Pre Descemet's endothelial keratoplasty (PDEK) is a kind of endothelial keratoplasty, where the pre descemet's layer (PDL) along with descemet's membrane (DM) and endothelium is transplanted. Conventionally in a corneal transplantation, doctors use a whole cornea or parts of the five layers of the cornea to perform correction surgeries. In May 2013, Dr Harminder Dua discovered a sixth layer between the stroma and the descemet membrane which was named after him as the Dua's layer. In the PDEK technique, doctors take the innermost two layers of the cornea, along with the Dua's layer and graft it in the patient's eye.
Definition
The normal cornea (Fig 1) has from the front to the back the following layers: 1. Epithelium 2. Bowman's membrane 3. Stroma 4. Pre Descemets layer 5. Descemet's membrane 6. Endothelium For the human eye to see, the cornea or the front window of the eye should be clear or transparent. For that to happen the inside corneal layer the endothelium pumps out water from the cornea so that the cornea remains transparent and light can pass into the eye and one can see. If the endothelium is bad the cornea starts retaining water and gets damaged which is called bullous keratopathy.Thus PDEK helps in replacing the non functioning endothelium in bullous keratopathy. PDEK is different from the whole cornea transplantation in which the transplantation of entire donor cornea to the recipient is done. Normal corneal thickness is about 520 to 540 microns in the centre and 600 to 620 microns in the periphery. Pre descemet's layer which is dissected in PDEK, measures about 10.15±3.6 microns thick. The descemet membrane (DM) measures about 16±2 microns (range 13-20μ) thick and the normal endothelium is about 5 microns thick. Hence, the overall thickness of the PDEK graft will be about 32 to 44 microns.
History Prof Amar Agarwal (India) in 2013, September 4 performed the first PDEK surgery technique in collaboration with Prof Harminder Dua (UK) and showed the significance of the Pre Descemets layer in corneal transplantation. The initial surgery was performed for pseudophakic bullous keratopathy. Though donor eyes of all age group were used in the initial PDEK cases; there was marked difference in eyes with young donor corneas which resulted in better corneal clarity and visual outcome. This paved the way for the difference of PDEK using young donors and the importance of the endothelial viability.
Indications PDEK surgery can be performed in patients with decompensated cornea like pseudophakic bullous keratopathy, aphakic bullous keratopathy, congenital endothelial decompensation like Fuch's dystrophy of cornea and post traumatic endothelial decompensation.
Donor age PDEK graft can be harvested from donor of any age. Easy dissection of PDL layer in infant (less than 1 year), pediatric (1– 15 years) and young donor (15 –40 years) is an added advantage in PDEK procedure, which helps in transfer of viable endothelial cells with maximum regenerating capacity from this group of donors.
Recipient age PDEK graft is transplanted so far for adult patients who have lost vision due to endothelial decompensation meaning the Endothelium is not working.
Surgical technique Graft can be obtained from a dissected corneoscleral button taken from the deceased. The procedure is performed in sterile operating theatre setup. PDEK graft is prepared initially and kept in the storage medium (Optisol or MK medium) till the recipient bed is prepared.
Graft preparation
The donor corneoscleral rim (donor cornea) is placed on the eye mount with the endothelial side facing the surgeon. A 30 gauge needle attached to a 5 ml syringe filled with sterile air is passed from the limbus (edge of the cornea) into the mid stroma (middle of the corneal layers) with the bevel of the needle facing upwards. Once the needle is stable in the stroma, controlled air injection is performed. Numerous tiny air bubbles are seen cleaving the stroma and finally they coalesce to form one large bubble or Big bubble (BB) (Fig 2A). Once the type 1 BB is formed in the centre, it is gradually enlarged to maximum size which is about 8 mm. Following this, the bubble is pierced with a trephine or knife (Fig 2B) and the graft is dissected meticulously. It is then stained with a dye (trypan blue) and delineated well (Fig 2C, D). The dissected PDEK graft is then placed in a storage medium till the recipient (patient) is ready for transplantation.
Host bed preparation After obtaining the informed consent from the patient, local anesthesia is given. Anterior chamber entry (eye entry) is made by a blade in the superior corneo-limbus area. Descemet's membrane along with endothelium in the patient is removed mechanically by reverse Sinskey hook by controlled stripping on the endothelial side (Fig 3A-C).
Graft Injection into host bed
The PDEK graft which is already preserved in the storage medium is then placed in an injector (Fig 3A). The graft takes a shape of scroll immediately after dissection and the surgeons maintains the integrity or orientation of the scroll throughout the procedure. The injector along with the graft is inserted via the corneal wound and the graft is injected in the anterior chamber (Fig 3C). Initially the graft is made sure that it is oriented with endothelium down and PDL against the host stroma. This can be confirmed by using an endoilluminator because in eyes with corneal decompensation, the clarity is usually poor and the visualization of graft is affected under naked eye.8 once the orientation is confirmed, the graft is unfolded under saline (Fig 3D). Then the graft is attached to the overlying host stroma pneumatically (Fig 3E,F). Air fluid pressure is maintained inside the chamber for 60 seconds and then minimal air is released. The wound is closed with 10-0 monofilament suture and subconjunctival antibiotic steroid injection is given.
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