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Careful study of ocular anatomy can help surgeons avoid problems related to glaucoma procedures and achieve better results for patients.
This article was reviewed by Dr Kuldev Singh.
When performing glaucoma surgeries, considering slightly different twists to the procedure may help avoid potential complications. Dr Paul Palmberg, a professor of ophthalmology at Bascom Palmer Eye Institute at the University of Miami Miller School of Medicine in Florida, United States, shared his view of the ocular anatomy, which he said helps him to avoid pitfalls and increase patient comfort.
When considering the conjunctiva, Tenon’s capsule and the sclera, careful techniques can be performed to avoid limbal leaks and bleeding in the conjunctival flap. Dr Palmberg explained that the ocular tissues are not islands but are attached to each other (i.e., the conjunctiva is attached to the limbus at the edge of the cornea and Tenon’s capsule is attached approximately 1–2 mm further to the posterior). He reminded surgeons of the blood supply in the conjunctiva and blood vessels on Tenon’s capsule and the scleral surface.
Bleeding inside Tenon’s capsule in the bleb can be avoided if incisions are made only at the insertions of the conjunctiva and Tenon’s capsule. The best route is to get under Tenon’s capsule and not inside it.
Dr Palmberg noted that the capsule is thicker at the 10 o’clock to 2 o’clock and 4 o’clock to 8 o’clock positions. When starting a surgery, he advised using a lateral incision through the conjunctiva and Tenon’s capsule and everting the tissues so that the tips of the scissors can be positioned sufficiently far from the limbus; this facilitates getting under both the conjunctiva and Tenon’s capsule and down to the bare sclera to take a bite with a suture.
As the operation progresses, the surgeon can continue around, everting the conjunctiva and Tenon’s capsule. “The scissors are passing through the potential space under Tenon’s capsule all the way around,” Dr Palmberg said.
A radial incision then can be made at the side and spread back through the intermuscular septum to each side of the superior rectus muscle. Sponges soaked with mitomycin C (MMC) can be placed through the openings in the intermuscular septum and along the limbus to apply the MMC widely and facilitate extensive spreading of the drug.
The technique described was developed by Prof. Sir Peng Khaw, who saw a marked decrease in bleb problems in paediatric cases from 20% to 0%. He used the fornix flap with wide MMC application instead of a limbus-based flap with local MMC application.
“The limbus-based procedure resulted in a focal, highly elevated, thin and highly vulnerable bleb,” Dr Palmberg said. He added that, following his use of the fornix-based procedure, his bleb-related complications decreased from approximately 8% to 1% at 5-year follow-up.
Another advantage of the fornix-based procedure involves aqueous drainage through Tenon’s capsule on each side of the superior rectus muscle; because the fluid can drain much more freely through lymphatics in the conjunctiva and Tenon’s capsule further back, a much more comfortable bleb forms.
In contrast, when a limbus-based flap is created, scars form at the incisions in the conjunctiva and Tenon’s capsule—the so-called ‘ring of steel’ that blocks posterior drainage. “Wide application of the MMC under a fornix flap avoids a ‘ring of steel’: that is, scarring, which can delimit the bleb posteriorly,” Dr Palmberg said.
Dr Palmberg uses 10-0 nylon to make a bite immediately peripheral to the cornea, down through the sclera and across and upwards; he also closes each side with a bite in the sclera and up through the conjunctiva (square wave bites). He pulls tightly down on this to impale over the needle, comes back out and takes another bite that is tied down under the conjunctiva and run out to the tip, and then takes a bite at the apex and passes it back underneath, tying a knot that is buried under the flap.
He repeats the same process on the other side. “In this way, the vector forces in this area are pulling to the sides—but not across where the conjunctiva should balloon up—without compressing Tenon’s capsule,” he said. “Fluid then can flow [backwards] through Tenon’s capsule.”
The manner with which the bites are made in the sclera is not to be taken lightly; these are square wave bites and not skimming bites. A vertical bite should be taken down with 10-0 nylon, moved across, and then brought back up as vertically as possible through the conjunctiva and down, with the knot tied underneath.
“The needle should not be parallel to the surface as it enters the sclera, because this is a skimming bite that is shallow upon entrance and exit,” Dr Palmberg said. “This puts tension on the thin sclera on the two sides and eventually tears and loosens, all of which cause leaking blebs 3 days postoperatively.”
Using a valve-like trabeculectomy with MMC can help avoid hypotony in the early and late phases postoperatively, when sutures dissolve. A valve-like incision when possible, and/or the scleral flap resistance, can be adjusted intraoperatively to set the intraocular pressure at equilibrium flow.
In this technique, Dr Palmberg makes a 3-mm-wide tunnel incision out to 1 mm in the cornea and then uses a Kelly Descemet Membrane Punch 0.75 mm to make a canal in the tunnel. He checks the pressure at equilibrium flow by filling the anterior chamber with balanced salt solution. At equilibrium, the pressure is approximately 4 to 6 mmHg, which is estimated by pushing on the cornea with a 30-gauge cannula.
After two 10-0 nylon stitches are placed in the flap and the flow is at equilibrium, the cornea is pressed again, and the pressure is approximately 8 to 12 mmHg. This avoids hypotony early and late when the sutures dissolve and avoids the need for an iridectomy. Additionally, there is less astigmatism because the sutures in the flap are not tight, there is no cost to the procedure and there is no net visual field loss.