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

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applied to the very individual ciliary processes.6,14 Endoscopic cyclophotocoagulation treatment encompassed 180 to 360 degrees of the ciliary body circumference and is performed through a limbal incision. An 810 nm pulsed continuous-wave diode laser capable of 1.2 W output is being used. Generally, 800 mW are being used for less than 1 second, for a total of 0.8 J per treatment. Early results suggest that endoscopic cyclophotocoagulation is a safe and effective therapeutic modality for refractory glaucomas.11

The postoperative care of patients undergoing cyclophotocoagulation consists of patching the eye for one day with topical steroids to control the postoperative inflammatory stage (Fig. 5.5). The steroids are gently tapered and antiglaucoma medication adjusted according to the postoperative examinations. The conjunctiva is frequently burned and uveitis is present after every laser treatment but subsides with steroids application. The pain might be moderate to severe, generally lasting no longer than a few days. The most severe complication implies prolonged hypotony and phthisis bulbi, a feared event that had already occurred in several cases after cyclocryocoagulation.5 These complications tend to be less frequent when using the diode laser cyclophotocoagulation.2

The pressure lowering effect of laser cyclophotocoagulation is variable but values of 15 to 30 mm Hg might be achieved, and results overtime remain quite stable. Nd:YAG and diode laser cycloablations are relatively safe and effective at controlling IOP in eyes with advanced refractory glaucoma in the short and medium term.4 Glaucoma medications are generally also reduced after one or more laser sessions. Repeated treatments are nevertheless required in some patients when the intended lowering effect is not reached at the first session.

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LASER TREATMENTS AFTER FILTERING SURGERY

After classical filtering surgery that has been performed without complications, it may happen that filtration progressively decreases. The IOP rises again to values measured before surgery. The main cause for such failure is an overscarring of the filtration bleb.

The trabeculectomy technique requires a good closure of the scleral flap to prevent excessive aqueous outflow resulting in hypotony and shallow anterior chamber. But a too tight closure of the scleral flap may result in excessive elevation in IOP in the postoperative period. To prevent prolonged elevation in the IOP, it is possible to selectively cut some sutures to release the tension of the scleral flap.7,9 Scleral flap sutures are made of nylon or polypropylene sutures that are either black or deep blue. The thermal effect of argon laser is efficient in lysis of such material through the conjunctiva. Technically, the procedure is performed with a contact glass applied to the conjunctiva, using the Hoskins or the Ritch lens.4,11 This allows a better visualization of the suture by flattening the Tenon and conjunctiva layers. It is better not to perform ocular massage immediately before the laser lysis, as the massage brings some subconjunctival fluid that will scatter laser light and reduce visibility and accessibility to the suture. The laser setting is 50 μm for the spot size, with a duration of 0.1 sec and power of 300 to 500 mW. The laser beam is focused on the suture and shots are applied until the suture is being cut. Intraocular pressure is then measured to ascertain the degree of efficiency reached. Gentle massage of the globe might also be performed to promote the outflow. Caution should be exercised not to excessively treat by cutting too many sutures resulting in hypotony, shallow anterior chamber, choroidal effusion, optic disk

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and macular edema.1 The ideal time to perform suture lysis is 1 to 2 weeks after surgery in case of elevated IOP. After this period, the result is less prone to be efficient, probably because of scarring of the scleral flap.3 Other causes of elevated pressure like iris incarceration into the internal sclerostomy or malignant glaucoma, should be investigated and eliminated accordingly.

Overfiltration on the other hand is a troublesome complication that requires efficient treatment. Overfiltrating bleb can be brought to a less effective size by enhancing the fibrosis. Invasive techniques like autologous blood injection in the bleb have been reported to be efficient. Noninvasive methods using laser techniques can also be developed to prevent persistent hypotony and create some local subconjunctival bleeding. By applying gentle laser spots onto the conjunctiva with a Nd:YAG laser, the conjunctival vessels are disrupted and bleeding occurs.2 The idea is to create adhesion scars between the conjunctival bleb and the Tenon shield that reduce the size and efficiency of the bleb. It is also possible to physically promote scarring of the subconjunctival space and Tenons and mechanically tighten the two layers. To achieve this goal, several burns are made at the edge of the bleb by using the Hoskins or the Ritch lens to depress the conjunctiva. The spot size is set between 50 and 100 μm, with a duration of 0.1 sec, a power of 200 to 400 mW. The beam is focused on the Tenon or slightly deeper to avoid perforation of the conjunctiva, and the spots are evenly spaced all around the bleb. Power should not be excessive as this will result in conjunctival buttonhole or formation of subconjunctival bubbles that will increase the bleb size instead of helping scar formation.6

Laser treatment in the postoperative period can also be used in conjunction with other surgical techniques used

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to treat refractory glaucoma. In this case, drainage devices are being used to create an effective outflow from the anterior chamber to the subconjunctival and sub-Tenon’s space. But this technique requires an adequate control of the outflow that can be achieved by placing a stent suture around the tube. In the postoperative period, the IOP is monitored and the suture may be released should the pressure be elevated.8 It may also happen that the tube opening in the anterior chamber becomes blocked by fibrin clots and this results in failure of the shunt technique. Nd:YAG laser membranectomy will clean the tube and thus restore a patent drainage to the external plate.12

In the new non-perforating techniques, like deep sclerectomy with collagen implants or viscocanalostomy, the trabecular meshwork remains untouched and only the inner wall of Schlemm’s canal and the juxtacanalicular meshwork are removed.5,13 A very thin membrane prevents the anterior chamber to collapse during the filtering surgery. But after a few weeks or months, the trabeculodescemetic membrane may become fibrotic because of elevated IOP and should be open to allow better outflow. This procedure is called goniopuncture, and can easily be done with a Nd:YAG laser. The goal of this technique is to perforate the remaining trabecular meshwork at the surgery site by performing an internal trabeculotomy or descemetomy. A gonioscopic contact glass is set on the eye of the patient after topical anesthesia to gain access to the anterior chamber angle. The trabecular meshwork at the surgery site appears as a thin less pigmented membrane, some light from the sclerectomy being transmitted through the wall (Fig. 5.6). The laser beam is pointed to this membrane and several impacts are shot. The power level is set at 5 mJ. Some air-bubbles may be generated during the treatment. It is sometimes difficult

Laser Treatment in Glaucomas 67

Mechanical techniques that involve the use of cutting blades, blunt dissection, tissue removal and titrated sutures promote inflammatory responses that will modulate the healing and scarring processes. Overacting fistulas or inefficient filtering blebs result from variation of standard procedure and lead to the need of complementary surgery or adjunctive treatment. Laser, by its physical property, is a very useful tool that can provide calibrated beams at various energy levels. By developing adequate delivery systems to the trabecular meshwork, it is possible to perform a direct fistula through the tissue, a so-called sclerostomy.7 The advantages of such techniques are that the fistula diameter can be better predicted, tissue trauma could be less extended, especially regarding the development of the filtering bleb.3

Three methods have been developed to achieve this goal. All are based on the kind of delivery system being used to apply laser energy to create a channel through the sclera and trabecular meshwork. Basically there are two approaches possible, from outside the eye, the ab-externo procedure, or from inside, the ab-interno procedure. The latter can further be divided into two categories, invasive or non-invasive procedure.

To perform a non-invasive ab-interno sclerostomy, a modification of the current laser technique has to be made before completion of the treatment. A pulse-dye laser is used to get a beam for the non-contact delivery system connected to the slit-lamp. A gonioscopy lens is applied on the eye to allow good visualization of the angle structures. The laser produces a beam at 660 nm, set at 200 μm spot size, 10 μsec duration and 200 to 400 mJ energy level. In order to get the most effects from the beam at that wavelength, the site of sclerostomy must be dyed with methylene blue dye with an iontophoresis marker.14 The

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beam is then directed to the blue patch that is visible in the gonioscopic lens. Shots are given repetitively until the whole sclera has been drilled and the anterior chamber shallows and the conjunctival bleb develops.8 The invasive ab-interno sclerostomy consists of introducing an endolaser probe in the anterior chamber to deliver laser beam directly to the trabecular meshwork.11 The main advantage consists of leaving the conjunctiva untouched avoiding scar formation. Disadvantages of this technique are those related to any intraocular approach; the infectious risks, trauma to the endothelium, the iris structures and the lens. Thermal damages due to accumulation of heat at the impact site are also related to this technique. To introduce the endolaser probe, a paracentesis is performed on the cornea and the anterior chamber is filled with a viscoelastic agent. The fiberoptic probe is introduced and the tip is brought into perpendicular contact with the corneoscleral tissue, taking care to avoid the posterior trabecular meshwork. Laser beam is actuated and tissue ablation begins. The probe is moved forward as ablation removes tissue leaving space for further ablation. The end of the procedure is determined when the tip is seen through the conjunctiva, and the aqueous humor and viscoelastic agent flow into the bleb.1

The ab-externo sclerostomy consists in creating a channel through the sclera to the anterior chamber.12,13 A contact probe is placed at the limbus to guide the laser beam. The conjunctiva is cut 15 mm away from the intended treatment site to allow the probe to be placed correctly thus allowing an adequate contact with the sclera. A holmium:YAG laser is used to produce the energy required to ablate the tissue. The energy level is set at 100 mJ/pulse. The beam is applied to the sclera and can be seen through the cornea into the anterior chamber. Progression of creation of the channel can therefore be

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monitored until completion. At that stage, aqueous humor flows through the fistula into the subconjunctival space.10 After removal of the probe, the conjunctival incision is closed with a single suture and topical antiinflammatory medication given.2,5,6,9

REFERENCES

Laser Principle and Introduction

1.Birngruber R. Thermal modeling in biological tissues. In Hillenkamp F, Praetesi R, Sacchi CA (Eds): Laser in Biology and Medicine Plenum: New York, 1980.

2.Dietlein TS, Jacobi PC, Krieglstein GK. Ab-interno infrared laser trabecular ablation: Preliminary short-term results in patients with open-angle glaucoma. Graef’s Arch Clin Exp Ophthalmol 1997;235(6):349-53.

3.Dietlein TS, Jacobi PC, Krieglstein GK. Erbium: YAG laser trabecular ablation (LTA) in the surgical treatment of glaucoma. Lasers Surg Med 1998;23(2):104-10.

4.Hillenkamp F. Interaction between laser radiation and biological systems. In Hillenkamp F, Praetesi R, Sacchi CA (Eds): Laser in Biology and Medicine. Plenum: New York, 1980.

5.L’Esperance FA Jr. Ophthalmic Laser: Photocoagulation, Photoradiation and Surgery (3rd edn): Mosby: St. Louis 1989.

6.Loertscher H, Shi WQ, Grundfest WS. Tissue ablation through water with erbium: YAG lasers. IEEE Trans Biomed Eng 1992;39:86-88.

7.Mainster MA, Sliney DH, Blecher CD, et al. Laser photodisruptors: Damage mechanisms, instrument design and safety. Ophthalmology 1983;90:973-91.

8.Mainster MA, Ho PC, Mainster KJ. Argon and krypton laser photocoagulators. Ophthalmology 1983;90: 48-54.

9.Ready JF. Industrial applications of lasers. Academic Press: New York, 1978.

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10.Schuman JS, et al. Experimental use of semiconductor diode laser in contact transscleral photocoagulator in rabbits. Arch Ophthalmol 1990;108:1152-57.

11.Schuman JS, et al. Energy levels and probe placement in contact transscleral semiconductor diode laser cyclophotocoagulation in human cadaver eyes. Arch Ophthalmol 1991;109:1534-38.

12.Sliney DH, Mainster MA. Potential laser hazards to the clinician during photocoagulation. Am J Ophthalmol 1987;103:758-60.

13.Wetzel W, Brinkmann R, Koop N, et al. Photofragmentation of lens nuclei using the Er: YAG laser: Preliminary report of an in vitro study. Ger J Ophthalmol 1996;5(5): 281-84.

14.Wieder M, Pomerantzeff O, Schnieder J. Retinal vessel photocoagulation: A quantitative comparison of argon and krypton laser effects. Invest Ophthalmol Vis Sci 1981;20:418-24.

Laser Iridotomy

1.Anderson DR, Forster RK, Lewis ML. Laser iridotomy for aphakic pupillary block. Arch Ophthalmol 1975;93:343-46.

2.Breingan PJ, Esaki K, Ishikawa H, et al. Iridolenticular contact decreases following laser iridotomy for pigment dispersion syndrome. Arch Ophthalmol 1999;117(3):32528.

3.Brooks AMV, Harper CA, Gillies W. Occurrence of malignant glaucoma after iridotomy. Br J Ophthalmol 1989;73:617-20.

4.Carassa RG, Bettin P, Fiori M, et al. Nd:YAG laser iridotomy in pigment dispersion syndrome: An ultrasound biomicroscopic study. Br J Ophthalmol 1998;82(2):150-53.

5.Fleck BW, Wright E, Fairley EA. A randomised prospective comparison of operative peripheral iridectomy and Nd:YAG laser iridotomy treatment of acute angle closure glaucoma: 3-year-visual acuity and intraocular pressure control outcome. Br J Ophthalmol 1997;81(10):884-88.