NON-PENETRATING GLAUCOMA SURGERIES

GUEST AUTHOR

RASHMI CHAUHAN

AJMAL KHAN TIBBIYA COLLEGE

ALIGARH, INDIA

INTRODUCTION

Trabeculectomy remains the standard surgery for intra-ocular pressure (IOP) control in cases of glaucoma.

It involves a full thickness penetration of sclera to allow aqueous seepage to the subconjunctival space, however, protected by a scleral flap.

This procedure, while being largely successful, carries the risk of serious bleb related complications, such as:

• Overfiltration.
• Hypotony
• Persistent choroidal detachment (Due to overfiltration).
• Shallow anterior chamber.
• Peripheral anterior synechiae.
• Decreased vision from hypotonous maculopathy.
• Bleb infection.
• Endophthalmitis.

Why NPGS?

Interest in non-penetrating glaucoma surgery (NPGS) developed in order to avoid the complications of full-thickness procedures.

NPGS evolved during late 1950s and early 1960s due to the pioneering work by Epstein and Krasnov.

However, its popularity as an effective treatment for IOP control was limited due to concurrent introduction of trabeculectomy as a relatively easier and more effective technique for IOP control.

NPGS mainly targets the outer trabecular meshwork at the site of maximum resistance to aqueous outflow.

Other possible mechanisms include:

Enhanced trans-scleral flow.

Enhanced Uveo-scleral outflow.

Opening of the non-functional areas of Schlemm’s canal.

SURGICAL TECHNIQUES

DEEP SCLERECTOMY

After adequate anesthesia, a superior rectus or corneal traction suture is placed and a fornix or limbus based conjunctival flap is created.

A partial thickness (1/3-1/2 thickness) limbus base scleral flap measuring 5mmx5mm is dissected 1.5 mm into the clear cornea. A deeper (upto 90% depth) second scleral flap measuring 4mmx4mm is then dissected forwards in the plane of scleral spur and Schlemm’s canal.

The Schlemm’s canal is identified and unroofed for approximately 3mm in length at the level of scleral spur.

Cleavage is taken forwards between the corneal stroma and Descemet’s membrane. This deeper scleral flap is subsequently excised to form a “scleral lake” for pooling of aqueous. This leads to the formation of “trabeculo-Descemet’s membrane” (TDM) as the outflow resistance for aqueous.

Evidence of flow through this TDM is mandatory at this point.

To improve outflow, inner wall of Schlemm’s canal can be removed and juxtacanalicular meshwork can be gently peeled with blunt forceps. Use of Mitomycin-c in the scleral bed and use of space maintainer implants (Aqua Flow, T Flux, SKGEL and PMMA) have been reported with varying results.

The scleral flap is loosely sutured with 10-0 nylon sutures and the conjunctiva and Tenon’s capsule are closed in layers.

Since deep sclerectomy involves filtration and formation of a bleb, modulation by means of post-operative 5-Fluorouracil injections has been described along with Nd:YAG goniopuncture of the TDM in postoperative phase, which converts it into a full-thickness procedure.

MECHANISM OF ACTION

Aqueous percolates through the TDM into the scleral lake and then subconjunctival space.

Resultant blebs are usually shallower and more diffuse.

Alternative pathways for drainage include into suprachoroidal space via the cut ends of Schlemm’s canal and through the intrascleral bleb.

VISCOCANALOSTOMY

It involves injecting high viscosity viscoelastic in the Schlemm’s canal, after unroofing the Schlemm’s canal.

Aqueous seepage from the TDM is thus forced to reach the ostia of Schlemm’s canal. Viscoelastic is also placed in the “aqueous lake” preventing collagen cross-linking.

The first scleral flap is tightly sutured as opposed to deep sclerectomy and thus there is no filtering bleb in this procedure.

The conjunctiva and Tenon’s capsule are closed in layers.

An alternative technique of dilation of Schlemm’s canal has been described using an illuminated optical fiber micro-catheter.

Following exposure of Schlemm’s canal, the catheter is passed circumferentially around the Schlemm’s canal under direct visualization.

High viscosity hyaluronic acid is injected to dilate the Schlemm’s canal every 2 clock hours.

A 10-0 nylon suture is affixed to the end of the catheter which is subsequently withdrawn. 

The suture is then tied resulting in 360⁰ dilation of the canal.

The conjunctiva and scleral flap are closed in layers.

MECHANISM OF ACTION

 

This procedure results in dilation of Schlemm’s canal and associated collector channels, resulting in enhanced drainage through this route.

In addition, damage to endothelium of Schlemm’s canal results in communication of juxtacanalicular zone to Schlemm’s canal. 

The superficial scleral flap is secured loosely with 10-0 nylon sutures and the conjunctiva and Tenon’s capsule are closed in layers.

INDICATIONS

Open angle glaucomas, whether primary or secondary (especially pigmentary and pseudo-exfoliative).

NPGS may be more suited when target IOP is in mid to high normal range.

It may be a procedure of choice in patients with high risk of choroidal hemorrhage or postoperative hypotony.

CONTRAINDICATIONS

• Angle closure glaucomas (as clinical decision depends on degree of synechial closure of the trabecular meshwork).
• Iridocorneal Endothelial Syndromes (ICE).
• Eyes with congenital or juvenile glaucomas with angle anomalies and scleral thinning.
• Neovascular glaucomas: Constitute an absolute contraindication (due to invasion of angle and trabeculum with neovascular vessels and subsequent loss of filtration).

POSTOPERATIVE COMPLICATIONS

In general, NPGS has been shown to be associated with fewer complications as compared to trabeculectomy.

Complications may be:

• Perforation of TDM (commonest post-opeartive complication).
• Descemet’s membrane detachment.
• Cataract.
• Scleral ectasia.
• Early post-operative IOP rise. (May be due to insufficient leakage through the TDM or may be viscoelastic induced.)
• Rupture of fragile TDM may also occur (In case of Valsalva maneuver, trauma or rubbing of eyes. It leads to iris prolapse and consequent raised IOP.)
• Peripheral anterior synechiae can also cause late rise in IOP.