Focus on Cornea

PDEK: A revolution in corneal transplantation

Our new procedure offers multiple advantages.

By Amar Agarwal, MS, FRCS, FRCOphth, and Priya Narang, MS

Descemet’s stripping endothelial keratoplasty (DSEK), Descemet’s membrane endothelial keratoplasty (DMEK) and their variants form a bulk of today’s endothelial keratoplasty procedures. Corneal transplant pioneer Gerrit Melles of The Netherlands described the harvesting of Descemet’s membrane and endothelium from a donor corneo-scleral disc as “a delicate procedure that is sometimes complicated by inadvertent tearing of the membrane.”¹

DMEK poses challenges such as difficulty in manual preparation of the donor tissue, undue rolling of the graft and subsequent difficulty in unrolling the graft when placed in the recipient eye, attributed to the elasticity of especially young donor tissue.

Because the tissue used in DMEK is more fragile, we see an inherent risk of complication greater than that with DSEK or DSAEK (Descemet’s stripping automated endothelial keratoplasty). Both the DSAEK and ultra-thin DSEK procedures entail the inclusion of corneal stroma in the donor graft, potentially limiting the speed of visual recovery; although the stability of the graft is achieved in the recipient eye.

This article will explore an alternate transplantation method that eliminates most of the disadvantages of DSEK, DSAEK and DMEK. We have pioneered this technique, which we call pre-Decemet’s endothelial keratoplasty, or PDEK.

USING ADDITIONAL TISSUE

Early findings

Indian ophthalmologist Harminder Dua first put forward the presence of the distinct pre-Descemet’s layer, proposing the addition of the pre-Descemet’s layer to the Descemet’s membrane-endothelial graft can help generate tissue for endothelial transplant.² Massimo Busin of Italy^3-5 and Siamak Zarei-Ghanavati⁶ were the first to propose that an injection of air could prepare tissue for DMEK. The authors, however, indicated that the harvesting technique yielded Descemet’s membrane and endothelium.

Others who examined the histology of the tissue that this method obtained reported the presence⁷ or absence⁸ of “residual stroma.” None of these studies made any attempt to characterize or investigate in detail the “residual stroma” or the “DMEK tissue” thus obtained.

What’s new in PDEK

Pre-Descemet’s endothelial keratoplasty is a technique that introduces the donor Descemet’s membrane along with the pre-Descemet’s layer, which are harvested directly from a corneo-scleral disc after the surgeon creates a type-1 (pre-Descemet’s layer) bubble.² This surgical technique requires injection of air in a corneo-scleral disc to create a type-1 bubble, which the surgeon then deflates before excising and/or trephining the Descemet’s layer and endothelium.

Easier harvesting and handling

The plane between the pre-Descemet’s layer and stroma can help to generate tissue for endothelial transplant, allowing for easier tissue harvesting, handling and insertion because it tends to not scroll as much as the Decemet’s membrane; the pre-Descemet’s layer splints Descemet’s membrane.

The surgeon can then use this tissue for PDEK, which provides the benefits of DMEK, such as speedy visual recovery and easier handling, while also reducing the risks associated with a thin and friable tissue. Here, we describe a step-by-step guide to performing the PDEK procedure.

PERFORMING PDEK

Surgical technique

We first dissect a corneo-scleral disc and insert a 30-gauge needle attached to a syringe from the limbus into the mid-peripheral stroma (Figure 1A). We then slowly inject air into the donor stroma until a type-1 big bubble forms — a well-circumscribed, central dome shaped elevation measuring 7 mm to 8.5 mm in diameter (Figures 1B and C).

FIGURE 1 – DONOR GRAFT PREPARATION
Preparation of the donor graft involves the following steps: A) Introduce a 30-G air-filled syringe from the limbus into the center of cornea. B) Formation of the air bubble. C) 8-mm type-1 air bubble extends from center to periphery. D) Injection of trypan blue dye to stain the graft. E) Completion of graft Trephination. F) Cut trypan blue-stained graft all around the air bubble with a corneo-scleral scissors.
COURTESY: AMAR AGARWAL, MD, AND PAIRA NARANG, MS

We then trephine the donor graft (Figures 1D and E) with penetration of the bubble wall at the extreme periphery. We inject trypan blue into the bubble to stain the graft, which the surgeon then cuts all around the trephine mark with a pair of corneo-scleral scissors (Figure 1F).

After we administer peribulbar anesthesia, we make a trephine mark on the recipient cornea (Figure 2A) respective to the diameter of Descemet’s membrane to be scored on the endothelial side. Then we make a 2.8-mm corneal tunnel incision, forming the anterior chamber and maintaining it with saline injection or infusion.

FIGURE 2 – DONOR GRAFT INSERTION
Steps for insertion of the donor graft: A) Pseudophakic bullous keratopathy in the recipient eye. B) Peeling of the DM after completion of descemetorhexis. C) Pupilloplasty for irregular shaped pupil. D) Round-shaped pupil is evident upon completion of pupilloplasty. E) Injection of donor graft. F) Air injected beneath the donor graft after proper orientation. Sutures are taken to ensure corneal wound integrity.
COURTESY: AMAR AGARWAL, MD, AND PAIRA NARANG, MS

Descemetorhexis

We perform descemetorhexis from the endothelial side with a reverse Sinskey hook (Figure 2B). Once you lift an adequate edge, you can use a non-toothed forceps to gently grab the Descemet’s membrane, which is peeled and is removed from the eye. Pupilloplasty ensures a round pupil in a case of pseudophakic bullous keratopathy (Figures 2C and D).

We load the graft into an injector of a foldable IOL when it is ready for insertion, removing the injector spring to prevent back suction and inadvertent damage to the donor graft. The donor graft should be injected into the anterior chamber in a controlled fashion and positioned onto the recipient posterior stroma by careful, indirect tissue manipulation with air and fluid (Figure 2E). Once the lenticule unfurls, an air bubble should be injected underneath the donor graft lenticule to lift it toward the recipient posterior stroma (Figure 2F).

Stripping the tissue

Descemetorhexis can be performed “under air” injected in the anterior chamber or with fluid to better monitor removal of the recipient’s Descemet’s membrane and to examine the quality of the recipient’s stromal bed.

Trypan blue can be helpful during this part of the procedure because it can stain the endothelium and enhance its visualization during descemetorhexis.

Pre-cut donor tissue with the stromal interface exposed to preservation media is known to show higher detachment rates than fresh (unpreserved) tissue, which rarely fails to adhere.⁹

The tissue in PDEK is freshly prepared and easier to unroll due to the splinting effect of the pre-Descemet’s layer, requiring less manipulation which lessens the risk of graft detachment compared with pre-cut tissue.

Preventing insertion failures

Compression of the donor lenticule during insertion can be one of the most traumatic steps during the operation.^{10, 11} To obviate this, various techniques and devices have emerged to aid in insertion and subsequent unfolding of the donor tissue, including cartridge devices and custom-made injector devices.

In our PDEK series, we used a foldable IOL injector for donor graft insertion. The injector spring was removed, as Francis W. Price Jr., MD, had described,¹² to prevent any damage to the donor graft.

Although endothelial cell survival may have improved with advances in tissue harvesting and implantation techniques, minimizing iatrogenic endothelial cell loss continues to be a challenge in endothelial keratoplasty. PDEK offers the promise of further advances, and studies to evaluate endothelial cells loss by this approach are under way.

PDEK minimizes haze

Interface haze is always an issue with lamellar keratoplasties. Dr. Busin and colleagues demonstrated less interface haze in Ultra-thin (UT) DSAEK than DSAEK, with comparable results to DMEK grafts.¹³

This may explain the importance of involvement of lesser stromal tissue in the surgery. Deep anterior lamellar keratoplasty (DALK) and UT-DSEK have been associated with fewer interface problems than anterior lamellar keratoplasty and DSEK.

In DALK, when a large central bubble arises, we now know that the recipient pre-Descemet’s layer is retained and the donor button, stripped off its Decemet’s membrane and endothelium, also retains its pre-Descemet’s layer. Despite this, the interface in DALK performed by the big-bubble technique almost always clears.

Hence, retention of the pre-Descemet’s layer with the Decemet’s membrane and endothelium in PDEK probably will not be associated with increased risk of interface haze and could match that of DMEK.

Smaller grafts; younger donors

The main shortcoming of PDEK is that the donor graft is smaller than other techniques. Though PDEK transplants fewer endothelial cells than DMEK, PDEK allows the use of younger donor eyes and enables the higher cell counts associated with younger donors to be exploited to the patient’s advantage.

To aid in graft attachment, the air fill of the anterior chamber at the end of surgery is extended. This helps pressurize the eye at the end of the surgery and appose the donor tissue to the recipient posterior stroma.

The case for PDEK

PDEK entails the inclusion of the pre-Descemet’s layer in the donor graft, providing the benefits of DMEK, such as speedy visual recovery and overcoming the disadvantages of DMEK.

Pre-Descemet’s endothelial keratoplasty takes UT-DSEAK to a “thinner level” while retaining its advantages, but not requiring sophisticated instrumentation such as an automated keratome. Moreover, the pre-Descemet’s layer bubble is easy to create in young eyes (age 50 or younger). These eyes, with a relatively higher count of endothelial cells, can successfully enable endothelial transplantation in PDEK, which should be a distinct advantage in expanding the range of donor tissue surgeons can use with confidence for corneal transplantation. OM

REFERENCES

About the Authors
	Amar Agarwal, MD, FRSC, FRCOPTH, (top) is founder of Dr. Agarwal’s Eye Hospital and Eye Research Centre in Chennai, India.
	Priya Narang, MS, is founder of the Narang Eye Care & Laser Centre in Ahmedabad, India.
	Disclosure: The authors have no conflicts to disclose.