William T. Ham

Basic mechanisms underlying the production of photochemical lesions in the mammalian retina.

Extended exposure (100s) of the macaque retina to blue light (400-500nm) produces a photochemical type or types of lesion. The basic mechanisms responsible for such photic damage are unknown but the toxic combination of light and oxygen leading to the free radicals O-.2, H2O2, OH., and O2(1 delta) have been suggested as a possible source of the phototoxicity. To test this hypothesis, the radiant exposure (J. cm-2) to short wavelength light (435-445nm) required for minimal damage in the macaque retina is under investigation as a function of oxygenation and after administration of substances known to either inhibit/scavenge radicals or act as anti-inflammatory/anti-oxidant agents. Substances under study include beta-carotene, steroids, catalase and SOD. Here we report radiant exposure in J.cm-2 needed to produce a minimal lesion vs oxygenation as measured by partial pressure of O2 in arterial blood (Po2). There is a sharp drop in the radiant exposure threshold with increase in the partial pressure of O2 in arterial blood, e.g. 30 J.cm-2 at 75 torr to 10 J.cm-2 at 271 torr, a factor of 3. Methylprednisolone injected intravenously one hour before exposure (125 mg) has been shown to raise the threshold for retinal damage in two macaques by a factor of approximately 2. Another animal fed beta-carotene (7.5 mg daily) over a period of 3 months has been exposed to blue light at several levels of oxygenation. The results suggest a protective effect.

SENSITIVITY OF THE RETINA TO RADIATION DAMAGE AS A FUNCTION OF WAVELENGTH

Abstract—Exposure of the retina of the rhesus monkey to visible and infrared radiation from CW optical sources like the Sun, xenon lamps, etc. produces small lesions or scotomata which may be classified as thermal or photochemical, depending on the wavelength and duration of exposure. The action spectrum for the production of retinal lesions has been determined for eight monochromatic laser wavelengths extending from 1064 to 441 nm. The corneal power required to produce a lesion decreases by three orders of magnitude in going from 1064 to 441 nm. Exposure to 1064 nm radiation for 1000 s produces a typical thermal lesion at elevated retinal temperatures. whereas a 1000 s exposure to 441 nm light produces a photochemical lesion at power levels too low to raise the retinal temperature by an appreciable amount (<0.1°). The two types of lesion have entirely different characteristics as will be discussed in some detail. The photopathology of thc photochemical lesion has been studied at postexposure times ranging from 1 h to 90 days and will be demonstrated in a number of histological slides. Moreover, this photopathology correlates well with monocular visual acuity tests in the rhesus monkey as defined by the Landolt ring technique.

The nature of retinal radiation damage: Dependence on wavelength, power level and exposure time

Abstract Wavelengths between 400–1400 nm are transmitted by the mammalian ocular media to the retina. There are at least three types of retinal injury in this waveband. They are mechanical, thermal and actinic in nature. Each type of damage is described briefly and it is suggested that melanin plays a key role in all three types of damage. Some of the peculiar properties of melanin are discussed briefly.

Ocular Hazard from Picosecond Pulses of Nd: YAG Laser Radiation

Seven rhesus monkeys (14 eyes) were exposed to 1064-nanometer radiation in single pulses of 25 to 35 picoseconds fromn a mode-locked Nd: YA G laser. Threshold injury resulted from single pulses with a mnean energy of 13 � 3 mnicrojoules. Electron microscopy of the retina revealed that damnage was highly localized in the photoreceptor and pigmented epithelial cells at the oluter retina. Membrane disruption, distorted outer segmtzents, and abnormnal melanin granules resembling fetal premelanosomnies were observed.

Ocular Hazard from Viewing the Sun Unprotected and Through Various Windows and Filters

An optical source simulating the sun at the top of the atmosphere has been constructed and used to obtain retinal burn thresholds in the rhesus monkey for image diameters corresponding to that of the solar disk on the human retina. Powers incident on the cornea and retinal irradiances required to produce threshold lesions are given for exposure times ranging from 1 s to 3 min. The ocular hazards associated with viewing the sun through aircraft window systems are assessed in terms of these data. Also, radiation in the near infrared is shown to the retina.

Evaluation of retinal exposures from repetitively pulsed and scanning lasers

Threshold damage in the macaque retina is shown to be equivalent for the argon-krypton (Ar-Kr) 647 nm and the helium-neon (He-Ne) 632.8-nm lines for exposures to continuous wave (CW) radiation from 1 to 1,000 s. This equivalence allows interpolation from experiments with 647-nm, exposures at power levels that are unavailable with the He-Ne laser. To simulate He-Ne laser scanner exposures, 40-microseconds pulses of 647-nm light transmitted through a revolving disk with holes in the periphery were used to expose the retinas of monkeys under deep anesthesia at pulse repetition frequencies (PRFs) of 100, 200, 400, and 1,600 Hz for exposure durations of 1, 10, 100, and 1,000 s. The thresholds between laser exposure at 488 nm (Ar-Kr) and between laser exposure at 647 nm (Kr) are compared to assess thermal versus photochemical effects on the retina. The threshold for 488-nm pulses was consistently lower than that for 647-nm pulses at all PRFs and exposure times. The difference in thresholds increased with exposure time and PRF. The sharp decreases in 488-nm thresholds at 100-s exposure times for each PRF can be interpreted as a basically photochemical effect. The radiant exposure required for damage at 647 nm was several orders of magnitude above the radiant exposure from typical He-Ne scanner applications. From the similarity of the macaque retina to the human retina, it is concluded that no realistic ocular hazard exists from exposure to scanning laser systems of 1 mW or less, operating at higher than 100 Hz.

Visual performance in the rhesus monkey after exposure to blue light

Abstract Rhesus monkeys trained to perform a visual task (Landolt ring discrimination) were exposed for 1000 sec to known amounts of 441 nm light by means of a 2500 W xenon lamp with narrow bandpass filter. Radiant exposures to the macula of 30 J/cm 2 did not impair vision, but 60 J/cm 2 produced a transient loss of 20/20 vision which lasted from 20 to 30 days. A radiant exposure of 90 J/cm 2 produced a permanent loss of 20/20 vision. These results, in addition to explaining solar retinitis and eclipse blindness, correlate well with the retinal photopathology of the short wavelength photochemical lesion.

Retinal Effects Of Blue Light Exposure

Recent research has shown that blue light exposure is an important factor in certain types of retinal injury. The mammalian ocular media transmits the spectral band 400-1400 nm to the retina. The short wavelengths (400-550 nm) produce a photochemical or actinic type of damage, while the longer wavelengths (550-1400 nm) produce thermal damage. Distinction between the two types of retinal damage are discussed briefly and the importance of the blue light effect for solar retinitis and eclipse blindness is emphasized. The significance of blue light retinal injury is summarized for various environmental and occupational exposures.

X-RAY AND PROTON INDUCED LENS CHANGES IN THE RABBIT.

Abstract Opacification of the rabbit lens was studied as a function of dose and time after irradiation with 1 MeV X-rays, 20 and 100 MeV protons. The degree of opacification was dependent upon time, radiation dose and LET. Twenty MeV protons produced the greatest relative lens damage. The relative biological effectiveness (RBE) was determined for the 20 and 100 MeV protons relative to 1 MeV X-ray induced lens changes. The RBE was found to increase with LET.

Ocular effects of GaAs lasers and near infrared radiation

Fiber optic communication systems present a possible or potential hazard to the eyes of engineers and technicians who work with or maintain these systems. To investigate this hypothesis, the retinas of macaque monkeys were exposed to near infrared cw radiation and to GaAs lasers modulated at 22 MHz and 1600 Hz. Trained animals (two) were exposed monocularly under normal physiological conditions to modulated GaAs lasers for several months, on a 5 day/week basis, 1000 sec/day. No loss of visual function or funduscopically visible damage was detected. One of these animals was sacrificed and examined histologically for damage. No differences were detected between the foveae of the exposed and control eyes in this monkey. The radiant exposure in J · cm−2 required to produce minimal lesions was determined on anesthetized animals for cw radiation at three wavelengths (820, 860, 910 nm) and for radiation at 830 nm from a GaAs laser modulated at a digital rate of 44 Mbit/sec. It required from 6 to 8.4 mW of GaAs radiation entering the eye for periods ranging from 400 to 3000 sec to produce a detectable lesion. Since the spot size on the retina was <50 μm in diameter, it is difficult if not impossible to imagine how the human eye could remain focused on such a source for an appreciable time, even if 8 mW were entering the pupil. Extrapolation to man is always dangerous, but these experiments do not suggest that engineers and technicians operating and maintaining fiber optic communication systems are subject to an ocular risk unless they use magnification optics.