Portal de Periódicos da CAPES

Retinal hazards during laser irradiation of the iris.

1971; BMJ; Volume: 55; Issue: 1 Linguagem: Inglês

10.1136/bjo.55.1.60

1468-2079

G.K. Watts,

Laser Applications in Dentistry and Medicine

Transpupillary pulsed laser radiation has been extensively studied to determine both the level at which it becomes a hazard to the retina (Ham, Williams, Mueller, Guerry, Clarke, and Geeraets, I966), and its effects on that tissue when such levels are exceeded (Marshall and Mellerio, I967a,b, I968).In spite of the development of various codes of practice to protect users, several accidents have been reported (Rathkey, I965; Zweng, I967; Curtin and Boyden, I968).More recently an attempt has been made to evaluate trans- scleral hazards (Smith and Stein, I968, i969), and safe levels have now been defined for both ruby and neodymium laser beams entering the eye through this tissue.No investigative work has so far been carried out on the possible transmission of laser beams impinging on the iris.Such an event might take place under two circumstances; first, with the eye open the iris may contribute up to half the exposed area of the globe and thus form a significant target for laser beams accidentally discharged towards the eye.Secondly, it may be necessary to fire laser beams directly at the iris for clinical reasons; peripheral iridotomy by laser has been discussed in the literature (Flocks and Zweng, I 964; Hallman, Perkins, Watts, and Wheeler, I 968), and retinal hazards commented upon (Snyder, i967), but no practical measurements of such hazards have been made.The aim of this work is to study the spectral characteristics of the iris to non-coherent light and (to a limited extent) laser light, and predict the possible hazards of trans-iridial laser radiation. Experimental methods A. NON-COHERENT LIGHTThe first problem in attempts to measure the spectral characteristics of the iris was the mounting of the specimens.The tissue stretches easily and thus required not only a firm base but careful control of its thickness.The thickness of the iris in vivo varies with the size of the pupil, and the following method was designed to give a preparation which could be handled easily and ensure that each iris was examined under similar conditions.Although the absolute values obtained with this method may not correspond to the iris in its normal state, comparative values for transmission and reflectance at different wavelengths and with different coloured irides should still be valid.The iris samples were prepared from freshly enucleated rabbit eyes; the dissection procedure was simple but, in view of the relatively fragile nature of the unfixed iris, extreme care was required.The posterior segment of the globe was cut away and the anterior segment was held face downwards in a dish while capsule forceps were slipped beneath the lens to lift it away from the underlying pigment epithelial layers of the iris.The iris was removed by seizing its periphery with a pair of toothed forceps and gently tugging.Provided that reasonable care was taken, the whole tissue could then be removed intact.A single radial cut from pupil to periphery enabled the iris to be laid flat on a microscope slide in a pool of saline and smoothed out.A second slide was then placed over the preparation and the tissue compressed until it had only the thickness of the single cover