Viscoelastics in Glaucoma Surgery
Which OVDs for which reasons?
BY ROBERT NOECKER, M.D.

Many ophthalmologists might associate the use of ophthalmic viscosurgical devices (OVDs) with cataract surgery, but when used properly, OVDs can be a valuable tool for increasing the success and safety of glaucoma surgery, as well. As in cataract surgery, it is important to first protect intraocular structures from damage that may occur during the procedure. Compared with cataract surgery, glaucoma procedures tend to be more variable and complicated, especially in a teaching situation or where an accomplished anterior-segment surgeon may be less familiar with the potential pitfalls inherent in glaucoma surgery. The use of OVDs can provide an extra degree of comfort and safety in these complicated procedures.

The use of OVDs can make glaucoma surgery safer during potential periods of hypotony. The utilization of newer intraocular procedures allows viscoelastics to greatly enhance visualization for placement of stents and probes into the angle and ciliary sulcus. However, at the same time, we must recognize that the optic nerves of glaucoma patients are much less tolerant of IOP spikes due to OVDs; therefore, close attention must be paid to the ease the removal of some of these compounds.

When using OVDs, the first step is understanding where and when to take advantage of the different viscoelastic properties; this is essential to help with your surgery. The next decision is which type of OVD to use in each situation to maximize effectiveness and safety. As someone who teaches novice surgeons glaucoma surgery, I take a conservative approach and emphasize safety first.

Breakdown of Glaucoma Surgeries

Advice for specific procedures:

Trabeculectomy. OVDs can help at several times throughout the procedure. I typically use a 6-0 vicryl corneal fixation suture; there are times when the globe may be soft or there is a perforation of the cornea and early placement of an OVD in the anterior chamber may be desirable. Also, early placement of OVD to create a firmer globe is helpful during dissection of the scleral flap, especially in a setting where the flap is thin -- either from prior surgery or the normal makeup of the eye.

In a typical case where early placement of the OVD is not necessary, I take the time after the antimetabolite is placed on Weck cell sponges underneath the conjunctiva to create a paracentesis. I create as small a temporal paracentesis as possible with a 15-degree blade beginning at the 2 o'clock position and aiming toward the 6 o'clock position. This is important especially in phakic eyes with shallower anterior chambers in which we want to avoid creating central lens trauma, which may impair the patient's vision. In the uncomplicated setting, I prefer to use a lower molecular weight cohesive OVD (around 0.2 ccs), which can easily be removed later. In this part of the procedure, the OVD serves several purposes. It helps to protect the corneal endothelium after the anterior chamber shallows following entry underneath the flap with the Descemet's punch. An OVD also helps to move the lens and iris posteriorly, thus minimizing the possibility of inadvertent trauma with the 15-degree blade or punch. Lastly, OVDs help to blunt the sudden drop in IOP that occurs during sclerostomy creation, minimizing the chance of suprachoroidal hemorrhage.

The most dangerous time during trabeculectomy is the period in which the sclerostomy has just been created and before sutures are secured on the scleral flap. At this time, it is possible to have the pressure drop to zero quickly and have the eye partially collapse. For this reason, I typically place a cohesive viscoelastic into the eye prior to performing this step in the operation. I create a temporal paracentesis and inject a small amount of OVD (around 0.2 ccs) into the anterior chamber to prevent collapse when the sclerostomy is created. This is especially important in eyes that have had prior vitrectomies, as the risk of suprachoroidal hemorrhage is significantly higher in these patients. I prefer cohesive viscoelastics in these cases because they can be easily removed from the eye through the paracentesis or the sclerostomy at the end of the case. In cases where we are worried about postoperative hypotony, we may choose to leave some viscoelastic in the eye with the option of removing it easily the next day in the office, if necessary.

Viscocanalostomy-deep sclerectomy. In these procedures, after the deep sclerectomy is performed, the viscoelastic is used to inflate Schlemm's canal and take up space. Sometimes the OVD can be visualized in the episcleral vessels confirming proper placement in the canal. In these cases, I prefer to use a higher molecular weight cohesive OVD because it tends to remain in place for a longer time, helping to maintain the space in the scleral pocket underneath the flap. This, in turn, appears to increase the success of surgery by preventing early adhesions from developing. While we do not plan to enter the anterior chamber during these procedures, having the OVD available can help to pressurize the eye and aid in dissection if necessary.

Endocyclophotocoagulation (ECP). Good visualization of the ciliary processes is essential in this procedure. Optimal exposure improves the ability to get a complete treatment of the ciliary processes, as well as protect other structures that should not be damaged. I typically perform ECP through one or two clear-cornea incisions. After the incision is made, I inflate the sulcus with a viscoadaptive OVD such as Healon5 (AMO). I prefer this type of OVD because there is a fair amount of manipulation of the incision site while performing the laser treatment, especially when treating the extreme ranges of the probe. I find that Healon5 is the best OVD in terms of displacing the iris anteriorly to provide maximum inflation of the ciliary space. By displacing the iris as far away form the ciliary processes as possible, the chance of undesirable iris burns is minimized. Also, there is minimal bubble formation to impair visualization of the ciliary processes during laser treatment. Healon5 is also helpful in breaking synechiae, which may impair access to and visualization of the ciliary processes.

Due to its viscoadaptive nature, Healon5 is also relatively easy to get out of the eye, which is important in ECP because the IOP-lowering effect of the treatment is not immediate. Therefore, there is a higher risk of IOP spikes if all of the viscoelastic is not removed. In cases of aphakic glaucoma and pediatric glaucoma, I choose to use an anterior-chamber irrigating cannula continuously infusing the eye with balanced salt solution to maintain pressure during the procedure. These eyes are at high risk for hypotony and collapse during the procedure. The infusion makes these cases safe and provides for excellent visualization and thorough treatment opportunities. However, it is still important to protect anterior-segment structures (i.e., the cornea and the endothelium) from possible damage. In these cases I choose to place a dispersive viscoelastic to coat the endothelium before the procedure. These OVDs tend to remain on the corneal endothelium throughout the case, despite copious irrigation.

Setons. The use of viscoelastics can be especially helpful during the tube insertion step of this procedure. Immediately before creating the sclerostomy with a 23-gauge needle, I create a temporal paracentesis (which is helpful to have present postoperatively for access to the anterior chamber) as in a trabeculectomy and inject 0.2 ccs of OVD. The OVD helps to push the lens-iris diaphragm back to allow tube placement as posteriorly as possible, and avoids iris trauma and incarceration. I am careful not to over inflate the eye as this can actually impair tube placement until the pressure is reduced and some OVD is removed. In a case for which a valved seton is used, it is important to use a lower molecular weight cohesive OVD because some of OVDs cause clogging of the valve and malfunction. In cases for which I use a valved seton, I try to irrigate the viscoelastic from the eye at the end of the case to avoid this complication.

In the case of nonvalved setons like the Baerveldt shunt, one does not have to worry about valve malfunction as I typically occlude the tube with a 3-0 prolene-stent suture and 8-0 vicryl-ligation suture in the immediate postoperative period. Also, in cases in which we are worried about leakage around the tube early in the postoperative period or in situations where a pre-existing trabeculectomy site may leak, we may choose to leave the OVD inside the eye (i.e., eyes with prior vitrectomies or aphakic eyes) to prevent hypotony and suprachoroidal hemorrhage.

Case Study

A 78-year-old phakic patient was referred to undergo placement of a seton to treat elevated IOP due to neovascular glaucoma after a failed trabeculectomy. Preoperative IOP was 38 mm Hg on maximal medical therapy, and there was extensive peripheral anterior synechiae in the angle, but no active neovascularization. Visual acuity was 20/80 in the presence of mild, nuclear-sclerotic cataract. The decision was made to place a Baerveldt glaucoma implant superotemporally into the anterior chamber. Immediately after the conjunctival peritomy, the old trabeculectomy site began leaking and a paracentesis was created and an OVD was placed in the eye to prevent hypotony. The case proceeded normally until iris trauma and some bleeding during the tube placement complicated placement of the tube. An OVD was once again used to move the iris/lens more posteriorly, and tube placement continued unimpaired. Because of the leak, the OVD was left in the eye. On postop day one, the pressure was 10 mm Hg, with no anterior chamber shallowing or choroidals present, but there was some conjunctival elevation consistent with some leakage from the pre-exisitng scleral flap. Postoperatively on day 8, IOP was 22 mm Hg and there was no evidence of anterior leakage.

OVDs play an important role in optimizing outcomes and improving safety in incisional glaucoma surgery. I use viscoelastics in every glaucoma procedure that I perform. They help to both improve access to intraocular structures and minimize hypotony both intraoperatively and postoperatively. I think every glaucoma specialist should have viscoelastics in their operating rooms. I always have OVDs available both during surgery and postoperatively to help minimize treatment complications and prevent problems.

We are lucky to have such a wide variety of viscoelastics at our disposal with different physical properties; this makes each one optimal to use in different situations, as well as being able to use certain ones together for better outcomes.

Robert Noecker, M.D., M.B.A., is vice chair and director of the glaucoma service at UPMC Eye Center at the University of Pittsburgh, Pittsburgh, Pa. He can be reached at (412) 647-2152 or by e-mail at noeckerrj@upmc.edu. Dr. Noecker has received research support from Advanced Medical Optics, Inc. in the past year, but has no financial interest in any of the products discussed in this article.