Descemet Membrane Endothelial Keratoplasty Surgical Technique

7 Descemet Membrane Endothelial Keratoplasty Surgical Technique


Christopher S. Sáles and Mark A. Terry


7.1 Introduction


Our collective efforts have advanced endothelial keratoplasty to the point where we can now attain a very nearly perfect anatomical replacement of the diseased endothelium and Descemet membrane with healthy donor tissue.1 To our patients, this means that we can deliver visual outcomes and rejection rates that are far superior to penetrating keratoplasty (PK) and even better than Descemet stripping automated endothelial keratoplasty (DSAEK). To the corneal transplant surgeon, it means that we have the opportunity to learn a new surgical skill set that is very different from that for DSAEK.


The procedure for Descemet membrane endothelial keratoplasty (DMEK) has many nuances that can be important for success. This chapter’s goal is to ease the learning curve of the novice DMEK surgeon by introducing the most critical details of our standardized surgical technique. It should not be considered an adequate substitute for a “hands-on” DMEK course, where one can assist an experienced DMEK surgeon at the operating microscope and gain experience in the DMEK in vitro wet lab.


7.2 Case Selection


7.2.1 Relative Contraindications for DMEK


Case selection plays a decisive role in the success of DMEK surgery, especially in the early days of one’s learning curve. We routinely use DMEK for cases of Fuchs dystrophy and pseudophakic bullous keratopathy (PBK) in the absence of any other complicating issues. Although others have achieved success in a number of these scenarios, we opt for DSAEK instead of DMEK if any of the following are present: anterior chamber intraocular lens (IOL), filtering tube, trabeculectomy, aphakia, extensive peripheral anterior synechiae, and a history of posterior vitrectomy. We also perform DMEK in eyes with a prior failed DSAEK or PK, but in the latter scenario the procedure can be more difficult than in eyes with a virgin cornea ( Table 7.1).


7.2.2 The Ideal First Case for DMEK


Two key components that make DMEK surgery easier during the manipulation of the graft are the presence of a small pupil (1–2 mm) and an easily shallowed anterior chamber. The ideal surgical candidate for the new DMEK surgeon is a nontriple case because it does not require dilation agents, which can prevent a complete and brisk response to Miochol (Bausch & Lomb). The ideal candidate also has an eye with a normal or small anterior chamber and minimal vitreous syneresis of the posterior segment.


All of these traits are most often found in younger patients between the ages of 30 and 50, who are frequently phakic. However, the novice DMEK surgeon must also take into account the additional stress of damaging the crystalline lens during anterior segment maneuvers when the chamber is shallow, which is why a young pseudophakic patient with a small- to average-sized anterior chamber is probably the best first case.


In addition, although we prefer to place a surgical inferior iridotomy at the time of DMEK surgery, it always presents a small risk of iris vessel bleeding. Thus, for the first few cases of DMEK, the novice DMEK surgeon should also consider placing a generous inferior iridotomy by laser applications 1 or 2 weeks prior to the scheduled DMEK surgery ( Table 7.2).


7.3 Ordering DMEK Tissue


7.3.1 Prestripped and S-Stamped Tissue


As was the case with their introduction of precut DSAEK tissue, eye banks have made DMEK more feasible to adopt by minimizing the risk of tissue loss. The introduction of prestripped tissue by eye banks has eliminated the risk of failed donor preparation by the surgeon. The “S” stamp has further eliminated iatrogenic primary graft failure caused by inadvertently implanting the graft upside-down.


We obtain all of our prestripped and S-stamped tissue from the Lions Vision Gift (LVG) Eye Bank in Portland, Oregon, a leading eye bank in the Eye Bank Association of America (EBAA). An increasing number of eye banks have begun offering DMEK tissue, but should your local eye bank not provide this service, their tissue can be shipped to LVG in Portland for prestripping the day before scheduled DMEK surgery. LVG has shipped DMEK tissue as far as Asia and Europe.


Table 7.1 Conditions to consider Descemet stripping automated endothelial keratoplasty (DSAEK) instead of Descemet membrane endothelial keratoplasty (DMEK)





























Relative contraindications to DMEK


Rationale


Posterior vitrectomy


Inability to shallow the anterior chamber


Anterior chamber intraocular lens


Trauma to the graft from the intraocular lens


Frequent coincidence of posterior vitrectomy


Aphakia


Risk of tissue loss into the posterior segment


Frequent coincidence of posterior vitrectomy


Glaucoma with a filtering tube


Trauma to the graft from the tube


Risk of graft separation due to postoperative hypotony and/or bubble migration


Glaucoma with a trabeculectomy


Risk of tissue loss into the sclerotomy


Risk of graft separation due to postoperative hypotony and/or bubble migration


Peripheral anterior synechiae


If extensive (e.g., in iridocorneal endothelial syndrome), difficulty unscrolling the graft without a smooth iris plane


Prestripped DMEK tissue is 90% stripped by a skilled eye bank technician, leaving a hinge of attachment of Descemet membrane to the peripheral posterior stroma. This allows the donor tissue to remain attached and still be shipped in Optisol GS (Bausch & Lomb) in the same plastic viewing chamber as standard tissue. A “notch” that is cut from the scleral rim designates the location of the hinge. Tissue is evaluated by slit lamp after prestripping, and specular microscopy is performed prior to and after preparation of the DMEK tissue ( Fig. 7.1).


Table 7.2 The ideal first case for the novice Descemet membrane endothelial keratoplasty (DMEK) surgeon





























Key factors to consider


Small pupil


Easily shallowed anterior chamber


Minimal risk of intraoperative hemorrhage


Desirable traits


Nontriple procedure


Short or average axial length


Preplaced laser peripheral iridotomy



Minimal vitreous syneresis (30–50 years of age)



Phakica


Ideal first case for the novice DMEK surgeon


Young pseudophakica patient with a small- to average-sized anterior chamber and a preplaced inferior laser peripheral iridotomy


a Phakic eyes are generally easier to shallow than pseudophakic eyes, but the additional stress of potentially damaging the crystalline lens during anterior chamber maneuvers must be weighed against the potential benefits. In general, it may be better to choose a pseudophakic eye for one’s first case.


7.3.2 Donor Age


Younger donor age correlates with more tightly scrolled DMEK tissue, which can be more difficult to unscroll intraoperatively, especially if the anterior chamber cannot be easily shallowed. Donor age is therefore a very important factor to consider during one’s initial cases, although in our experience it becomes less of a consideration as the surgeon becomes more adept at the procedure. As of 2016, we routinely implant tissue from donors that are 50 years of age or older using a tapping technique, and we suspect that this age may continue to decrease as new techniques emerge. One such example is the specialized Fogla DMEK Cannula (Storz Bausch & Lomb Surgical), which has horizontally oriented orifices on each side of a closed-ended cannula to “jet” the leaflets of a tight scroll into a double-scroll configuration ( Fig. 7.2).



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Fig. 7.1 (a) Specular microscopy. (b) View of the prestripped and S-stamped tissue in the viewing chamber, with a notch in the scleral rim to designate the location of the hinge. (c,d) Slit lamp biomicroscopy of the Descemet membrane endothelial keratoplasty tissue in the viewing chamber. Figure courtesy of Philip Dye, Lions VissionGift, Portland OR.



7.4 Preoperative Medications


It is very important that the pupil is as small as possible for the insertion and unscrolling of the DMEK tissue; therefore we prefer not to use any cycloplegic drops preoperatively, even when concurrent cataract surgery is planned. In “triple” procedures, we use 1 to 3 drops of phenylephrine 2.5% to augment the dilation attained from a well-placed retrobulbar block, and we avoid the use of nonsteroidal drops to prevent persistent dilation. If the eye already has a posterior chamber IOL, we do not routinely use pilocarpine to constrict the pupil preoperatively because it may cause longer-term miosis that can either lead to posterior synechiae in the presence of a gas bubble or interfere with pupil dilation postoperatively should it be needed.


Nonsteroidal drops in DMEK cases that are performed without cataract surgery do not interfere with the miotic effects of Miochol, and can be given based on the surgeon’s preference. Other preoperative drops such as antibiotics can also be given according to the surgeon’s preference.


Discontinuation of systemic anticoagulants is ideal but not mandatory if a surgical peripheral iridotomy is planned.


7.5 Anesthesia


DMEK surgery is easiest in the first few cases with the patient under general anesthesia, as the stress and frustration level of the novice DMEK surgeon can be high, and it is always better when the patient does not hear the surgeon curse! Otherwise, DMEK can be performed under retrobulbar/peribulbar anesthesia quite easily and comfortably. Well-placed blocks can cause temporary mydriasis, which can help with some parts of the case (e.g., cataract surgery, stripping of Descemet membrane), but must be reversed with Miochol prior to injecting the tissue.



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Fig. 7.3 The shape of the Straiko injector creates a Venturi effect that slows the tissue as it is aspirated from the tube’s narrower, high-flow orifice into its wider, low-flow base–this makes the injector safer. A fully assembled injector consists of a 3 ml BD syringe (BD Worldwide), 14 French tubing, and a modified Jones tube (Gunther Weiss Scientific Glass Blowing) used off-label for Descemet membrane endothelial keratoplasty surgery.


7.6 Preparing the Injector


The Straiko injector consists primarily of a modified glass Jones tube that has been reshaped and polished by the Gunther Weiss Scientific Glass Blowing Company to resemble the profile of a medicine dropper—tapered distally, and bulb shaped in its midsection ( Fig. 7.3). Dr. Mike Straiko chose this design because it creates a venturi effect that slows the tissue as it is aspirated from the tube’s narrower, high-flow orifice into its wider, low-flow base. The risk of inadvertently damaging the tissue by sucking it into the syringe is therefore reduced. The modified Jones tube is coupled to a 3 mL syringe with a short piece of 14 French single-lumen suction tubing, and used off-label for DMEK surgery. Dr. Straiko designed the injector to be a closed system, which makes injection of the tissue highly controlled by ensuring that there is absolutely no reflux of fluid from the anterior chamber into the injector once its tip fully occludes the main incision ( Fig. 7.4).


The Straiko injector is constructed in the operating room by cutting a 14 French single-lumen suction tube anywhere along its length with heavy scissors, wetting the cut end with balanced salt solution (BSS), and twisting it onto a 3 mL Luer-lock syringe until it is firmly seated. The tubing is then trimmed to approximately 15 mm in length at a 45-degree angle, and this end is twisted onto the proximal end of the modified Jones tube. A well-constructed Straiko injector should require very little movement of the plunger to move the tissue into and out of the Jones tube. Care should be taken to minimize the gap between the syringe’s orifice and the proximal end of the Jones tube, which ensures that the injector responds predictably to inputs made with the plunger. The injector must also be watertight and purged of all air, both of which can make the injector less responsive to actions on the plunger.


A modified Jones tube can be obtained from Gunther Weiss Scientific Glassblowing Company, Inc., in Portland, Oregon (503–644–4056, info@guntherweiss.com). Although these glass injectors may be labeled for single use due to local regulatory requirements, they are easily sterilized with standard methods and can be used multiple times as long as the tip remains smooth and unchipped.



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Fig. 7.4 (a) Assembling the Straiko injector. (b) Filling the 20% SF6 cannulas. (c) Making the incisions. (d) Making the descemetorhexis. (e) Widening the main incision to fit the Straiko injector. (f) Rinsing the anterior chamber and Straiko injector of all viscoelastic. (g) Preparing the donor at the back table. (h) Injecting, unscrolling, and supporting the tissue with a bubble. (i) Closing the case. (j) Contingency instruments. (k) Making a pseudophakic peripheral iridotomy (PI). (l) Making a phakic PI. (m) Accounting of all of the syringes opened for a Descemet membrane endothelial keratoplasty case.


7.7 Preparing the Recipient


The description of our surgical technique is from the perspective of the surgeon sitting temporally.


7.7.1 Paracentesis Sites


Calipers set at 3.2 mm are used to mark the clear corneal limbus at the 180-degree meridian for the main wound, and this is embellished with a surgical marking pen on the epithelium. Two additional ink marks are made in the superior and the inferior temporal limbal cornea to mark the locations of the paracentesis sites. Square-shaped, self-sealing stab incisions are then made at the two sites with a 1 mm keratome. The anterior chamber is filled with cohesive viscoelastic (Healon, Abbott Medical Optics, Inc.).


7.7.2 Descemetorhexis


The central surface of the cornea is marked with an 8 mm circle indentation, and this is embellished with multiple spots of marking pen ink to establish a template for the descemetorhexis. Care is taken to ensure that the surface template does not overlap the paracentesis or the main wound openings into the anterior chamber ( Fig. 7.5). With the chamber filled with cohesive viscoelastic, a reverse Terry-Sinskey hook is used to strip Descemet membrane, making sure to score exactly along the circular mark so that a full 8 mm area of stroma is bared. The goal is to attain a recipient stromal bed that is 0.5 mm larger than the donor DMEK tissue so that there is minimal, and ideally no, overlap between the donor and recipient Descemet membranes because any overlap can increase the risk of graft edge separation. Kruse and colleagues have termed this technique overstripping.


More so than in DSAEK surgery, extra care is taken during the descemetorhexis to ensure that the overlying stromal fibers are not disrupted because a rough stromal bed can increase the risk of graft separation in DMEK. Specialized instruments designed to protect the stroma during this step of the procedure are available, including the Sáles Paddle Stripper ( Fig. 7.6) and the Fogla Separator ( Fig. 7.7) (Storz Bausch & Lomb Surgical). Unlike DSAEK surgery, scraping the stroma is not performed in the DMEK procedure.


The chamber is filled again with Healon to pressurize the eye, and the main wound is created. A guarded diamond knife is set for 325 µm and a vertical 3.2 mm incision is made at the temporal limbus where previously marked. A 2.8 mm diamond or steel microkeratome is placed through the vertical incision edge to create a beveled entrance wound about 1.5 mm in length. The stripped Descemet membrane is then removed with forceps, or if available, Straiko Twin Ring Forceps (Storz Bausch & Lomb Surgical), which have been designed to grasp the tissue without engaging the stroma ( Fig. 7.8).


7.7.3 Inferior Peripheral Iridotomy


The inferior peripheral iridotomy is then performed if it has not already been made preoperatively with a laser. To make the iridotomy as peripheral as possible, a few microliters of Miochol are placed directly onto the iris surface beneath the Healon to constrict the pupil. In phakic eyes, we perform the iridotomy by excising iris that has been externalized from the eye through an overlying vertical paracentesis with forceps. In pseudophakic eyes, Healon is then injected into the anterior chamber and also placed between the iris pigment epithelium and the anterior capsule inferiorly. A needle driver is used to make two bends in a half-inch 30-gauge needle, one at the beveled tip and one at the hub, to fashion an ergonomic reversed hook that has a sharp cutting edge. The bent needle tip is placed through the superior paracentesis site, passed through the pupil, walked along the posterior iris inferiorly, and then pressed upward against the far peripheral iris pigment epithelium. At the same time, a straight Sinskey hook is placed through the inferior paracentesis or the main incision and onto the iris stroma. The hook is then used to scrape down onto the location of the needle tip that is tenting up the iris tissue in the inferior periphery. Once the tip of the needle perforates the iris from behind, this creates a hole where the tip of the Sinskey hook can then be placed. Using the needle and the hook, the small iridotomy can be stretched to open it further. Intraocular MST Scissors (Micro-Surgical Technology) are then used to excise a full-thickness wedge of iris from the inferior periphery to ensure that the peripheral iridotomy remains patent ( Fig. 7.9). When making the peripheral iridotomy, care should be taken to place it as peripheral and as close to 6 o’clock as possible to allow for a large bubble, and to avoid lacerating iris stromal vessels and pulling radially on the iris root, which can result in hemorrhage. In addition, the surgeon must be absolutely certain that iris pigment is excised (and not just iris stroma) to ensure patency of the peripheral iridotomy.


7.7.4 Main Incision


The 2.8 mm main incision is then widened and custom-fit to the injector tip. A crescent blade is used to widen the incision incrementally to approximately 3.2 mm, or until the injector tip fits snugly into the incision. One or two test-fits with the injector may be necessary. When widening the main incision, care should be taken to make its shape trapezoidal such that the injector’s tapered midsection completely occludes the internal and external wounds. A watertight main incision is imperative to safe injection of DMEK tissue into the anterior chamber. Care must also be taken not to widen the incision excessively, which can make it more difficult to disengage the tissue because the injector must be inserted further into the anterior chamber to maintain a watertight seal, thereby reducing the space available between the angle and the distal tip.


7.7.5 Evacuation of Viscoelastic


The anterior chamber is then evacuated with an irrigation/aspiration tip, preferably with a silicone sleeve to help to occlude the enlarged main incision and prevent iris prolapse. The Straiko injector is also flushed at least three times with BSS to ensure that there is absolutely no viscoelastic in the injector system.


7.7.6 Final Check


The eye is brought to a normal to slightly hypertensive pressure with BSS to prevent any bleeding from the iridotomy or from episcleral reflux. As a systemic measure to prevent mishaps in the operating room, we review the following checklist before moving to the donor table: (1) completion of the descemetorhexis, (2) completion of the inferior peripheral iridotomy, (3) widening of the main incision, and (4) evacuation of the anterior chamber and the Straiko injector of viscoelastic.


7.8 Preparing the Donor


Our methods of handling the donor tissue ensure that the DMEK graft is touched only once at its far periphery during the entire transplant surgery. All preparation of the donor and loading of the injector is performed under an operating microscope.


The donor corneoscleral rim is removed from the viewing chamber with forceps by grasping the scleral notch placed by the eye bank. The tissue is then placed endothelial side up onto the donor table, where trypan blue is dripped onto the endothelium and left to stain for 1 minute. The trypan blue is wicked away using a polyvinyl-alcohol (Merocel) sponge placed at the edge of the donor Descemet flap opposite its hinge, while raising the hinged side of the corneoscleral rim with forceps. Care should be taken to seat the prestripped Descemet membrane flat against the underlying posterior stroma when draining the fluid. If the membrane does not lie flat, especially along any of its edges, refloat it with BSS and wick the fluid again until the folds are gone.


May 28, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Descemet Membrane Endothelial Keratoplasty Surgical Technique
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