Experimental retinochoroidal tissue vaporization with a CO2 laser probe

International Ophthalmology 11:163-169 (1988) 9 Kluwer Academic Publishers, Dordrecht - Printed in the Netherlands

Experimental retinochoroidal tissue vaporization with a CO2 laser probe Gholam A. Peyman & Antimo Candel The LSU Eye Center, Louisiana State University Medical Center School of Medicine, New Orleans, Louisiana, USA Accepted 9 January 1988

Key words: CO2 laser, tissue vaporization, retina, choroidal tumors Abstract

The immediate and chronic effects of an intraocular carbon dioxide C O 2 laser probe used for tissue vaporization were studied in cynomolgus monkeys' eyes. Acute lesions produced necrosis (from superficial to deep) of the retina and choroid, depending on the amount of energy used. Chronic lesions showed disorganization of the retina and choroid, with scar formation. Extensive tissue vaporization produced tractional retinal detachment in the chronic stage. Tractional detachment was prevented if the area of CO2 laser application was previously encircled with argon laser scars.

Introduction

Materials and methods

Although the use of CO, laser for vaporization of the vitreous and coagulation of the retina via an endoprobe has been extensively studied [1-6], there is little information on its use for vaporization of the retina and choroid. We previously reported on a technique for ab-interno removal of retinochoroidal tissue using a vitrectomy instrument [7-9]. The purpose of those investigations was to evaluate a surgical method for internal vaporization of choroidal tumors located close to the optic nerve, for which no satisfactory treatment presently exists. In this study, the effect of a CO~ laser on retinochoroidal tissues was evaluated (specifically. the effect of the CO~ laser when used for vaporization of these structures in primate eyes).

A small, air-cooled CO~ laser (CooperVision, Santa Clara, CA), which was capable of producing 6 to 10 watts of output in a continuous mode, was used in this study. The intraocular portion of the handpiece measured 25 mm long and 2 mm wide, and could be connected to an air pumping system. Six adult cynomolgus monkeys (12 eyes) were used in this study. The eyes were divided into four groups of three eyes each. In Group I, the immediate effects of COn laser on the retina and choroid were evaluated. Group II was used to evaluate the long-term effects of CO~ laser on the retina and choroid. In Group III, CO~ vaporization of an area of retina and choroid measuring 2 to 4 disk diameters (DD) was studied. In Group IV, eyes were treated first with an encircling application of photocoagulation with an argon laser, then tissue vaporization with a CO~ laser was performed. The pupils were dilated with 1% phenylephrine hydrochloride (Neo-synephrine| and 1% Cvclogyl| While under general anesthesia by intramus-

This work was supported in part by PHS grant EYO 2377 from the National Eye Institute. National Institutes of Health, Bethesda, MD.

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cular injection of thiamylal (Surital'-~), the lids were retracted with a lid speculum. A conjunctival peritomy was performed. The recti muscles were isolated and a 4-0 silk suture was passed under them to provide traction. An infusion cannula was sutured at the site of the pars plana (2 o'clock hour position) and was connected to an infusion bottle. Another sclerotomy was performed at the pars plana (10:30 o'clock hour position) for insertion of the vitrophage. A lensectomy and vitrectomy were performed. After removal of the vitrophage, the infusion cannula was connected to an air pump and, using a flute needle, a complete fluid/air exchange was performed. At this time, the intraocular portion of the CO2 probe (connected to an additional air pump) was inserted inside the eye. The air pump connected to the laser probe delivered air at 50 to 60 mm Hg, while air was delivered to the infusion cannula at a pressure of 30 to 40 mm Hg. The differential pressure forced the air from one side to exit through the air pump connected to the infusion cannula. The purpose of this arrangement was to prevent the smoke produced during tissue vaporization from entering the CO~ laser probe and damaging its optics. In addition, it prevented collapse of the eye during removal of the CO, laser probe. The CO~ laser probe was placed close to the retina and the power was adjusted to range between 2 and 6 watts (continuous application). Various lesions were produced in the eyes of the animals. In Group I animals, the eyes were enucleated immediately after laser application and processed for histology. Group II received laser application as did Group I; however, the eyes were observed for a period of one month, and the animals were sacrificed by an overdose of intravenous pentobarbital. The eyes were then enucleated and processed for histological evaluation. In Group III animals, tissue coagulation and vaporization was performed in areas ranging from 2 to 4 DD. In Group IV eyes, the area of tissue vaporization was surrounded with multiple argon laser coagulation four weeks before the CO~ laser application. Otherwise, the procedure was the same as for Group III. The animals were kept alive for a period of 6 to 8 weeks. Eyes were examined

and fundus photographs were taken. Then these animals were sacrificed with an overdose of pentobarbital. The eyes were enucleated and fixed in a solution containing 2% formaldehyde and 2% glutaraldehyde. After an overnight fixation, the eyes were sectioned with a sharp razor blade and the lesions were examined under the dissecting microscope. The laser lesions were refixed in osmium tetroxide and embedded in plastic. The eyes of Group II to IV animals were fixed, but embedded in paraffin for light microscopy. The electron microscope sections were cut and stained with Azure blue and paraffin sections were stained with hematoxylin/eosin (H.E.) and studied under a light microscope.

Results

Clinically, CO~ laser application was accompanied by smoke from the coagulated tissue. The charring effect was a consistent observation after laser application. Histologically, the CO2 laser application involved superficial or deeper layers of the retina and choroid depending on the amount of energy and on the time of application (Figs. 1 and 2). Chronic lesions showed complete disorganization of the retina (Fig. 3). Extensive tissue vaporization without prior photocoagulation of that area produced tractional retinal detachment within two to six weeks (Fig. 4A, B, C). When the tissue vaporization was preceded by an encircling laser photocoagulation, tractional detachment was prevented (Fig. 5A, B).

Discussion

In this experimental study, the short- and longterm effects of CO, laser on the retina and choroid were evaluated. The lesions produced immediately after CO,,_ laser application demonstrated coagulative necrosis of the retina and choroid. Proportional to the amount of energy used, either or both superficial and deeper structures of the retina and choroid were coagulated. The reactions were visible clinically as a whitish discoloration of the reti-

Tissue vaporization with a COe laser probe

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Fig. 1. Light micrograph of primate retina after CO, laser application. Note tissue loss and charring (arrow). R, retina. Azure blue. x212.5.

Fig. 2. Light micrograph of primate retina after CO2 laser application. Note the extent of charring of the retina (arrow). Azure blue. x 238.

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Fig. 3. Lightmicrographof primate retina 30 daysafter CO2 laser application. Note completedisorganizationof the retina (arrow). Azure blue. x340. na, then smoke production and charring of the tissue. The structures of the retina and choroid were coagulated with no predilection for tissue pigmentation. These data are comparable to those described by Miller [3-6], who advocated the use of CO2 laser for vaporization of the vitreous, coagulation of the detached retina, and coagulation of surface neovascularization. The chronic lesions showed disorganization of the retinochoroidal structures, with loss of tissue. Interestingly, these lesions were always accompanied by contraction of the normal retina adjacent to the lesion or contraction of the attached vitreous, with production of tractional retinal detachment adjacent to the lesion. The latter finding was more prominent when tissue vaporization involved large areas (3 DD). In fact, all retinas of Group III eyes detached by the end of the observation period (eight weeks). This data is at variance with previously published data [1-6], which indicated that chronic lesions did not produce tractional detachment. The findings of this study are explained on the basis of the extent of tissue coagulation and

vaporization. In previous reports [1-6], the lesions were seldom larger than 1.5 disk diameters and the coagulation may not have been as intense as in this study. When the area of tissue vaporization was initially surrounded by encircling argon laser coagulation, and when a scar was allowed to form before tissue vaporization with CO2 laser, tractional retinal detachment was prevented. Our study describes a technique for proper application of CO2 laser when used for tissue vaporization in the eye. It was discovered that a complete vitrectomy is important to prevent adhesion of the vitreous to the edges of the vaporized tissue. Preoperative argon laser application around the areas intended for vaporization stabilizes the tissue and resists tractional forces in this area which develop during the scar formation process. The method of using two air pumps having differential pressure forces the smoke away from the CO2 laser probe (which can be damaged by smoke) and is of importance in preventing collapse of the eye when the CO2 laser probe is removed. In the latter situation, the air pump connected to the infusion cannula

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Fig. 4. (A) Fundus photograph of primate retina after CO2 laser vaporization (arrow) two weeks after laser application. (B) Fundus photograph of the same eyes six weeks after laser application. Note development of tractional retinal detachment around the lesion. (C) Light micrograph of a chronic CO2 lesion. Note central chorioretinal scar (arrow) and detachment of the retina. H.E. x85.

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G.A. Peyman & A. Candel Fig. 5. (A) Fundus photograph of the primate eye after tissue vaporization with prior encircling coagulation of the area. Note attached retina surrounding the lesion (arrow). (B) Histological section of the chronic lesion of a CO2 laser application with prior argon laser application. Note chorioretinal scar (arrows) and attached intact retina. R. retina. H.E. x51.

maintains the intraocular pressure. With perfection of this technique, it is believed that the use of the CO~ laser in ophthalmology can be expanded.

Vaporization of small tumors located close to the optic nerve would be possible in cases that do not permit conventional therapy (such as radiation).

Tissue vaporization with a C02 laser probe References 1. Karlin DB, Patel CKN, Wood OR II et al. CO, laser in vitreoretina[ surgery: Quantitati~ e investigation of the effects of CO~ laser radiation on ocular tissue. Ophthalmology 1979; 86:290. 2. Meyers SM, Bonner RF, Rodrigues MM et al. Phototransection of vitreal membranes with the carbon dioxide laser Ln rabbits. Ophthalmology 1983: 90:563. 3. Miller JB, Smith MR, Boyer DS. Intraocular carbon dioxide laser photocautery Preliminary report of clinical trials. Arch Ophthalmol 1979: 97:2123. 4. Miller JB, Smith MR, Boyer DS. Miniaturized intraocular carbon dioxide laser photosurgical system for multi-incision vitrectomy. Ophthalmology 1981: 88:440/ 5. Miller JB, Smith MR, Boyer DS. Intraocular carbon laser photocautery: Indications and contraindications at vitrectomy. Ophthalmology 1980; 87:1112.

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6. Miller JB, Smith MR, Pincus F et al. Transvitreal carbon dioxide laser photocautery and vitrectomy. Ophthalmology 1978; 85:1195. 7. Peyman GA. Barrada A. Retinochoroidectomy ab interno. Ophthalmic Surg 1984: 15: 749-751. 8. Peyman GA, Hindi M. Ab interno retinochoroidectomy in primates. Arch Ophthalmol 1985: 103: 572-575. 9. Peyman GA, Cohen S. Ab interno resection of uveal melanoma. Int Ophthalmol 1986: 9: 29-36.

Addressfop offprints: Dr. G.A. Peyman LSU Eye Center 2020 Gravier Street Suite B New Orleans, LA 70112-2234 USA