Proper Pars Plana Placement of Glaucoma Shunts

Success requires teamwork between two skilled surgeons

BY ANDREW RABINOWITZ, MD

Given a significant boost by the data released last year in the important Tube vs. Trabeculectomy (TVT) study, glaucoma shunts are now being viewed as more of a front-line option in glaucoma surgery. Though glaucoma specialists are more familiar with anterior placement of these shunts, excellent results can be achieved in certain types of cases with the more demanding but highly rewarding pars plana placement performed in conjunction with vitrectomy.

Appropriate techniques for placement of the three widely used aqueous shunts (Baerveldt, Ahmed and Molteno3) in the anterior chamber have been well described in the literature. Similarly, observations and recommendations regarding what to expect in the peri- and postoperative timeframe with anterior placement have also been well documented. However, placement of these devices in the eye through a pars plana site requires two highly skilled surgeons, as well as an awareness of many pearls and pitfalls that have not been as abundantly described in the literature.

Pars plana placement is, in my opinion, a series of unique surgical procedures with only limited commonality with standard anterior segment placement. Indeed, it is two unique surgeries performed in one setting by both a retinal specialist and a glaucoma surgeon working together. Each surgeon has to be at the cutting edge of his or her specialty to provide the patient with the greatest degree of safety and probability of a successful outcome. This article will strive to share with the reader my observations and suggestions regarding pars plana placement of aqueous glaucoma shunts.

Pars Plana Placement: Why and When

We should consider pars plana placement of shunts when there are contraindications to placement of the tube into the anterior chamber. First and foremost, patients with active or quiescent corneal pathology are not good candidates for anterior chamber placement. Published studies have shown a decrease in endothelial cell counts in patients who have previously undergone tube placement into the anterior chamber.

Optimal pars plana placement of the Baerveldt device.

Experience has taught me that in patients with moderate to severe rheumatologic disorders there is a significant chance of the conjunctiva, Tenon's and a scleral or pericardial graft melting at the region of the limbus overlying the tube's insertion into the anterior chamber. This “melting” can occur as rapidly as six months and as remote as decades following the initial surgery. Patients whose rheumatologic status predisposes to inflammatory disease should not have tubes placed in the anterior chamber. In fact, when evaluating a potential shunt candidate, I look at their hands. If there is moderate to severe disruption of the joints in the hands, I do not advocate anterior chamber placement. In these cases, I recommend pars plana placement.

Additional indications for pars plana placement include patients with florid rubeosis, as well as patients with anterior chamber inflammatory conditions such as iritis. A further advantage to the patient is that pars plana placement with vitrectomy virtually eliminates the problem of annoying floaters.

Two Surgeons: One Goal

The technique for placing the plate onto the sclera is identical for anterior chamber placement and pars plana placement. Once the plate has been secured to the sclera, the procedures diverge.

In order to place the shunt properly, the patient must undergo a complete pars plana vitrectomy by a fellowship-trained vitreoretinal surgeon. When I perform combined procedures with the retinal surgeon, I often start the surgery. I will perform a standard peritomy and then place the shunt under the superior and lateral recti. The plate is then anchored to the sclera with 7-0 prolene. For non-valved shunts such as the Baerveldt and the Molteno, the tube is ligated with a 7-0 Vicryl suture. This step is not used with the Ahmed device. Next, the retinal surgeon will take the surgical chair and perform a complete pars plana vitrectomy, including shaving of the vitreous base. Once the vitrectomy is complete, the retinal surgeon will remove all instruments with the exception of the infusion.

I will then reassume the surgeon's chair. I place the tube into the pars plana through a 25-gauge tract. The tube is trimmed with the bevel facing posteriorly as opposed to anteriorly. This prevents the iris from incarcerating the tube's lumen.

The tube length is calculated to allow the tube to be just visible on dilated indirect ophthalmoscopy. This allows both the glaucoma and retinal specialists to easily assess the tube postoperatively. The tube is anchored to the sclera with a 9-0 nylon suture. This step is performed to prevent the tube from sliding out of the eye. I will often fenestrate the tube with three passes of the needle attached to the 7-0 Vicryl suture. This allows for some fluid egress while the tie-off dissolves.

I then place a scleral or pericardial patch graft over the tube from its insertion to the base of the plate. This step is performed to prevent melting of the overlying conjunctiva, which could leave that patient vulnerable to endophthalmitis. The graft is sutured to the sclera with four 7-0 Vicryl sutures. I would suggest that a patch be used in all cases as I have seen late erosion of the conjunctiva years after the initial surgery. The conjunctiva and Tenon's layer are then closed as separate layers in a watertight fashion. Once these layers are securely closed, the infusion cannula can be removed. I would not advocate removal of the infusion until the case is complete.

It is not possible to perform the post-vitrectomy steps I have described unless the infusion is present and running. Attempting to complete the glaucoma steps described subsequent to the complete vitrectomy without an infusion can be challenging and dangerous. The eye often softens profoundly following the vitrectomy and the infusion is needed to prevent collapse of the globe during the final steps of the tube insertion, anchoring, graft placement and wound closure. The final step is removal of the infusion cannula. The eye is then treated with atropine 1% eye drops and Tobradex ophthalmic ointment followed by gentle patching.

Ideal tube length and angulation for pars plana placement of the Baerveldt aqueous shunt.

For the surgery to be successful, it is vital to ensure that tube length is adequate to allow postoperative visualization as well as to minimize the chance of the tube slipping out of the eye. It is also vital to maintain a flowing infusion until the surgical steps are completed. If a surgeon fails to adhere to the above steps, he or she will undoubtedly encounter unpleasant peri- and postoperative complications.

The 25-Gauge Technique

With regard to the pars plana vitrectomy, using a 25-gauge technique provides profound advantages over larger-gauge surgery. Because the 25-gauge technique does not involve opening of the conjunctiva, the healing of the surgery is dramatically quicker than historical vitrectomy techniques. When coordinating the glaucoma surgery (shunt placement and insertion) with the retinal surgeon who performs the vitrectomy, it is ideal for the retinal surgeon to place the infusion cannula temporally. A 25-gauge trocar can be placed at roughly the 12 o'clock position. The retinal surgeon can use this incision to place a chandelier light. Once the retinal surgery is completed, the trocar at this site can be removed and the tube placed into the pars plana through this tract. This site is ideally placed when lined up with the middle of the plate, along a perpendicular line created by the tube as it approaches the limbus. This alignment allows for optimal tube dynamics by ensuring linear flow from the pars plana to the surface of the plate.

Avoiding Hypotony

In cases in which hypotony must be avoided at all costs, a gas-bubble of C₃F₈ or SF₆ can be injected into the pars plana at completion of the case. This bubble can serve to minimize the occurrence of choroidal effusions or choroidal hemorrhage.

In my experience, hypotony with choroidal effusion is more likely with pars plana placement than with anterior chamber placement. Additionally, the size and volume of the choroidals are much greater with pars plana placement. Every effort should be taken to minimize postoperative hypotony as a result of these choroidals.

Care should be taken to ensure that there is no leaking around the tube at its insertion site. The 7-0 Vicryl tie-off used with the non-valved Baerveldt and Molteno shunts should be carefully inspected to ensure that no flow can reach the plate over the first month following surgery. Absolute closure can be verified by cannulating the distal end of the tube with a 30-gauge cannula attached to a 5 cc syringe filled with BSS.

Once the tube is cannulated, a 0.12 forceps can be used to hold the tube while BSS in forcefully injected toward the plate. If the tie-off is complete, no BSS will pass through the tie off. If any BSS reaches the plate, the 7-0 Vicryl should be re-tied and checked in a similar fashion to ensure water-tight ligation of the tube.

The postoperative course in cases which involve pars plana placement appears to be more variable with regard to IOP fluctuations. It is not clear to me at this time why this is the case. Pressures tend to reveal greater fluctuations with pars plana placement of the tube.

In cases of elevated postoperative pressure, the impact of acetazolamide 1000 mg/day (the maximum recommended dose in adults) appears to be dramatically blunted compared to cases with anterior chamber placement. Similarly, the IOP reduction achievable with topical glaucoma medications also appears less vigorous than would otherwise be expected.

In cases of postoperative hypotony, the first goal is to ensure that there is no wound leak. If a wound leak is found, it must be closed immediately. If no leak is found, then the cause of the hypotony should be assessed. The most likely sources of postoperative hypotony are fluid leakage around the tube insertion or failure to completely ligate the tube effectively.

In a moderate number of Baerveldt and Molteno cases, the slit or fenestrations in the tube, which are intentionally created to allow for some IOP control while the 7-0 Vicryl tie-off dissolves, allows too much fluid to leave the eye. In these cases, it is helpful to place the patient on oral steroids such as a Medrol Dose Pack to try to pressurize the globe. If choroidals are present, they must be monitored closely. If they are not “kissing,” they can be observed without surgical intervention. If they develop appositional “kissing,” they must be drained. A gas bubble — if not already present — can help restore normal physiology following external drainage of the choroidals.

Postoperative Medications

Unlike cases of anterior chamber tube placement, tubes placed in the pars plana should be seen postoperatively by both the glaucoma surgeon and the retinal surgeon.

The standard pharmacologic regimen deployed includes a topical steroid such as prednisolone acetate 1% four to six times daily and tapered over a six- to eight-week period. A fluoroquinolone drop is used three to four times daily for the first four to six weeks until all wounds are sealed. Finally, atropine 1% drops are used once daily for the first four to eight weeks to prevent aqueous misdirection both initially, and again (with the Baerveldt and Molteno) when the 7-0 Vicryl ligature dissolves and the tube opens.

It is vital to have adequate atropinization throughout the postoperative course. Failure to do so will leave the eye vulnerable to malignant glaucoma or aqueous misdirection. Should this occur, emergent surgery involving a vitreo-irido-capsulo-cyclectomy should be performed. To properly break an attack of misdirection, aggressive cycloplegia may be effective if the attack is caught in the first 24 hours. If, however, the attack has been present for beyond 24 hours, the vitrector must be placed in the posterior segment though a pars plana incision and then brought anterior through all structures encountered, including the vitreous, vitreous face, ciliary body, posterior capsule, zonules, anterior capsule and iris.

The mouth of the vitrector needs to be clearly visible in the anterior chamber, thereby creating a truly uni-cameral eye. Placement of an anterior chamber infusion at the beginning of the case is also crucial to performing the procedure. Once the eye has been rendered uni-chambered, the anterior chamber will deepen back to physiologic depth. Again this untoward event, aqueous misdirection or malignant glaucoma, can be avoided by establishing adequate cycloplegia starting at the completion of the case by instilling atropine 1% eye drops, and continuing to instill atropine 1% once daily to the surgical eye until the tube has completely opened by virtue of dissolution of the 7-0 Vicryl ligature.

Possible Complications

With the Baerveldt and Molteno, once the tube is opened at about four to six weeks, the IOP should remain in an ideal level between 10 and 16 mm Hg. Occasionally, the IOP can rise precipitously at some time following dissolution of the ligature and opening of the tube. This IOP rise can be caused by obstruction of the tube by residual vitreous left behind at the time of the original surgery. If this occurs, a simple vitrectomy can remove the obstruction and allow for proper fluid egress. It is not always possible to visualize vitreous in the lumen of the tube on clinical exam. It is possible to determine if this is the cause of IOP elevation by evaluating the height of the conjunctiva overlying the plate. If there is no elevation of the conjunctiva and adequate time has passed to allow the 7-0 Vicryl tie-off to dissolve, then the only possible cause of IOP elevation is obstruction of the tube. Clinical exam can easily reveal if there is any iris or uveal tissue obstructing the lumen. If no iris or uveal tissue is noted, then the most likely cause of lumen obstruction is retained vitreous. At the time of surgical removal, the remaining vitreous can be more easily identified by injecting triamcinolone into the posterior segment.

Summarizing the Procedure

In conclusion, it is important for the surgeon to understand that pars plana placement of the tube from an aqueous shunt is not a simple, easily learned derivation of anterior chamber tube insertion. Rather, it constitutes a series of unique surgical procedures with only limited commonality with standard anterior segment placement. Pars plana placement is basically two separate surgeries performed in one setting by both a retinal specialist and a glaucoma surgeon. Each surgeon must possess a highly developed skill set to offer the patient both a high degree of safety and the probability of a successful outcome.

The retinal surgery is most effective with modern 25-gauge technique and technology due to its minimally invasive and intrusive methodology and instrumentation. The glaucoma surgery is most effective when performed by a facile glaucoma surgeon with a good knowledge of management of both hypotony as well as elevated IOP in the postoperative setting.

Choroidal hemorrhage in the early postoperative period can be devastating. Aqueous misdirection or malignant glaucoma that occurs in the setting of glaucoma surgery and that abruptly lowers intraocular pressure must be rapidly identified and treated if seen. The probability of both choroidal hemorrhage and aqueous misdirection can be minimized by keen awareness of their antecedent contributing factors. On occasion, these events can occur even in spite of all the best efforts of the surgeon and patient. Rapid identification followed by appropriate intervention offer the patient the best chance of salvaging the globe.

Surgeons wishing to offer pars plana aqueous shunt placement to patients must be willing to encounter unexpected peri- and postoperative surprises. Patients who will undergo this surgery should ideally meet both the glaucoma and retinal surgeons preoperatively so as to be aware of the complexity of what they are about to undergo.

In my experience, the best order for this surgery is to have the glaucoma surgeon make the opening incisions and place and anchor the plate in the supero-temporal quadrant. The tube should be temporarily tucked under one of the rectus muscles. The retinal surgeon should then perform pars plana vitrectomy with shaving of the vitreous base to remove all possible vitreous. All the cannulas except the infusion can be removed and the glaucoma surgeon will then place the tube into the pars plana and close all of the wounds and incisions. Once the conjunctiva and Tenon's layer are closed in a water-tight fashion, the infusion cannula can be removed by the glaucoma surgeon. The eye should then be aggressively cyclopleged with atropine 1% eye drops and then covered with Tobradex ophthalmic ointment prior to being patched closed.

In cases in which hypotony must be avoided at all costs, the retinal surgeon can inject a gas-bubble into the posterior segment prior to patching the eye shut. If a gas bubble is injected, patients should be informed at the time of discharge so that when the patch is removed they are not startled by the obstruction in their visual axis caused by the bubble.

Successful surgery requires comprehensive and constant communication between the two surgeons as well as between the patient and each of the surgeons. In general, the glaucoma surgeon will quarterback the case but the retinal surgeon must be intimately involved in the patients postoperative care if the intraocular pressure deviates above or below the expected levels. Preoperative consultation with both of the subspecialists serves to make the patient aware of the magnitude of what they are about to undergo. OM

Andrew Rabinowitz, MD, is the glaucoma specialist for Barnet, Dulaney and Perkins, a multi-location practice based in Phoenix. Dr. Rabinowitz's techniques for performing glaucoma surgery have been used in educational videos and films for a number of years. Dr. Rabinowitz can be reached via e-mail at andrewrabinowitz@aol.com.