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Nuances involved in creation of type 1 bubble in pre-Descemet’s endothelial keratoplasty
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Pre-Descemet’s endothelial keratoplasty is a variant of endothelial keratoplasty and involves the creation of a type 1 bubble with a 30-gauge needle attached to an air-filled 5 mL syringe. This procedure has a theoretical advantage of safer manipulation of tissue and greater ease of deployment than has been encountered during traditional Descemet’s membrane endothelial keratoplasty.
Type 1 bubble
A type 1 bubble is characterized by a central dome-shaped bubble with distinct edges and a diameter of about 7.5 mm to 8 mm that characteristically spreads from the center to the periphery. Its extension is limited in the extreme periphery by the adhesions between the collagenous layers. Creating a type 1 bubble is the key to PDEK. The most crucial part of the bubble creation is the level at which the 30-gauge needle is inserted. Normally, the needle enters in a bevel-up position from the rim of the corneoscleral button to the center of the corneal tissue, and the needle injects air in the plane beneath the pre-Descemet’s layer (Dua’s layer) and the stroma, leading to separation of the pre-Descemet’s layer-Descemet’s membrane-endothelium complex from the remaining stromal layer (Figure 1).
Figure 1. PDEK graft preparation. A 30-gauge needle attached to a 5-mL air-filled syringe with bevel-up position is introduced from the corneoscleral rim and up to the mid-periphery or the center of the cornea. Air is injected, and a type 1 bubble is formed (a). The peripheral rim of the bubble is punctured with a blade, and trypan blue is injected to stain the graft (b). The graft is obtained by cutting around the edges of the big bubble with corneoscleral scissors (c). The graft is loaded onto the cartridge (d).
Images: Narang P, Agarwal A
When inserted superficially, the needle is in between the pre-Descemet’s layer and the Descemet’s membrane. Often the plane at which the surgeon enters is correct, but non-maintenance of the same plane during the procedure often leads to misdirection of the needle toward the endothelium. Depending on the extent of change of plane, a type 2 bubble may accidentally be created, or in extreme cases, the endothelium may be perforated.
Type 2 bubble
When the air enters the plane between the pre-Descemet’s layer and Descemet’s membrane, a type 2 bubble, with a larger diameter than a type 1 bubble, is created and spreads from periphery to center. The PDEK procedure must then be converted into a DMEK procedure.
Figure 2. Viscoelastic-assisted PDEK graft preparation to seal the punctured endothelium. A 30-gauge needle is introduced, and as air is injected, small bubbles are formed (a). As more air is injected, the tip of the needle is seen to puncture the endothelium because the needle has become superficial and small bubbles are coming out of the endothelial rupture area. This is a sign of endothelium rupture (b). The syringe is withdrawn, the 30-gauge needle attached to a viscoelastic syringe is re-introduced, and the bubble creation is attempted again with the viscoelastic. A small puncture in the endothelium gets sealed with the viscoelastic and allows the formation of a type 1 bubble (c). A well-formed type 1 bubble made from viscoelastic is seen (d).
Figure 3. PDEK procedure converted into a DMEK procedure. A 30-gauge needle with an air-filled syringe is introduced. A type 1 bubble not created (a). A trephine marks the area of the endothelium to make the DMEK graft (b). Donor Descemet’s membrane is scored (c). Curled DMEK graft kept in storage media (d).
Perforation of the endothelium with the tip of the needle may lead to corneal tissue that can no longer be used and must be discarded. However, if the endothelium is perforated with the tip of the needle and the surgeon sees small air bubbles escaping, then the air injection should be withheld (Figure 2). A cannula attached to a syringe filled with viscoelastic is re-introduced to form a bubble. In other words, the bubble now is created with viscoelastic. The viscoelastic material plugs the small hole in the endothelium and facilitates creation of the type 1 bubble (Figure 2).
When a type 1 bubble cannot be created, then it is necessary to convert to a DMEK procedure (Figure 3). This helps avoid discarding the donor tissue. For this conversion of PDEK preparation to DMEK, an 8- to 8.5-mm trephine is used to make a mark in the endothelium. A stripper then begins to strip the endothelium from the Dua’s layer. Finally, one gets a DMEK graft ready.
Occasionally, a small bubble is formed after air injection. In such a scenario, viscoelastic can facilitate the breakdown of superficial adhesions between the pre-Descemet’s layer and the stromal layer, as it tends to overcome resistance offered by the collagenous layer better than air.
Despite all the surgical challenges in creating a bubble, surgical competence and the art of handling the corneal tissue effectively save the loss of corneal tissue, render economic practicality and enhance productivity by retaining the clinical merits of the PDEK procedure.