BLOG: Ab externo microshunt offers possibilities for glaucoma surgical treatment

ByAhmad A. Aref, MD

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Data in support of a novel ab externo microshunt, the Preserflo from Santen, are mounting.

The device is designed for implantation underneath a sub-Tenon’s flap for filtering of aqueous to the subconjunctival and sub-Tenon’s space to allow for IOP lowering in glaucomatous disease. Several features of the device are intended to combat some of the issues associated with conventional trabeculectomy surgery. These include a standardized lumen diameter of 70 µm and length of 8.5 mm to minimize risk for postoperative hypotony and a composition of a novel SIBS material that is biologically inert and less likely to cause an inflammatory/fibrotic tissue response.

The device was compared head on with trabeculectomy in 527 patients, and results were recently published. Patients randomly assigned to receive the microshunt experienced a lowering in IOP from a baseline of 21.1 ± 4.9 mm Hg to 14.3 ± 4.3 mm Hg (29.1% decrease, P < .01) at 1 year on a mean of 0.6 ± 1.1 glaucoma medications. These are impressive results given that 30% of patients had an IOP less than 18 mm Hg at baseline, and a large proportion had advanced glaucomatous disease based on visual field testing. Using strict criteria for surgical success, a higher percentage of patients succeeded with trabeculectomy compared with the microshunt, and trabeculectomy resulted in a lower IOP at 1 year (11.1 ± 4.3 mm Hg).

Ahmad A. Aref

I interpret the results of this well-conducted trial to be very favorable for the novel microshunt. Investigators in the study were limited to use of a standardized mitomycin C concentration of 0.2 mg/mL and application for 2 minutes for all patients. Prior studies suggest that higher concentrations of MMC are associated with lower postoperative IOPs in patients undergoing microshunt implantation. Furthermore, the risk for postoperative hypotony in patients treated with the microshunt was indeed minimized compared with trabeculectomy, with a 28.9% vs. 49.6% risk of IOP less than 6 mm Hg at any time (P < .01), respectively. This is important because it is often the hypotony-related complications — choroidal effusions, anterior chamber shallowing, hypotony maculopathy — that are most vision-threatening after glaucoma filtering operations.

As with all decision-making in medicine, glaucoma surgical choice comes down to risk vs. benefit. For patients in need of high single-digit IOPs, as for some cases of normal tension glaucoma, trabeculectomy is likely the initial procedure of choice. However, in many other cases in which the risk for postoperative hypotony is not acceptable, the novel ab externo microshunt will provide a wonderful option. The device will nicely complement our existing microinvasive glaucoma surgeries, which typically offer more modest degrees of IOP lowering for earlier disease states.

• Reference:
• Baker ND, et al. Ophthalmology. 2021;doi:10.1016/j.ophtha.2021.05.023.

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