PDEK BUBBLE CHALLENGES

PDEK (Pre-Descemet’s endothelial keratoplasty) is a new endothelial keratoplasty technique described by Professors Amar Agarwal and Harminder S Dua referring to transplantation of Pre-Descemet’s layer (PDL; Dua’s layer), Descemet’s membrane (DM) and endothelium.

As discussed in EuroTimes Vol 19, Issue 5, p36, the big bubble formed on injecting air into donor cornea can be either Type 1 – the PDEK graft (well circumscribed, central dome-shaped elevation expanding towards the periphery; tough and resistant to tears and achieving a size of between 7 to 8.5mm); Type 2 – the Descemet membrane endothelial keratoplasty (DMEK) graft (poorly circumscribed bubble starting in periphery and expanding towards centre, achieving a size up to 10.5mm and thin-walled and delicate); or Type 3 – mixed, with both types forming.

This article discusses challenges and solutions for creating a good Type 1 bubble. Some personal techniques to avoid complications in bubble creation are described below. Uncontrolled sudden expansion of the bubble should be avoided to prevent bubble rupture. Multiple, small, well-defined, discrete bubbles are indicative of a Type 1 bubble forming.

These are expanded very slowly and carefully by either slow injection of air or by exchanging the air-filled syringe with one filled with storage medium/ viscoelastic while retaining the needle in its position, followed by slow, controlled injection. (Figure A)

Expansion should be done with care beyond 7mm and grafts larger than 8.5mm should not be attempted. With central endothelial perforation, a more viscous material is needed to prevent leakage from perforation and therefore viscoelastic (HPMC 2%) is preferred. (Figure B)

Older donors are more likely to form a Type 2 bubble. If a Type 2 bubble forms instead of Type 1, it can be prevented from expanding further by puncturing it and allowing air to escape. A small Type 2 bubble at the periphery can be sequestered from an expanding central Type 1 by applying pressure between the two with a thin blunt instrument and not allowing the Type 1 to meet it. (Figure C)

This prevents a split graft which can be very challenging to handle. A split graft generally occurs in older corneas and generally starts at one edge. With large splits, it may be more preferable to separate a DMEK graft and proceed with DMEK. (Figure D)

Bubble enlargement is directed towards the desired side by linear pressure on the endothelium on opposite side during expansion. Only endothelium beyond intended graft should be touched.

Mark Soper’s technique of first scoring the donor rim as for DMEK before injecting air also prevents Type 2
bubble formation.

PDEK graft preparation has advantages of not requiring special instruments unlike DSAEK (Descemet stripping automated endothelial keratoplasty).

PDEK does not induce hyperopia as stroma is not transplanted. It has advantages over DMEK in being able to use younger donor tissues with higher cell counts. DMEK graft preparation is difficult in donor eyes younger than 50 because of firm attachment of DM to PDL, however PDEK graft is possible in young donors of any age as air cleaves the plane between stroma and PDL.

PDEK graft is also tougher and more resistant to tears than DMEK. It is therefore easy to centre under pressurised air infusion with a reverse Sinskey hook after floating it without causing wrinkling or graft tears.

Disadvantages of PDEK are similar to DMEK and include the learning curve for preparing, unfolding and floating the graft. In the author's experience, there is also greater likelihood of Type 2 bubble forming in older corneas as compared to younger ones. Long-term results still need to be evaluated as well as the incidence of haze and rejection as compared to DMEK.

PDEK can be easily performed in combination with phaco in Fuchs’ dystrophy. Cataract surgery with intraocular lens (IOL) implantation is performed first. Care should be taken to avoid a posterior capsular rent as a vitrectomised, soft eye may not provide adequate postoperative air tamponade to support the PDEK graft. It is therefore more preferable to perform all cataract manipulations anteriorly closer to the endothelium than to the posterior capsule. Viscoelastic is completely removed after IOL implantation and this is followed by host Descemet’s stripping and PDEK.

It may also be performed for pseudophakic bullous keratopathy with a well-positioned IOL. In cases with widely dilated, atonic or floppy pupil, an iridoplasty may be required to prevent air from going behind the iris. In aphakic eyes, the ideal location of the IOL is in the sulcus with, if possible, an optic capture.

My personal second choice is to combine with glued IOL because of less pseudophakodonesis. Sclerotomies for haptic exteriorisation may be made slightly closer to the limbus than normal and an iridoplasty done in order to get a stable iris-IOL diaphragm to allow good postoperative air fill.

To determine the need for iridoplasty, the bubble test may be performed after IOL implantation by injecting air into the AC and checking adequacy of air fill. If air goes behind the iris, an iridoplasty should be done.

Once the PDEK graft is injected into the AC, my personal technique of endoilluminator assisted PDEK or E-PDEK helps keep all further intraocular manipulations to a minimum.

Oblique external illumination from a vitreo-retinal light pipe helps define intraocular structures and the PDEK graft with great clarity and three-dimensionality. This helps verify orientation of the graft in a non-touch manner. It also helps during the remainder of surgery by allowing better graft perception, thus making surgery easier and faster.

To conclude, PDEK is an effective alternative to DMEK for modern endothelial keratoplasty and offers distinct advantages over DMEK. Techniques for harvesting the PDEK graft are continuing to evolve in the form of automated graft preparation, femtosecond assisted dissection etc, which will result in increased safety and repeatability. Ready-to-order grafts from eye banks will ultimately make PDEK grafts easily available for the EK surgeon without fear of tissue loss during preparation.

* 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