Flammer, Josef

Shaarawy, Tarek

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Received: 14 April 2003Published online: 20 August 2003© Springer-Verlag 2003HistoryFew would recall the operation pro-posed by Krasnov in the late 1950s,sinusotomy [16, 17, 18, 19], which is in fact the first seed of non-pene-trating surgery. Believing that themaximum resistance to outflow resides at the level of sclera in glau-coma patients, Krasnov proposed excising the bulk of sclera overlyingSchlemm’s canal (SC). The obviousdifficulty of the surgical technique,together with the lack of modern mi-croscopes at that time, hindered thepopularization of his operation. Also,in what would later be a stigma ofnon-penetrating surgery, he abstractlyreported his success rates withoutfully explaining his success criteriaor offering solid arguments on howhis technique functioned.Many years later Zimmerman [31, 32], benefiting from an im-proved understanding of outflowmechanisms, proposed what hetermed ab-externo trabeculectomy.The procedure differs from sinusoto-my in that Zimmermann advocatedthe removal of the inner wall of SCand the juxtacanalicular trabecularmeshwork (JCT), as well as the creation of a superficial scleral flapguarding the sinusotomy, in the samemanner as in trabeculectomy.Kozlov [14, 15] suggested ex-tending the dissection anteriorly toexcise a portion of corneal stromatogether with the deep sclera, expos-Graefe’s Arch Clin Exp Ophthalmol(2003) 241:699–702DOI 10.1007/s00417-003-0715-8G U E S T  E D I T O R I A LTarek ShaarawyJosef FlammerPro: Non-penetrating glaucoma surgery—a fair chanceT. Shaarawy (✉) · J. FlammerUniversity Eye Hospital,Mittlere Strasse 91, 4012 Basel, Switzerlande-mail: tshaarawy@glaucoma-eg.comT. ShaarawyGlaucoma Department,Memorial Research Institute of Ophthalmology,Giza, Egypting Descemet’s membrane, with theaim of at creating percolation throughthe membrane as well. Stegmann[29], on the other hand, augmentedthe operation by injecting viscoelas-tic material in the two cut ends ofSC, thus dilating it, and termed theprocedure viscocanalostomy.Thus the saga of non-penetratingsurgery was forged by visionariessuggesting ideas, but not offering evidence-based medicine to substan-tiate their concepts. It is a fact thatwe are currently overwhelmed bystudies reporting on the results ofone technique or another, often withconflicting results, in a virtual vacu-um of studies examining mecha-nisms of function.The common conceptThe principal common concept ofnon-penetration is to create filtrationthrough a naturally occurring mem-brane that acts as an outflow resis-tance site [23], allowing a progres-sive decrease in intraocular pressure(IOP) and avoiding postoperative ocular hypotony. This membrane, thetrabeculo-Descemet’s membrane(TDM), consists of the trabecularmeshwork and the peripheral Descemet’s membrane. To exposethe membrane a deep sclerokera-tectomy should be performed, there-by also providing a postoperativescleral space. This space may act asan aqueous reservoir and as a filtra-tion site that may reduce the need fora large subconjunctival filtrationbleb thus reducing the risk of latebleb-related complications.Mechanisms of functionThere are several points of interestwhen studying the mechanisms offunction of non-penetrating surger-ies. Namely, the removal of the innerwall of SC together with adjacenttrabecular tissue, the aqueous humorflow through the TDM, the aqueousresorption after its passage throughthe TDM, and the SC dilatation byviscoelastic injection.Flow through the TDMGrant [8], more than 40 years ago,demonstrated that maximum outflowresistance resides within the innerwall of SC together with the JCT.Various studies have attempted toidentify the nature of the “plug” inthe trabecular meshwork, but to noavail.The JCT contains a mixture ofcells, extracellular matrix, and aqueous pathways that appear asempty spaces under electron micros-copy. The JCT extracellular matrix is a combination of glycosamino-glycan and proteoglycan. With itsmorphological structure containingrelatively small openings and tortu-ous pathways, many studies [7, 8]have hypothesized that the JCT isthe principal site of outflow resis-tance.Passing through the JCT, aqueousis confronted with an anatomic barri-er, namely the endothelial lining ofSC. In its normal physiologic stateSC is large enough not to generatesignificant outflow resistance. AsIOP increases, the trabecular mesh-work expands into the SC lumen[10], causing a concomitant signifi-cant narrowing.In non-penetrating glaucoma sur-gery (NPGS) the canal is unroofed.As numerous collagenous septa be-tween the inner and outer walls exist,it has been demonstrated [27] thatthe act of unroofing results in dam-age to the inner wall as the septa pullon it. Such damage allows aqueousaccess to the canal.Furthermore, in NPGS the innerwall of the canal is peeled (ab-exter-no trabeculectomy). Analysis of thepeeled membrane provided conclu-sive evidence [9] that includes asidefrom the inner wall of SC, the JCT,and to a lesser extent the corneo-scleral trabecular meshwork. In thatrespect the procedure attempts to re-move the external portion of the tra-becular meshwork responsible forthe main aqueous outflow resistancein a glaucomatous eye.The TDM offers resistance toaqueous humor outflow that allowsfor a slow decrease in IOP duringsurgery and will account for the reliable and reproducible IOP on the first postoperative day [23].Thus the main advantage of theTDM is to reduce the immediatepostoperative complications such ashypotony, flat anterior chamber,choroidal detachments and inducedcataract [3].In an experimental model [30], thegradual decrease in IOP was studiedand the resistance of the TDM calcu-lated. Experiments were performedon enucleated human eyes unsuitablefor keratoplasty. The mean IOP de-crease speed was 2.7±0.6 mmHg/min. The ocular aqueous outflowresistance dropped from a mean of5.34±0.19 ml/min/mmHg preopera-tively to a mean of 0.41±0.16 ml/min/mmHg postoperatively. Thesame study [30] also reported that the outflow facility increased from0.19±0.03 to 24.5±12.6 µl/min/mmHgafter deep sclerectomy.The TDM resistance thus appearslow enough to ensure a low IOP andyet sufficient to maintain the anteriorchamber depth and avoid the postop-erative complications in relation tohypotony.The same study histologically[30] examined the surgical site using700ocular perfusion with ferritin, dem-onstrating that the main outflowthrough the TDM occurred at thelevel of the trabecular meshwork,rather than Descemet’s membrane.This is in line with studies [28] re-porting on the very limited perme-ability of Descemet’s membrane,which by itself is not enough to re-lieve the elevated pressure in glauco-ma. In fact, most proponents ofNPGS advocate [25] the creation ofthe Descemet’s portion of the TDMonly as a window that can be punc-tured by Nd:YAG goniopuncture incases of elevated postoperative intra-ocular pressure.Aqueous resorptionAfter aqueous humor passagethrough the TDM, multiple mecha-nisms of aqueous resorption havebeen proposed: a subconjunctivalbleb, an intrascleral bleb (intrasclerallake), suprachoroidal filtration, andan episcleral vein outflow via SC.As after trabeculectomy, almostall patients undergoing NPGS have adiffuse, conjunctival bleb on the firstpostoperative day [12]. As demon-strated by ultrasound biomicroscopy(UBM) studies, successful casesshow a low-profile and diffuse sub-conjunctival bleb even years aftersurgery [13, 21]. However, this blebtends to be shallower and more dif-fuse than the one seen after trabecul-ectomy.Although this has not been stud-ied, subconjunctival blebs probablyoccur more commonly with deepsclerectomy than with viscocanalos-tomy. The reason for this is that inviscocanalostomy [29] the superfici-al scleral flap is tightly sutured, so asto force percolating aqueous into thetwo surgically created ostia of SC,while in deep sclerectomy, the flap isonly loosely sutured [26] and a dif-fuse shallow bleb is desired.In NPGS, a certain volume ofsclera is removed, ranging between 5and 8 mm3. Provided the superficialscleral flap does not collapse, thisscleral volume may be considered asan intrascleral bleb. The value of in-trascleral bleb was ascertained byRoters and co-workers [24], whoshowed that absence of intrascleralbleb in postoperative UBM examina-tions correlates with lack of IOPcontrol in viscocanalostomy.The importance of intrascleralbleb has prompted research intoways to keep it patent. The idea ofusing a collagen implant was pro-posed by Kozlov [14, 15]. The con-cept is simply to occupy this spaceduring the period of maximal healingresponse. After the scarring processfalters, the implant is slowly re-sorbed 6–9 months postoperatively[2], leaving behind an aqueous lake.The intrascleral bleb was ob-served [13] in more than 90% of pa-tients who received a collagen im-plant and the mean volume of the in-trascleral bleb was 1.8 mm3. In theintrascleral bleb the aqueous resorp-tion mechanism may be differentfrom that in the subconjunctivalspace. The aqueous might be re-sorbed by new aqueous drainagevessels, as demonstrated by Delariveand co-workers [4]. This studyshowed that in the scleral space cre-ated after deep sclerectomy, regard-less of whether or not a collagen im-plant was used, new aqueous humordrainage vessels were growing andresorbing the aqueous flowingthrough the TDM.By thinning the sclera by 90%,aqueous humor outflow into the su-prachoroidal space may occur; infact on UBM, it is possible to ob-serve fluid between the ciliary bodyand the remaining sclera in 45% ofthe patients studied years after thedeep sclerectomy [13]. However,this could also indicate chronic lo-calized ciliary body detachment withsubsequent reduction of the aqueousproduction [23]. Further studies onaqueous dynamics following non-penetrating filtering surgery areneeded to better understand the exactmechanisms of aqueous drainage andtheir respective importance in termsof success and complications.When performing the deep scler-ectomy dissection, SC is opened andunroofed. On either side of the deepsclerectomy the two surgically creat-ed ostia of SC may drain the aqueoushumor into the episcleral veins. Thismechanism is probably more impor-tant after viscocanalostomy, duringwhich the ostia and SC are dilated.Injection of viscoelastic materialinto the two surgically created ostiacauses multiple ruptures in both in-ner and outer endothelial walls of thecanal, as has been demonstrated [27]in human and monkey eyes. Theseruptures extend into the JCT andmay also rupture some of the mesh-work itself. In that respect, viscoca-nalostomy seems to function as a“delicate” trabeculotomy.The futureAlthough we have taken some stepsin our understanding of how NPGSfunctions, we are still hindered bylimited interest in NPGS basic re-search, as well as lack of overall un-derstanding of the pathological pro-cess of glaucoma.On the clinical side, NPGS hasbeen proved to be a safer techniquethan other available surgical modali-ties [1, 3, 6, 22, 26], includingtrabeculectomy and drainage devic-es. The question of efficacy, never-theless, is far from resolved.Controversial, often contradictory[3, 6] results have been published.As one browses between results,though, one should keep in mind thatit is all about technique. Issues relat-ed to which technique is superior towhich in the wide spectrum of NPGSare of paramount importance. Thefact of an existing long learningcurve can not be overstated. It is nei-ther meaningful nor scientificallysound to compare one’s last 20 casesof trabeculectomy to one’s first 20deep sclerectomies [5, 11, 20].With its apparent mechanisms offunction that seem to target specificpathological structures in glaucoma,NPGS deserves a fair chance.701References1. Ambresin A, Shaarawy T, Mermoud A(2002) Deep sclerectomy with collagenimplant in one eye compared withtrabeculectomy in the other eye of thesame patient. J Glaucoma 11:214–2202. Chiou AG, Mermoud A, Underdahl JP,Schnyder CC (1998) An ultrasoundbiomicroscopic study of eyes afterdeep sclerectomy with collagen im-plant. Ophthalmology 105:746–7503. Chiselita D (2001) Non-penetratingdeep sclerectomy versus trabeculecto-my in primary open-angle glaucomasurgery. Eye 15:197–2014. Delarive T, Schnyder CC Shaarawy T.Mermoud A (2001) An animal modelfor the study of non-penetrating glau-coma surgery. University of Lausanne5. 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Johnstone MA, Grant WM (1973)Pressure-dependant changes in thestructures of the aqueous outflowsystem of the human and monkey eyes.Am J Ophthalmol 75:365–38311. Jonescu-Cuypers C, Jacobi P, KonenW, Krieglstein G (2001) Primary visco-canalostomy versus trabeculectomy inwhite patients with open-angle glauco-ma: a randomized clinical trial. Oph-thalmology 108:254–25812. Karlen ME, Sanchez E, Schnyder CC,Sickenberg M, Mermoud A (1999)Deep sclerectomy with collagen im-plant: medium term results. Br JOphthalmol 83:6–1113. Kazakova D, Roters S, Schnyder C,Achache F, Jonescu-Cuypers C, Mermoud A, Krieglstein G (2002) Ultrasonic biomicroscopy images:long-term results after deep sclerectomywith collagen implant. Graefes ArchClin Exp Ophthalmol 240:918–92314. Kozlov VI, Bagrov SN, Anisimova SY,Osipov AV, Mogilevtsev VV (1990)Deep sclerectomy with collagen. EyeMicrosurgery 3:44–4615. 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Pro: Non-penetrating glaucoma surgery—a fair chance

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Pro: Non-penetrating glaucoma surgery—a fair chance

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