.28 (0.09–0.89)	0.031
Preoperative number of medications	0.86 (0.77–0.95)		0.003	0.90 (0.67–1.20)	0.46	1.00 (0.75–1.32)	0.98
Glaucoma type (XFG vs POAG)	0.90 (0.70–1.16)		0.43	0.75 (0.26–2.18)	0.59	1.01 (0.44–2.29)	0.99

IOP intraocular pressure, XFG exfoliation glaucoma, POAG primary open-angle glaucoma, HR hazard ratio, CI confidence interval

Hypertensive limits: (A) IOP ≤12 mmHg; (B) IOP ≤15 mmHg; and (C) IOP ≤18 mmHg plus an IOP reduction of ≥ 20% from baseline. Reoperation, including trabeculectomy, drainage device implantation, or bleb revision, was deemed a hypertensive failure

Table 3.

Multivariable Cox proportional hazards model for factors associated with surgical failure by the hypotensive limit

Variable	HR	95%CI	P value
Age (per year)	1.05	1.01–1.09	0.01
Axial length (per mm)	1.25	0.87–1.79	0.23
Preoperative IOP (per mmHg)	0.93	0.78–1.10	0.39
Presence of filtration toward the corneal limbus	1.58	0.18–13.7	0.68
Preoperative number of medications	1.07	0.68–1.66	0.78
Glaucoma type (XFG vs POAG)	3.75	1.47–9.53	0.006

IOP intraocular pressure, XFG exfoliation glaucoma, POAG primary open-angle glaucoma, HR hazard ratio, CI confidence interval

Hypotensive limit: IOP ≤ 5 mmHg on two consecutive visits or hypotony maculopathy persisting for > 3 months after surgery

Postoperative Interventions and Adverse Events

Postoperative complications are summarized in Table 4. Numerical hypotony (<6 mmHg) occurred in 13 eyes (14.8%). Of the 13 eyes (14.8%) that developed hypotony maculopathy, 9 experienced spontaneous resolution within 1–2 months. Of the remaining four, one persisted for 3 months and three for 6–10 months. OVD injection was required in 7 eyes (8.0%) and needling in 12 eyes (13.6%). Ciliary body detachment occurred in 51 eyes (58.0%), whereas choroidal detachment occurred in 17 eyes (19.3%). Most cases were mild, transient, and resolved spontaneously during follow-up without additional surgical intervention. Additional PMS implantation was performed in 3 eyes (3.4%) and trabeculectomy in another 3 (3.4%). No eyes lost light perception.

Table 4.

Postoperative complications and additional interventions

	n (%)
Shallow anterior chamber	8 (9.1)
Hyphema	26 (29.5)
Aqueous leakage at the conjunctival incision site	2 (2.3)
Numerical hypotony (< 6 mmHg)	13 (14.8)
Hypotony maculopathy	13 (14.8)
Ciliary body detachment	51 (58.0)
Choroidal detachment	17 (19.3)
Additional interventions
Needling	12 (13.6)
OVD injection	7 (8.0)
Trabeculectomy	3 (3.4)
Additional PMS	3 (3.4)

OVD ophthalmic viscosurgical device, PMS PreserFlo MicroShunt

Discussion

In this study, PMS implantation significantly reduced IOP, with the median decreasing from 19 mmHg at baseline to 10 mmHg at 12 months. The multivariable Cox proportional hazards models identified several independent factors associated with a lower risk of surgical failure by the hypertensive limits at 12 months: the presence of a fluid space near the corneal limbus on AS-OCT at 1 month, longer axial length, and older age. In contrast, older age and XFG increased the risk of surgical failure beyond the hypotensive limit, indicating a higher likelihood of hypotony-related complications. The prospective design with predefined postoperative AS-OCT evaluation strengthened the consistency of imaging assessment.

Among the variables influencing surgical outcomes, the presence of a fluid space near the limbus was consistently associated with a reduced risk of failure across all hypertensive criteria. Several studies have examined filtering bleb morphology after PMS implantation using AS-OCT. Ibarz et al. reported that functioning PMS blebs commonly exhibit hyporeflective thick walls or expansive filtering surfaces with thin capsules [15], features that correspond to sustained filtration. Hasan et al. found a positive correlation between bleb-to-limbus distance and postoperative IOP [16], showing that blebs located closer to the limbus are associated with lower postoperative pressure. These findings align with our observation that a fluid space near the limbus reflects enhanced aqueous outflow and better postoperative IOP control.

Understanding device-specific bleb morphology is essential for interpreting our findings. The PMS is an 8.5-mm implant inserted via an ab externo approach that drains aqueous humor into the subconjunctival space. Comparative AS-OCT studies have shown that PMS blebs often display a prominent episcleral lake, larger internal cavities, and a more posterior location than blebs after XEN Gel Stent implantation or trabeculectomy [11, 17]. Although the device is designed to promote posterior, non-limbal bleb formation, limbal-proximal blebs are frequently observed in clinical practice. One possible explanation is that subconjunctival outflow from the distal tip of the PMS tracks anteriorly toward the limbus. Such anterior extension may be influenced by individual tissue biomechanics and surgical factors, including Tenon’s capsule mobility and the extent of conjunctival dissection.

Bleb maturation over time is another key consideration. Gambini et al. reported that PMS blebs evolve during the first postoperative months, progressing from an initial “multiple internal layers” pattern to microcystic multiform and, eventually, to uniform configurations, with functional blebs typically exhibiting an episcleral cavity extending posterior to the limbus [18]. Schneider et al. quantified bleb vessel density at 2–4 weeks after PMS implantation and found that greater early vascularity predicted the need for surgical revision within 12 months [19]. These findings suggest that bleb morphology dynamically changes during early healing, reflecting tissue remodeling and fibrosis that may affect long-term outcomes. Importantly, AS-OCT findings should be interpreted as complementary postoperative information rather than definitive predictors of surgical success.

Baseline clinical factors have been identified as potential predictors of surgical success in previous studies. Rabiolo et al. reported that lower preoperative IOP, higher numbers of preoperative antiglaucoma medications, and PMS combined with cataract surgery were risk factors for surgical failure [20]. In contrast, Theilig et al. found similar postoperative IOP levels between eyes with high preoperative IOP (≥ 30 mmHg) and those with moderate preoperative IOP (≤ 25 mmHg) after PMS implantation [21]. In the present study, higher preoperative IOP was associated with a reduced risk of failure under the less stringent hypertensive criterion (IOP ≤ 18 mmHg) but with an increased risk under the strict criterion (IOP ≤ 12 mmHg). These results suggest that eyes with higher baseline IOP are less likely to achieve low-teen IOP, despite showing favorable outcomes at high-teen levels.

Although a greater number of preoperative glaucoma medications was associated with a lower risk of hypertensive failure under criterion A in the present study, this finding should be interpreted cautiously. Previous studies, including those by Rabiolo et al. [20] and Mieno et al. [22], have reported that greater preoperative medication use was associated with surgical failure using composite surgical failure criteria that included both hypertensive failure and hypotony-related events after PMS implantation. Of note, Mieno et al. reported that a greater number of preoperative medications was associated with a higher risk of hypotony-related complications after PMS implantation [23]. Therefore, differences in outcome definitions may partly explain the apparent discrepancy. Furthermore, the association with preoperative medication use was no longer significant after additional adjustment for glaucoma stage (Supplementary Table 1). Likewise, when overall surgical failure was analyzed using a composite outcome that included both hypertensive and hypotony-related failure, the association was no longer significant. Taken together, these findings suggest that the observed association should be interpreted cautiously.

Longer axial length was linked to a significantly reduced risk of failure according to the hypertensive thresholds of Criterion B and C. A previous case report demonstrated favorable IOP control with PMS implantation in a highly myopic eye [24]; however, consistent evidence linking high myopia with surgical success remains lacking. Possible mechanisms include anatomical differences in highly myopic eyes, such as thinner sclera or altered tissue compliance, that may influence bleb morphology and outflow dynamics. The prevalence of myopia is significantly higher in Asian populations than in non-Asian populations [25, 26], and its global prevalence is expected to increase substantially [27]. Because myopia is a known risk factor for glaucoma [28], the number of patients with both conditions will likely rise, increasing the need for effective surgical options in this subgroup. High myopia is frequently associated with scleral thinning, which can complicate scleral flap creation during trabeculectomy. Thus, the favorable outcomes observed in eyes with longer axial length in our study support the potential advantages of PMS in these patients. As PMS implantation does not require a scleral flap, it may represent a technically safer and more suitable option for individuals with glaucoma and high myopia.

Recently, Iemura et al. reported apparently contrasting findings in a retrospective study comparing PMS outcomes between eyes with and without high myopia [29]. In their study, high myopia, defined as an axial length > 26.5 mm, was associated with lower 6-month surgical success rates, and longer axial length was identified as a risk factor for surgical failure. These findings differ from the present results, in which longer axial length was associated with a lower risk of hypertensive failure over 12 months under criteria B and C. Several differences between the studies may account for this discrepancy, including differences in follow-up duration, axial length distribution, baseline IOP, surgical techniques, success definitions, and postoperative management. Notably, 0.04% MMC was applied for 4 min in the present study, whereas Iemura et al. used the same concentration for 2 min. Previous studies have suggested that greater MMC exposure may enhance antifibrotic effects and improve surgical outcomes after PMS implantation [30]. Therefore, differences in wound-healing modulation may have contributed, at least in part, to the differing associations between axial length and surgical outcomes observed between the studies. Such effects may be particularly relevant in eyes with longer axial length, which may exhibit distinct subconjunctival and scleral tissue characteristics. Further studies are needed to clarify the role of axial elongation and wound-healing modulation in surgical response following PMS implantation.

In this study, numerical hypotony (IOP < 6 mmHg) occurred in 14.8% of eyes, comparable to previously reported PMS rates (10–30%) [7, 31, 32]. XFG and older age were identified as risk factors for failure by the hypotensive limit. Nobl et al. reported that XFG increased the risk of postoperative hypotony and choroidal detachment after PMS implantation [33]. The increased risk of hypotony-related failure in older patients may reflect reduced aqueous humor production and/or altered wound-healing responses associated with aging. Several studies have shown that placing a nylon stent within the PMS lumen to modulate aqueous outflow significantly reduces postoperative hypotony [34, 35]. Our study included only patients who underwent PMS implantation without stent placement; therefore, further research is needed to clarify the effect of stent placement on postoperative bleb morphology. Postoperative OVD injection into the anterior chamber may also influence AS-OCT evaluation. In this study, AS-OCT imaging was performed approximately 2 weeks after OVD injection in all such cases. Because OVD typically clears within several days, filtration from the PMS at the time of imaging was unlikely to be affected by residual viscoelastic material.

From a clinical perspective, the identification of both axial length and postoperative filtration patterns as independent factors associated with favorable outcomes highlights the multifactorial nature of surgical response following PMS implantation. Axial length is a readily available preoperative anatomical parameter, while AS-OCT provides noninvasive postoperative assessment of filtration patterns. Considering these factors together may assist clinicians in postoperative assessment and risk stratification and in tailoring individualized follow-up strategies.

Limitations

This study has some limitations. First, it was conducted at a single institution and included only Japanese patients, which may limit generalizability. Second, the follow-up period was relatively short (12 months). Third, although the concentration and exposure time of MMC were consistent across the three surgeons, variations in suturing and incision techniques remained. These factors may affect the external validity of the findings and contribute to residual confounding. Fourth, because 98.9% of the study eyes were pseudophakic, it remains unclear whether lens status may influence the efficacy and outcomes of PMS implantation. Finally, although the multivariable models included clinically relevant covariates, the limited number of events may have increased the risk of model overfitting. Further large-scale studies are needed to validate the associations identified in the present study.

Conclusions

PMS implantation achieved sustained IOP reduction over 12 months. The presence of a fluid space near the corneal limbus on AS-OCT at 1 month was independently associated with favorable outcomes according to hypertensive criteria. Longer axial length and older age were also associated with better outcomes under the hypertensive limits, whereas older age and XFG were associated with failure under the hypotensive criteria. Integrating baseline ocular anatomy with early postoperative AS-OCT findings may provide complementary information for understanding postoperative surgical response following PMS implantation. Future studies should investigate how surgical techniques and perioperative management strategies promote favorable bleb morphology, particularly filtration toward the corneal limbus, and whether these approaches improve long-term surgical outcomes.

Supplementary Information

Below is the link to the electronic supplementary material.

Author Contributions

Tomomi Higashide contributed to the conception and design of the study and data management. Takahiro Minami, Sachiko Udagawa, and Yoko Yamashita contributed to data collection. Takahiro Minami, Daisuke Takemoto, and Tomomi Higashide contributed to data analysis. Takahiro Minami drafted the manuscript. Takahiro Minami, Daisuke Takemoto, Taro Muroya, Sachiko Udagawa, Shunsuke Tsuchiya, Kimikazu Sakaguchi, Ryotaro Wajima, Yoko Yamashita, Kazuhisa Sugiyama, and Tomomi Higashide contributed to data interpretation, were involved in revising the manuscript, critically reviewed the manuscript, and approved the final version for publication.

Funding

Open Access funding provided by Kanazawa University. No funding or sponsorship was received for this study. The Rapid Service and Open Access fees were funded by the authors.

Data Availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Conflict of Interest

Takahiro Minami, Daisuke Takemoto, Taro Muroya, Sachiko Udagawa, Shunsuke Tsuchiya, Kimikazu Sakaguchi, Ryotaro Wajima, Yoko Yamashita, Kazuhisa Sugiyama, and Tomomi Higashide have nothing to disclose.

Ethical Approval

This was a single-center, prospective observational cohort study. The study was approved by the Medical Ethics Committee of Kanazawa University Hospital (2020-008) and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all participants.