Walter J. Geeraets

An equilibrium thermal model for retinal injury from optical sources.

A uniform absorption thermal model is described which allows the calculation of the temperature rise in the retina due to steady state or continuous optical irradiation. Temperature rises of 9-10 degrees C are found to correspond to the production of threshold lesions. For a worst case approximation, a power of 1-2 mW entering the eye and focused onto a 10-micro diam area for 250 msec or longer can be shown as sufficient to cause irreversible damage.

OPTICAL MASERS (LASERS).

The possibility of constructing an oscillator using Microwave Amplification by means of Stimulated Emission of Radiation MASER was discussed at a symposium at the University of Illinois in May 1951 by A. H. Nethercot, a colleague of C. H. Townes of Columbia University. A year later, independently and almost simultaneously, J. Weber of the University of Maryland and N. G. Basov and A. M. Prokhorov of the Lebedev Institute discussed the basic ideas and theoretical possibilities of making a molecular generator ( 1 ) . The first molecular oscillator was reported by Townes and his colleagues of Columbia University in 1954 (2). Further contributions by Schawlow and Townes (3), demonstrated the feasibility of extending the maser principle to shorter wavelengths by using solid state devices; Maiman (4) achieved a notable technological wbreak through<< in 1960 when he succeeded in producing stimulated optical emission in a ruby crystal. This stimulated radiation in the visible region of the electromagnetic spectrum (694.3 mp) suggested the acronym LASER (L for light) as an alsternate for the nomenclature, optical maser, suggested by Townes. Rapid advances in the applications of lasers to military problems, communicattions, medicine, and basic research ( 5 ) have resulted in a need to investigate the biological effects of intense beams of photons, especially the ocular hazards associated with the widespread use of laser sources in laboratories throughout the world. Solon (6), (7) and Zaret (8), (9) have published

THE EFFECT OF Q-SWITCHED RUBY LASER ON RETINAL PIGMENT EPITHELIUM IN VITRO

From the time that powerful, man-made sources of light became a reality, the problem of ocular hazards derived from their spectral emission has become of increasing concern. Though retinal lesions in man, caused by eclipse gazing, had been detected for centuries by loss of visual acuity, Verhoeff, Bell and Walker (11), in their classical paper on the pathological effects of radiant energy on the eye, were the first to scientifically show that this type of ocular damage was the result of thermal injury causing destruction of the retinal pigment epithelium and choroid. Buettner and Rose (3) were the first to point to the vulnerability of the eye to atomic explosions. These investigators stressed the physical and optical fact that the image of the fireball, when focused on the retina, has an irradiance which tends to negate the “inverse square law”. This holds true out to distances where ocular resolution fails. Due to the optical properties of the eye, mainly that of focusing light on the retina, thermal injury to the retina presents a hazard at distances generally considered safe from the immediate effects of atomic fireballs. With this background information, a series of critical studies were undertaken by Ham and coworkers (4, 5 , 7, 8) in these laboratories, studying the

RETINAL DAMAGE FROM HIGH INTENSITY LIGHT

The results of the preceding papers raise the question as to the mechanism involved in retinal damage from exposure to high intensity light. After reviewing the data obtained from histochemical techniques and tissue electrophoresis in particular, it should be stressed that the reported results and interpretations have to be studied carefully since there is a lack of adequate comparison in this relatively unexplored field. The observed changes in the proteinogram of the retina by tissue agar electrophoresis, for instance, have to be examined very critically. I t should be realized that with this method alterations were recorded at the site of a calculated thermal density of 1.0 X 104 cal/cm* and this even if no loss by absorption would have occurred within the zone between center and 2 mm distance. However, it remains to explore the basis of this observation to rule out alterations in membrane changes rather than true protein denaturation. Another observation requiring caution is ,the reduction of B wave amplitude in ERG recordings after light exposure. I t should be stressed that this alteration should not be equaited with functional defect, since the two might not be related at all. Moreover, direct thermocouple measurements as a means of accurate temperature recording in biological structures presents a serious problem. The insertion of a thermocouple into biological structures in itself gives rise to artefacts of reflectance, absorption, and conduction, at the very site in the tissue where measurements are being carried out. The relative size of the thermocouples with respect to image size of the incident light beam on the retina and with regard to the dimensions of the biological structures is of great importance. Temperature rise and thermal conduction within the retina and choroid is an extremely rapid process and can easily be missed or underrated if the response time of the measuring device is of the same order as the time of actual

AGING, ANOMALIES AND RADIATION EFFECT OF THE RABBIT LENS

A causal relationship between the effects of aging on biological structures and that of ionizing radiation has been suggested by various investigators (1-4). If this suggestion is valid, one might expect to find: a) that tissue subjected to doses of ionizing radiation would develop prematurely certain changes which one customarily recognized as aging effects; b) that these “aging effects” might be present to a more pronounced degree than is usually apparant; and c) that a continuum might be detectable between the changes which are ascribed to aging and those which are known to follow ionizing radiation. The mammalian lens represents an ideal “medium” for providing points of comparison between the two “insults” i.e. age and radiation effect. The lens is easily studied in vivo by means of biomicroscopy. It naturally develops changes with age, and characteristic changes take place after exposure to ionizing radiation. In order that lens variations and anomalies which may occur can be distinguished from changes due to aging and radiation, such observations are described first. The present study is therefore divided into two sections. 1) Normal lens, variations and anomalies 2 ) Aging and lens irradiation effects The reported findings were obtained by slitlamp examinations ( X 16). Detailed drawings of each lens were made showing changes in an anterior-posterior

ENZYME ACTIVITY IN THE COAGULATED RETINA: A MEANS OF STUDYING THERMAL CONDUCTION AS A FUNCTION OF EXPOSURE TIME

A short definition of .retinal thermal threshold lesions< seems justified because the data presented in the following papers in this supplement have to do with such lesions. Since the introduction of the term .retinal thermal threshold lesions<< by Ham and his coworkers (I) a considerable number of misunderstandings in regard to this term have occurred, both in the literature and in the minds of investigators concerned with the problem of retinal thermal injury. The original definition of a threshold burn was an ophthalmoscopically observable lesion barely visible five minutes after exposure. A burn of this type is of course *threshold<< only in the sense that it fits this particular definition, but it is not *threshold<< according lto other examination methods. The ultimate in threshold lesions, therefore, is a retinal burn of such degree as to just perceptibly irreversibly impair retinal function. In order to evaluate a retinal burn of this type, obviously, the most sensitive method would be a subjective one; i. e. visual acuity and visual fields testing in intelligenit human volunteers following light coagulation. For obvious reasons this approach is not feasible. However it has been suggested by Newton (12) that primates trained by psychologists to perform certain tests might be utilized as likely substitutes, and in the future, data obtained from such experiments may prove valuable. Our basic research, as previously reported and as outlined in this supplement, has been carried out on rabbits. This readily available animal is quite

The Effect of Threshold Macular Lesions and Subthreshold Macular Exposures on Visual Acuity in the Rhesus Monkey

The purpose of this research program was to evaluate retinal threshold burns and subthreshold exposures of the mammalian macula in terms of visual acuity. Rhesus monkeys {Macaca mulatto) were trained by a reward system to respond to the automated presentation of Landolt rings. After appropriate training, these animals were exposed to threshold and subthreshold levels of retinal energy density ranging from 3.2 to 10.7 J/cm2, exposure time approximately 135 ms. spectral quality approximately that of color temperature 6000° K with wavelengths above 900 nm removed,, and image sizes on the retina of about 1 mm in diameter, covering a major portion of the monkey macular area. Results, in terms of visual acuity decrement (monocular), indicated that energy densities on the retina below 5 J/cm2 were not statistically significant, whereas energy densities greater than 5 J/cm2 produced losses in visual acuity (monocular) which were significant. These results indicate that, at levels of energy density on the retina w...

Effects of acute and protracted ionizing radiation on the rabbit lens.

With the advancement of manned space exploration, potentially hazardous exposure levels to ionizing radiation deriving from the continuous fields of the van Allen belt to less predictable exposures from solar flares have been of concern. While the former can be avoided to some degree by selected orbital flight patterns, solar flares may present some problems. Since the eye is a superficially located organ and its lens is a relatively sensitive biological structure, responding to ionizing radiation with characteristic changes easily observable under in vivo biomicroscopy, it lends itself particularly well to studying effects of selected dose and energy levels. T o accomplish this goal, the following investigations were conducted:

ELECTRORETINOGRAPHY AFTER LIGHT COAGULATION.

The term ,threshold burn* has produced a number of contradictory arguments and a number of misquotations, as has been pointed out previously (1). The question of what happens foIlowing extensive light exposure is not only of academic interest but also has a bearing on the clinical use as well as the development of adequate devices. To obtain exact data with regard to the functional status of the human retina after light exposure, presents, for obvious reasons, many problems. Conventional in vitro experimentation however is usually difficult to apply to in vivo situation. In the past the transfer mechanisms in visual function have been studied extensively in the horseshoe crab and the frog. However, even with very precise experimentation, it is exceedingly difficult to transfer those data to the human eye. Preliminary studies on chincilla rabbits (2) have shown that B wave changes may be obtained after light exposure with energies about 50 per cent below that required for production of ophthalmoscopically visible retinal lesions if a sufficient area of the retina was involved. These observations led to more extensive studies which are presented in this paper. Although the rabbit eye does not have a fovea, it has been situdied more than any other with regard to thermal injury. Quite accurate data have been accumulated from such studies and marly comparisons and correlations exist between the rabbit and human eye with respect to light coagulation. Several approaches to measuring electrical responses from the retina after light coagulation have been tried; the one described here is probably the simplest one of these and has provided the most accurate and reproducible data.

THE EFFECT OF ASCORBIC ACID ON THE FACILITY OF OUTFLOW IN NORMAL AND BUPHTHALMIC RABBITS

In the past decade some interest has been shown toward the possible use of ascorbic acid as a tension-lowering agent in glaucoma. This concept arose with the discovery of the presence of mucopolysaccharides within the trabecular meshwork of the eye, for it had been well known that ascorbic acid was essential for the syntheses of glucuronic acid, a constituent of hyaluronic acid, and, in addition, that it acted as a spreading factor by depolymerizing hyaluronic acid. (1) The fact that the normal concentration of ascorbic acid in the aqueous humor was extraordinarily high, ten to twenty times that in the serum, also motivated research in this field. Clinical and laboratory investigations of the tension-lowering effect of ascorbic acid have generally been favorable, however, they have not been conclusive in demonstrating the mechanism of action. Virno et al. (2) (1967) were most optimal when they reported that oral administration (0.5 GrniKg body weight daily) significantly decreased the intraocular pressure in both normal and glaucomatous eyes. Indeed, with ascorbic acid they were able to normalize pressures that had been uncontrolled by miotics and carbonic anhydrase inhibitors. Mo-