The PreserFlo MicroShunt, manufactured by Santen, Inc. of Osaka, Japan, is a small gel-like stent classified as a minimally invasive bleb-forming procedure. It is implanted through an ab externo approach: the surgeon creates a conjunctival flap, applies the antifibrotic agent mitomycin C to the sub-Tenon’s space, and tunnels the tube through the sclera so that its distal tip sits beneath the conjunctiva approximately six millimeters behind the corneal limbus, the junction between the clear cornea and the white sclera. Aqueous humor then drains through the device into a filtering pocket, or bleb, that forms over the shunt. Compared with trabeculectomy, the long-standing gold-standard filtration surgery, the microshunt has demonstrated effective pressure reduction with a more favorable safety profile and less demanding postoperative care. Yet trabeculectomy remains indispensable for achieving very low target pressures in advanced disease, and the two procedures serve different clinical indications.

Glaucoma itself is the leading cause of irreversible blindness worldwide, and lowering intraocular pressure, or IOP, remains the only proven way to slow its progression. The rationale for studying bleb behavior is straightforward: the success of any filtration surgery depends on whether the drained fluid can spread and be absorbed without being walled off by scarring. Previous studies of microshunt outcomes had examined bleb height, area, and internal reflectivity using anterior segment optical coherence tomography, or AS-OCT, a light-based imaging technique that resolves microscopic structures of the anterior eye without touching it. But the clinical significance of one particular pattern—filtration extending forward toward the corneal limbus—had never been clearly established. The Kanazawa team set out to determine whether that pattern, along with baseline anatomical factors such as axial length, was associated with durable one-year success.

The study was a single-center, prospective observational cohort conducted at Kanazawa University Hospital and approved by its medical ethics committee. Between March 2023 and October 2024, the investigators consecutively enrolled patients with primary open-angle glaucoma or exfoliation glaucoma whose pressure remained inadequately controlled on maximally tolerated medication, or whose disease progression warranted surgery. Only the first operated eye was included for each patient, and eyes that received an intraluminal stent or combined cataract extraction were excluded to keep the analysis homogeneous. The final cohort comprised 88 eyes from 88 patients, the vast majority—87.5 percent—with primary open-angle glaucoma and 12.5 percent with exfoliation glaucoma. Notably, 63.6 percent of the eyes had advanced glaucoma by visual field criteria, and 98.9 percent were already pseudophakic, meaning the natural lens had previously been replaced with an artificial implant.

Surgery was performed by three experienced glaucoma surgeons using a standardized technique: fornix-based conjunctival dissection, application of 0.04 percent mitomycin C sponges for four minutes, creation of a double-step scleral tunnel beginning three millimeters behind the limbus, and insertion of the shunt without an intraluminal nylon stent. One month after surgery, each patient underwent AS-OCT imaging with the Casia 2 system (Tomey, Japan), with multiple radial sections perpendicular to the limbus and tangential sections along it. Filtration toward the corneal limbus was defined as the presence of fluid between the limbus and the scleral entry site of the shunt, evidenced by an episcleral fluid space, subconjunctival microcysts, or multiple internal hyporeflective layers within the bleb wall. Two independent observers graded the images in a masked fashion and achieved excellent agreement, with a kappa statistic of 0.88; disagreements were adjudicated by a third senior grader.

The results were striking on several fronts. Median IOP fell from 19 mmHg at baseline to 10 mmHg at twelve months, and the median number of glaucoma medications dropped from four to zero, both highly significant changes. At the one-month scan, 76.1 percent of eyes showed a fluid space extending toward the corneal limbus. Using World Glaucoma Association consensus criteria—with hypertensive thresholds of 12, 15, and 18 mmHg (criteria A, B, and C), each requiring at least a 20 percent IOP reduction and each bounded below by a hypotensive limit of 6 mmHg—qualified success rates at one year were 56.8, 70.5, and 73.9 percent, respectively. Complete success, defined as success without any antiglaucoma medications, ranged from 46.6 to 55.7 percent depending on the criterion.

The multivariable Cox proportional hazards models, fitted with standard errors clustered by surgeon to account for intrasurgeon correlation, revealed that limbus-directed filtration on AS-OCT was independently associated with a substantially lower risk of hypertensive failure across all three criteria, with hazard ratios of 0.57, 0.35, and 0.28 for criteria A, B, and C. In other words, eyes whose early blebs extended fluid forward toward the limbus were roughly two to three and a half times less likely to fail by pressure criteria during the following year. Longer axial length also emerged as protective, with hazard ratios of 0.70 and 0.77 per millimeter under criteria B and C, and older age was similarly associated with a lower risk of hypertensive failure. Preoperative IOP and medication count showed criterion-dependent effects, reflecting the statistical interplay between baseline severity and the fixed thresholds used to define success.

The story was different at the low-pressure end of the distribution. Numerical hypotony, defined as IOP below 6 mmHg, occurred in 14.8 percent of eyes, consistent with the 10 to 30 percent range previously reported for the microshunt. In the Cox model for hypotensive failure, older age (hazard ratio 1.05 per year) and exfoliation glaucoma (hazard ratio 3.75 relative to primary open-angle glaucoma) were identified as independent risk factors. This aligns with prior reports that exfoliation glaucoma increases the risk of postoperative hypotony and choroidal detachment after microshunt implantation, and the authors suggest that reduced aqueous humor production and altered wound-healing in older patients may contribute. Notably, the study excluded eyes that received an intraluminal nylon stent, an adjunct shown in other work to significantly reduce early hypotony, leaving open the question of how stenting might interact with bleb morphology.

The axial length finding carries particular weight given an ongoing scientific debate. Myopic eyes—those with long axial lengths—are overrepresented among glaucoma patients, particularly in East Asia, where the prevalence of pathologic myopia is projected to rise substantially in the coming decades. A Japanese multicenter study by Iemura and colleagues, and a separate report by Mieno and colleagues, had suggested that longer eyes fared worse after microshunt surgery, with axial lengths exceeding 26.5 millimeters associated with lower six-month success. The Kanazawa team’s results point in the opposite direction at twelve months, and they attribute the discrepancy at least in part to differences in mitomycin C exposure: their protocol applied 0.04 percent mitomycin C for four minutes, double the two-minute exposure used in the earlier work. Greater antifibrotic exposure is known to enhance bleb survival, and longer eyes may possess distinct subconjunctival and scleral tissue characteristics that respond differently to wound-healing modulation. Longer follow-up also matters, since fibrotic failure tends to declare itself over time.

The authors are careful about their study’s limitations. It was conducted at a single Japanese institution, which may limit generalizability to other populations; the follow-up was limited to twelve months; minor variations in suturing and incision technique persisted among the three surgeons despite the standardized mitomycin C protocol; nearly all eyes were pseudophakic, leaving lens status unexamined; and the modest number of failure events raised the possibility of model overfitting. Additional analyses using advanced glaucoma status as a covariate, however, left the main findings materially unchanged, lending robustness to the core conclusions about axial length and limbus-directed filtration.

Clinically, the study’s message is one of complementary information streams. Axial length is cheap, fast, and universally available preoperatively through optical biometry, while AS-OCT offers a radiation-free, noninvasive window into early postoperative bleb function. A surgeon could, in principle, identify a short-eyed patient or one with exfoliation glaucoma before the operation and counsel that patient accordingly, then use a one-month scan to decide whether the bleb is behaving as hoped—informing decisions about follow-up intensity, the timing of postoperative interventions such as needling or viscoelastic injection, and the potential value of intraluminal stenting in hypotony-prone eyes. The authors call for future research into which surgical techniques and perioperative strategies actively promote limbus-directed filtration, and whether deliberately sculpting that morphology improves long-term outcomes. For now, the study adds a compelling data point to the growing recognition that glaucoma surgery is not one-size-fits-all: the geometry of the eye before the knife and the physics of fluid flow after it both matter, and imaging may let clinicians see success or failure forming weeks before it announces itself in the tonometer reading.