Deferring PI in filtering surgery does not increase risks

By Cheryl Guttman

ORLANDO, FL - With small modifications in filtering surgery technique, both trabeculectomy and phacotrabeculectomy can be performed safely and effectively in many eyes without routine peripheral iridectomy (PI).

In a retrospective study, researchers at the John Moran Eye Centre, University of Utah, Salt Lake City, US reviewed all cases of trabeculectomy and phacotrabeculectomy performed by three glaucoma surgeons between January 1998 and August 2000.

They extracted data on patients' eyes aged 40 years and older diagnosed with primary open angle glaucoma, pseudoexfoliation glaucoma or primary angle closure glaucoma and a minimum postoperative follow-up of six months.

The investigators divided the eyes that underwent filtering surgery alone or combined with phacoemulsification into two groups according to whether or not a PI was performed. They then compared the data for development of complications and intraocular pressure (IOP) control.

In the phacotrabeculectomy group, PI was performed only if there was significant iris prolapse. There were 38 eyes in the series with a PI and 120 eyes without.
Among eyes undergoing trabeculectomy alone, PI was performed routinely in the presence of angle closure glaucoma, hyperopia, a short axial length (less than 22.5 mm) or if there was significant iris prolapse.

The study included 36 eyes that had trabeculectomy with a PI and 76 eyes that had the filtering surgery without a PI.
A 24-month follow-up analysis showed that safety and efficacy in both the trabeculectomy and phacotrabeculectomy groups was as good or better in eyes not undergoing PI.

There were no complications potentially related to lack of a PI, for example pupillary block or iris incarceration to the internal ostium.
Moreover, avoiding a PI reduced the risk of postoperative hyphaema and in the phacotrabeculectomy group was associated with less postoperative inflammation.
IOP control over time was generally comparable in eyes with as opposed to without a PI, although there was a trend for the filtering surgery success rate to be higher in eyes without a PI, Ike K. Ahmed MD reported.

Dr Ahmed reiterated that a PI has been considered an essential part of filtering surgery because it can relieve and even prevent pupillary block as well as prevent iris incarceration into the internal ostium of the trabeculectomy.
Furthermore, it was felt to be mandatory when filtering surgery was combined with cataract removal using intracapsular or extracapsular extraction techniques.

However, PI can potentially be associated with a number of complications and the question has been raised that it might even reduce the success rate of trabeculectomy and increase the risk of posterior capsule opacification (PCO) after phacotrabeculectomy secondary to disruption of the blood-aqueous barrier and induction of inflammation and flare.

"Our preliminary findings suggest that it may not be necessary to perform a PI routinely in eyes undergoing filtering surgery alone or combined with phacoemulsification and that it may in fact be beneficial to defer the PI in selected low risk eyes," he reported.

However, he cautioned that this study did not address eyes with secondary glaucoma. He said that a PI is likely to be needed when performing trabeculectomy for phakic eyes with angle closure glaucoma and in hyperopes with a shorter axial length.

In addition, Dr Ahmed emphasised that it behoved the surgeon to perform an iridectomy if significant iris prolapse occurs intraoperatively, if there is premature entry into the anterior chamber and in nanophthalmic eyes.

Rates of postoperative hyphaema and increased inflammation in the PI group were significantly higher than among eyes undergoing trabeculectomy without a PI.
Shallow anterior chamber was encountered more often in the non-PI group compared to eyes having surgery with a PI, although the difference was not statistically significant.

Dr Ahmed emphasised that when a PI is deferred, the approach to surgery must be modified using strategies that will increase the distance of the ostium from the iris and reduce the chance of early postoperative shallowing of the anterior chamber.
Those principles are illustrated by the technique used by the Moran Eye Centre surgeons.

The procedures were performed under topical or retrobulbar anaesthesia and began with the creation of a fornix-based conjunctival flap.
Then a 4.0 mm x 3.0 mm rectangular or trapezoidal scleral flap was fashioned with the dissection taken relatively far anteriorly into clear cornea, past the vascular arcades.

If indicated, an antimetabolite soaked sponge was then applied under the scleral and conjunctival flaps. After irrigation with a balanced salt solution, the anterior chamber was entered.

"Creation of a fistula anteriorly in clear cornea increases the distance between the ostium and the iris and creates a valve-like effect, reducing the likelihood of anterior chamber shallowing," Dr Ahmed explained.

Next, a Crozafon punch was used to create a 1.0 mm x 2.0 mm sclerostomy.
Two 10-0 nylon sutures were placed as slipknots and their tension was adjusted to allow a slow ooze from the scleral flap.

Careful intraoperative titration of aqueous flow through the scleral flap helps avoid early postoperative shallowing of the anterior chamber, and that goal can also be facilitated with the use of early suture lysis or releasable sutures, he noted.

Additional sutures were used if needed and the conjunctiva was closed with a running horizontal mattress suture using 10-0 Vicryl. Some patients were prescribed cycloplegics to use postoperatively for one week.

In eyes undergoing combined surgery, a two-site approach was used with implantation of a polyacrylic foldable posterior chamber IOL. Viscoelastic was removed from the anterior chamber after the scleral sutures were placed.

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