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Early results of stab incision glaucoma surgery promising

The technique involves the creation of a compromised leaking corneoscleral tunnel with minimal to no conjunctival dissection.

Issue: July 10, 2014

BySoosan Jacob, MS, FRCS, DNB

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Glaucoma is a potentially blinding disease that affects 2.3 million Americans age 40 years and older, and in a subset of patients, it may not be adequately managed by medical therapy alone and may necessitate surgical intervention in an effort to control glaucomatous optic nerve damage and vision loss.

Although various glaucoma procedures are currently available, trabeculectomy remains the gold standard for advanced glaucoma that requires significant reduction in IOP. However, trabeculectomy may be associated with significant postoperative complications in some cases. The combination of significant IOP lowering offered by trabeculectomy combined with possibly eliminating or decreasing postoperative complications would make a modified trabeculectomy procedure a more attractive choice.

In this column, Dr. Jacob describes a guarded filtration procedure that promises effective lowering of IOP without the usual potential complications that are known to be associated with a trabeculectomy procedure. This procedure is named stab incision glaucoma surgery. Continued monitoring and evaluation of this new surgical procedure over time are mandatory to fully determine its overall effectiveness and safety as a filtration surgical procedure.

Thomas “TJ” John, MD
OSN Surgical Maneuvers Editor

Soosan Jacob

I started stab incision glaucoma surgery (SIGS) as a new option for a guarded filtration procedure (Figure 1). It essentially involves the creation of a compromised leaking corneoscleral tunnel with minimal to no conjunctival dissection.

Surgical technique

The conjunctiva is pushed forward toward the limbus as much as possible, and a 2.8-mm bevel-up keratome is used to enter lamellar sclera transconjunctivally, starting about 2 mm behind the limbus. The keratome is carefully dissected forward through superficial lamellar sclera 1 mm into clear cornea. At this point, the keratome is pushed forward to pierce through the cornea and enter the anterior chamber. It is entered horizontally, taking care to be parallel to the iris plane and therefore avoiding the chance of touching the crystalline lens or Descemet’s membrane. The keratome is entered to its broadest extent and then withdrawn. During the keratome pass, the globe should be held firmly at the limbus with strong toothed forceps in order to gain good control over the position of the globe and to allow an easy pass of the keratome while creating the SIGS tunnel.

 

Figure 1. A: Conjunctiva is slid forward, and a 2.8-mm bevel-up keratome enters the lamellar sclera transconjunctivally starting about 2 mm behind the limbus. B: Keratome is carefully dissected forward through superficial lamellar sclera 1 mm into clear cornea. C: Keratome is pushed forward to pierce through the cornea and enter the anterior chamber. D: A 1-mm Kelly’s Descemet’s punch is introduced through the SIGS tunnel to punch the inner posterior lip of the corneal tunnel. E: Additional punches are taken in a longitudinal manner until reaching the limbus. F: Adequacy of the ostium is assessed by balanced salt solution irrigation from the side port. Once free flow of fluid is seen, the conjunctiva is closed with a single running suture, and the bleb is elevated by balanced salt solution irrigation from the side port.

Image: Jacob S

Viscoelastic is then instilled into the anterior chamber through the SIGS tunnel, and a 1-mm Kelly’s Descemet’s punch is slid along the sclera to be introduced into the anterior chamber through the SIGS tunnel. It is then turned posteriorly, engaged on the inner posterior lip of the corneal tunnel and a punch taken. Additional punches are taken in a longitudinal manner up to the limbus. The anterior chamber should be filled with viscoelastic while punching to prevent shallowing of the anterior chamber and iris damage on inserting the punch. Viscoelastic is then gently washed out, and balanced salt solution is irrigated from the side port.

Egress of fluid from the SIGS tunnel is sought because a free flow of fluid without having to apply excess irrigating pressure signifies adequacy of the ostium. If a free flow of fluid is not seen, it indicates an inadequate ostium, and additional punches are taken toward the limbus. Once this has been tested, the conjunctiva is closed with a single running suture. The bleb is then elevated by balanced salt solution irrigation from the side port. This differs from the bleb that is elevated after a trabeculectomy because it is a physiological, non-scar-inducing, hydrostatic expansion of the subconjunctival drainage channels. Postoperative management is similar to that of trabeculectomy except that there is no micromanagement in the form of adjustable or releasable sutures.

Challenges

Scleral tunnel: The ideal depth is automatically attained if one passes the keratome in such a way that the keratome is just seen through the overlying sclera and conjunctiva. Our postoperative optical coherence tomography studies have shown that this is the right depth for the tunnel. Too deep a pass results in a premature entry. The length of the scleral part of the tunnel should not be too long to prevent tunnel occlusion.

Premature entry: Too deep a pass results in the keratome directly entering into the anterior chamber, which results in the lack of a posterior lip to punch out. In this case, one of two options may be chosen. Because the tunnel is self-sealing and the single conjunctival cut is small, it may be abandoned and surgery continued by creating a new tunnel at a fresh site. But if the surgeon does not want to do this or if there is not adequate mobile conjunctiva to allow a fresh site, the SIGS tunnel may be converted into a flap, and surgery may be continued as a conventional trabeculectomy.

Conversion to conventional trabeculectomy: This is done by minimally enlarging the conjunctival incision to either side. The conjunctiva overlying the tunnel is dissected bluntly off the roof of the tunnel, and the tunnel is then converted to a flap by cutting along either side. The tunnel is dissected further forward into clear cornea, and a punch is taken that includes the bridge of tissue as well as the posterior lip if required. A peripheral iridectomy is then performed, and the flap and conjunctiva are sutured as in trabeculectomy.

Trapdoor hinging: This occurs with a thin posterior corneal lip (again occurring if the scleral tunnel is created deeper than normal) if excessive posterior pressure is applied while entering the anterior chamber. This can be prevented by not being too deep in the cornea, and if deep, by entering with the keratome directed horizontally without applying any downward pressure. If this occurs, the hinged trapdoor can be excised bimanually with Vannas scissors and Utrata forceps or with a vitrector.

Peripheral iridectomy: A peripheral iridectomy is not normally needed in cases of primary open-angle glaucoma or secondary open-angle glaucoma because the ostium is situated adequately anterior enough to the iris base to avoid the iris from plugging the ostium. It is, however, required in cases with angle closure, shallow anterior chamber or peripheral anterior synechiae or when the iris is seen peaking into the ostium intraoperatively. It may also be required in case of a short corneal component of the tunnel where the ostium may be more posteriorly located than normal. The peripheral iridectomy is performed by sliding angled McPherson forceps into the SIGS tunnel while the assistant retracts the conjunctiva. The iris is then grasped near the base with the forceps and gently pulled out and excised with curved Vannas forceps, as normally done for a peripheral iridectomy. Care should be taken to reposit the iris completely into the anterior chamber and to not leave it partly incarcerated within the tunnel.

SIGS with MMC: It is relatively easy to combine SIGS with mitomycin C when the surgeon so desires. The corneoscleral tunnel is created; however, it is stopped short of entering the anterior chamber. A sponge soaked in 0.03% MMC is held with forceps under the scleral tunnel for 2 minutes, after which a thorough irrigation is carried out. Lifting up the conjunctiva while washing helps prevent ballooning of the conjunctiva. Viscoelastic is then applied into the tunnel, the keratome is slid back in, and the anterior chamber is entered. The rest of the surgery is completed as discussed previously. For those surgeons who prefer to inject MMC subconjunctivally, it may be possible to do the same; however, I personally do not have experience with this technique.

SIGS with phacoemulsification: SIGS may easily be combined with phaco. The SIGS tunnel is made first, followed by creation of the main port and side port to either side of the SIGS tunnel. Phaco is then carried out. As the SIGS tunnel is a self-sealing tunnel at this stage, it does not interfere with phaco in any way. The IOL is implanted, and the SIGS ostium is created before viscoelastic removal. Leakage is tested for, and if adequate, the conjunctiva is sutured followed by viscoelastic removal. The bleb can be seen to elevate during irrigation and aspiration.

Contraindications for SIGS: Contraindications, or rather cases in which the success rate may be less, include eyes in which the conjunctiva is already scarred and virgin conjunctiva is not available over a sufficient area to allow filtration, eg, post-retinal detachment surgery or a patient who has already undergone previous trabeculectomies.

Advantages of SIGS

SIGS has numerous advantages, which include almost complete elimination of subconjunctival dissection, thereby decreasing the risk of a failed filter from scarring; maximized virgin conjunctiva, which is important for any glaucoma patient; and the presence of only a single 2.8-mm conjunctival incision located well away from the scleral tunnel. The scleral tunnel is biplanar and less likely to seal than a triplanar incision. A controlled, posteriorly directed flow is obtained, which decreases the chance of an overhanging bleb, bleb dysesthesia and other bleb abnormalities.

In our experience, the incidence of postoperative shallow anterior chamber is low; there is no need to routinely perform a peripheral iridectomy except in indicated cases, and postoperative inflammation is therefore less. Hydrostatic bleb elevation facilitates physiological expansion of subconjunctival drainage channels that are almost completely intact. The lack of scleral sutures avoids suture-related complications as well as induced astigmatism.

SIGS is economically advantageous because it does not require any expensive devices or instrumentation as in microincision glaucoma surgery. It is easy, rapid and less traumatic and, if required, can be easily converted to conventional trabeculectomy. In the extreme event of an expulsive hemorrhage, it is easier to rapidly close the SIGS tunnel as compared with a trabeculectomy flap.

I started performing SIGS about 15 months ago, and as a group, we have now performed approximately 130 procedures so far, either in isolation or combined with MMC/phaco. The results have been promising so far, and it is gratifying to see the acceptance of SIGS by all the surgeons in our group of hospitals as well as international surgeons.