Introduction: The aim of this study was to describe and evaluate double PreserFlo MicroShunt implantation as a modified micro-invasive glaucoma surgery technique and to retrospectively compare the outcomes in a cohort of glaucoma patients with single or double implantation. Materials and Methods: A retrospective data analysis of 57 glaucoma patients who consecutively underwent PreserFlo implantation was performed. Medical records were examined for patients’ demographics, glaucoma type, intraocular pressure (IOP), medication, complications, and re-interventions. Two groups with single (n = 29) or double (n = 28) implantation were formed, and the outcomes were compared. In cases of two-stage double implantation (n = 17), the courses of the initial and the second implantations were compared. Results: Mean preoperative IOP was significantly higher in the double compared to the single implantation group (29.4 ± 10.0 mm Hg; 21.7 ± 8.2 mm Hg; p = 0.003). Postoperatively, IOP was significantly lower in the double implantation group at various time-points (day 1, week 1, months 3 and 6; all p < 0.021). In the subgroup with two-stage procedures, mean preoperative IOP was 24.5 ± 8.5 mm Hg and 29.8 ± 10.1 mm Hg, respectively (p = 0.128). While immediately postoperatively, mean IOP lowering was clinically significant and similar following both procedures, the longer sustainable effect was observed after the second procedure (month 12: 25.5 ± 7.5 mm Hg; 12.4 ± 4.8 mm Hg; p = 0.001). No serious complications were observed. Discussion/Conclusion: Double PreserFlo implantation appears safe and efficient for lowering IOP in glaucoma patients. Our preliminary findings suggest that double is superior to single implantation in terms of IOP lowering and the need for additional topical medication. Patients with insufficient IOP lowering following single implantation may benefit from a second implantation. Further research is warranted to evaluate double implantation as a first-line, one-stage procedure.

Glaucoma is a progressive, irreversible neurodegenerative disease affecting the retinal ganglion cells, which make up the optic nerve and project the electrophysiological signal of the visual circuit to the brain. Clinically, the disease manifests with deterioration of visual sensitivity, progressive visual field deficits and ultimately blindness [1]. Projected to affect up to 110 million patients until 2040, glaucoma is among the most prevalent irreversible blinding disorders worldwide and is a significant health and economic burden [2]. With elevated intraocular pressure (IOP) being the most important glaucoma risk factor, current treatment strategies aim at lowering IOP, either pharmaceutically, by laser treatment or surgically [1, 3, 4]. Introduced in 1968, trabeculectomy has long been considered the gold standard among surgical IOP-lowering procedures [5]. However, with the emergence of the so-called micro-invasive glaucoma surgery (MIGS) techniques and a number of IOP-lowering aqueous humor drainage implants over the past years, treatment options have expanded [6‒9]. Many of these techniques have shown promises for significantly lowering IOP while minimizing the risk for complications [6, 10]. The PreserFlo MicroShunt (Santen, Miami, FL, USA) is one of these new implants, consisting of an 8.5 mm long tube with a 70 µm lumen composed of poly(styrene-block-isobutylene-block-styrene). This device is implanted into the anterior chamber angle using an ab externo approach, creating an aqueous humor outflow path from the anterior chamber to a bleb underneath the posterior conjunctiva and Tenon’s capsule [11]. The procedure may be augmented by intraoperative application of mitomycin C to lower the risk of bleb scarring [12].

According to the Hagen-Poiseuille equation outflow resistance depends on the length and width of the tube lumen [13]. The drainage tubes used in MIGS have been designed taking this into account, in order to allow a steady aqueous humor outflow whilst avoiding postoperative hypotony [14]. However, in some patients a pronounced aqueous humor outflow deficiency from the bleb may occur, and it has been hypothesized that multiple drainage devices could be beneficial in such patients. Recently, based on the residual aqueous humor outflow after 14 days of washout of IOP-lowering pharmaceutical therapy, a theoretical mathematical model was developed with which to predict the number of XEN45 Gel stents (Allergan, Dublin, Ireland) – another MIGS drainage implant with a 45 µm wide lumen – needed to sufficiently lower IOP in an individual patient [13]. Today, growing evidence supports that the PreserFlo MicroShunt is efficient and safe in lowering IOP and many experts consider this the preferable MIGS drainage tube that is currently available [15‒17]. Furthermore, recent studies have shown that the device creates a novel type of bleb with a distinct morphology in particular with the posterior episcleral fluid lake [12]. Possibly, this novel type of bleb morphology renders the standard single PreserFlo MicroShunt-produced bleb sensitive to flow oscillations, e.g., due to temporary congestion with intraluminal debris, and hence bleb volume reduction, leading ultimately to bleb closure and scarring. The presence of an additional implant feeding the same bleb from a different vector angle could provide for more constant bleb volume. Nevertheless, the concept of multiple implantations of this device has not yet been debated in the scientific community, either as a one-stage or as a two-stage surgical procedure.

After the initial successes in patients requiring surgical bleb revision after the first PreserFlo implantation, where we implanted the second implant in an attempt to facilitate the filtration and to reduce the risk of second bleb failure, we went on with implementing this double PreserFlo technique in further patients with initial bleb failure. This was an off-label application, based on individual clinical decision in each patient, and all known options were discussed with each patient prior to reaching the decision.

Thus, the aim of this analysis is to report on feasibility of double PreserFlo MicroShunt implantation as an innovation and expansion of the known MIGS technique and to retrospectively compare the IOP-lowering efficacy and outcomes in a cohort of glaucoma patients with single or double implantation.

Study Design and Ethics

This present analysis is as a retrospective cohort study. The retrospective data analysis was conducted in accordance with the Declaration of Helsinki and was approved by the appropriate Ethics Committee (Ethikkommission Nordwest-und Zentralschweiz EKNZ/Kantonale Ethikkommission Zürich KEK, approval number 2020-00121).

Surgical Procedure

All surgeries were performed by the same surgeon (KG). First, local anesthesia was applied, after which mitomycin was applied as a subconjunctival injection in the concentration of 0.3 mg/mL and in volume of 0.2 mL injected under the conjunctiva at the intended site. Then, after 3 min a conjunctival incision was made at the superior corneal limbus. The conjunctiva and Tenon’s capsule were carefully dissected from the sclera using blunt tipped scissors. Bipolar diathermy was used to stop bleeding when needed. Lateral paracentesis was made and anterior chamber filled with viscoelastic, which was flushed out at the end of the surgery. Next, a needle tract reaching from the sclera to the iridocorneal angle was made using an ab externo approach. In case of single implantation, one PreserFlo MicroShunt implant was inserted through the needle tract using forceps, until the wedge fins of the device locked into the scleral pocket. In case of one-stage double implantation, another needle tract was made and another PreserFlo MicroShunt device was implanted in an identical manner parallel and with a distance of maximal 2–3 mm to the first one, in fact as close as possible, so that the same bleb space is supplied through both implants. A gonioscopy mirror was used to visualize the iridocorneal angle and to check for correct positioning of any implant. To check for patency of any implanted tube, saline fluid was administered through the tube into the anterior chamber using a thin-wall cannula. Subsequently, a relatively loose scleral suture was made using a rapid resorbable Vicryl 6-0 to tie the implant onto the sclera (to fix it/them below the Tenon’s capsule) and the conjunctiva was closed using Vicryl 8-0 sutures.

In case of two-stage double implantation, secondary surgery was performed as follows: the conjunctiva and the Tenon’s capsule were re-incised and re-dissected using blunt tipped scissors. Any tissue adhesions were removed, the conjunctival end of the first originally implanted PreserFlo MicroShunt was freed and the implant brought to function again by flushing it with saline. The second PreserFlo MicroShunt device was implanted and the operative site was closed as described above. As described previously, also here mitomycin was applied in the concentration of 0.3 mg/mL and in volume of 0.2 mL injected under the conjunctiva at the intended site of the filtering bleb.

Subjects and Retrospective Medical Records Reviewing

All patients who consecutively underwent PreserFlo MicroShunt implantation at the University Hospital Basel between October 2018 and April 2022 (n = 57) were identified using our clinical information software. Patients’ medical records were extracted and examined for patients’ age, sex, glaucoma type, intraocular pressure (IOP), co-morbidities, topical and systemic medication, intra- and postoperative complications and re-interventions.

The cohort was divided into two groups of with either single (n = 29) or double (n = 28) PreserFlo MicroShunt implantation and the groups were compared to each other. In addition, in the group with double implantation those patients who underwent a two-stage procedure (n = 17) were identified and the course of the initial implantation was compared with the course of the second procedure.

Statistical Analysis

Differences in preoperative IOP as well as differences in postoperative IOP at day 1, week 1 (±3 days), month 3 (±1 month), month 6 (±2 months), month 12 (±2 months), month 18 (±2 months), and month 24 (±2 months) between the single and double PreserFlo implantation group, as independent samples, were defined as primary outcomes. Within the subgroup with two-stage double implantation, differences in IOP between the first and the second PreserFlo implantation, as dependent samples, at the same time-points from the operation as described above were also defined as primary outcomes. Numbers of topical IOP-lowering compounds between the same (sub-)groups and at the same time-points as described above are reported. All statistical analysis was performed in Statistica StatSoft programme. Descriptive analysis with calculation of mean values, standard deviation, median values and ranges was performed. Data were tested for normality with a Shapiro Wilk and Kolmogorov-Smirnov test, and since normally distributed, they were compared by a non-paired and paired Student’s t-test. p < 0.05 was considered statistically significant.

Patient Characteristics

A total number of 57 patients (n = 57) were included in this study. The group with single PreserFlo MicroShunt implantation comprised 29 patients (n = 29) while the group with double implantation consisted of 28 patients (n = 28). Within the group with double implantation, 11 patients (n = 11) had undergone a one-stage procedure and 17 patients (n = 17) had undergone a two-stage procedure. Overall mean age at the time of (first) surgery was 69.7 years. Twenty-seven (47.4%) of all patients were female. Twenty-seven (47.4%), 21 (36.8%), and 6 (10.5%) patients had primary open-angle glaucoma, secondary open-angle glaucoma and juvenile glaucoma, respectively. One patient (1.8%) had normal tension glaucoma. Distributions of age and sex were similar among groups and subgroups. We did not find statistically significant differences in glaucoma types between the groups. Patients’ characteristics and distribution among the different subgroups are displayed in Table 1.

Table 1.

Patient characteristics

Single PreserFlo (n = 29)Double PreserFlo (n = 28)Overall (n = 57)
one-stage procedure (n = 11)two-stage procedure** (n = 17)
Mean age, years 73.1 63.7 67.7 69.7 
Female, n (%) 18/29 (62.1) 3/11 (27.3) 6/17 (35.3) 27/57 (47.4) 
Glaucoma type, n (%) 
 Primary open-angle glaucoma 20/29 (69.0) 2/11 (18.2) 7/17 (41.2) 27/57 (47.4) 
 Secondary open-angle glaucoma 7/29 (24.1) 6/11 (54.5) 8/17 (47.1) 21/57 (36.8) 
 Juvenile glaucoma 2/29 (6.9) 2/11 (18.2) 2/17 (11.8) 6/57 (10.5) 
 Normal tension glaucoma None 1/11 (9.1) None 1/57 (1.8) 
Previous glaucoma surgery, n (%) 
 None 21/29 (72.4) 2/11 (18.2) 4/17 (23.5) 27/57 (47.4) 
 Laser trabeculoplasty (SLT or ALT) 3/29 (10.3) 1/11 (9.1) 8/17 (47.1) 12/57 (21.1) 
 Cyclophotocoagulation 1/29 (3.4) 1/11 (9.1) 2/17 (11.8) 4/57 (7.0) 
 Filtering glaucoma surgery 4/29 (13.8) 7/11 (63.6) 3/17 (17.6) 14/57 (24.6) 
Pseudophakia, n (%) 18/29 (62.1) 4/11 (36.4) 9/17 (52.9) 31/57 (54.4) 
Single PreserFlo (n = 29)Double PreserFlo (n = 28)Overall (n = 57)
one-stage procedure (n = 11)two-stage procedure** (n = 17)
Mean age, years 73.1 63.7 67.7 69.7 
Female, n (%) 18/29 (62.1) 3/11 (27.3) 6/17 (35.3) 27/57 (47.4) 
Glaucoma type, n (%) 
 Primary open-angle glaucoma 20/29 (69.0) 2/11 (18.2) 7/17 (41.2) 27/57 (47.4) 
 Secondary open-angle glaucoma 7/29 (24.1) 6/11 (54.5) 8/17 (47.1) 21/57 (36.8) 
 Juvenile glaucoma 2/29 (6.9) 2/11 (18.2) 2/17 (11.8) 6/57 (10.5) 
 Normal tension glaucoma None 1/11 (9.1) None 1/57 (1.8) 
Previous glaucoma surgery, n (%) 
 None 21/29 (72.4) 2/11 (18.2) 4/17 (23.5) 27/57 (47.4) 
 Laser trabeculoplasty (SLT or ALT) 3/29 (10.3) 1/11 (9.1) 8/17 (47.1) 12/57 (21.1) 
 Cyclophotocoagulation 1/29 (3.4) 1/11 (9.1) 2/17 (11.8) 4/57 (7.0) 
 Filtering glaucoma surgery 4/29 (13.8) 7/11 (63.6) 3/17 (17.6) 14/57 (24.6) 
Pseudophakia, n (%) 18/29 (62.1) 4/11 (36.4) 9/17 (52.9) 31/57 (54.4) 

Mean for continuous variables and n (%) for categorical variables.

Distributions of age and sex were similar among groups and subgroups.

We did not find statistically significant differences in glaucoma types between the groups.

*Patients shown in the subgroup of laser trabeculoplasty were previously treated with SLT (selective laser trabeculoplasty) and/or ALT (argon laser trabeculoplasty) only, whereas patients shown in the subgroup of cyclophotocoagulation may have been previously treated with SLT and/or ALT in addition; patients shown in the subgroup of filtering glaucoma surgery (including trabeculectomy, XEN gel stent implant, Ex-Press shunt) may have been previously treated with both of the other options in addition.

**At time-point of first PreserFlo MicroShunt implantation.

***The following types of secondary open-angle glaucoma occurred (n = 21/57 over all subgroups): pseudoexfoliation glaucoma (n = 14), pigment dispersion glaucoma (n = 2), Posner-Schlossmann (n = 2), uveitic secondary glaucoma (n = 2), uveitis-glaucoma-hyphema syndrome (n = 1).

Intraocular Pressure, Medication, and Complications

Mean preoperative IOP was significantly higher in the group with double PreserFlo MicroShunt implantation compared to the group with single implantation (29.4 ± 10.0 mm Hg [mean ± standard deviation] vs. 21.7 ± 8.2 mm Hg; p = 0.003). Postoperatively, IOP was significantly lower in the double implantation group at various time-points (day 1, week 1, months 3 and 6; all p < 0.021). At later postoperative time-points (months 12, 18 and 24), IOP was still lower in the double implantation group, yet data is available only for a limited number of patients and the differences did not reach statistical significance (month 24: 15.8 ± 6.4 mm Hg vs. 11.7 ± 2.7 mm Hg; p = 0.145).

Preoperatively, mean number of topical IOP-lowering compounds was similar in the single and the double implantation group (3.5 ± 1.2 vs. 3.0 ± 1.2). Postoperatively, mean number of topical IOP-lowering medications was nominally lower in the double implantation group at all assessable postoperative time-points.

Out of the 29 patients with single PreserFlo MicroShunt implantation, 13 patients had reached the end of the observation period. Nine have been lost to follow up during the observation period (i.e., patients were referred back in the care of his/her private ophthalmologist and no further data were available), and 7 eyes underwent additional operations during the observation period (3 bleb revisions, 2 cyclophotocoagulations and 2 non glaucoma related operations; mean time-point after the PreserFlo surgery: 12.5 ± 8.2 months; 5–26 months). Out of the 11 patients who underwent one-stage double PreserFlo implantation, 2 patients had reached the end of observation period. Two have been lost to follow up during the observation period, and in 7 patients the time-point of operation was less than 24 months prior to the point of retrospective analysis. In the 17 patients who underwent two-stage procedure, the average time between the first and the second PreserFlo Implantation was 57 weeks (±47.6; 5–164 weeks). Out of these 17 patients, after the second PreserFlo implantation in two-stage procedure, 4 patients had reached the end of the observation period. Two have been lost to follow up during the observation period, and in 11 patients the time-point of operation was less than 24 months prior to the point of retrospective analysis. Complete data on pre- and postoperative IOP and number of topical IOP-lowering medications in the groups with single and double PreserFlo MicroShunt implantation are shown in Table 2 and Figures 1 and 2.

Table 2.

Intraocular pressure and number of topical intraocular pressure lowering medications before and following single or double PreserFlo MicroShunt implantation

Single PreserFloDouble PreserFlo
intraocular pressure (IOP)number of IOP-lowering medicationsintraocular pressure (IOP)number of IOP-lowering medications
Preoperative 
 Available data for n patients 29 29 Available data for n patients 28 28 
 Mean (±SD; range) 21.7 (±8.2; 11–47) 3.5 (±1.2; 1–5) Mean (±SD; range) 29.4 (±10.0; 13–55) 3.0 (±1.2; 0–5) 
    p value single versus double PreserFlo 0.003  
Postoperative; day 1 
 Available data for n patients 29 29 Available data for n patients 27 27 
 Mean (±SD; range) 14.1 (±3.6; 9–21) 0.0 (±0; 0-0) Mean (±SD; range) 7.5 (±3.5; 2–15) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo <0.001  
Postoperative; week 1 
 Available data for n patients 29 29 Available data for n patients 27 27 
 Mean (±SD; range) 10.7 (±4.0; 4–22) 0.1 (±0.6; 0–3) Mean (±SD; range) 8.0 (±2.9; 2–16) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo 0.005  
Postoperative; month 3 
 Available data for n patients 27 27 Available data for n patients 24 24 
 Mean (±SD; range) 14.0 (±9.2; 5–43) 0.4 (±1.0; 0–4) Mean (±SD; range) 8.3 (±3.4; 2–14) 0.3 (±0–7; 0–3) 
    p value single versus double PreserFlo 0.005  
Postoperative; month 6 
 Available data for n patients 18 18 Available data for n patients 18 18 
 Mean (±SD; range) 14.8 (±7.1; 6–30) 0.6 (±1.2; 0–4) Mean (±SD; range) 10.3 (±3.5; 5–17) 0.2 (±0.7; 0–3) 
    p-value single versus double PreserFlo 0.02  
Postoperative; month 12 
 Available data for n patients 18 18 Available data for n patients 17 17 
 Mean (±SD; range) 15.4 (±5.3; 8–25) 0.6 (±1.0; 0–4) Mean (±SD; range) 12.0 (±5.0; 4–22) 0.2 (±0.7; 0–3) 
    p value single versus double PreserFlo 0.054  
Postoperative; month 18 
 Available data for n patients 16 16 Available data for n patients 15 15 
 Mean (±SD; range) 13.8 (±5.3; 7–26) 1.3 (±1.6; 0–4) Mean (±SD; range) 12.3 (±3.8; 6–18) 0.4 (±0.6; 0–2) 
    p value single versus double PreserFlo 0.38  
Postoperative; month 24 
 Available data for n patients 13 13 Available data for n patients 
 Mean (±SD; range) 15.8 (±6.4; 7–28) 1.6 (±1.6; 0–4) Mean (±SD; range) 11.7 (±2.7; 8–16) 0.7 (±0.7; 0–2) 
    p value single versus double PreserFlo 0.145  
Single PreserFloDouble PreserFlo
intraocular pressure (IOP)number of IOP-lowering medicationsintraocular pressure (IOP)number of IOP-lowering medications
Preoperative 
 Available data for n patients 29 29 Available data for n patients 28 28 
 Mean (±SD; range) 21.7 (±8.2; 11–47) 3.5 (±1.2; 1–5) Mean (±SD; range) 29.4 (±10.0; 13–55) 3.0 (±1.2; 0–5) 
    p value single versus double PreserFlo 0.003  
Postoperative; day 1 
 Available data for n patients 29 29 Available data for n patients 27 27 
 Mean (±SD; range) 14.1 (±3.6; 9–21) 0.0 (±0; 0-0) Mean (±SD; range) 7.5 (±3.5; 2–15) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo <0.001  
Postoperative; week 1 
 Available data for n patients 29 29 Available data for n patients 27 27 
 Mean (±SD; range) 10.7 (±4.0; 4–22) 0.1 (±0.6; 0–3) Mean (±SD; range) 8.0 (±2.9; 2–16) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo 0.005  
Postoperative; month 3 
 Available data for n patients 27 27 Available data for n patients 24 24 
 Mean (±SD; range) 14.0 (±9.2; 5–43) 0.4 (±1.0; 0–4) Mean (±SD; range) 8.3 (±3.4; 2–14) 0.3 (±0–7; 0–3) 
    p value single versus double PreserFlo 0.005  
Postoperative; month 6 
 Available data for n patients 18 18 Available data for n patients 18 18 
 Mean (±SD; range) 14.8 (±7.1; 6–30) 0.6 (±1.2; 0–4) Mean (±SD; range) 10.3 (±3.5; 5–17) 0.2 (±0.7; 0–3) 
    p-value single versus double PreserFlo 0.02  
Postoperative; month 12 
 Available data for n patients 18 18 Available data for n patients 17 17 
 Mean (±SD; range) 15.4 (±5.3; 8–25) 0.6 (±1.0; 0–4) Mean (±SD; range) 12.0 (±5.0; 4–22) 0.2 (±0.7; 0–3) 
    p value single versus double PreserFlo 0.054  
Postoperative; month 18 
 Available data for n patients 16 16 Available data for n patients 15 15 
 Mean (±SD; range) 13.8 (±5.3; 7–26) 1.3 (±1.6; 0–4) Mean (±SD; range) 12.3 (±3.8; 6–18) 0.4 (±0.6; 0–2) 
    p value single versus double PreserFlo 0.38  
Postoperative; month 24 
 Available data for n patients 13 13 Available data for n patients 
 Mean (±SD; range) 15.8 (±6.4; 7–28) 1.6 (±1.6; 0–4) Mean (±SD; range) 11.7 (±2.7; 8–16) 0.7 (±0.7; 0–2) 
    p value single versus double PreserFlo 0.145  
Fig. 1.

Intraocular pressure before and after single and double PreserFlo MicroShunt implantation. Graph diagram showing intraocular pressure before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) single and double PreserFlo MicroShunt implantation.

Fig. 1.

Intraocular pressure before and after single and double PreserFlo MicroShunt implantation. Graph diagram showing intraocular pressure before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) single and double PreserFlo MicroShunt implantation.

Close modal
Fig. 2.

Number of topical intraocular pressure lowering medications before and after single and double PreserFlo MicroShunt implantation. Graph diagram showing number of topical intraocular pressure lowering medications before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) single and double PreserFlo MicroShunt implantation.

Fig. 2.

Number of topical intraocular pressure lowering medications before and after single and double PreserFlo MicroShunt implantation. Graph diagram showing number of topical intraocular pressure lowering medications before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) single and double PreserFlo MicroShunt implantation.

Close modal

We generally performed no bleb needlings in either of the patient groups. No serious intraoperative or postoperative complications were documented. No relevant hyphema was observed postoperatively. There were temporary choroidals (3/29 in the single PreserFlo group, and 3/28 in the double PreserFlo group), all of them in <3 quadrants respectively, which receded either spontaneously within 3 weeks or after a one-time viscoelastic instillation in the anterior chamber (dispersive viscoelastic, Johnson&Johnson Vision EndoCoat).

First versus Second PreserFlo MicroShunt Implantation in Patients with Two-Stage Procedures

In the subgroup with two-stage double PreserFlo MicroShunt implantation, mean preoperative IOP was 24.5 ± 8.5 mm Hg before the first implantation and 29.8 ± 10.1 mm Hg before the second implantation (p = 0.128). While immediately postoperatively, mean IOP-lowering was clinically significant and similar following both procedures (day 1: 8.2 ± 3.8 mm Hg vs. 7.2 ± 3.4 mm Hg; p = 0.465), the effect was sustainable only after the second procedure (month 12: 25.5 ± 7.5 mm Hg; 12.4 ± 4.8 mm Hg; p = 0.001).

Before the first and the second PreserFlo MicroShunt implantation, the number of topical IOP-lowering medications were similar (3.5 ± 0.9 vs. 3.0 ± 1.2). Postoperatively, the number of medications was nominally lower following the second implantation at all assessable postoperative time-points (month 24: 2.5 ± 0.5 vs. 0.8 ± 0.8).

No serious intra- or postoperative complications were observed. Complete data on pre- and postoperative IOP as well as number of topical IOP-lowering compounds in the subgroup of two-stage double PreserFlo MicroShunt implantation are shown in Table 3 and Figures 3 and 4. Figure 5 shows the timeline with each intervention performed between October 2018 and April 2022.

Table 3.

Intraocular pressure and number of topical intraocular pressure lowering medications before and following implantation of the first and the second PreserFlo MicroShunt in patients with two-stage double implantation

First PreserFloSecond PreserFlo
intraocular pressure (IOP)number of IOP-lowering medicationsintraocular pressure (IOP)number of IOP-lowering medications
Preoperative 
 Available data for n patients 17 17 Available data for n patients 17 17 
 Mean (±SD; range) 24.5 (±8.5; 15–47) 3.5 (±0.9; 2–5) Mean (±SD; range) 29.8 (±10.1; 16–55) 3.0 (±1.2; 0–4) 
    p value single versus double PreserFlo 0.128  
Postoperative; day 1 
 Available data for n patients 17 17 Available data for n patients 17 17 
 Mean (±SD; range) 8.2 (±3.8; 3–14) 0.2 (±0.7; 0–3) Mean (±SD; range) 7.2 (±3.4; 2–15) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo 0.465  
Postoperative; week 1 
 Available data for n patients 17 17 Available data for n patients 17 17 
 Mean (±SD; range) 11.8 (±5.9; 2–22) 0.3 (±0.8; 0–3) Mean (±SD; range) 7.6 (±3.2; 2–16) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo 0.014  
Postoperative; month 3 
 Available data for n patients 14 14 Available data for n patients 16 16 
 Mean (±SD; range) 24.0 (±12.2; 10–55) 1.1 (±1.3; 0–4) Mean (±SD; range) 7.7 (±3.8; 2–14) 0.2 (±0–7; 0–3) 
    p value single versus double PreserFlo 0.0003  
Postoperative; month 6 
 Available data for n patients 10 10 Available data for n patients 10 10 
 Mean (±SD; range) 24.1 (±7.6; 11–37) 1.7 (±1.5; 0–4) Mean (±SD; range) 9.5 (±3.2; 5–16) 0.3 (±0.8; 0–3) 
    p value single versus double PreserFlo 0.0005  
Postoperative; month 12 
 Available data for n patients Available data for n patients 10 10 
 Mean (±SD; range) 25.5 (±7.5; 16–40) 1.3 (±1.0; 0–3) Mean (±SD; range) 12.4 (±4.8; 7–20) 0.3 (±0.8; 0–3) 
    p value single versus double PreserFlo 0.001  
Postoperative; month 18 
 Available data for n patients Available data for n patients 10 10 
 Mean (±SD; range) 26.2 (±9.7; 13–40) 2.0 (±0.9; 1–3) Mean (±SD; range) 12.1 (±4.3; 6–18) 0.5 (±0.7; 0–2) 
    p value single versus double PreserFlo Small sample size  
Postoperative; month 24 
 Available data for n patients Available data for n patients 
 Mean (±SD; range) 19.5 (±0.7; 19–20) 2.5 (±0.5; 2–3) Mean (±SD; range) 12.3 (±12.5; 11–16) 0.8 (±0.8; 0–2) 
    p value single versus double PreserFlo Small sample size  
First PreserFloSecond PreserFlo
intraocular pressure (IOP)number of IOP-lowering medicationsintraocular pressure (IOP)number of IOP-lowering medications
Preoperative 
 Available data for n patients 17 17 Available data for n patients 17 17 
 Mean (±SD; range) 24.5 (±8.5; 15–47) 3.5 (±0.9; 2–5) Mean (±SD; range) 29.8 (±10.1; 16–55) 3.0 (±1.2; 0–4) 
    p value single versus double PreserFlo 0.128  
Postoperative; day 1 
 Available data for n patients 17 17 Available data for n patients 17 17 
 Mean (±SD; range) 8.2 (±3.8; 3–14) 0.2 (±0.7; 0–3) Mean (±SD; range) 7.2 (±3.4; 2–15) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo 0.465  
Postoperative; week 1 
 Available data for n patients 17 17 Available data for n patients 17 17 
 Mean (±SD; range) 11.8 (±5.9; 2–22) 0.3 (±0.8; 0–3) Mean (±SD; range) 7.6 (±3.2; 2–16) 0.0 (±0.0; 0-0) 
    p value single versus double PreserFlo 0.014  
Postoperative; month 3 
 Available data for n patients 14 14 Available data for n patients 16 16 
 Mean (±SD; range) 24.0 (±12.2; 10–55) 1.1 (±1.3; 0–4) Mean (±SD; range) 7.7 (±3.8; 2–14) 0.2 (±0–7; 0–3) 
    p value single versus double PreserFlo 0.0003  
Postoperative; month 6 
 Available data for n patients 10 10 Available data for n patients 10 10 
 Mean (±SD; range) 24.1 (±7.6; 11–37) 1.7 (±1.5; 0–4) Mean (±SD; range) 9.5 (±3.2; 5–16) 0.3 (±0.8; 0–3) 
    p value single versus double PreserFlo 0.0005  
Postoperative; month 12 
 Available data for n patients Available data for n patients 10 10 
 Mean (±SD; range) 25.5 (±7.5; 16–40) 1.3 (±1.0; 0–3) Mean (±SD; range) 12.4 (±4.8; 7–20) 0.3 (±0.8; 0–3) 
    p value single versus double PreserFlo 0.001  
Postoperative; month 18 
 Available data for n patients Available data for n patients 10 10 
 Mean (±SD; range) 26.2 (±9.7; 13–40) 2.0 (±0.9; 1–3) Mean (±SD; range) 12.1 (±4.3; 6–18) 0.5 (±0.7; 0–2) 
    p value single versus double PreserFlo Small sample size  
Postoperative; month 24 
 Available data for n patients Available data for n patients 
 Mean (±SD; range) 19.5 (±0.7; 19–20) 2.5 (±0.5; 2–3) Mean (±SD; range) 12.3 (±12.5; 11–16) 0.8 (±0.8; 0–2) 
    p value single versus double PreserFlo Small sample size  
Fig. 3.

Intraocular pressure before and after first and second PreserFlo MicroShunt implantation. Graph diagram showing intraocular pressure before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) first and second PreserFlo MicroShunt implantation.

Fig. 3.

Intraocular pressure before and after first and second PreserFlo MicroShunt implantation. Graph diagram showing intraocular pressure before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) first and second PreserFlo MicroShunt implantation.

Close modal
Fig. 4.

Number of topical intraocular pressure lowering medications before and after first and second PreserFlo MicroShunt implantation. Graph diagram showing number of topical intraocular pressure lowering medications before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) first and second PreserFlo MicroShunt implantation.

Fig. 4.

Number of topical intraocular pressure lowering medications before and after first and second PreserFlo MicroShunt implantation. Graph diagram showing number of topical intraocular pressure lowering medications before and following (1 day, 1 week, 3 months, 6 months, 12 months, 18 months, 24 months) first and second PreserFlo MicroShunt implantation.

Close modal
Fig. 5.

Timeline with PreserFlo MicroShunt implantations between October 2018 and April 2022. Timeline showing the chronological distribution of single PreserFlo MicroShunt implantations and one-stage double PreserFlo MicroShunt implantations as well as the first and the second procedures of two-stage double PreserFlo MicroShunt implantations between October 2018 and April 2022.

Fig. 5.

Timeline with PreserFlo MicroShunt implantations between October 2018 and April 2022. Timeline showing the chronological distribution of single PreserFlo MicroShunt implantations and one-stage double PreserFlo MicroShunt implantations as well as the first and the second procedures of two-stage double PreserFlo MicroShunt implantations between October 2018 and April 2022.

Close modal

Next, we compared the analog follow-up results between the two-stage-double PreserFlo group and the one-stage-double PreserFlo group. There were no relevant differences in either the IOP course or the medication use after the last operation. The results of this analysis are shown in Table 4.

Table 4.

Intraocular pressure and number of topical intraocular pressure lowering medications before and following two-stage or single-stage double PreserFlo MicroShunt implantation

Two-stage double PreserFloOne-stage double PreserFlo
intraocular pressure (IOP)number of IOP lowering medicationsintraocular pressure (IOP)number of IOP lowering medications
Preoperative 
 Available data for n patients 17 17 Available data for n patients 11 11 
 Mean (±SD) 29.8 (±10.0) 3 (±1.2) Mean (±SD) 28.7 (±10.4) 3 (±1.3) 
    p value one-stage versus two-stage 0.782 0.9 
Postoperative; day 1 
 Available data for n patients 17 17 Available data for n patients 11 11 
 Mean (±SD) 7.2 (±3.4) 0 (±0.0) Mean (±SD) 7.9 (±3.7) 0 (±0.0) 
    p value one-stage versus two-stage 0.64 
Postoperative; week 1 
 Available data for n patients 17 17 Available data for n patients 11 11 
 Mean (±SD) 7.6 (±3.2) 0.2 (±0.7) Mean (±SD) 8.7 (±2.1) 0.4 (±1.3) 
    p value one-stage versus two-stage 0.366 0.51 
Postoperative; month 3 
 Available data for n patients 16 16 Available data for n patients 
 Mean (±SD) 7.7 (±3.8) 0.2 (±0.8) Mean (±SD) 9.4 (±2.3) 0.1 (±0.4) 
    p value one-stage versus two-stage 0.26 0.74 
Postoperative; month 6 
 Available data for n patients 10 10 Available data for n patients 
 Mean (±SD) 9.5 (±3.2) 0.3 (±0.9) Mean (±SD) 11.4 (±3.7) 0 (±0.0) 
    p value one-stage versus two-stage 0.27 0.397 
Postoperative; month 12 
 Available data for n patients 10 10 Available data for n patients 
 Mean (±SD) 12.4 (±4.8) 0.5 (±0.7) Mean (±SD) 11.4 (±5.5) 0.3 (±0.5) 
    p value one-stage versus two-stage 0.71 0.6 
Postoperative; month 18 
 Available data for n patients 10 10 Available data for n patients 
 Mean (±SD) 12.1 (±4.3) 0.8 (±1.0) Mean (±SD) 13 (±2.2) 0.5 (±0.7) 
    p value one-stage versus two-stage 0.694 0.765 
Postoperative; month 24 
 Available data for n patients Available data for n patients 
 Mean (±SD) 12.3 (±2.5) n/a Mean (±SD) 10.5 (±3.5) n/a 
    p value one-stage versus two-stage 0.51  
Two-stage double PreserFloOne-stage double PreserFlo
intraocular pressure (IOP)number of IOP lowering medicationsintraocular pressure (IOP)number of IOP lowering medications
Preoperative 
 Available data for n patients 17 17 Available data for n patients 11 11 
 Mean (±SD) 29.8 (±10.0) 3 (±1.2) Mean (±SD) 28.7 (±10.4) 3 (±1.3) 
    p value one-stage versus two-stage 0.782 0.9 
Postoperative; day 1 
 Available data for n patients 17 17 Available data for n patients 11 11 
 Mean (±SD) 7.2 (±3.4) 0 (±0.0) Mean (±SD) 7.9 (±3.7) 0 (±0.0) 
    p value one-stage versus two-stage 0.64 
Postoperative; week 1 
 Available data for n patients 17 17 Available data for n patients 11 11 
 Mean (±SD) 7.6 (±3.2) 0.2 (±0.7) Mean (±SD) 8.7 (±2.1) 0.4 (±1.3) 
    p value one-stage versus two-stage 0.366 0.51 
Postoperative; month 3 
 Available data for n patients 16 16 Available data for n patients 
 Mean (±SD) 7.7 (±3.8) 0.2 (±0.8) Mean (±SD) 9.4 (±2.3) 0.1 (±0.4) 
    p value one-stage versus two-stage 0.26 0.74 
Postoperative; month 6 
 Available data for n patients 10 10 Available data for n patients 
 Mean (±SD) 9.5 (±3.2) 0.3 (±0.9) Mean (±SD) 11.4 (±3.7) 0 (±0.0) 
    p value one-stage versus two-stage 0.27 0.397 
Postoperative; month 12 
 Available data for n patients 10 10 Available data for n patients 
 Mean (±SD) 12.4 (±4.8) 0.5 (±0.7) Mean (±SD) 11.4 (±5.5) 0.3 (±0.5) 
    p value one-stage versus two-stage 0.71 0.6 
Postoperative; month 18 
 Available data for n patients 10 10 Available data for n patients 
 Mean (±SD) 12.1 (±4.3) 0.8 (±1.0) Mean (±SD) 13 (±2.2) 0.5 (±0.7) 
    p value one-stage versus two-stage 0.694 0.765 
Postoperative; month 24 
 Available data for n patients Available data for n patients 
 Mean (±SD) 12.3 (±2.5) n/a Mean (±SD) 10.5 (±3.5) n/a 
    p value one-stage versus two-stage 0.51  

When pharmaceutical and laser treatment fail to lower IOP sufficiently, surgery is the sole remaining therapeutic option [1, 3]. Trabeculectomy has long been considered the gold standard in glaucoma surgery, yet is associated with significant risks [5]. MIGS and especially implantable aqueous humor drainage devices have shown promises for lowering IOP whilst reducing the risk for complications [6‒8]. However, both postoperative hypotony and hypertension remain an issue. While the first is commonly due to over-drainage, the latter may be caused either by insufficient outflow capacity provided either by the drainage tube or bleb scarring. Creating the right amount of outflow and avoiding postoperative tissue fibrosis is the surgeon’s challenge and key to success at the same time.

The Hagen-Poiseuille equation states that the pressure differential for a non-compressible Newtonian fluid under laminar flow across a cylindrical tube of constant width is proportional to resistance to flow, which itself is directly proportional to the length and inversely proportional to the radius of the lumen to the fourth power [13]. Taking this into account, modern MIGS drainage tubes have been designed with a specific width and length in order to allow for a relevant amount of aqueous humor outflow while also avoiding over-drainage and postoperative hypotony [14]. On the other hand, it has been hypothesized that a specific tube with a certain length and width may provide only insufficient additional aqueous humor outflow capacity for a certain individual. For the PreserFlo MicroShunt for example, the resistance to flow is predicted to be 2 mm Hg/L/min [18], which may be too much in certain cases. Thus, in patients with more pronounced aqueous humor outflow deficiency, theoretically either a shorter/wider tube or multiple tubes may be required. De Gregorio et al. developed a theoretical mathematical model with which to predict the number of XEN45 Gel stents needed to reach the target IOP, based on preoperative IOP values after 14 days of IOP-lowering medical therapy washout. In their study, the authors found that higher preoperative outflow resistance (that is, higher IOP) after medication washout, is associated with higher postoperative IOP. Given this, they concluded that using these parameters it is possible to predict the number of XEN45 Gel stents needed to sufficiently lower IOP in a certain patient [13].

Many experts consider today the PreserFlo MicroShunt the preferable MIGS drainage device [15‒17]. However, double implantation of this type of tube has not been debated yet. Here, for the first time, we describe double PreserFlo MicroShunt implantation as an innovation and expansion of the known MIGS technique and assess the efficiency and safety of this technique.

In our cohort of glaucoma patients, both in the single and the double PreserFlo implantation group, Patients had well established topical IOP-lowering therapy before surgery. However, the mean preoperative IOP was significantly higher in the patients who ultimately required two stents compared to the patients in whom single stenting was able to provide sufficient IOP-lowering. This is consistent with the above mentioned theoretical mathematical model, according to which the number of stenting devices needed for reaching the target IOP is higher the higher the preoperative IOP is [13]. Following surgery, mean IOP was significantly lower in the double PreserFlo group at various postoperative time-points, while requiring nominally less topical IOP-lowering compounds. These findings support that double stenting can be more efficient than single stenting and they strengthen the rationale that there is an individual need for a certain number of stents depending on the required total humor outflow capacity. In none of our patients, serious intraoperative or postoperative complications were documented, supporting the rationale that double PreserFlo MicroShunt implantation is a safe procedure. Notably, no long lasting postoperative hypotony occurred.

In our cohorts, the immediate postoperative IOP levels were slightly higher than reported in the literature; whereas a first day IOP can be explained by the insufficient flushing of the viscoelastics used during the surgery, higher level of IOP after the first week in the first stage double implantation group can be explained by the negative selection bias. However, when assessing the postoperative course of those patients with double PreserFlo implantation as a two-stage procedure, it becomes apparent that in many patients IOP dropped significantly immediately following implantation of the first stent, yet rose again in the further course. Given this, it appears suggestive that in these patients the first stent initially provided enough outflow capacity and that only in the further course this outflow capacity was impaired.

Bleb scarring has been recognized as the most common reason for postoperative hypertension [19‒24]. Trabeculectomy is known to form distinct types of bleb morphologies. Recent studies have shown that MIGS drainage devices lead to similar bleb morphologies [25]. However, as the external end of the PreserFlo MicroShunt is placed under the Tenon’s capsule, this type of MIGS drainage type has been shown to cause a large and distinct fluid accumulation between the sclera and Tenon’s capsule in addition to the pre-known appearance of intra-/subconjunctival fluid distributions. Gambini et al. [12] who first described this phenomenon, termed it posterior episcleral fluid lake. Since this episcleral fluid lake appears like a homogenously fluid-filled balloon, we believe it is easier with this type of bleb to keep the tissue layers apart from each other and hence prevent tissue fibrosis compared to diffuse intra-/subconjunctival blebs commonly seen after trabeculectomy, provided that it is constantly supplied by aqueous humor. Regarding our study, it is conceivable that bleb revision, getting rid of the adhesions and freeing the conjunctival end, in the second of the two-stage-double PreserFlo operations may have improved the bleb functionality in the further course. However, as shown in the subanalysis in the Table 4 with the lack of difference between the outcomes of the two-stage-double PreserFlo group and the one-stage-double PreserFlo group this was not relevant. In our opinion, it seems that the effect of two PreserFlos, instead of one, had more impact on the IOP course.

MIGS drainage stents have been designed to create a steady aqueous humor outflow [12]. Yet, a growing number of reports on stent failure due to intraluminal cellular debris have been published [26‒29]. These intraluminal occlusions cannot be visualized by gonioscopy and arguably are thus an underrecognized problem. According to reports, fluid shockwaves provoked by neodymium-doped yttrium aluminum garnet (Nd:YAG) laser application to the anterior chamber were successful for dislodging the intraluminal debris, restoring stent patency and aqueous humor outflow [26, 27]. We believe it is possible that intraluminal occlusion may resolve spontaneously over time and that outflow through a specific tube may temporarily suspend also due to other factors like local pressure fluctuations and changes in the rheologic conditions within the anterior chamber. During these phases of suspended outflow, the posterior episcleral fluid lake may dry out, causing the filtering bleb to collapse and letting the tissue layers stick to each other, which may allow for fibrosis and ultimately bleb scarring. We hypothesize that the probability that at all times at least one stent is creating outflow is higher the higher the number of stents is, lessening the risk of bleb collapse and subsequent tissue fibrosis. Additionally, we believe it is plausible that the chances to keep the filtering bleb open and hence to keep the tissue layers apart from each other are better when having two different impact sites exerting forces onto the filtering bleb from two different angles, providing another advantage of double stenting over single stenting procedures. Yet, these hypotheses remain to be definitely tested.

Our study is limited by its retrospective nature and the relatively small sample size, as well as the partially missing long-term follow-up data. An additional limitation is the heterogeneity of the Double PreserFlo group, consisting of the two subgroups of single- and two-stage procedures. Furthermore, in our patients, bleb morphology was not specifically assessed and documented, neither biomicroscopically nor by optical coherence tomography. The exact mechanism of action, by which double stenting possibly enhances filtering capacity, remains elusive and at this point speculative. In the present analysis, there were no patients undergoing a revision-only procedure without implantation of a second PreserFlo MicroShunt. Telling whether revision alone is the cause of the more favorable postoperative IOP course, and not the second stent, is without direct comparison between such two groups in the present analysis not possible. Besides, a certain learning effect may have biased the results, although this effect was at most a minor one. In case of a considerable learning effect, the timeline of the procedures (Fig. 5) would show a clear pattern with the patients who received a second implant during the first part of the observational period and those with single implantation during the later part. The chronological distribution of the procedures does not show such a pattern and hence invalidates the idea of a considerable learning effect.

In conclusion, double PreserFlo MicroShunt implantation appears as a safe and efficient micro-invasive procedure to lower IOP in glaucoma patients. Our preliminary findings suggest that double implantation is superior to single implantation in terms of IOP lowering and the need for additional topical medication. Patients with insufficient IOP lowering following single PreserFlo implantation may benefit from a second implantation. Further research is warranted to determine the exact mechanism of action by which double stenting promotes enhanced and sustained drainage capacity as well as to evaluate whether double PreserFlo implantation may also be considered a first-line, one-stage surgical procedure.

The authors would like to thank Professor Jost B. Jonas for his expertise and support in analyzing and interpreting the data.

This study was conducted in accordance with the Declaration of Helsinki and the study protocol “Clinical outcome of PreserFlo Microshunt” was approved by the appropriate Ethics Committee (Ethikkommission Nordwest-und Zentralschweiz EKNZ/Kantonale Ethikkommission Zürich KEK) with the reference number 2020-00121. All patients signed a general consent with which they have given permission for further use and analysis of their anonymized health-related personal data for scientific purposes. The above mentioned, competent ethics committees decided that given this, consent is not required for this study in accordance with local and national guidelines.

Dr. Scholl is a member of the Scientific Advisory Board of the following: Boehringer Ingelheim Pharma GmbH & Co; Claris Biotherapeutics Inc.; Eluminex Biosciences; Gyroscope Therapeutics Ltd.; Janssen Research & Development, LLC (Johnson & Johnson); Novartis Pharma AG (CORE); Okuvision GmbH; ReVision Therapeutics Inc.; and Saliogen Therapeutics Inc. Scholl is a consultant of the following: Alnylam Pharmaceuticals Inc.; Gerson Lehrman Group Inc.; Guidepoint Global, LLC; and Intergalactic Therapeutics Inc. Scholl is member of the Data Monitoring and Safety Board/Committee of Belite Bio (CT2019-CTN-04690-1), F. Hoffmann-La Roche Ltd. (VELODROME trial, NCT04657289; DIAGRID trial, NCT05126966; HUTONG trial) and member of the Steering Committee of Novo Nordisk (FOCUS trial; NCT03811561). All arrangements have been reviewed and approved by the University of Basel (Universitätsspital Basel, USB) and the Board of Directors of the Institute of Molecular and Clinical Ophthalmology Basel (IOB) in accordance with their conflict of interest policies. Compensation is being negotiated and administered as grants by the USB, which receives them in its proper accounts. Scholl is co-director of the Institute of Molecular and Clinical Ophthalmology Basel (IOB), which constitutes a non-profit foundation and receives funding from the University of Basel, the University Hospital Basel, Novartis, and the government of Basel-Stadt. Alphabetical List: Alnylam Pharmaceuticals Inc., Belite Bio, Boehringer Ingelheim Pharma GmbH & Co., Claris Biotherapeutics Inc., Eluminex Biosciences, F. Hoffmann-La Roche Ltd., Gerson Lehrman Group Inc., Guidepoint Global, LLC, Gyroscope Therapeutics Ltd., Intergalactic Therapeutics Inc., Janssen Research & Development, LLC (Johnson & Johnson), Novartis Pharma AG (CORE), Novo Nordisk, Okuvision GmbH, ReVision Therapeutics Inc., and Saliogen Therapeutics Inc. All other authors declare no conflict of interest. All other authors have no conflicts of interest to declare.

This work was funded by the department of the authors. Dr. Scholl is supported by the Swiss National Science Foundation (Project funding: “Developing novel outcomes for clinical trials in Stargardt disease using structure/function relationship and deep learning” #310030_201165, and National Center of Competence in Research Molecular Systems Engineering: “NCCR MSE: Molecular Systems Engineering (phase II)” #51NF40-182895), the Wellcome Trust (PINNACLE study), and the Foundation Fighting Blindness Clinical Research Institute (ProgStar study).

Study design and ethical approval: K.G., Z.G., and T.J.E. Data collection: T.D. Data management and interpretation: T.D., K.G., and T.J.E. Statistical analysis: K.G. Manuscript drafting: T.J.E. Manuscript reviewing and approval: T.D., K.G., H.P.N.S., Z.G., and T.J.E. Allocation of resources: H.P.N.S.

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.

1.
Jonas
JB
,
Aung
T
,
Bourne
RR
,
Bron
AM
,
Ritch
R
,
Panda-Jonas
S
.
Glaucoma
.
Lancet
.
2017
;
390
(
10108
):
2183
93
.
2.
Tham
YC
,
Li
X
,
Wong
TY
,
Quigley
HA
,
Aung
T
,
Cheng
CY
.
Global prevalence of glaucoma and projections of glaucoma burden through 2040: a systematic review and meta-analysis
.
Ophthalmology
.
2014
;
121
(
11
):
2081
90
.
3.
Theventhiran
AB
,
Kim
G
,
Yao
W
.
Fornix-based versus limbal-based conjunctival trabeculectomy flaps for glaucoma
.
Cochrane Database Syst Rev
.
2021
8
8
CD009380
.
4.
Gopesh
T
,
Camp
A
,
Unanian
M
,
Friend
J
,
Weinreb
RN
.
Rapid and accurate pressure sensing device for direct measurement of intraocular pressure
.
Transl Vis Sci Technol
.
2020
;
9
(
3
):
28
.
5.
Cairns
JE
.
Trabeculectomy
.
Am J Ophthalmol
.
1968
;
66
(
4
):
673
9
.
6.
Lim
R
.
The surgical management of glaucoma: a review
.
Clin Exp Ophthalmol
.
2022
;
50
(
2
):
213
31
.
7.
Mathew
DJ
,
Buys
YM
.
Minimally invasive glaucoma surgery: a critical appraisal of the literature
.
Annu Rev Vis Sci
.
2020
;
6
:
47
89
.
8.
Birnbaum
FA
,
Neeson
C
,
Solá-Del Valle
D
.
Microinvasive glaucoma surgery: an evidence-based review
.
Semin Ophthalmol
.
2021
;
36
(
8
):
772
86
.
9.
Xin
C
,
Wang
H
,
Wang
N
.
Minimally invasive glaucoma surgery: what do we know? Where should we go
.
Transl Vis Sci Technol
.
2020
;
9
(
5
):
15
.
10.
Fujimoto
T
,
Nakashima
KI
,
Watanabe-Kitamura
F
,
Watanabe
T
,
Nakamura
K
,
Maki
K
.
Intraocular pressure-lowering effects of trabeculectomy versus MicroShunt insertion in rabbit eyes
.
Transl Vis Sci Technol
.
2021
;
10
(
9
):
9
.
11.
Sadruddin
O
,
Pinchuk
L
,
Angeles
R
,
Palmberg
P
.
Ab externo implantation of the MicroShunt, a poly (styrene-block-isobutylene-block-styrene) surgical device for the treatment of primary open-angle glaucoma: a review
.
Eye Vis
.
2019
;
6
:
36
.
12.
Gambini
G
,
Carlà
MM
,
Giannuzzi
F
,
Boselli
F
,
Grieco
G
,
Caporossi
T
.
Anterior segment-optical coherence tomography bleb morphology comparison in minimally invasive glaucoma surgery: XEN gel stent vs. Preserflo MicroShunt
.
Diagnostics
.
2022
;
12
(
5
):
1250
.
13.
De Gregorio
A
,
Montali
M
,
Stevan
G
,
Pedrotti
E
,
Morselli
S
.
Theoretic scientific rationale of double XEN 45 Gel Stent implant in severe glaucomatous ocular hypertension
.
Int Ophthalmol
.
2023
;
43
(
5
):
1629
38
.
14.
Sheybani
A
,
Reitsamer
H
,
Ahmed
II
.
Fluid dynamics of a novel micro-fistula implant for the surgical treatment of glaucoma
.
Invest Ophthalmol Vis Sci
.
2015
;
56
(
8
):
4789
95
.
15.
Majoulet
A
,
Scemla
B
,
Hamard
P
,
Brasnu
E
,
Hage
A
,
Baudouin
C
.
Safety and efficacy of the Preserflo® Microshunt in refractory glaucoma: a one-year study
.
J Clin Med
.
2022
;
11
(
23
):
7086
.
16.
Burgos-Blasco
B
,
García-Feijóo
J
,
Perucho-Gonzalez
L
,
Güemes-Villahoz
N
,
Morales-Fernandez
L
,
Mendez-Hernández
CD
.
Evaluation of a novel αb εxterno MicroShunt for the treatment of glaucoma
.
Adv Ther
.
2022
;
39
(
9
):
3916
32
.
17.
Kuet
ML
.
Will the PRESERFLO™ MicroShunt impact the future of trabeculectomy practice? A UK and Éire Glaucoma Society National Survey
.
Eye (Lond)
.
2022
1
5
.
18.
Ibarz Barberá
M
,
Hernández-Verdejo
JL
,
Bragard
J
,
Burguete
J
,
Fernández
LM
,
Rivero
PT
.
Evaluation of the ultrastructural and in vitro flow properties of the PRESERFLO MicroShunt
.
Transl Vis Sci Technol
.
2021
;
10
(
13
):
26
.
19.
Wells
AP
,
Crowston
JG
,
Marks
J
,
Kirwan
JF
,
Smith
G
,
Clarke
JCK
.
A pilot study of a system for grading of drainage blebs after glaucoma surgery
.
J Glaucoma
.
2004
;
13
(
6
):
454
60
.
20.
Ciancaglini
M
,
Carpineto
P
,
Agnifili
L
,
Nubile
M
,
Lanzini
M
,
Fasanella
V
.
Filtering bleb functionality: a clinical, anterior segment optical coherence tomography and in vivo confocal microscopy study
.
J Glaucoma
.
2008
;
17
(
4
):
308
17
.
21.
Napoli
PE
,
Zucca
I
,
Fossarello
M
.
Qualitative and quantitative analysis of filtering blebs with optical coherence tomography
.
Can J Ophthalmol
.
2014
;
49
(
2
):
210
6
.
22.
Tominaga
A
,
Miki
A
,
Yamazaki
Y
,
Matsushita
K
,
Otori
Y
.
The assessment of the filtering bleb function with anterior segment optical coherence tomography
.
J Glaucoma
.
2010
;
19
(
8
):
551
5
.
23.
Kokubun
T
,
Kunikata
H
,
Tsuda
S
,
Himori
N
,
Maruyama
K
,
Nakazawa
T
.
Quantification of the filtering bleb’s structure with anterior segment optical coherence tomography
.
Clin Exp Ophthalmol
.
2016
;
44
(
6
):
446
54
.
24.
Meziani
L
,
Tahiri Joutei Hassani
R
,
El Sanharawi
M
,
Brasnu
E
,
Liang
H
,
Hamard
P
.
Evaluation of blebs after filtering surgery with en-face anterior-segment optical coherence tomography: a pilot study
.
J Glaucoma
.
2016
;
25
(
5
):
e550
8
.
25.
Kudsieh
B
,
Fernández-Vigo
JI
,
Canut Jordana
MI
,
Vila-Arteaga
J
,
Urcola
JA
,
Ruiz Moreno
JM
.
Updates on the utility of anterior segment optical coherence tomography in the assessment of filtration blebs after glaucoma surgery
.
Acta Ophthalmol
.
2022
;
100
(
1
):
e29
37
.
26.
Fellman
RL
,
Grover
DS
,
Smith
OU
,
Kornmann
HL
.
Rescue of failed XEN-45 gel implant by Nd:YAG shock wave to anterior chamber tip to dislodge hidden intraluminal occlusion
.
J Glaucoma
.
2021
;
30
(
7
):
e338
43
.
27.
Seo
JH
,
Lim
SH
.
Recanalization of XEN-45 gel stent occlusion with cortical material after phaco-XEN surgery using Nd: YAG laser treatment: a case report
.
Medicine
.
2021
;
100
(
34
):
e27010
.
28.
Mustafa
M
,
Shoham-Hazon
N
,
Reiss
GR
,
Samuelson
TW
,
Condon
G
,
Ahmed
IIK
.
Neodymium laser treatment of IOP rise following ex-press glaucoma device implantation: a retrospective review from 4 institutions
.
J Glaucoma
.
2020
;
29
(
2
):
92
6
.
29.
Gillmann
K
,
Bravetti
GE
,
Mansouri
K
.
Delayed obstruction of XEN gel stent by cell debris in primary open-angle glaucoma: a new insight into the pathophysiology of filtration device failure
.
J Curr Glaucoma Pract
.
2019
;
13
(
3
):
113
5
.