SIGS AND GLAUCOMA SURGERY

Trabeculectomy has remained essentially unchanged since it was first described by Cairns in 1968. However, bugbears such as sub-conjunctival fibrosis and failure as well as flap-related problems such as uneven flap; maceration/tears in the flap; irritation, foreign body reaction and rarely, abscess secondary to flap sutures; sealing of the triplanar flap etc, are problems associated with it.

Creation of either a limbal- or fornix-based conjunctival flap for any kind of procedure invariably induces sub-conjunctival fibrosis which can lead on to filtration failure secondary to scarring. Wound modulators such as Mitomycin-C or 5-Flurouracil are used intraoperatively for decreasing the fibrotic tendency, however, these may be associated with other problems such as thin, avascular blebs, blebitis etc.

A new technique

Stab Incision Glaucoma Surgery (SIGS) is a new guarded filtration procedure that I described which aims at getting rid of many of the disadvantages of trabeculectomy and tunnel trabeculectomy, at making bleb function less dependent on scarring while at the same time making the anti-glaucoma procedure easier and faster to perform. The technique consists of using a 2.8mm bevel-up keratome in a single step to create a sclero-corneal tunnel which is then intentionally compromised by punching the posterior corneal lip.

The technique consists of first making the anterior chamber (AC) firmer by instilling viscoelastic with a 26 G needle introduced at the limbus. A site with mobile conjunctiva is selected and the conjunctiva slid forwards with a blunt instrument. The keratome is used to make a stab incision through the conjunctiva and into lamellar sclera starting about 2.5mm behind the limbus and avoiding major blood vessels. It is passed in a single movement through conjunctiva into lamellar sclera. While holding the globe firmly at the limbus with a toothed forceps, a superficial lamellar scleral tunnel is then dissected forwards with the keratome with careful side-to-side movements up to the limbus. The ideal plane of dissection is such that the blade is just visible through the conjunctiva. The keratome then enters about 1mm into lamellar cornea and is pushed further forwards into the anterior chamber.

Downward push

Downwards push on the posterior corneal lip should be avoided while entering the AC as it may cause a trapdoor hinging of the posterior lip of the corneal tunnel. The blade is then withdrawn gently in a single smooth movement without allowing aqueous leak through the incision.

The entire tunnel is thus created in a single step with a single instrument. Viscoelastic is again injected into the AC through the paracentesis/SIGS tunnel. While holding the globe rotated downwards, a Kelly's Descemet's punch (1mm) is slid along the tunnel into the AC and the internal lip of the corneal section engaged and punched.

Additional punches are taken posteriorly in the clear cornea up to the limbus. Care should be taken that the punch faces downwards while punching and that the iris is pushed away with sufficient viscoelastic. The AC is then gently irrigated through the tunnel to wash away excess viscoelastic and the now punched and thereby compromised SIGS tunnel is checked for leakage by irrigating through the side port. The end point is a free flow of fluid on irrigation. Additional punches towards the limbus are taken in case of inadequate leak. However, it is not extended beyond the limbus to avoid excessive leak and postoperative shallow AC. A peripheral iridectomy (PI) need not be done in cases of open angle glaucoma.

In cases with angle closure, peripheral anterior synechiae, shallow AC or cases showing tendency to intraoperative iris prolapse into the SIGS tunnel on irrigation, a PI is done by simply grasping the iris with a non-toothed forceps and cutting with a Vannas scissors. The single, small, 2.8mm conjunctival cut is then sutured with a running or figure-of-eight suture.

BSS is again injected through the sideport to cause physiological hydrostatic ballooning of the bleb. SIGS can also be combined with Mitomycin-C (MMC) by dissecting the tunnel into lamellar cornea, applying intra-tunnel MMC, rinsing well and then entering the AC. Sub-conjunctival MMC is not required as there is no subconjunctival dissection in SIGS.

SIGS may also be combined easily with phaco by first making the SIGS stab incision, followed by phaco and IOL implantation. THE SIGS tunnel is self-sealing before it is intentionally compromised by punching it, hence it does not interfere with phaco in any way. The posterior corneal lip of the tunnel is punched before viscoelastic removal. Care should be taken to construct the SIGS incision at a site where it does not intersect with phaco incisions.

Advantages of SIGS include complete absence of sub-conjunctival dissection and thereby lesser risk of failure from scarring. Creation of the tunnel is easy and one step. Problems associated with flap tearing, laceration etc, that sometimes occur in trabeculectomy are done away with. Bleb elevation is by sub-conjunctival dissection via hydrostatic pressure of fluid from side port irrigation or from the I/A probe when combined with phaco rather than by scissor dissection as in other procedures, hence there is lesser chance of scarring. Virgin conjunctiva is maximised for any future procedures that may be required as compared to trabeculectomy and other procedures. It allows faster surgery, is less traumatic and may be combined easily with phaco or MMC if required. Surgery is easier and as conjunctival drainage channels are almost intact, trans-conjunctival absorption of aqueous is likely to be better.

Dr Soosan Jacob is a senior consultant ophthalmologist at Dr Agarwal's Eye Hospital, Chennai, India and can be reached at: dr_soosanj@hotmail.com