Fellman, Ronald L.

Grover, Davinder S.

Sheybani, Arsham

English

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Washington University School of Medicine Digital Commons@Becker Open Access Publications 2020 Critical response to ab interno versus ab externo surgical approach on outflow resistance of a subconjunctival drainage device Arsham Sheybani Washington University School of Medicine in St. Louis Davinder S. Grover Glaucoma Associates of Texas Ronald L. Fellman Glaucoma Associates of Texas Follow this and additional works at: https://digitalcommons.wustl.edu/open_access_pubs Recommended Citation Sheybani, Arsham; Grover, Davinder S.; and Fellman, Ronald L., ,"Critical response to ab interno versus ab externo surgical approach on outflow resistance of a subconjunctival drainage device." Translational Vision Science & Technology.,. . (2020). https://digitalcommons.wustl.edu/open_access_pubs/8941 This Open Access Publication is brought to you for free and open access by Digital Commons@Becker. It has been accepted for inclusion in Open Access Publications by an authorized administrator of Digital Commons@Becker. For more information, please contact engeszer@wustl.edu. Letter to the EditorCritical Response to Ab Interno Versus Ab Externo SurgicalApproach on Outflow Resistance of a SubconjunctivalDrainage DeviceArsham Sheybani1, Davinder S. Grover2, and Ronald L. Fellman21 Washington University in St. Louis School of Medicine, St. Louis, MO, USA2 Glaucoma Associates of Texas, Dallas, TX, USACorrespondence: Arsham Sheybani, Washington University in St. Louis School of Medicine, 660 South Euclid Ave, Campus Box 8096,St. Louis, MO 63110, USA. e-mail: arsham.sheybani@gmail.comWe read with interest the recent paper by Lee et al.1published in TVST entitled “The Implications of anAb Interno Versus Ab Externo Surgical Approachon Outflow Resistance of a Subconjunctival DrainageDevice for Intraocular Pressure Control.”We would like to present a few important consid-erations that may help readers better understandpatient outcome predictors for ab interno and abexterno procedures. Wound healing in a clinical settingcompared to microfluidics in a non-healing rabbit eyeis clearly very different.First, the title of the manuscript creates confusionbecause recent trends in surgery for subconjunctivaldrainage devices include ab interno (through a clearcorneal incision that then tracks through sclera) orab externo (from the sclera to the anterior chamber).This study does not look at the difference in howdevices are placed but rather the difference in surgicallydissecting conjunctiva versus no surgical dissection inan ex vivo model without blood supply or inflam-matory mediators. The complete absence of woundhealing and conjunctival closure and the fact that theproximal portion of the microstent does not rest inthe anterior chamber as in the surgical eye severelylimit this study from being clinically applicable. Theresistance to outflow can be significantly manipulatedor altered by these techniques, as well as by the useof perioperative antifibrotic agents. In addition, theauthors state that the flow resistance of the stand-alone Xen45 gel stent is 10.3 mm Hg at a flow rateof 2 μL/min. The actual resistance for the Xen45 gelstent at 2 μL/min has been measured at 6.28 mm Hgand calculated via the Hagen–Poiseuille equation to be6.05 mm Hg at 37°C.2This article seems to inappropriately position theXen gel stent and the InnFocus/PreserFlo microshuntagainst one another based purely on surgical technique,which is likely neither appropriate nor accurate,especially because the Xen gel stent can and is beingplaced ab externo with an open conjunctival technique.To our knowledge, there are no peer-reviewed studiesdirectly comparing the Xen45 gel stent and the InnFo-cus microshunt.The discussion section contains several statementsthat are concerning, likely emanating from the differ-ence between a microfluidics ex vivo rabbit model andwhat is seen in a clinical research setting with variablewound healing:1. The authors compared various peer-reviewedarticles on the Xen45 gel stent and the InnFo-cus microshunt; however, these quoted studiesare vastly different in terms of methods, patientpopulation, and follow up, as well as dose ofmitomycin C (MMC). The authors also did notcite the US Food and Drug Administration pivotaltrial on the Xen gel stent nor did they cite theinternational comparative study of the Xen gelstent versus trabeculectomy.3,4 The conclusionmade by the authors is that one implant is perhapssuperior at lowering intraocular pressure thananother implant. Although this may eventuallyturn out to be true, there are no studies supportingthis statement, and the current experiment was notdesigned to test that hypothesis.2. The authors refer to the Moorfields Safer SurgerySystem to make statements about the Xen gel stentand InnFocus microshunt. Although this systemremains a tremendous innovation for improvingsafety for trabeculectomy, there is no study confirm-ing that the system is applicable to subconjunctivalfiltration with microshunt devices. Moreover, thereCopyright 2020 The Authorstvst.arvojournals.org | ISSN: 2164-2591 1This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.Downloaded from tvst.arvojournals.org on 03/22/2020Critical Response Ab Interno Versus Ab Externo Procedures TVST | February 2020 | Vol. 9 | No. 3 | Article 14 | 2is no study indicating that this system is applicablewhen one injects MMC in the far posterior fornixduring subconjunctival filtration procedures.3. The authors also make a statement that perform-ing a conjunctival dissection allows for greaterpredictability of directing aqueous flow posteriorly;however, their paper does not directly support thisobservation.4. The authors postulate on a valve-like mechanismof the conjunctiva but make no comment regardingthe position of the stent with reference to Tenon’scapsule, which clearly has a major impact on alltypes of filtration surgery, including microshuntsurgery. This theory is likely difficult to test or evalu-ate in their ex vivo model when tissues are not alive,vascularized, or even closed.5. The authors make a statement thatMMC injectionsare inherently more random and less predictable.Granted there is less long-term information regard-ing MMC injections and bleb morphology, butthere is plenty of evidence to demonstrate improvedbleb morphology and trabeculectomy success rateswith MMC injection versus sponge use.56. The authors make a statement that there is“unpredictability of the bleb development withdevices implanted via the ab interno approachwith preimplantation subconjunctival injection ofMMC.” With the recent advances in subconjunc-tival minimally invasive glaucoma surgical (MIGS)surgery, including proper microstent placementand MMC application, we have seen significantlyimproved predictability, contrary to the authors’laboratory work.Although this is a well-designed study to evalu-ate the outflow resistance of a specific subconjunc-tival drainage device in an ex vivo model, we areconcerned that some of the authors’ conclusions arenot supported by this manuscript. SubconjunctivalMIGS surgeons, especially nascent ones, must bevigilant in incorporating both laboratory and clinicalfindings in order to develop a meaningful perspectiveof subconjunctival filtrationwhen approaching both abinterno and ab externo surgery.AcknowledgmentsDisclosure: A. Sheybani, Allergan (C), Santen Inc(R); D.S. Grover, Allergan (C); R. Fellman, NoneCitation: Sheybani A, Grover DS, Fellman RL. Criti-cal response to ab interno versus ab externo surgicalapproach on outflow resistance of a subconjunctivaldrainage device. TransVis Sci Tech. 2020;9(3):14, https://doi.org/10.1167/tvst.9.3.14References1. Lee RMH, Bouremel Y, Eames I, Brocchini S,Khaw PT. The implications of an ab interno versusab externo surgical approach on outflow resistanceof a subconjunctival drainage device for intraocu-lar pressure control. Transl Vis Sci Technol. 2019;8:58.2. Sheybani A, Reitsamer H, Ahmed II. Fluid dynam-ics of a novel micro-fistula implant for the surgicaltreatment of glaucoma. Invest Ophthalmol Vis Sci.2015;56:4789–4795.3. Grover DS, Flynn WJ, Bashford KP, etal.Performance and safety of a new ab internogelatin stent in refractory glaucoma at 12 months.Am J Ophthalmol. 2017;183:25–36.4. Schlenker MB, Gulamhusein H, Conrad-HengererI, et al.Efficacy, safety, and risk factors for failure ofstandalone ab interno gelatin microstent implanta-tion versus standalone trabeculectomy.Ophthalmol-ogy. 2017;124:1579–1588.5. Pakravan M, Esfandiari H, Yazdani S, etal.Mitomycin C-augmented trabeculectomy:subtenon injection versus soaked sponges:a randomised clinical trial. Br J Ophthalmol.2017;101:1275–1280.Downloaded from tvst.arvojournals.org on 03/22/2020

Critical response to ab interno versus ab externo surgical approach on outflow resistance of a subconjunctival drainage device

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Critical response to ab interno versus ab externo surgical approach on outflow resistance of a subconjunctival drainage device

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