American Academy of Ophthalmology Web Site: www.aao.org
Clinical Update: Glaucoma
The Limbus, the Fornix and the Options for Trabeculectomy
Around eight years ago, Peng T. Khaw, PhD, MD, began publicizing the Moorfields Safer Surgery system, a technique for achieving good bleb morphology. The system was inspired by one of the first patients he’d treated with mitomycin C, a young child, who, after several years of well-controlled pressure, lost his sight after developing endophthalmitis from a thin avascular cystic bleb.
That child’s case was a turning point for Dr. Khaw, who said that though com plications had been accepted as an inevitable consequence of using MMC, “I felt we really had to improve the way we carried out filtration surgery, particularly with antimetabolites like mitomycin C.”
Through clinical observation and laboratory experiments, Dr. Khaw, professor of glaucoma and ocular healing at Moorfields Eye Hospital in London, hoped to discover why cystic blebs formed. He concluded that these blebs always had two features: 1) a ring of scar tissue surrounding the bleb, the so-called ring of steel, which restricts flow and creates a higher more cystic bleb, and 2) a point of anterior limbal drainage of aqueous.
Dr. Khaw’s Technique
To achieve healthier blebs, to reduce the tendency for blebs to migrate down to the cornea, and to avoid complications such as blebitis and endophthalmitis, Dr. Khaw changed his treatment technique. To prevent the ring of scar tissue, he uses a much broader application of MMC, rather than a smaller sponge of MMC, which others were advocating in an attempt to reduce complications from the antimetabolite.
Dr. Khaw also makes a larger scleral flap not cut all the way to the limbus, thus directing aqueous flow posteriorly. And he has converted completely to fornix-based flaps for good exposure of the sclera and to avoid a posterior scar restricting the posterior flow of aqueous.
Also, rather than manually cutting with a blade and scissors, he performs a sclerostomy with a 0.5-mm scleral punch. He said the result is a dramatic improvement of bleb appearance and a reduction from 20 percent to 0.5 percent in long-term bleb-related problems in high-risk patients. And for better postoperative aqueous flow and intraocular pressure control, Dr. Khaw uses an intraoperative infusion and adjustable sutures to secure the scleral flap.
Liberal MMC, tight closure. Both Alan S. Crandall, MD, and Garry P. Condon, MD, speak of a paradigm shift that occurred at the time Dr. Khaw began advocating the Moorfields system. “Some of the pictures Peng Khaw published made us rethink trabeculectomy technique,” said Dr. Condon, professor and chairman of ophthalmology and director of the glaucoma service at Allegheny General Hospital in Pittsburgh. The bottom line, he said, is creating a cut that allows for a diffuse application of MMC and the absolute, watertight closure of the fornix-based flap. “Without watertight closure in these cases, you shouldn’t be doing them,” he said. “You’re going to lose.”
Dr. Crandall said that when doctors first used MMC in the early ’90s, they applied it with one or two small sponges over a small surface area around the half-scleral thickness site. But late bleb leaks often occurred, and thinning blebs developed, and it was hard to achieve good pressures. “Then Peng rediscovered opening up a diffuse area and putting in eight or nine sponges,” said Dr. Crandall, professor and vice chairman of ophthalmology at the University of Utah in Salt Lake City.
Variations on a Theme
Conjunctival closure. The Moorfields Safer Surgery system hasn’t rendered all other surgical techniques obsolete. “There are as many ways to do trabs as there are surgeons,” Dr. Crandall said. “My closure, and variations of it, are still easily doable with the new shift.” Dr. Crandall’s conjunctival wound closure technique is designed to decrease the incidence of wound leakage following trabeculectomy. He leaves a small edge of conjunctiva adher ent to the limbus to facilitate conjunctival closure. Using a 10-0 vicryl vascular needle, he creates a running mattress suture to close the limbal conjunctiva in a routine fornix-based flap.
In 2001, Dr. Condon switched from limbus- to fornix-based flaps following a presentation by Dr. Khaw. “I did that with trepidation because the whole operation depends on watertight closure,” Dr. Condon said. But he has been convinced that fornix-based surgery produces a better bleb, with fewer complications.
Crimp and stretch. Today, to achieve watertight closure, Dr. Condon uses a modified technique proposed in the early 1990s by James B. Wise, MD, in which he leaves a lip of conjunctiva along the edge of the limbus. When making the anterior limbal incision, Dr. Condon leaves approximately 0.5 mm of limbal conjunctiva at the corneoscleral junction. Then he uses the Wise mattress suturing technique to attach the edge of the conjunctival flap directly to the corneoscleral tissue beneath this anterior lip of residual conjunctiva. The anterior lip acts as a bolster, reducing the tendency for early leakage.
All of Dr. Condon’s conjunctival and scleral suture bites are longer than any distance between them. This produces alternate crimping and stretching of the conjunctival edge, tightly applying it to the sclera. “I fell in love with this procedure because I had as predictable a watertight closure as I did with a limbal- based approach,” he said.
Dr. Condon stressed that he has modified his trabeculectomy technique by using the Ex-Press mini shunt (Optonol), which he inserts under the scleral flap in hopes of better outflow control and fewer hyphemas. The 400-µm wide by 3-mm long, stainless-steel device is placed in a microscopic opening inside the scleral flap, probed with a 27-gauge needle. The minimally invasive procedure avoids the need for an iridectomy and lowers the incidence of hyphema, Dr. Condon said.
The Case for the Limbus-Based Flap
For more than two decades, ophthalmologists have been debating which flap yields better results. “The disadvantage of limbal-based flaps is that visibility of the scleral area is more difficult because there is a large conjunctival flap present while you are doing the surgery,” said Dr. Condon.
But a substantial number of ophthalmologists still use the limbus-based approach, said Kuldev Singh, MD, MPH, who is among that group. “I have greater faith that my patient will not have an early postoperative wound leak with a limbus-based approach,” said Dr. Singh, professor of ophthalmology and director of the glaucoma service at Stanford University.
Pratap Challa, MD, assistant professor of ophthalmology at Duke University, is another limbus-based proponent. “The fornix-based approach tends to have more wound leaks postoperatively because it can be difficult to get the conjunctiva tacked down immediately after surgery,” he said.
Dr. Khaw acknowledged that concern. “The main reason that people don’t use the fornix-based approach is they are understandably worried about leaks, which you can see as the cut is at the limbus,” he said. “But with newer techniques of fornix-based flap closure, such as corneal conjunctival closure, or the short conjunctival frill method described by Alan Crandall, leaking is not really an issue.”
While leaks are not common when the fornix-based flap is properly closed, Dr. Singh said that when leaks do occur, the postoperative course is generally rocky and the long-term success of trabeculectomy is significantly reduced.
There are three key elements to Dr. Singh’s limbus-based technique:
Regarding the third point: He explained that incorporating the anterior Tenon’s capsule in the closure may cause the incision line to migrate anteriorly in the postoperative period, thus decreasing the size of the bleb and increasing the likelihood it will become cystic. On the other hand, incorporating the posterior Tenon’s capsule anchors the incision posteriorly and prevents the anterior migration associated with cystic blebs.
The fornix-based technique is appropriate in some circumstances, Dr. Singh said. If there is inadequate ocular exposure to allow an incision 10 mm posterior to the limbus, or if the surgeon is not comfortable with his closure technique, it might be better to take the higher (but not necessarily high) risk of wound leak age with fornix-based trabeculectomy.
And the limbus-based flap is not for everyone. It’s a more technically challenging approach and often requires an assistant, Dr. Singh said. But he said the greater technical difficulty is more than justified because it almost completely eliminates the risk of early wound leakage at the incision site.
Dr. Singh added that, for years, he has been applying MMC broadly over three to four clock hours extending approximately 10 mm posterior to the limbus, with limbus-based flaps.
Surgeon skill and meticulous attention to detail are more important than the type of flap one uses, said Dr. Singh.