How to optimize refractive outcomes after trab, tube shunts

By Laird Harrison

Careful attention to detail can optimize refractive outcomes in trabeculectomy and large aqueous shunts, reported Cynthia Mattox, MD.

Dr. Mattox, director of the Glaucoma and Cataract Service, New England Eye Center, Tufts University, Boston, described state-of-the art techniques for the procedures.

Cynthia Mattox, MD, described state-of-the art techniques to optimize refractive outcomes in trabeculectomy and large aqueous shunts procedures.

“Frankly, not only do our patients have higher expectations of what their postoperative outcomes are going to be, we have our own high expectations,” she said.

The first step is to evaluate carefully the refractive status of both eyes, she said. Does the patient have cataracts or astigmatism? High myopia or hyperopia?

Next, consider the current level of IOP, and the desired level of postoperative IOP. This is particularly important in a combination of phacoemulsification with trabeculectomy, because going from a very high to a very low IOP will shorten the axial length, said Dr. Mattox.

“So for patients where you’re planning to go with a very low pressure postoperative in a combined procedure, you may want to pick the lens implant to be a little bit on the myopic side and you’ll probably end up just right,” she suggested.

Case emphasizes point

She gave the example of a 70-year-old female with advanced primary open-angle glaucoma and IOP at 18 mm Hg, who was continuing to lose vision. The patient had a 20/40 cataract and a pre-existing astigmatism of 4 D against the rule.

The patient opted for a toric IOL. “I felt it was best to do a trabeculectomy first, so that I could plan a sequential phaco,” she said. This gave Dr. Mattox the opportunity to use an intraoperative aberrometer.

“It takes longer,” she said.  “It’s a slower course. But the patient is ultimately going to be happier now that you’ve dealt with her astigmatism. It also allows you to do it sequentially, so you can adjust for any induced astigmatism that occurred from your trabeculectomy.”

On the other hand, if physicians are contemplating cataract surgery after a trabeculectomy, it’s important to assess the bleb first, she said.

“There are things that will clue you in to the fact that maybe that bleb is not as functional as it used to be,” Dr. Mattox said. “That changes your counseling for the patient and what you might do.”

Causes for concern about the bleb include:

• A short interval between the trabeculectomy and the phaco.

• The patient’s return to multiple medications after the trabeculectomy.

• The bleb appears low, fibrotic, or localized.

• Bleb needlings were required after the trabeculectomy.

Another consideration in doing a phaco after a trabeculectomy is that the IOP may rise. The mean increase is 3 mm HG.

“That’s certainly tolerable if your current pressure is at a low level and the target pressure is still attainable, but it still may be a surprise,” Dr. Mattox said. “It’s really important to counsel patients that their bleb may become less functional after your cataract surgery.”

It’s also important to counsel patients that their vision may not recover swiftly. In one study, 64% of eyes lost 3 or more lines of vision after trabeculectomy. The eyes typically recover in two to three months, with a median of one month. However, patients who have very advanced disease may not regain their vision for a longer period. About 2% had severe permanent vision loss, Dr. Mattox pointed out.

Small study segment

So why do patients have slow recovery after trabeculectomy? “We have only small studies to look at,” said Dr. Mattox. “The general consensus is that there’s steepening in the surgical meridian that resolves and then relaxes within three months.”

Some studies suggest that this initial with-the-rule astigmatism will relax over time, even sometimes to against-the-rule, she said. The superior cornea actually becomes flatter in somewhat of an irregular way, which may be related to surgical technique.

Dr. Mattox shared some of her tricks to minimize astigmatism:

• Use minimal, but effective, cautery.

• Create a small sclera flap at 12 o’clock. She prefers a tunnel to a free-hand dissection on the belief that this will result in greater stability. She makes side cuts to convert the tunnel to a limited flap, stopping the cuts posterior to the limbal ring. She noted the need for a larger scleral flap if an Ex-PRESS shunt (Alcon Laboratories) is used.

• Use a small punch for a sclerostomy into peripheral cornea.

• Use two sutures in her flap, then tensions them equally to avoid astigmatism. She then closes the conjunctiva with the Condon closure, which distributes the forces across the limbus.

With large aqueous shunts, she noted, the incidence of diplopia is about 5%. “So if you have a binocular patient, it’s important to counsel them that this is a possibility,” she said. “Fortunately, most of it is correctable with prisms and surgical correction is really fairly rare.”