Pre-Descemet’s endothelial keratoplasty (PDEK) is a new type of endothelial keratoplasty (EK) procedure where the surgeon uses the Pre-Descemet’s layer (PDL) or Dua’s layer. The surgeon separates the Descemet’s membrane endothelium complex (DM-endothelium complex) and PDL from the remaining donor stroma using a type 1 bubble. The procedure resembles Descemet's membrane endothelial keratoplasty (DMEK), one of the differences being that younger corneas, including those of infants, can be used.
PDEK is gaining popularity because of the global shortage of corneas. The use of younger corneas means increased availability of corneas. The use of younger corneas also means increased endothelial cell density leading to longer graft survival periods.
The success of a PDEK depends on the use of air dissection and air bubble techniques. Air dissection involves using a syringe filled with air to inject the stroma with the endothelial side up. Air dissection is cost-effective because the surgeon does it manually. However, it requires a skilled surgeon since the injection needle is introduced at the correct depth beneath the DM-endothelium complex.
Before the Procedure
Three types of bubbles can be created in the donor graft preparation stage. The dome-shaped Type 1 bubble is the desired one, extending from the centre to the periphery. Type 2 bubble extends from the periphery to the centre and is formed with the entry of the air into the plane between the DM-endothelium complex and PDL. Type 2 bubble further extends up to the extreme periphery due to the absence of adhesions between the DM and PDL. It is often recommended that if type 2 bubble is formed, then the surgeon should proceed with a DMEK procedure instead of PDEK. Type 3 bubble is a challenge to the surgeon because it requires to be handled and manipulated delicately to avoid rupturing the bubble.
The concern of PDEK surgery is that the surgeon creates a good type 1 bubble from the beginning. The first phase involves proper donor graft harvesting, where a type 1 bubble is created. The surgeon places the corneo-scleral rim with the endothelial side up. S/he introduced a needle attached to a syringe filled with air from the rim into the corneal stroma’s mid-periphery. The surgeon injects in the air to form a type 1 bubble. S/he uses a special blade to puncture the bubble's edge and injects trypan blue inside the bubble to stain the graft. The surgeon cuts the graft all around it to harvest it.
After the patient has received either local or general anesthesia, the surgeon begins to prepare the patient’s bed and transplants the tissue. The procedure essentially is the same as in other EK procedures. To prepare the bed, the surgeon:
- Makes a paracentesis and injects air into the anterior chamber
- Uses a Sinskey hook to perform a descemetorhexis to strip and remove the patient’s diseased DM-endothelium complex
- Loads the donor graft onto a foldable intraocular lens’ cartridge and injects it into the anterior chamber
- Uses fluids and air to unroll the donor lenticule
- Ensures the graft is settled well before s/he injects air under the graft so that the graft adheres to the recipient’s bed
- Uses a suture to seal the corneal incision
Risks & Complications
The risks and complications depend on the surgeon’s skills and experience.
Intraoperative complications may include:
- A small graft which can result in graft failure
- Reverse graft unfolding with the graft curled away from the host cornea
- Bubble burst during pneumatic dissection caused by the surgeon pushing excess air into a tiny space
- Failure to create a type 1 bubble due to the surgeon’s failure to reach the correct plane of dissection. This failure can lead to double bubble formation. It also means that the procedure must be converted to a DMEK surgery
Postoperative complications soon after the procedure may include:
- Loss of air bubble
- Fibrin or sterile hypopyon
- Hyphema which is usually mild and will resolve spontaneously
- Descemet’s folds due to excess tissue manipulation that occurred during the procedure
- Graft detachment when the pressure inside the chamber is insufficient for graft adhesion
- Lenticule drop which occurs due to insufficient chamber pressure or hindrances to graft attachment
- Ocular hypertension in eyes without preoperative signs of glaucoma caused by the high-pressure bubble filling the anterior chamber
Late postoperative complications may include:
- Graft failure
- Graft rejection
- Graft-host interface
- Corneal epithelial changes
- Intraocular lens opacification
Aftercare & Recovery
After the procedure, the patient should lie in a supine position (lie flat) for about three hours. It is also advisable for the patient to continue lying supine for most of the day or longer. The surgeon uses a slit lamp exam to ensure that the graft is centred and well-located.
Topical antibiotics and corticosteroids will be prescribed and gradually tapered over four months.
The first follow up visit is scheduled for the first day after the procedure. The patient will then be seen on the second day of the procedure where intraocular pressure is assessed. During this second visit, the surgeon also confirms the patency of the inferior iridectomy. An anterior segment optical coherence tomography is used at every follow-up to check graft positioning.
PDEK is an effective technique to help patients with a compromised endothelium to achieve good visual rehabilitation. It allows the use of young donor tissue that’s usually not possible with other EK procedures.