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Editors Selection IGR 17-3

Surgical Treatment: Ab-externo Microshunt vs. Trabeculectomy

Comment by Kaweh Mansouri on:

112812 Ab-Externo MicroShunt versus Trabeculectomy in Primary Open-Angle Glaucoma: Two-Year Results from a Randomized, Multicenter Study, Panarelli JF; Moster MR; Garcia-Feijoo J et al., Ophthalmology, 2023; 0:

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Panarelli et al. report two-year results of a multicenter pivotal study comparing the Preserflo Microshunt to trabeculectomy in patients with mild to severe POAG. This study is valuable as it is one of the few RCTs comparing a minimally invasive glaucoma technique to the old gold standard of trabeculectomy and both the sponsor and the investigators are to be congratulated for it. Its findings largely confirm the previously published one-year results.¹

The efficacy endpoint was defined as a 20% reduction from baseline in mean diurnal IOP at the month 24 visit without increasing the number of preoperative IOP-lowering medications, consistent with WGA guidelines. The study included 527 subjects with a 1:3 allocation to trabeculectomy vs Microshunt. All surgeries were augmented with 0.2 mg/ml MMC soaked sponges. At two years, a mere 6% (34 subjects) were lost to follow-up, signifying an excellent study management.

At two years, surgical success was higher in the trabeculectomy eyes (64.4% vs 50.6%) with 61.1% of eyes in the Microshunt group and 79.8% of eyes in the trab group being medication-free. In fact, from month three onward, mean IOP was lower in the trab group, by approximately 3 mmHg, at each time point. Mean IOP from month three to year two was approximately 14 mmHg in the Microshunt group and 11 mmHg in the trab group. Interestingly, when subjects with low baseline IOP (< 18 mmHg) were analyzed, there was no statistically significant difference between the two groups, although trab eyes still seemed to be doing better.

Interestingly, when subjects with low baseline IOP (< 18 mmHg) were analyzed, there was no statistically significant difference between the two groups, although trab eyes still seemed to be doing better

The rate of success with the Microshunt was substantially different between the USA (47.5%) and Europe (69%), possibly due to a higher percentage of Black patients in the US cohort. It has been argued that higher concentrations of MMC (e.g., 0.4 mg/ml) than used in the current study could contribute to improved surgical success of the Microshunt, as the device dimensions limit flow and manipulating the bleb may be harder than with trabeculectomy. In fact, higher success rates of Microshunt have been seen in studies using 0.4-0.5 mg/ml MMC.²

Failure to achieve an IOP reduction of 20% from baseline was the most common reason for failure in the Microshunt group, whereas persistent hypotony was the leading cause of failure in the trabeculectomy group.

One finding of this study that was surprising to many of us, this author included, was the lack of significant advantages in terms of safety for Microshunt compared to trabeculectomy. It should still be noted that persistent hypotony occurred more frequently in trab eyes (15.2% vs 3.8%). This is an important finding for me, as it may place the Microshunt as a better option than trabeculectomy for patients with sight-threatening glaucoma, monocular patients, younger patients, and patients at higher risk of vision loss as due to hypotony, such as high myopes.

The study was too short to study the question of endothelial cell loss, a potential risk of the Microshunt, between the groups in a comprehensive manner.

In conclusion, while the results of this study seem to demonstrate superiority of trabeculectomy to Microshunt, it has some limitations that may preclude wider generalization of its results. It would be interesting to conduct a comparison of the two methods using higher concentrations of MMC and in different populations. Another limitation of the study was the lack of washout IOP as a baseline inclusion criterion.

Despite these shortcomings, this study demonstrates one more time that being able to perform classic filtration surgery remains a mainstay of glaucoma management and should continue to be an important part of a training curriculum for aspiring glaucoma surgeons

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