Transscleral application of a semiconductor diode laser

Lasers in Surgery and Medicine 10:56!3-575 (1990)

Transscleral Application of a Semiconductor Diode Laser Gholam A. Peyman, MD, Khaled S. Naguib, MD, and Douglas Gaasterland, MD LSU Eye Center, Louisiana State University Medical Center School of Medicine (G.A.P., K.S.N.), New Orleans 701 12; Center for Sight, Georgetown University Medical Center (D.G.), Washington, D.C. 20007

We used a diode laser with an output power of 1 W through a fiberoptic light pipe (200 Fm diameter) to deliver laser energy through the sclera of pigmented rabbits. Ciliary body destruction occurred with energy levels of 300-400 mW and exposure time of 0.5 sec. Retinal photocoagulation was achieved with energy levels of 200500 mW in 0.5 sec. Histologic examination of acute lesions demonstrated thermal destruction of ciliary body processes and retina. Chorioretinal scar formation was observed clinically and histologically within !2-3 weeks. Our data indicate that the transscleral diode laser may be used for destruction of the ciliary body processes or peripheral retinal coagulation in pigmented eyes. Key words: ciliary body coagulation, diode laser, retinal photocoagulation, transscleral

INTRODUCTION

MATERIALS AND METHODS

The recent availability of semiconductor lasers has focused great interest on their application in ophthalmology [1-61. The characteristics of semiconductor lasers which make them attractive for ophthalmic use are high electrical-optical efficiency, small size, and the fact that water cooling is not necessary. Previous reports of the use of this laser have described its application as an endophotocoagulator [1,61 and as a transpupillary application with the slit lamp microscope [2-41. The 810-nm wavelength of this laser is mainly absorbed by melanin present in the pigment epithelial cells and the choroid, producing its thermal effect. A retina photocoagulated with a semiconductor application was shown to have clinical and angiographic characteristics similar to those exposed to shorter wavelengths, such as argon lasers [1,5]. These characteristics include the whitish appearance of the lesion in an early stage of photocoagulation and subsequent pigmentation within 2 weeks. We describe the effect of this laser, when applied through the sclera, on the retina and ciliary body structures.

Experiments were performed in five eyes of three dwarf Netherlands rabbits, each weighing an average of 1kg. The animals were anesthetized intramuscularly by using ketamine hydrochloride (50 mg/kg) and xylazine (2 mg/kg) and with topical proparacaine. The eyes were dilated 30 minutes preoperatively with 1%tropicamide hydrochloride and 2.5%phenylephrine hydrochloride. Photocoagulation was performed by using an OculightTMSL diode laser endophotocoagulator (Iris Medical Instruments, Inc., Mountainview, California). This device utilizes high-power diode laser to deliver 1.0 W of CW power at the 810-nm wavelength. The probe size is 20 gauge (200 pm

0 1990 Wiley-Liss, Inc.

Accepted for publication July 31, 1990. Address reprint requests to Gholam A. Peyman, M.D., LSU Eye Center, 2020 Gravier St., Suite B, New Orleans, LA 70112. This work was supported in part by US.Public Health Service grants EY07541 and EY02377 from the National Eye Institute, the National Institutes of Health Services, Bethesda, MD, and by grants from Research to Prevent Blindness to the Georgetown Center for Sight.

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Fig. 1. Fundus photograph demonstrating acute (white) lesions after transscleral application of diode laser. Small arrow indicates a 200 mW lesion, and large arrow a 500 mW lesion. Bent arrow indicates an explosive choriodal hemorrhage when the power setting approached 700 mW.

fiberoptic) by 35 mm. Exposure duration is userselected from 100 to 5,000 msec. Transscleral photocoagulation of the retina and ciliary body was performed by using different output powers (200 mW to 700 mW). Exposure duration of 0.5 sec was kept constant for all burns applied to the retina and ciliary body. Fundus and anterior segment photographs were obtained (using Topcon fundus camera TRCFET). Two rabbits were sacrificed 30 minutes after laser application by using an overdose intravenous injection of pentobarbital sodium. Eyes were enucleated and fixed in a mixture of 2% paraformaldehyde and 3% glutaraldehyde, dehydrated in graded series of ethyl alcohol (7080% and 95%), embedded in glycol methacrylate (GMA), sectioned at 1 pm, and stained with toluidine blue. One rabbit was followed for 21 days. After obtaining fundus pictures, the animal was killed

and eyes were enucleated for histology as described. RESULTS

Transscleral coagulation of the retina was achieved by using powers of 200-300 mW (Fig. 1). Higher powers (400-600 mW) produced intense reactions. At powers approaching 700 mW, retinal photocoagulation was accompanied by explosive hemorrhage from the choroid (Fig. 1).The lesions became clinically pigmented within 2 weeks and their appearance did not change thereafter (Fig. 2). Histological evaluation of acute lesions (300 mW) demonstrated necrosis of the retina (Fig. 3A,B) without damage t o Bruch’s membrane. The chronic lesion showed chorioretinal scar formation, and disorganization of the retina (Fig. 4A,B). Transscleral ciliary body coagulation was achieved by using powers of 300-400 mW. How-

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Fig. 2. Fundus photograph of the same eye as in Figure 1,demonstrating pigmented chronic lesions after transscleral application of diode laser.

ever, the reaction was not predictable. Higher laser powers produced explosive reaction at the site of the ciliary body. Histological evaluation of the acute lesions demonstrated necrosis at the site of the iris root and the ciliary processes located behind the iris. However, the lesions produced were variable in intensity and reaction (Fig. 5 ) . The inner portion of the sclera was also involved in this thermal reaction. Chronic lesions demonstrated disorganization of the ciliary body processes (Fig. 6) and scar formation. Because of the anterior location of ciliary body processes in the rabbit eye (behind the iris), peripheral anterior synechiae was observed in some eyes (Fig. 6). DISCUSStON

Advances in semiconductor technology have allowed the development of diode lasers which emit near-infrared beams of 810 nm. Early work by Puliafito et al. [l],Brancato et al. [2-41, and McHugh et al. [ 5 ] have shown the ability of this

laser to produce chorioretinal photocoagulation and its potential use in clinical applications. Histological studies on enucleated human eyes have shown similarities between lesions produced with semiconductor laser and those previously described after krypton laser application [5]. Subsequently, this laser has been used to treat retinal vascular diseases such as proliferative diabetic retinopathy, and branch and central retinal vein occlusion [71. In this study we evaluated the transscleral application of diode lasers. We found that coagulation of the retina was achieved at low power settings of 200-400 mW/0.5 sec. At these powers, the sclera at the site of photocoagulation was not significantly damaged. Similarly, although the reaction was not constant, ciliary body coagulation and coagulation of ciliary processes were possible using this laser. Explosive reactions could be produced at higher power both at the retina and ciliary body area. These reactions were at times unpredictable. Similar variability occurs, despite

Fig. 3. A: Histological section through an acute lesion after transscleral application of diode laser demonstrating necrosis of the entire retina. Note normal-appearing sclera under the

lesion (arrow). Toluidine blue, x 150. B (sideturned) Higher magnification of A demonstrating intact Bruch’s membrane (arrow). Toluidine blue, x 400.

Fig. 4.A Histologic section through a chronic lesion after transscleral application of diode laser, demonstrating full thickness disorganization of the retinal structure with pigment migration. Toluidine blue, x 120. B: Histologic section, higher magnification, of A, demonstrating glial scar and pig-

ment migration. Note also thickness and scarred choroid beneath this lesion. Bruch’s membrane is repopulated with regrowth of retinal pigment epithelial cells (arrow). Toluidine blue, x 350.

Fig. 5. Acute lesions after transscleral application of diode laser over a ciliary body demonstrating destruction of the iris root and the ciliary processes (arrows), defect in the iris and

ciliary body processes, and presence of fibrin in the anterior chamber. Note also ablation of the posterior peripheral part of the cornea (curved arrow). Toluidine blue, x 40.

Fig. 6. As in Figure 5, demonstrating chronic lesions after transscleral application of diode laser over ciliary body. Note disorganization and scar formation a t the site of ciliary body

processes (large arrow) and peripheral anterior synechia to a retrocorneal membrane (small arrow). Toluidine blue, x 40.

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ACKNOWLEDGMENTS

This work was presented in part at the 1990 ARVO meeting in Sarasota, Florida.

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