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

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the market and it transmits a beam of 5-15 W through an articulated arm. A scanner was attached to the CO2 laser to enable the surgeon to delimit the area to be treated and to provide even and regular distribution of the laser energy over this area. The laser probe was attached to the surgical microscope, thus maintaining the probe at a predetermined fixed distance from the ablated tissue when the microscope was in focus. In practice, tissue ablation is done while the surgeon is looking through the microscope, accurately targeting the scanner marks with the proper pattern and dimensions over the desired treatment location.

PRECLINICAL STUDIES

Initial studies were done on enucleated cow and sheep eyes at the Laboratory for Intraocular Microsurgery and Implants, Goldschleger Eye Research Institute, Sheba Medical Center, Israel, and at the Laboratory for Intraocular Microsurgery and Implants, Meir Medical Center, KfarSaba, Israel. The intraocular pressure was maintained at a constant predetermined pressure of 38 mm Hg by using anterior chamber maintainer (ACM) with the bottle placed at 50 cm above the tested eye. Following dissection of a scleral flap the tissue underneath was laser ablated until fluid was seen percolating in the treated area without evidence of penetration. These studies proved the validity and feasibility of the concept and help determine the laser parametes for clinical use.

The second set of experiments was done on rabbit eyes. Rabbits are known to be very reactive to any surgical procedure and even full thickness trabeculectomy is often closed and ineffective within days after surgery. Nevertheless, in order to investigate the pure effect of the laser treatment we did not use any tissue spacers, viscoelastic substances or antimetabolites to enhance and

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prolong the surgical effect. The IOP was measured by pneumotonometry and compared to the fellow, untreated eye. In one case perforation into the anterior chamber was accompanied by iris prolapse. In the rest of the cases the IOP decreased immediately after the laser surgery (a mean of 10 mm Hg on day 1). The intraocular pressure was significantly lower than the fellow eye for the first 3 weeks. It should be emphasized that these rabbits did not have glaucoma and the pressure eventually stabilized at the preoperative normal levels.

The third set of experiment was done on human cadaver eyes at the Center for Research on Ocular Therapeutics and Biodevices, Storm Eye Institute, Medical University of South Carolina, Charleston, SC, USA (Director: David J. Apple, MD). A 4 × 5 mm scleral flap was dissected and laser pulses of moderate power (10 W) were applied over the exposed scleral wall. Since it was evident from the rabbit study that tissue charring interferes with the laser effect, the charred tissue was removed after every 5-7 laser shots with a wetted sponge. Upon approaching the trabecular tissue, or when the first signs of fluid percolation were seen, the laser power was lowered to 5 W and the application rate was reduced. The treated zone was dried with a sponge, and the next shot was applied only after a delay of 2-3 seconds to allow localized wetting by the percolating fluid. This way only the dry area was further ablated, whereas over the wetted area, where more ablation was not required, the laser energy was absorbed by the percolating fluid. If the treated area seemed to be too small, laser dissection of the tissue was extended laterally to a desired width until satisfactory aqueous percolation was achieved. Histopathological studies confirmed the deep ablation down to the trabecular meshwork and Descemet’s membrane, leaving a micro-thin wall 30-50 m thick, with

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Hydrabad, India (Prof R. Thomas). Twenty-three patients were treated using the protocol determined in the preclinical studies. We decided that on our initial cases we would study only the net effect of the laser treatment and would not use any adjunctive treatment. Even though we knew that we might reduce the chances of low pressure and filter survival we did not use spacers under the external flap, apply antimetabolic agents to reduce tissue scarring, inject viscoelastics into the Schlemm’s canal (viscocanalostomy) or perform YAG laser goniopuncture in failed cases.

Surgical procedure succeeded in all cases and the pressure dropped dramatically on the first postoperative day from a mean of 27.4 to 5.2 mm Hg. There was no case of flat anterior chamber or any significant postoperative complication. In one eye prolapse of the iris base into the treated area was seen on gonioscopy and the iris was surgically repositioned. The mean pressure after the first week was 11.3 mmHg, however from the two-weeks visit on, two distinct groups were evident: those patients whose IOP was low at the two-weeks visit maintained a low pressure thereafter (half of the cases), whereas increased pressure at two weeks was usually associated with longterm elevated pressure that required additional medications, and in 3 cases re-operations (Fig. 18.3).

The clinical studies confirmed that the CO2 laser can effectively ablate the dry tissue without tissue perforation by a relatively simple procedure. Satisfactory fluid percolation was achieved in all cases and no significant complications were seen in any of the eyes. The immediate postoperative results indicate that by applying the laser alone, long-term pressure drop without medications can be achieved in half of the cases. The late failure in the other cases is probably secondary to the localized tissue heating,

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Fig. 18.3: Ultrasonic biomicroscopy of a clinical case. The filtration “pool” is open, even though no spacers were used. A thin active sub-conjunctival bleb is seen above the scleral flap. Compare with the histology on Figure 18.2

which causes tissue irritation and inflammatory reaction. Anterior synechia and localized fibrosis were typically seen in the failed cases. The optimal laser parameters that would provide the desired tissue effect with minimal heating still need to be determined. We speculate that by using adjunctive therapy, such as placing spacers and applying Mitomycin C under the scleral flap, frequent application of local steroids and using external lasers for suturelysis and goniopuncture will increase the success rate of the procedure.

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SUMMARY

In summary the CO2 laser assisted NPFS utilizes the unique qualities of this far-infrared laser, i.e. the ability to ablate dry tissue and the almost complete absorption by water. This promising procedure enables accurate dissection of the scleral wall and unroofing of the Schlemm’s canal without penetration into the anterior chamber. The technique is, therefore, practically extraocular, relatively simple and requires only a short learning curve. Further modifications of the surgical procedure, and more controlled clinical studies are still required.

Video

A clinical case of CO2 laser assisted non-penetrating filtration surgery.

A large area under the scleral flap is first ablated in order to create a filtration “lake”. Then, the scanner pattern is narrowed to create a slit over the area above the trabecular meshwork. Treatment is applied until fluid easily exits through the thinned wall. Note that when the treated area is wet, repeated laser application are not effective and the remaining scleral “membrane” is not perforated. Two 10-0 nylon sutures are used to close the external flap and a single suture is sufficient to close the conjunctiva. Anterior chamber maintainer was used in this case to maintain constant intraocular pressure, however it is not necessary in a routine case.

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HISTORY

The goal of all surgery for glaucoma is to lower the ocular pressure in order to reduce postoperative risk as much as possible.

Surgical techniques using perforation have several postoperative disadvantages.

A certain number produce complications such as hyphema, flat anterior chamber, choroid detachment, cataract, and endophthalmitis.

Non-penetrative surgical techniques do not have the postoperative complications of the first, penetrative, surgery, but they are more difficult to carry out. An exact knowledge of the micro-anatomy of the region is important and a learning curve necessary.

The main drawbacks with these new techniques are that they are less effective with weak hypertensions, and, importantly, they increase the number of times the operation needs to be performed due to poor healing and flap collapse.

•1909 Elliott’s trepanation

•1960 Burian’s trabeculectomy,7 Sugar 196126

•1968 Cairn’s trabeculectomy for open angle glaucoma8

•1962 Krasnov’s Sinusotomy: this technique aims to remove the external wall of the Schlemm canal15

•1984 Zimmermann: Non-penetrating trabeculectomy28

•1984 Fiodorov and Koslov suggest the term ‘nonpenetrating deep sclerectomy’ (NPDS)12

•1990 Koslov improves his technique by adding a collagen implant into the base of the flap. At the same time, many others tried different types of implants to increase the duration of the drainage life of the aqueous humor14

•1991 Arenas: Archila Trabeculotomy ab externo. Areans uses the same technique but employs the help of a trepan to open the Schlemm canal1

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•1999 Stegman: Viscocanalostomy The author proposes a dissection and an injection of viscous fluid into the Schlemm canal.20

ANATOMOPHYSIOLOGY

Goldman13 demonstrated by using manometric experiments, the main point of resistance to the drainage of the aqueous humor was situated between the anterior chamber and the Schlemm canal.23 Nowadays, it is generally accepted that 75 percent of drainage resistance is situated at the level of the endothelium of the Schlemm’s canal and of the trabeculum network.21

New Techniques

The new techniques of filtering non-perforating are all connected with dissection or injection of the Schlemm’s canal.

Krasnov15 proposes the sinusotomy removing the endothelium of the Schlemm canal using microdissection.

The principle of deep sclerectomy is to dissect the internal wall of the Schlemm canal where the greatest resistance to drainage of the aqueous humor is situated, thus allowing a physiological filtration, since the external wall is kept intact.

The technique of using high viscosity to widen the Schlemm canal, allows better filtration of aqueous humor.

The biggest drawback of these techniques, above all deep sclerectomy, is the formation of fibrous tissue on the sclerotic flap.

Numerous authors have described a multitude of implants that would keep the space of the second flap

free.10,11,14,16,18,19,24

We submit a new surgical technique which allows a synthesis of the three main surgical techniques for non-