Ординатура / Офтальмология / Английские материалы / Step by Step Minimally Invasive Glaucoma Surgery_Garg, Melamed, Bovet, Pajic, Carassa, Dada_2006

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meshwork. The laser application results in a biological and mechanical reaction in the trabecular meshwork to open the previously blocked meshwork and increases the flow of aqueous fluid from the eye.

3.Selective laser trabeculoplasty: Reduces intraocular pressure by enhancing drainage of excess aqueous fluid. The laser increases drainage by selectively treating certain cell tissue of the trabecular meshwork.

4.Non-penetrating deep sclerostomy (Viscocanalostomy):

This is a modification of trabeculectomy, wherein a fullthickness hole in the eye is avoided. Instead, a very deep dissection is performed in the sclera and trabecular meshwork.

5.Tube-shunt: These are synthetic drainage devices particularly useful in neovascular glaucoma, aphakic glaucoma and advanced developmental glaucoma. In most cases the treatment is palliative.

NPDS – AN EVOLUTIONARY TECHNIQUE

The ideal glaucoma surgery is that which can create adequate drainage to enable a controlled reduction of IOP without the risk of over-filtration while ensuring long-term patency of the filtration channel. Hence, we believe that the model around which all glaucoma surgeries should be conceptualized is the non-penetrating deep sclerectomy (NPDS) or viscocanalostomy.1-4 In this procedure the intraocular pressure is lowered as fluid oozes through a permeable thin layer of tissue, the trabeculo-Descemet’s membrane. A bleb may be formed, but it is usually smaller than one that would be formed following trabeculectomy. The main advantage of this procedure is that it minimizes the chances of overfiltration. This avoids the complications of filtration blebs and the shallow anterior chamber seen

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after trabeculectomy.17 In Viscocanalostomy, the cut ends of the Schlemm’s canal are expanded with Sodium Hyaluronate. However, the IOP lowering capacity of this procedure is less than that of conventional trabeculectomy and the chances of closure are high. Some surgeons place a lake of viscoelastic such as sodium hyaluronate or a collagen implant5,6 under the scleral flap to reduce the chance of closure due to the natural process of healing.

NPDS is fast gaining popularity among surgeons due to decreased incidence of postoperative complications when compared with conventional trabeculectomy. However, most surgeons find dissection of the trabecular meshwork and the scleral bed difficult to perform. The meticulous tissue excision is challenging even to the most skillful and experienced surgeon. Additionally, it is commonly reported by surgeons, that once aqueous percolation starts to occur during the course of tissue dissection, a significant amount of hypotony sets in, making the excision of tissue even more difficult. From the variable reports of clinical success, it is clear that this technique has a long learning curve. Nevertheless, the potential opportunity to create a filtration procedure which successfully controls intraocular pressure in the absence of a prominent bleb is prompting innovators to improve upon the technique of NPDS.

Accelerated advancement in laser technology in the late 90s has been responsible for the widespread use of this hitech modality in ophthalmology and it is no surprise that investigators are aggressively exploring the potential of lasers to help evolve a more reliable version of NPDS. Technically, Lasers with predominant ablative properties have the advantage of helping the surgeon remove precise amount of tissue with relative ease. This appears to be the critical factor responsible for ensuring consistency in the surgical technique.

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LASER-ASSISTED TECHNIQUES FOR THE FUTURE

Many investigators evaluated the use of of the Holmium laser,7 Nd: YAG,8,9 Erbium YAG laser10-12 and the Excimer laser in an attempt to create a minimally invasive controlled glaucoma procedure. The technique of nonpenetrating filtering procedure, called sinusotomy was first introduced by Krasnov in 1969.1,2 Zimmerman et al investigated non-penetrating trabeculectomy in 1984 and demonstrated a success rate of 83.7 percent after 1 year.3,4 An increased success rate was demonstrated with collagen implants or sodium hyaluronate.1-4 Injection of sodium hyaluronate into the canal of Schlemm was recommended to make the procedure more effective and the procedure was called viscocanalostomy.16,17 Non-penetrating techniques reported lower rate of complications1-4 as compared to conventional Trabeculectomy or Sclerostomies. However, it has been observed that most surgeons require extensive experience to master the technique of accurate dissection of the trabacular meshwork and the scleral bed. It is also difficult to control the amount of tissue to be excised even with the aid of specially designed surgical instruments. A pulsed laser, capable of ablating ocular tissue with high precision, is the obvious tool that can overcome this challenge. The laser should have cutting properties ideally suited for corneal and scleral tissues and should be preferably available to the surgeon at the tip of an ergonomically designed handpiece.

Partial excimer laser sclerostomy and trabeculectomy has not gained wide acceptance due to inconsistent scleral ablative properties and lack of control arising from the nonavailability of a suitable delivery system through a surgical handpiece. Pulsed Erbium: YAG laser was used to ablate scleral and corneal tissue layer by layer to create filtration

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channels.18,19 Our own experience with the Er:YAG laser (unpublished studies) showed that the laser did produce significant colateral thermal damage. This would often lead to formation of coagulum at the tip of the fiber in the contact mode thus reducing the efficiency of subsequent ablation.

CUSTOMIZED LASER ASSISTED FILTRATION SURGERY (CLAFS) WITH THE PR-270

The PR-270 Pulsed laser (Fig. 17.2) uses as its source, the Nd:YAG laser crystal and nonlinear crystals to generate the 4th harmonic at a UV wavelength of 266 nm. It is very efficient in ablating tissues with high water content such as cornea and sclera. The laser is delivered through a specifically designed articulated arm coupled to a handpiece which delivers the UV laser energy via a focusing lens (Fig. 17.3). The 5-nanosecond short pulsed laser is focused to a spot size of about 0.6 mm on the treated area with energy per pulse of 5 to 7 mJ and operates at 10 to 20 Hz. Both, the pulse energy and frequency, are adjustable. We note that this 5 ns frequency of the laser is much shorter than the typical excimer laser (about 10 to 20 ns), Ho:YAG laser (about 200 microsecond) or Er:YAG (about 500 microsecond). Therefore, it offers minimal thermal damage with effective tissue ablation. Furthermore, the focused UV laser spot may be as small as 0.3 mm if needed, an attribute which is not available with IR lasers. Being non-contact in its operation the laser overcomes two major disadvantages of the contact fiberbased lasers. Firstly, unlike contact ablative systems, one does not encounter progressive decrease in the efficiency of ablation due to accumulation of coagulum at the end of the fiber tip. Secondly, there is no apprehension about damage to the tips which, in the fiber-based system, adds significantly to running cost.

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Depending on the initial IOP and the target IOP we recommend different ab externo techniques of ablation:

1.Linear radial sclerostomy (Fig. 17.4A): After creating a conjunctival flap, preferably fornix based, a pair of linear ablative scleral grooves is created about 85 percent of scleral depth. The starting point of the groove is about 0.5 mm from the limbus and is about 4 mm in length. The limbal end of the groove is deepened in a controlled fashion until aqueous percolates. This provides an open non-penetrative aqueous drainage pathway via the subconjunctival space. The expected reduction in intraocular pressure is between 2 to 4 mmHg.

2.Extended linear radial sclerostomy: The opposite end of the groove is deepened until a small brownish dot, about 1 mm in size, appears indicating exposure of underlying choroidal tissue. This will provide additional suprachoroidal drainage for the aqueous. The IOP lowering effect of this modification is expected to be about 4 to 6 mm Hg.

3.Laser-assisted non-penetrating deep sclerostomy (Fig. 17.4B): The initial steps are same as that of the surgical NPDS procedure. After an initial 5 × 5 mm partial thickness scleral flap, the scleral bed overlying the canal of Schlemm is ablated in order to “De-roof” the canal. This ablative excision is carried on anteriorly as a partial trabeculectomy and further anteriorly to expose the Descemet’s membrane. The endpoint is the percolation of aqueous. This procedure may be combined with collagen implants sutured to the bed under the partial thickness scleral flap or a lake of Sodium Hyaluronate may be placed under the flap. Viscocanalostomy using Sodium Hyaluronate may also be carried out for additive effect. Judicious use of sponge soaked in 5-FU

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or Mitomycin placed over the scleral bed is advocated in select cases or at the discretion of the operating surgeon. The expected IOP lowering capacity of this procedure is between 4 and 10 mm Hg and more.

4.Extended laser-assisted non-penetrating deep sclerostomy: An additional step of ablative dissection in the posterior part of the scleral bed in order to expose a small dot of brown choroidal tissue would be useful in providing additional suprachoroidal drainage for the percolating aqueous humor. The IOP lowering capacity of this modification in expected to be about the same as 3. However, the chances of short-term as well as longterm success are expected to be better.

5.Combination procedure: The procedure described above may be combined in a single operation or separately for additive effect.

6.Modified scleral bed ablation: In order to facilitate the drainage of the percolated aqueous under the flap, the surgeon can conveniently create ablative grooves connecting the trabeculectomized regions to the posterior parts. Creation of such multiple drainage channels will also decrease the chances of closure of the drainage channel and the grooves will facilitate fixation of the collagen implant.

7.Penetrating filtration procedure: Any of the above mentioned procedures can be converted to penetrating trabeculectomy combined with iridotomy if desired.

From the above description it can be appreciated that

the surgeon can customize the type of ablative excisions to suit the requirement of individual cases. Hence, we propose the name “Customized Laser Assisted Filtration Surgery” (CLAFS) to encompass all the variation of the procedures described. The multiple filtration channels are well illustrated in Figure 17.5. The procedure is preferably

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Fig. 17.5: CLAFS - Aqueous drainage through trabeculo-Descemet’s membrane and then via subconjunctival as well as through suprachoroidal spaces

carried out under peribulbar anesthesia, although, topical anesthesia with subconjunctival infiltration over the surgical site is also possible.

As shown in Table 17.1, the PR-270 using a solid-state UV laser has the advantages of minimal thermal effects, small energy per pulse and intermediate range power required for efficient ablation. The unique feature of beam spot adjustable of 0.2 to 0.8 mm also offers better controllable precise ablation. One of the major innovations employed in the new laser unit is an optical delivery system through an articulated arm in place of a fiber-based system. Any fiber-based delivery system inherently demonstrates a fairly early and consistent deterioration of energy output due to progressive fiber damage during usage. Moreover, the beam spot size in such a system is limited by the size of