Newer Surgical modalities for treating Glaucoma

Generally the mechanism of all glaucoma filtering procedures is the creation of a fistula or opening, at the limbus thereby allowing aqueous humor to drain freely from the anterior chamber. The aqueous flows directly or indirectly into subconjunctival spaces and is then removed by various routes. Here are brief descriptions of some procedures of surgical procedures for treating glaucoma.

The supratemporal quadrant of the eye is exposed with a traction suture; 2.5mm conjunctival peritomy is performed without cutting the tenons capsule near the limbus. A 1/3-1/2 partial thickness incision is made at the limbus & a scleral pocket is dissected posteriorly.The sub-conjunctival space is entered with an Alcon Crescent knife (bevel up) passed through the scleral pocket (fig ) and balanced salt solution (BSS) is injected forming a sub-conjunctival bleb. The anterior chamber is entered at the initial limbal incision and the Vannas scissors is used to excise a 1.5mm x 1mm fragment of the floor of the pocket followed by peripheral iridectomy. The scleral wound as well as the conjunctiva is closed with separate 10-0 running / interrupted sutures.

Advantages:
1) Places the incision between the insertion of the conjunctiva and Tenon’s capsule avoiding manipulation of the latter and the subconjunctival space.
2) Thus obviates the stimulus for episcleral fibrosis without using pharamacological wound modulation
3) Minimal cautery is used thus avoiding further stimulus for post-operative scarring.
4) Low cost and highly efficacious.
5) As the cornea is not dissected, there occurs no visual disturbance d/t astigmatism or corneal oedema.

Principle: Removal of deep scleral flap, external wall of SC, corneal stroma behind the anterior trabeculum and the DM, thus creating a scleral lake →Aqueous leaves the AC through the intact Trabeculo-Descemet’s membrane (TDM)

Indications: Indications are summarised in figure below

Contraindications:

DEEP SCLERECTOMY:
Limbal-based conj flap created 8mm from the limbus. A 5x5mm rectangular, 1/3 scleral thickness flap is made, dissected 1mm anteriorly into clear cornea with a disposable crescent knife. A 2nd 3x3 mm deep scleral flap approx 90% scleral thickness is dissected leaving behind a thin layer of deep sclera over the choroids. The scleral spur is identified as organized scleral fibres forming a ligament parallel to the limbus. The SC lies just anterior to this which is then deroofed .The sclerocorneal dissection is then carried forward for 1mm to remove sclerocorneal tissue in front of the anterior trabeculum and the DM taking due care not to perforate the thin TDM thereby avoiding accidental entry into the AC. Aqueous ooze is verified by a dry merocel sponge and the deep scleral flap is excised using a Vanna’s scissors. The sclerocorneal space thus created is prone to collapse and fibrosis, hence a 10*6mm amniotic membrane with the epithelial side up is placed over the scleral bed or an implant (collagen implants, reticulated cross-linked Na hyluronate implants ) is then sutured in position under the superficial scleral flap which is then sutured with loose interrupted 10-0 nylon and knots buried. Conjunctiva and Tenon’s capsule sutured with continuous 8-0 nylon and checked for any wound leak. Mechanism of Action of the procedure is described in figure below

VISCOCANALOSTOMY:
Proposed by Robert Stegman 1991.Surgical steps being the same as in Deep sclerectomy upto Schlemm’s canal being deroofed.Then by a paracentesis the IOP is lowered , the 2 cut ends of the SC are cannulated by a 165mico-metre blunt needle and a high molecular sodium hyluronate is slowly injected into the canal. Upto 1-2 clock hrs of the canal is a-traumatically dilated. The slow injection is repeated 6-7 times on each side. A 2nd site of injection is between superficial scleral flap and deep scleral bed to display the potent anti-inflammatory properties of the high viscoelastic device. The outer scleral flap is tightly secured with 10-0 nylon sutures to ensure that the intra-scleral chamber is created. Conjunctiva and the Tenon’s capsule are then sutured in a similar fashion with 8-0 nylon sutures.

Mechanism of Action: Targets the melanin-containing cells in the trabecular meshwork, without incurring collateral thermal damage to adjacent non-pigmented trabecular meshwork cells and underlying trabecular beams. A biological response is induced involving the release of cytokines that triggers macrophage recruitment and other changes leading to IOP reduction. Thus unlike Argon laser trabeculoplasty , SLT is repeatable

Laser technique: Pulse duration - 3 nanoseconds, Spot size of 400 microns, Energy levels between- 0.70mJ and 1.10mJ, 50 spots placed over 180 degrees.

4) Intrastromal Holomium Laser Keratostomy:

Procedure: A laser (Holomium) canal is created intrastromally in the cornea anterior to Scwalbe’s line in the floor of corneo-scleral tunnel incision, made with a knife from the corneal site which acts as a valved mechanism. Mitomycin C is used intra-operatively by subconjunctival injection over the proposed surgical site to prevent sub-conjunctival fibrosis.
Advantages: Post-op blebs are pale, diffuse and low-lying. No occurrence of flat AC is seen. Similarly low incidence of transient shallowing of AC (25%) while achieving IOP < 20mmHg without medication and re-operation in 63% cases over a mean follow up of 22.5months
Dis-advantages: Need for expensive laser equipments with costly probes that should not be used more than 5 times.

5) Glaucoma surgery using Fugo blade

Dr. Richard Fugo invented the Fugo Blade, which employs plasma energy for ablating incisional pathways in tissues in such a manner that it creates a smooth wall along the ablation pathways. Long-term results however remain unknown.Following procedures can be done:

The most common causes of failure of filtration surgery are-blockage of fistula by tissue in-growth, adhesion of scleral flap to the scleral bed, fibrous breakdown of conjunctival filtering bleb. Thus in order to halt this process, mechanical barriers have been tried out. A reservoir portion made of rigid materials is needed to facilitate aqueous outflow and prevent prevent blockage of distal end of the tube, which requires a large incision through the conjunctiva and Tenon’s tissue for its insertion. Promising results have been seen with the use of double layered Expanded polytetra-fluroethylene (E-PTFE) membrane, a newer implant, soft and malleable as a reservoir portion. This obviates the need for a large incision and the related complications such as wide scar, erosion ad extrusion of implant and extra-ocular muscle movements.

In a recent study, retinectomy was performed to lower IOP in pts. With uncontrolled IOP(> 35mm Hg for more than 4 months) despite conventional filtration surgery and drug treatment. Pars plana vitrectomy was performed and the peripheral retina was surgically excised to various degrees. The procedure was concluded by an intraocular gas temponade of 60% C3F8. Retinotomy may be alternative to enucleation in otherwise intractable glaucoma. It has the advantage of a lack of initial post operative hypotony together with a sufficient long term drainage effect. The posterior aqueous routing via retinectomy depends on the integrity of the choroids and thus the size and area of retinectomy should be chosen accordingly. The reduction in systemic and topical medications after retinectomy is considered beneficial for the patients.

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