Quest for the Ideal Bleb

To avoid blebitis risks and any possible leakage, glaucoma surgeons tend to aim for posterior, diffuse-filtering blebs in primary open-angle glaucoma (POAG) treatment – a goal usually achieved through trabeculectomy. However, though trabeculectomy is generally considered to be an effective procedure, bleb morphology can vary considerably and the postoperative course can be somewhat unpredictable. Furthermore, potential serious risks such as hypotony, shallow AC, choroidal detachment, hyphema , and decompression retinopathy can still occur in a small number of patients (1).

The PRESERFLO MicroShunt – performed using an ab-externo approach, with the patient under at least topical anesthesia – is an innovative, minimally invasive bleb-filtering device that allows surgeons to perform controlled (and thus more predictable) intraocular pressure (IOP) lowering and subconjunctival drainage of aqueous humor, increasing their chances of achieving ideal bleb morphology. Built from poly(styrene-block-isobutylene-block-styrene), or “SIBS,” the device is a 100 percent synthetic, biocompatible material first employed for coronary stents in 2002 and features an 8.5-mm tube with an outer diameter of 350 μm and 67-μm diameter lumen. This diameter is ideal for preventing both clogging and chronic hypotony. Additionally, the biocompatible and biostable SIBS components have been found to produce minimal scar tissue formation (2), leaving no biodegradation byproducts, positioning the PRESERFLO MicroShunt as a predictable, controlled and safe alternative to trabeculectomy when operating on POAG patients.

The view from the top

The PRESERFLO MicroShunt’s unique set of features have drawn leading industry names – such as MIGS trailblazer, allround surgical innovator, and our number one Power Lister, Ike Ahmed – to participate in clinical evaluations of the device. Since 2015, Ahmed has been conducting follow-up studies examining the efficacy of the implantation outcomes and risk factors, documenting his team’s findings in patients over one-year (3) and three-year (4) periods.

These studies have “[guided] us in what to expect with the results of this procedure in a wide range of patients and settings,” says Ahmed. “We had a high degree of followup, which is important.” In retrospective studies, patients can get lost in followup and the data remains incomplete, Ahmed explains, but in his PRESERFLO MicroShunt studies “around 90 percent of patients had follow-up after one year, which is very good, and we were able to really look into those patients.”

The two studies showed promising results for the device’s efficacy. Initially, the one-year follow-up data – taken from a large and diverse cohort of 436 glaucoma patients who underwent ab-externo SIBS surgery with Mitomycin C (with or without previous subconjunctival surgery) – indicated “promising rates of qualified and complete surgical success with decreased medication burden and few postoperative complications and interventions” (3). The three-year follow-up study built on these promising rates, with the authors encouraging further comparisons to other filtering surgeries be made to “further facilitate integration of the SIBS microshunt into the surgical treatment paradigm” (4). The study results show comparable low IOPs within the same range achieved by trabeculectomy. Ahmed says, “Although we did not do any head-to-head comparisons, we had substantial pressure lowering – IOPs that were typically down to 12, to the low teens – and we had success rates that were fairly consistent with what we saw in other studies looking at trabeculectomy. We felt these results showed a significant IOP drop coinciding with medication reduction.” Ahmed believes that these trials also point to the safety of the procedure: “The results for PRESERFLO demonstrate a fairly good adverse event profile.”

Tips and tricks

A key factor in the IOP-lowering success of the implant is when it’s used in conjunction with chemotherapeutic agent, Mitomycin C. “Based on our Cox analysis, patients who received a higher dose of Mitomycin C were found to have a higher success rate,” notes Ahmed. “We started off at 0.2 mg/ml MMC, then moved to 0.4 mg/ml MMC [in] a timeinterrupted fashion. So, we feel we have a reasonable data set to also confirm the benefit of using Mitomycin C at higher doses, without really increasing the risk of patient complications.”

Alongside selecting the most effective device for the type of surgery being performed, there are also various critical surgical factors involved in reliably reaching low IOP. “We’ve been very meticulous about how we handle the Tenon’s capsule,” explains Ahmed. “Because bleb formation with this device occurs in the sub-tenon space, and, because this is a single lumen device, placement of the device so that it doesn’t get trapped under Tenon’s is really important.” The Tenon’s capsule also needs to be managed atraumatically. “We take great caution during the opening and dissection, being careful not to adulterate the Tenon’s with any kind of injection or trauma,” Ahmed continues. “And when we close, we [suture the] Tenon’s in a two-layer closure – meaning that we close the Tenon’s first and then the conjunctiva – to really ensure adequate positioning.”

The physiological placement of the device is crucial; the surgeon should ensure it is placed away from the cornea and not kinked, where it might get either blocked or else reduce the lumen size. Additionally, says Ahmed, the device should be placed in the superior quadrants, rather than at 12 o’clock, to encourage posterior bleb formation and provide ideal morphology. Of course, surgery can be an unpredictable endeavor at the best of times. “There are some patients that still get hypotony, so we need to understand why they do,” he says. “There might be some device differences, patient differences, healing differences, technique differences… We’re seeing people using rip cord sutures in the lumen to protect from hypotony – so it’s about understanding whether this is necessary. We don’t do it routinely; we do it for high-risk patients who are at risk of hypotony.”

Ahmed believes the number one benefit of the PRESERFLO MicroShunt is the controlled pressure drop and flow that the device offers, which helps reduce the risk of hypotony complications and provides a smooth postoperative course for the majority of patients. He adds that the device “also achieves substantial pressure lowering – probably some of the most powerful pressure lowering of our new devices – in a way that’s done with minimal postoperative intervention, with reduced needling rates compared to other procedures, and also bleb morphology being optimal, meaning again that posterior diffuse bleb, which is ideal.”

Which patients?

For ophthalmologists choosing to use the PRESERFLO MicroShunt for the first time, Ahmed lists a number of considerations one should bear in mind. First is patient selection – “making sure we have healthy conjunctiva in the area of surgery, and that we have a good open angle to place a device deep in the anterior chamber to protect the cornea.”

In terms of pressure targets, he adds that while the PRESERFLO MicroShunt typically results in pressures down to 12, if patients do require a more substantial drop to “six or seven or eight mmHg”, he would recommend still performing a trabeculectomy to reach these numbers, adding, “but otherwise, if we want to get the pressure down typically into the low teens, and most patients are on no medications, then that’s where the sweet spot is.” Plus, “post-op bleb management is of course crucial to overall patient outcome,” he adds, reminding us that sometimes patients will still need to have a post-operative 5-FU, and that ample steroids should also be used for the healing process.

Ahmed is optimistic about how the PRESERFLO MicroShunt might further evolve in clinical practice, especially when surgeons remain focused on accurate patient selection. It’s important to understand the right patient profile, he explains. “For example, we looked at primary standalone procedures compared to combined procedures with phaco, and we looked at refractory patients who have had previous surgery, and we found the highest success rates were in patients who have had standalone primary surgery.”

He also suggests that, moving forward, this patient population should be expanded into pediatrics, as well as older patients. “There is a learning curve, both with patients and surgeons who are learning this procedure,” he says. “Hopefully the pearls I’ve covered here will shorten that learning curve. Around the world, we’re seeing fairly good adoption – with surgeons using the device we’re finding fewer postoperative visits, less variation in pressure, faster visual recovery, which is important for return to function – and overall good experience.” All of which, Ahmed believes, will hopefully lead to “fewer sleepless nights for glaucoma surgeons.”

The PRESERFLO MicroShunt offers surgeons a range of benefits not previously seen in minimally invasive devices designed to lower IOP for glaucoma and reduce the need for further medication. Using the device, surgeons may find that they are able to fully optimize their outcomes, increasing their surgical success rates, and provide their carefully selected patient cohort with the utmost level of care.

Santen does not endorse off-label use of PRESERFLO^® MicroShunt. Long term effects of MMC with the use of this device have not been evaluated. Necessary precautions and interventions on the use of MMC are highly recommended.

The PRESERFLO^® MicroShunt glaucoma drainage system is intended for reduction of intraocular pressure in eyes of patients with primary openangle glaucoma where IOP remains uncontrollable while on maximum tolerated medical therapy and/or where glaucoma progression warrants surgery. Please read the PRESERFLO^® MicroShunt Instructions for Use carefully and complete the relevant training before using the device.

^{PMS-EMEA-240031 | Date of Preparation: June 2024}