EXCELLENT VISIBILITY

Corneal transplantation for endothelial decompensation is shifting predominantly to endothelial keratoplasty in suitable patients. Descemet stripping automated endothelial keratoplasty (DSAEK) has received widespread acceptance amongst corneal surgeons mainly because of ease of surgery and ability to procure prepared tissue from eye banks. However, Descemet membrane endothelial keratoplasty (DMEK) though better in terms of visual acuity achieved and decreased risk of graft rejection has not yet become widely practised. This is mainly because of greater difficulty in graft preparation and the more challenging nature of surgery as far as graft unrolling, orientation and flotation are concerned.

DMEK also has the disadvantage of inability to harvest grafts from donor corneas less than 40 to 50 yrs old because of the high probability of damage to the graft while harvesting. Pre-Descemet endothelial keratoplasty (PDEK) is a new technique for endothelial keratoplasty recently introduced by Profs Amar Agarwal and Harminder Dua which combines many of the advantages of DSAEK and DMEK while avoiding many disadvantages that each individually has.

CORNEAL ANATOMY
To understand this new technique better, let us take a re-look at some previous reports as well as at corneal anatomy. Air separation of a DMEK graft has been reported earlier by Busin et al and Jafarinasab et al reported additional tissue as “residual stroma” attached to Descemet’s membrane when separation is caused with injection of air in deep anterior lamellar keratoplasty.

Prof Dua presented evidence regarding a distinct pre-Descemet’s layer (PDL) of tissue in 2007. The cornea was described as having six layers instead of the conventionally accepted five layers. A new fourth layer was introduced called the Dua’s layer or the PDL. In DMEK, only the Descemet’s membrane and healthy donor endothelium without the PDL are transplanted onto the recipient cornea. PDEK also includes the PDL as part of the grafted tissue along with the Descemet's membrane and endothelium.

The PDEK graft is prepared by injecting air into the donor cornea to induce separation between the stroma and PDL by creating a Type 1 Big bubble. This is in a manner very similar to the Anwar's Big Bubble created for DALK except air is injected from the endothelial side of the corneo-scleral rim just outside the limbus.

In PDEK, the objective is to retain the PDL with the endothelium-Descemet’s membrane complex, thereby providing additional support to the graft tissue used for the procedure. The presence of this layer, with its characteristics of relative rigidity and toughness, allows easy intraoperative handling and insertion of the tissue because it is not as flimsy and does not tear as easily as the Descemet’s membrane alone.

Profs Agarwal and Dua said: "The most popular Descemet’s membrane-baring technique is the big bubble method, in which the big bubble forms a cleavage plane, leaving the Descemet’s membrane bare for the dissection in lamellar keratoplasties. The Descemet’s membrane is truly laid bare only when a type 2 (pre-Descemetic) bubble is created between the PDL and the Descemet’s membrane. In the PDEK procedure, a type 1 big bubble, which typically lies between the PDL and the posterior corneal stroma, is formed, thereby creating a dome of PDL-Descemet’s membrane-endothelial complex above the air bubble. Including the PDL with Descemet’s membrane in endothelial keratoplasty would add tissue rigidity and potentially facilitate the procedure."

CLASSIFICATION OF BIG BUBBLE
When air is slowly injected with a 30-gauge needle attached to a 5ml syringe inserted from the limbus into midperipheral stroma, it can form either a Type 1 or Type 2 big bubble. Type 1 big bubble is a well-circumscribed, central dome-shaped elevation measuring 7mm to 8.5mm in diameter (Figure 1A) which always starts in the centre and enlarges centrifugally, retaining a circular configuration. Type 2 big bubble appears as a larger bubble expanding to fill the space between the Descemet's membrane and the Dua's layer. Sometimes, a combination of Type 1 and Type 2 bubble can be obtained.

Donor preparation: After achieving a Type 1 big bubble, trephination of the donor graft is done along the margin of the big bubble. The bubble is entered at the extreme periphery, and trypan blue injected into the bubble to stain the graft. The PDEK graft is then cut around the trephine mark with a pair of Vannas scissors and covered with tissue culture medium. The graft is loaded into an injector when ready for insertion. Once the donor is prepared, the rest of the surgery is similar to that of DMEK, wherein the host Descemet's membrane is stripped and the PDEK graft injected into the AC, oriented, unrolled, opened up and floated against the stroma with an air bubble. The Endoilluminator assisted DMEK (E-DMEK) technique which has been described by the author (SJ) and was discussed in the last issue of this series is translated into this surgery as Endoillumintaor assisted PDEK (E-PDEK) (Figure 1B, C).

Says Prof Agarwal: "E-PDEK is an extremely useful technique for achieving excellent visibility in these cases and helps me immensely in being able to be certain about my graft orientation and positioning during the entire surgery just as it also helps in E-DMEK." At the end of surgery, light perception and IOP are checked for and the patient maintains a face-up position for 24 hours.

"PDEK thus provides the benefits of DMEK, such as speedy visual recovery (Figure 1D) while overcoming the disadvantages posed by DMEK. PDEK takes ultra thin-DSEK to a “thinner level” while keeping its advantages, at the same time getting rid of the requirement for sophisticated instrumentation and keratome. Spectral-domain optical coherence tomography in vivo analysis of PDEK grafts showed mean graft thickness after one month to be 28 ± 5.6 μm, which is larger than the conventional DMEK graft and less than the ultra-thin DSAEK graft. It also gives a faster visual recovery," Profs Agarwal and Dua said (Figure 1E).

* Dr Soosan Jacob is a senior consultant ophthalmologist at Dr Agarwal's Eye Hospital, Chennai, India and can be reached at: dr_soosanj@hotmail.com