DMEK: The Future of Cornea Replacement

Descemet’s membrane endothelial keratoplasty is the newest, most exciting form of corneal grafting.

BY THOMAS JOHN, MD

The cornea may be considered the final common pathway to eyesight. The final visual outcome for any surgical procedure performed within the eye, in the anterior or posterior segment, depends on the quality and clarity of the cornea. As such, the cornea becomes an important focus from a visual standpoint. Any compromise in corneal clarity may have a deleterious effect on the way an individual perceives the world around him. Permanent structural damage to the cornea usually demands surgical intervention for restoration of vision, provided the posterior segment is functional.

History of Lamellar Keratoplasty

In the past, such a corneal replacement was heavily titled toward full-thickness, penetrating keratoplasty in which the entire cornea was replaced with a healthy corneal graft that was sutured in place to re-establish the ocular anatomy with the added negative impact of iatrogenic corneal astigmatism and sutures. However, in the recent past, there’s been a deliberate move by cornea surgeons worldwide toward selective corneal transplantation (SCT),¹ where only the diseased part of the patient’s cornea is replaced with similar, healthy donor corneal tissue, while retaining the unaffected corneal parts. Such procedures take on the well-established terminology of lamellar keratoplasty (LKP) with often better corneal biomechanics as compared to full-thickness corneal replacement. LKP procedures performed in the anterior or posterior domain of the human cornea are referred to as anterior lamellar keratoplasty (ALK) and posterior lamellar keratoplasty (PLK) respectively. The dividing boundary line between ALK and PLK (Descemet’s stripping endothelial keratoplasty [DSEK], Descemet’s stripping automated endothelial keratoplasty [DSAEK] and Descemet’s membrane endothelial keratoplasty [DMEK]) is the Descemet’s membrane.

Role of Interface

It’s important to note that with the choice of LKP procedures, one introduces a new zone, namely the interface where the donor and recipient cornea tissues meet. And since it encompasses the visual axis, the patient looks through this interface, which in many instances may degrade the quality of vision. This interface, in some cases, may be quite hazy, especially in the immediate post-operative period. Over time, however, the haziness will decrease or resolve almost completely. With this interface, the visual recovery can be slow in the immediate post-surgical period, especially if this interface is more anteriorly located, as in some of the ALK procedures. The deeper this donor-recipient interface, the better the quality of vision and this may explain why visual recovery is fastest with DMEK procedures, where this interface is in the deepest possible location or the zero-point. Hence, it may be theorized that it has very little effect, if any, on final vision after DMEK procedure. In contrast to lamellar corneal procedures, the full-thickness penetrating keratoplasty (PKP) doesn’t have any interface in the visual axis.

ALK Procedures

The ALK procedures have different terminologies depending on the depth of corneal tissue that is replaced with similar healthy donor tissue. ALK procedures include superficial-ALK (SALK) (<160 microns), mid-ALK (MALK) (160-400 microns), deep-ALK (DALK)(400-490 microns) and total-ALK (TALK) (>490 microns) (John-Malbran classification).⁶ Often, the complexity of these procedures exponentially increases as we go from the more superficial lamellar procedures to the deepest ALK procedures, such as DALK and TALK, in which there’s the added risk of potentially perforating the Descemet’s membrane and entering the anterior chamber, which is to be avoided in these ALK procedures.

Figure 1. Left: Extensive primary pterygium completely blocking the visual axis; Right: Day 1 post-operative appearance with a clear cornea and an amniotic membrane transplantation.

It’s important to emphasize that corneal astigmatism isn’t eliminated with ALK procedures since there is a circular corneal wound and for most ALK procedures, sutures are an integral part of the procedure to retain the donor tissue in its docked position on the recipient cornea. The elimination of iatrogenic astigmatism and corneal sutures occurs only when the anterior corneal surface of the recipient cornea isn’t violated as in PLK procedures such as DSEK and DMEK (and the almost extinct DLEK).

Simplifying ALK Procedures

The face of corneal transplantation has changed completely with PLK. DSEK and DMEK have almost completely replaced PKP for permanent endothelial decompensation. ALK procedures are still in a state of flux with peaked interest from a surgical standpoint, but dampened somewhat with the complexities associated with ALK procedures. Clearly, simplification of a surgical procedure is the catalyst to universal acceptance of the procedure that allows for easy duplication by the vast majority of surgeons globally, and thus will become the accepted standard over time. Some ALK procedures (especially DALK and TALK) and DMEK need further simplification for mass acceptance and conversion to these procedures, while DSEK has already achieved, for the most part, a significant degree of simplification. Hence, the noted paradigm shift from PKP to DSEK for endothelial decompensatory procedures.

The Wonders of DMEK

DMEK is a “pie-in-the-sky” type procedure. It’s on the horizon, waiting for simplification, at which point it’s projected to replace DSEK. DMEK is truly a somewhat fascinating surgical procedure, performed in the back part of the human cornea. DMEK brings back the “wow-effect” that’s often seen with LASIK. Similar to LASIK, DMEK also attains 20/20 vision more often than other PLK procedures. Historically, none of the lamellar procedures had a consistent promise of 20/20 vision until DMEK arrived on the scene. DMEK has elevated visual results to a new high in the world of lamellar keratoplasty procedures. DMEK restores the human cornea to near normal anatomy, even in the post-transplanted state, and it has, for the first time, brought the “woweffect” to posteriorly performed corneal procedures. A 20/20 DMEK result may be attributed to the restoration of this near-normal anatomy of the transplanted human cornea with an almost non-detectable donor-recipient interface. With this seemingly eliminated interface, the visual result has been elevated to 20/20 in many DMEK cases. Additionally, DMEK has been shown to have the least amount of corneal graft rejection (1%-yr1, 1%-yr2) as compared to DSEK (8%-yr1, 12%-yr2) and PKP (14%-yr1, 18%-yr2) .⁷

With this new era of DMEK surgery, we enter a new way of doing business, so to speak. There’s no stromal tissue in the donor disk, and this translates to significantly increased complexity of the procedure that often tests the surgical skill-set of the DMEK surgeon. The donor Descemet’s membrane rolls on itself with the healthy donor endothelial cells on the outside. The surgeon has to unroll this DM-endothelial scroll of tissue within the recipient AC using for the most part, a no-touch technique to limit potential damage to endothelial cells that can occur with a direct touch technique. Hence, fluidics are utilized and fluid currents are created within the enclosed space in the AC to gently unroll the DM. Newer instrumentation facilitates this procedure. We have recently introduced a DMEK surgical set (Storz, Bausch+Lomb) that helps in the simplification process of DMEK surgery. This is a continuum, and until we reach the “sweet spot” where easy duplication is possible, DMEK will continue to be a pie-in-the-sky procedure, for the most part. My projection is that we will reach this goal as we continue to take baby steps in the simplification process of DMEK surgery. Further challenges of DMEK surgery include variations in the AC space. If the AC is too shallow or too deep, it adds to the complexity. I’ve performed DMEK in the presence of an open-loop anterior chamber IOL that’s firmly adhered to the angle tissue structures. This becomes even more challenging since the available space where the donor DM needs to be unrolled and attached to the recipient cornea is significantly reduced. Further, endothelial touch to an IOL can also be deleterious for overall endothelial survival. Similarly, it becomes difficult to perform DMEK with a Seton tube or a trabeculectomy. DMEK can be performed in such instances, but one has to have adequate experience to perform this procedure in such instances. Also, I have elected to go the DMEK route in a one-eyed patient with trabeculectomy, since it offers a closed system for transplantation and removes the dreaded complication of an expulsive hemorrhage with an open-sky PKP procedure.

Figure 2. Slit lamp view showing clear cornea 1 month after after DMEK in an eye with an AC IOL.

Donor tissue preparation for DMEK surgery is also challenging. If the donor DM is accidentally torn beyond a usable state, there is a step-up cost for the procedure as one will require an additional donor cornea to procure the donor DM disc with healthy donor endothelial cells. The DMEK set of instruments (Storz, Bausch+Lomb) also helps in the donor tissue preparation.

It’s important to note that with the acceptance of the new way of doing PLK, namely, DSEK and DMEK, we as corneal surgeons have accepted an unavoidable casualty of these newer procedures, namely, a lower donor endothelial cell count. We have seen corneas that are clear and compact with significantly reduced endothelial cells. This begs the questions: What happens to these newly created corneas over time? Will they fall short over time? Will these newly revived corneas with no iatrogenic astigmatism and no corneal sutures meet their demise earlier than the traditional corneal transplants, namely, PKP with a higher post-transplant endothelial cell count?

The answers to these questions and others are unknown, and we need to inject time into the equation of DMEK corneal transplantation to fully understand and unravel the answers to these questions. However, the present looks good and these newer corneal transplantation procedures are here to stay, giving our patients improved vision, faster visual recovery, faster return to their normal lifestyles and more 20/20 vision than ever achieved in the past. We are finally bringing the “wow-effect” back to PLK procedures. ■

References

1. John T. Selective tissue corneal transplantation: A great step forward in global vision restoration. Expert Rev Ophthalmol. 2006;1:5-7. Available at: http://www.expert-reviews. com/doi/pdf/10.1586/17469899.1.1.5.

2. John T, Regis-Pacheco LF, Pecego JG, Terry MA. History of lamellar and penetrating keratoplasty. In: John T (Ed.): Corneal endothelial transplant (DSAEK, DMEK and DLEK). Ch. 14. New Delhi: Jaypee-Highlights Medical Publishers, Inc., 2010:143-153.

3. Barraquer C. Central lamellar keratoplasty: From past to present. In: John T (Ed.): Surgical techniques in anterior and posterior lamellar corneal surgery, New Delhi: Jaypee-Highlights Medical Publishers, Inc. 2006: 23-35.

4. Ko WW, Feldman ST, Frueh BE, Shields CK, Costello ML. Experimental posterior lamellar transplantation of the rabbit cornea. Invest Ophthalmol Vis Sci. 1993; 34(suppl): p. 1102.

5. Melles GRJ, Lander F, Beekhuis WH, Remeijer L, Binder PS. Posterior lamellar keratoplasty for a case of pseudophakic bullous keratopathy. Am J Ophthalmol. 1999; 127:340-341.

6. John T. Surgical management of diffuse corneal opacities. In: Becker MD, Davis JL (Eds.): Surgical management of inflammatory eye disease. Springer-Verlag Berlin Heidelberg, pp. 2008: 67-83.

7. Anshu A, Price MO, Price FW. Risk of corneal transplant rejection significantly reduced with Descemet’s membrane endothelial keratoplasty. Ophthalmology. 2012 ;119:536-540.

Thomas John, MD is a cornea specialist and clinical associate professor at Loyola University at Chicago and in private practice in Oak Brook, Oak Lawn and Tinley Park, Illinois.

Financial Disclosure: Dr. John is a consultant for Bausch + Lomb.