Photic Injury Research Articles

Correlation of Phospholipid Hydroperoxide Glutathione Peroxidase Activity to the Sensitivity of Rat Retinas to Photic Injury

We hypothesize that the differential susceptibility to photic injury among different strains of rat retinas may depend on the levels of phospholipid hydroperoxide glutathione peroxidase (PHGPX) activity, one of the endogenous antioxidant enzymes in the retina. The retinas of four inbred strains of albino rat (Fischer, Wistar, Buffalo and Lewis) were analyzed for glutathione peroxidase activity using H2O2, cumene hydroperoxide, and phospholipid hydroperoxide as assay substrates. In all four strains of rat, PHGPX was observed only in the high salt extract of the retina, while peroxidases determined by H2O2 or cumene hydroperoxide were observed mainly in the low salt extract. PHGPX was highest (66.7 mU/mg) in the most light-resistant Fischer strain and lowest in the most light-sensitive Lewis strain (31.9 mU/mg), while the activity levels in the moderately light-sensitive Buffalo and Wistar strains were 46.6 and 38.5 mU/mg, respectively. In contrast, there was no significant difference in peroxidases determined by H2O2 or cumene hydroperoxide among the four strains. These observations suggested that, in rat retina, the membrane-associated PHGPX may have an important role in the defense against light-induced free radical damage.

An immunohistochemical study of opsin in photoreceptor cells following light-induced retinal degeneration in the rat.

Light-induced retinal degeneration has been hypothesized to be rhodopsin-mediated. However, the alterations induced in the opsin moiety of the rhodopsin molecule and its distribution in the rod cell after a photic insult have not been definitively established. We used light and electron immunohistochemistry to study the alterations in retinal opsin immunoreactivity in a rat model of retinal photic injury. In normal unexposed rat retinas, opsin immunoreactivity was restricted to the rod outer segments. At 6 h after a 24-h light exposure, opsin immunoreactivity was present in the rod outer segments in both the superior and inferior retina, but in addition marked immunoreactivity was present in the inner segments in the superior quadrant of the light-damaged retina. At 6 days after exposure, intense immunoreactivity was noted around the severely degenerating rod nuclei and inner segments. However, at 21 days following light exposure, opsin immunoreactivity in areas of recovery was again restricted to the short regenerated rod outer segments. It appears that, despite severe light-mediated retinal degeneration, anti-opsin immunoreactivity persisted in the photoreceptor cells but with an altered pattern in damaged rod outer segments and photoreceptor perikarya. However, opsin immunoreactivity relocated to the regenerated rod outer segments in the recovery phase.

Amelioration of Retinal Photic Injury by a Combination of Flunarizine and Dimethylthiourea

Free radical scavengers and a calcium overload blocker have been demonstrated separately to ameliorate light-induced retinal degeneration, suggesting that both free radical formation and increased intracellular calcium levels are involved in the pathologic changes of this disease process. To understand the relationship between these two mechanisms, we studied the ameliorative effects of combined treatment with flunarizine and dimethylthiourea as well as individual treatment with either drug in a rat model of light-induced retinal degeneration. At 6 hr and 6 and 14 days after light exposure, morphologic and morphometric studies of the retinas from the rats receiving the combined treatment demonstrated better-preserved retinal pigment epithelial cells, photoreceptor elements, and nuclei than did retinas from rats receiving treatment with either flunarizine or dimethylthiourea alone. Rhodopsin level measurements at 6 and 14 days further substantiated the results of the protective effects on the photoreceptor outer segments. Because we used a saturating dose for dimethylthiourea, the enhanced ameliorative effect of the combination treatment suggested that free radical formation and elevated intracellular calcium levels were two separate mechanisms in light-induced retinal degeneration.

Macular Phototoxicity Caused by Fiberoptic Endoillumination During Pars Plana Vitrectomy

Three cases of retinal phototoxicity were caused by light from the endoilluminator used in vitrectomy. Preoperative clinical examination, color photography, and fluorescein angiography in all three cases failed to disclose retinal pigment epithelial changes. Postoperative clinical findings, photographs, and fluorescein angiograms are highly suggestive of the presence of retinal phototoxicity. The characteristics of these lesions and surgical conditions implicate the endoilluminator as the source of photic injury. The macular lesions were noted within one week of the surgical procedure, measured between 2 and 5 disk diameters in size, involved the fovea in two eyes, and resulted in marked decrease in visual acuity in two of three eyes, with persistence in one eye. Initially, whitening of the outer retina was present, but was replaced by pigmentary mottling at the level of the retinal pigment epithelium within a few weeks. Preventive measures to avoid macular phototoxicity associated with vitrectomy are discussed.

Methylprednisolone ameliorates retinal photic injury in rats.

Since lipid peroxidation induced by oxygen free radicals is believed to play an important role in retinal photic injury and high doses of certain steroids have been demonstrated to inhibit lipid peroxidation, we evaluated the effect of high-dose methylprednisolone sodium succinate on retinal injury inflicted by green light (490 to 580 nm, 160 to 180 foot-candles [1721.6 to 1936.8 lux]) for 24 hours in 36 rats. Animals received either intraperitoneal injections of the drug (80 mg/kg twice daily) or saline as a control, twice daily for 2 days, commencing with light exposure. The retinal photic injury was assessed 6 hours, 6 days, and 14 days after light exposure by light and electron microscopy, morphometric study of outer nuclear layer thickness and count of subretinal macrophages, and spectrophotometric measurement of rhodopsin. A beneficial effect of high-dose methylprednisolone was observed in retinal photic injury.

Dexamethasone ameliorates retinal photic injury in albino rats

The effect of dexamethasone in two regimens on retinal photic injury was studied in Lewis albino rats that were exposed to 24 hr of continuous green fluorescent light. Under regimen 1, dexamethasone was given at a daily dosage of 1 mg kg −1 for 8 days, starting 6 days before light exposure. Under regimen 2, dexamethasone was given at the same daily dosage for 3 days, started 1 day before light exposure. Pathologic study of the light-exposed retina, morphometric evaluation of the photoreceptor cell loss, cell counts of the macrophages in the subretinal space, and measurements of rhodopsin levels were undertaken in the dexamethasone-treated and control retinas at various times. The administration of dexamethasone in both regimens did not produce pathologic changes in the retina before light exposure, but rhodopsin levels were significantly lowered in both treated groups when compared to corresponding vehicle treated control animals. Under regimen 1, at 6 hr after light exposure, both the treated and the control groups showed comparable loss of photoreceptor cells, degeneration of the photoreceptor elements and retinal pigment epithelium, but a significantly lowered level of rhodopsin in the treated group was noted. At 6 days after exposure, the outer nuclear layer thickness, and the outer and inner segments showed significant preservation in the treated group. Also in the treated group, the number of macrophages was significantly reduced and the retinal pigment epithelial (RPE) vacuolation was markedly less. However, there was no difference in rhodopsin levels. At 14 days after exposure, the outer nuclear layer thickness and rhodopsin levels of the treated rats had significantly higher values than the controls. Under regimen 2, however, at 6 days after exposure, and ameliorative effect in the RPE was observed but there were no differences of rhodopsin levels, the outer nuclear thickness and number of macrophages between the treated and control groups. Regimen 1 was associated with a significantly higher retinal level of dexamethasone when compared with regimen 2. The ameliorative effect of dexamethasone on rat retinal photic injury may be through inhibition of lipid peroxidation, in which a high retinal level of the steroid is required.

Photic Retinal InjurjrFrom Endoillumination During Vitrectomy

We treated a patient who developed a paramacular area of light-induced retinal damage after endoscopic epimacular membrane removal. Postoperative color photographs showed complete absence of the membrane, but fluorescein angiography demonstrated a previously absent superior paramacular lesion consistent with a photic injury. Operative microscope illumination had been eliminated by corneal shielding, which implicated endoillumination as the source of injury. We recommend the following procedures to avoid this complication: careful planning of vitreous surgery for epimacular membrane removal; using filters; minimizing the length of surgery; keeping the light output low; maintaining maximal light pipe distance from the retina; eccentric orientation of the light pipe; and use of intermittent and variable site illumination techniques.

A pathologic study of photoreceptor cell death in retinal photic injury

In the studies of retinal photic injury in the rat model, about 14-47% of the photoreceptor cell loss occurs in the first 24 hours. To understand the mechanism of this massive cell death and subsequent dissolution, we studied the early morphologic changes and examined the effect of cycloheximide, a protein synthesis inhibitor, on photic injury in rats. Groups of 2 dark-adapted albino Lewis rats were sacrificed immediately after 8, 16 or 24 hr of continuous exposure to green fluorescent light (intensity, 160-180 foot-candles; wavelength, 490-560 nm). An additional 2 rats were sacrificed 8 hr after a 24 hr light exposure, and 2 animals served as unexposed controls. The morphologic findings of the degenerating photoreceptor cells were assessed by light and electron microscopy. The integrity of the nuclear chromatin was investigated using a monoclonal anti-DNA antibody. Most of the photoreceptor cell loss was observed between 16 and 24 hr of exposure. No inflammatory or macrophagic cells were seen. Different stages of nuclear condensation and chromatin margination could be defined. The chromatin showed a progressive decrease in DNA labelling density. Scattered photoreceptor cells showed early cytoplasmic densification. To study the effect of cycloheximide, 4 rats were treated with 5 mg/kg subcutaneously at the start of a 24 hr exposure period and were sacrificed 6 hr after the exposure. Four untreated animals served as exposed controls for morphometric comparison of the outer nuclear layer (ONL). The control rats showed a 24% decrease in the thickness of the ONL when compared to cycloheximide-treated rats (p less than 0.001). The observations of mitochondrial changes and early DNA digestion were consistent with necrosis as the mechanism of cell death. However, in scattered photoreceptor cells, cytoplasmic densification, margination of nuclear chromatin, the lack of associated inflammation and the protective effect of cycloheximide were suggestive of apoptosis as another mechanism of cell death.

Desferrioxamine ameliorates retinal photic injury in albino rats

Albino rats were given intraperitoneal desferrioxamine before exposure to intense fluorescent light. Morphometric and morphologic studies of the retina indicated that there was better preservation of photoreceptor nuclei and fewer subretinal macrophages in rats treated with desferrioxamine than in the control rats. The results of the study imply that iron and hydroxyl radicals may be important mediators in photic retinal injury.

Amelioration of Retinal Photic Injury in Albino Rats by Dimethylthiourea

Light-induced formation of oxygen free radicals has been proposed as the underlying mechanism of photic retinal injury. We investigated the role of hydroxyl radical in retinal photic injury by treating dark-adapted albino rats with intraperitoneal dimethylthiourea 3 hours before exposure to intense fluorescent light. Dimethylthiourea is a specific antioxidant against hydroxyl radical. We demonstrated that dimethylthiourea penetrates well into retinal tissue and has a half-life of approximately 19 hours. Morephologic differences between the control and dimethylthiurea-treated rats were not remarkable 6 hours after light exposure, but they became significant 6 and 14 days after light exposure. Morphometric studies showed that there was significantly better preservation of photoreceptor nuclei in dimethylthiourea-treated rats 6 and 14 days after light exposure. Rhodopsin levels were significantly higher in the dimethylthiourea-treated rats 6 hours and 14 days after light exposure, while rhodopsin levels were comparable in the control and dimethylthiourea-treated rats 6 days after exposure. The differences in morphometry and rhodopsin levels between the control and dimethylthiourea-treated rats were statistically significant in relationship to dimethylthiourea treatment. The superior and temporal retinal quadrants appeared most vulnerable to photic injury in control rats 6 and 14 days after light exposure. These findings indicate that dimethylthiourea ameliorates retinal photic injury, and that hydroxyl radical plays an important role in mediating retinal photic injury.

Effects of Repeated Photic Exposures on the Monkey Macula

The posterior pole of the eyes of rhesus monkeys was exposed to the light of an indirect ophthalmoscope. The tissue changes caused by repeated (three to five) photic injuries were compared with those produced by a single photic insult. The ophthalmoscopic and angiographic examinations showed a progressive involvement of the foveal area after multiple exposures to light. Histopathologic study showed that after one exposure the retinal pigment epithelium appeared focally atrophic. Plaques of proliferated retinal pigment epithelium could also be seen, and on the anterior surface of these plaques a layer of relatively cuboidal hypopigmented retinal pigment epithelial cells was present. With multiple exposures, focal areas of total absence of retinal pigment epithelium were seen in these eyes. The eyes that received only one exposure showed partially regenerated photoreceptor outer segments. After repeated photic exposures, the photoreceptor outer segments failed to regenerate even 6 to 8 months after the last exposure. In one eye exposed five times for 60 minutes, areas of total absence of photoreceptor cells could be seen, and the inner nuclear layer was directly apposed to Bruch's membrane. These experimental exposures to the light of an indirect ophthalmoscope were 60 to 120 minutes long and were not comparable to clinical exposures of the ophthalmoscope in patient examination. Our observation highlighted the cumulative effects of repeated light injury to the retinal pigment epithelial and photoreceptor cells and also showed the similarity between this animal model of photic maculopathy and the changes present in the eyes of human patients with atrophic agerlated macular degeneration.

A comparative study of photic injury in four inbred strains of albino rats

The effects of light damage were studied in four inbred strains of albino rats, Fischer (F344), Lewis (L), Wistar (W), and Buffalo (BUF). Thirty-five-day-old rats (3 from each strain) were kept in cyclic light for two weeks, after which they were exposed to constant fluorescent light (180-190 foot-candles, 490-580 nm) for 24 hours. The rats were sacrificed 6 days after exposure. Photic injury to the exposed eyes was evaluated morphometrically by measuring the mean outer nuclear layer (ONL) thickness and qualitatively by light microscopy. Statistical analysis of the quantitative data revealed that the Lewis and Buffalo strains were more severely affected than the Wistar and Fischer strains. The effects of injury were more severe in the superior and temporal quadrants. Taking both quantitative and qualitative assessment into consideration, it appeared that, among the strains studied, the rats from the Lewis strain were most sensitive to photic damage. These findings support previous findings of differential light sensitivity and further suggest that similar variability occurs among inbred strains with type 1 light damage.

Blue light hazard in rat

Rats have been extensively used in light damage studies. Retinal damage threshold for white light were found at 1–10 J/cm 2, and the action spectrum resembled the absorption spectrum of visual pigment. We wished to answer the question whether a different class of light damage, the “blue light hazard”, with white light damage thresholds at about 300 J/cm 2, and an action spectrum peaking in the ultra-violet, could also be demonstrated in rat. To that purpose 5 deg patches of retina were exposed to white xenon light with exposure times between 10 sec and 1 hr. We found that for funduscopic threshold damage the product of irradiance and exposure time was constant at a level of 315 J/cm 2. Thereafter, the action spectrum was measured by exposing rat eyes to narrow band spectral lights. Threshold irradiant dose ranged from 4 J/cm 2 at 379 nm to 2000 J/cm 2 at 559 nm. Thus, susceptibility for damage sharply increased towards the ultra-violet, just like in earlier monkey studies. We conclude that in similar experimental conditions susceptibility to photic injury in rat is comparable to that in primates. Rat is the first species for which two different action spectra of photochemical damage have been established.

Pathogenetic Factors of Aging Macular Degeneration

In order to explore the pathogenetic factors of aging macular degeneration, risk factors from epidemiologic studies and causative factors from clinical and laboratory studies were reviewed. From the epidemiologic studies, the risk factors of senile macular degeneration included: (1) personal characteristics such as age, sex, race, height, family history, and strength of hand grip; (2) ocular characteristics such as hyperopia, color of iris and senile cataract; and (3) systemic diseases and environmental exposure such as cardiovascular diseases, smoking, lung infections, and chemical exposures. The causative factors suggested from clinical and laboratory studies include: drusen, choroidal vascular diseases, photic injury, and vitamin C deficiency. It is postulated that aging macular degeneration is a multifactorial syndrome. Different etiologic factors may inflict damaging effect on the macula, resulting in a common set of clinical manifestations that are interpreted as aging macular degeneration.

Effect of Photic Injury on the Retinal Tissues

In order to study the tissue damages in photic injury of the retina, the macula and posterior pole of the rhesus monkey were exposed to the light of an indirect ophthalmoscope. During the acute phase of the injury, disruption of the blood-retinal barrier appeared to be an early and sensitive indicator of retinal pigment epithelial damage and cyclic-GMP-phosphodiesterase of the photoreceptor lamella was inactivated. These lesions were followed for 2 to 5 years. Focal chronic decompensation of the blood-retinal barrier and subretinal pigment epithelial neovascularization were noted in the chronic degenerative phase of this photic maculopathy. A possible protective role of ascorbic acid in mild photic injury was proposed.

Retinal Protection from Solar Photic Injury

Samples of protective filters commonly used to observed the partially eclipsed sun were obtained and subjected to spectrophotometric analysis over the wavelength interval 330 to 2500 nm. Performance of these filters was compared with criteria for adequate ocular protection which have appeared in the literature. It appears that the greatest safety is provided by those devices incorporating a monatomic metallic coating as the filtering agent.

Photic injury to the human retina.

Photic injury to the human retina.

Comment on ‘Morphologic Changes in Photoreceptor Outer Segments following Photic Injury’

Comment on ‘Morphologic Changes in Photoreceptor Outer Segments following Photic Injury’

Morphologic Changes in Photoreceptor Outer Segments Following Photic Injury

Morphologic changes characteristic of mild and severe cell injury produced in retinal photoreceptors by a xenon arc coagulator were studied by light and electron microscopy. Mild or reactive cell changes consisting of a tubulovesicular rearrangement or breakdown of cell membranes differed considerably from the production of myriad focal densifications of cell membranes and associated ground substances occurring in the severe injuries. The latter alterations are considered to beirreversible and, therefore, a characteristic of necrosis.

Recovery of the rod and cone cells after photic injury.

Recovery of the rod and cone cells after photic injury.