McHugh, John Dominic Anthony; (1991) An investigation into the clinical potential and applications of ophthalmic diode lasers. Doctoral thesis (M.D.), University College London.

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Abstract

A study has been carried out of transpupillary retinal and of trabecular photocoagulation with infrared diode lasers. There were three phases to the study: A. Design and construction of instrumentation. B. Histopathological studies of lesions produced by diode lasers in rabbit retina, human peripheral retina and macula and on human trabecular meshwork. The appearances of the lesions were compared with those produced by lasers which emitted at other wavelengths. C. Pilot clinical trials in which diode lasers were used in the treatment of the following conditions: 1. Proliferative diabetic retinopathy 2. Exudative diabetic retinopathy 3. Branch retinal vein thrombosis complicated by neo- vascularisation of the optic disc or of the retina 4. Central retinal vein thrombosis complicated by established or threatened rubeosis iridis, or by optic disc neovascularisation 5. Chronic open angle glaucoma RESULTS A. INSTRUMENTATION For initial histopathological and clinical studies, a diode laser emitting at 810 nm and with a power output of 800 mW was constructed, which was incorporated in a modified hand-held direct ophthalmoscope. In a subsequent phase of development, a diode laser was assembled that could be attached to a standard slit lamp microscope and which had an eventual power output of 1.4 W. The project was completed using this modality, as it was felt that there was greater flexibility and ease of use in the treatment of patients. B. HISTOPATHOLOGICAL STUDIES In both the animal and in the human studies of peripheral retinal irradiation with a diode laser, the retinal burns were found to be similar to those produced by argon, and more particularly krypton photocoagulation. Macular photocoagulation produced burns in which damage was confined to the outer retina, retinal pigment epithelium and the choroid. This was in contrast with the appearances seen following argon blue-green photocoagulation 15 of the macula, in which inner retinal damage was observed, which was associated with absorption by macular pigments. Trabecular photocoagulation with a diode laser produced a pattern of damage to trabecular beams. The histological appearances were similar to those seen in relation to argon blue-green exposures, although the diode laser lesions extended more deeply into the trabeculum. C. PILOT CLINICAL STUDIES WITH DIODE LASERS 106 eyes in 85 patients were given diode laser therapy for proliferative diabetic retinopathy, exudative diabetic retinopathy, branch and central retinal vein thrombosis and chronic simple glaucoma. Regression of neovascularisation was observed in 33 of 47 eyes (70%) with proliferative diabetic retinopathy, and in all 11 eyes treated for branch vein thrombosis. Six eyes were successfully treated for established or incipient rubeosis iridis, following central vein thrombosis. Focal photocoagulation applied to 22 eyes for exudative diabetic maculopathy resulted in a reduction in the number of microvascular abnormalities and partial resorption of exudates. Laser trabeculoplasty carried out on 20 eyes for glaucoma resulted in a mean ocular hypotensive effect of 10.2 mm Hg, at 2 weeks following treatment and of 9.55 mm Hg, at 6 months. This thesis will detail the methodology and results associated with each phase of the study. In the context of the histological and clinical results, the discussion will consider the relative advantages of diode laser irradiation compared with treatment with lasers of other wavelengths. This will allow an assessment of the implications of the development of the diode laser with regard to its place in ophthalmic therapy.

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