Pre-Descemet's Layer

Descemet’s membrane endothelial keratoplasty (DMEK) is now considered the gold standard keratoplasty procedure for eyes with endothelial damage. However, pre-Descemet’s endothelial keratoplasty (PDEK) has now emerged as a promising alternative.

PDEK uses a graft comprised of Descemet’s membrane (DM) and endothelium splinted by the pre-Descemet’s layer (PDL or “Dua’s layer”), which was discovered by Harminder S Dua MD, PhD, Professor of Ophthalmology, University of Nottingham, UK.

Speaking at the 6th EuCornea Congress in Barcelona, Spain, Dr Dua reviewed outcomes in an initial PDEK case series demonstrating viability of the new technique. In addition, he described potential advantages of PDEK and new instrumentation for PDEK graft harvesting designed to overcome one of the challenges of this new procedure.

The outcomes paper reported on five eyes, reported by Dr Dua with Amar Agarwal MD, Chennai, India (Agarwal A et al. Br J Ophthalmol. 2014;98(9):1181-5). Preoperatively, best corrected visual acuity (BCVA) ranged from 0.02 to 0.17 and central corneal thickness was between 604μm and 790μm.

Postoperative follow-up showed successful graft attachment, and no interface abnormalities. Corneal clarity was achieved with good visual acuity, and at one month after surgery, corneal thickness values ranged from 508μm to 591μm.

POSSIBLE BENEFITS

Compared with the DMEK graft that is comprised of DM and endothelium only, the presence of the PDL in the PDEK graft improves handling ease, said Dr Dua.

“The additional 10μm to 20μm of stromal support from the PDL reduces the scrolling tendency of the Descemet’s membrane, making the PDEK graft easier to unroll in the eye. It is also easier to centre the PDEK graft, which can be done by stroking the graft on the PDL with a blunt spatula,” said Dr Dua.

Endothelial cell survival is also expected to be better after PDEK, in part because much of the endothelial cell loss after DMEK is attributed to the time and handling spent with graft unscrolling. In addition, findings from a study performed by Dr Dua and colleagues measuring endothelial cell density before and after PDEK and DMEK grafts by pneumodissection suggest there may be better endothelial cell survival after preparation of the PDEK lenticule. (Altaan SL, et al. Br J Ophthalmol. 2015;99(5):710-3)

“This was an ex vivo study, and the difference between PDEK and DMEK was not statistically significant. However, we can say that corneal endothelial cell loss after tissue handling and processing is at worst the same for PDEK compared with DMEK and possibly lower for PDEK,” Dr Dua said.

He noted that the opportunity to use younger donors, which bring higher endothelial cell counts, is another advantage of PDEK compared with DMEK. As reported by Dr Agarwal, PDEK has been performed successfully using tissue from donor eyes of infants as young as nine months. (Agarwal A et al. Cornea. 2015;34(8):859-65)

“In contrast, most surgeons performing DMEK prefer using tissue from donors older than age 50 because of the strong adhesion of the DM to the underlying stroma (PDL) and increased risk of tearing,” Dr Dua said.

Whereas a large graft, up to 9mm in diameter, can be obtained when harvesting the DMEK lenticule, PDEK graft size is limited to 7mm to 8.5mm which represents the maximum diameter of a Type 1 bubble. If the Type 1 bubble that forms does not expand to the desired diameter, the margins can be extended using a blunt spatula because the PDL offers a surgical cleavage plane. Formation of a Type 2 bubble, however, necessitates conversion to DMEK graft harvesting.

Aiming to avoid the latter situation, Dr Dua has been working to develop instrumentation for consistently achieving a Type 1 bubble. He reported that a third prototype device was in final testing and expected to become commercially available by the end of 2015.

“This instrument should almost completely stop a Type 2 bubble from forming and should give users greater confidence to perform PDEK because they will know they can obtain the tissue they need,” he said.

harminder.dua@nottingham.ac.uk