4 RESULTS AND DISCUSSION
4.3 Discussion
4.3.4 LED-induced RPI is wavelength-dependent
Excessive LED light exposure presents a potential hazard to retinal function 4. The previous study reported that a white light LED is more likely to induce RPI than is a CFL 77. In the present study, the same irradiance level (102 μW/cm2) for three light sources was used to conduct a more sophisticated experiment. This study analyzed the three major components of white light. As shown in Figure 35 (ERG), Figure 36 (H&E), and Figure 37 (TEM), the functional and morphological results suggested the blue light contributed the most to the RPI. The wavelength-dependent effect from the recent studies by Jaadane et al. 45 Bennet et al. 37 and Knels et al. 38 have similar results regardless of the different LED exposure intensities, durations, and experimental settings.
Furthermore, the results agreed with the claim that wavelength is the determining factor rather than the total light irradiance 38 in terms of RPI.
to 70 mmHg 78, which is ideal for ROS formation. Absorption spectra explain how light absorption changes by wavelength 14. The exposure of blue light increased the vulnerability to oxidative stress. The defense function in the retina creates high oxidative stress, which makes it extremely susceptible to RPI as reported by many previous studies 23, 37, 38, 41, 43, 45-49, 53, 57, 58, 61, 79-82.
This study results demonstrated that ROS accumulation was involved in RPI. ROS caused cellular injury by attacking the macromolecules within the cells when the light exposure duration exceeds its threshold 83. The oxidative stress markers such as 8-OHdG, acrolein and nitrotyrosine noticeably increased in the blue light-exposed retinas. The caspase-independent apoptotic marker, PARP-1, was significantly up-regulated in blue and green light-induced photoreceptor cell apoptosis after light exposure. This indicated the initiation of the retina cell apoptotic pathway. Furthermore, the antioxidant enzymes were produced to defend the insult, and the phagocytosis of toxicants or damaged debris by macrophage leading to the retina thickness decrease.
Apoptosis plays a major role in retinal visual cell loss 57, 84-88. The caspase-dependent and caspase-independent pathways are the two major apoptotic pathways associated with retinal light injury have been reported 41, 45, 49, 57, 58, 76, 89-96. There are varies reports regarding caspase involvement in retinal light injury. As previously reported, prolonged white light exposure does not activate caspase-3 protein expression 89, while blue light exposure increases its expression and leads to its activation 95. The classical caspase did not show a consistent wavelength-dependent expression in the experiment, but the caspase-independent marker has in another report 96. PARP-1 can induce apoptotic cell
The cleavage of PARP-1 inhibits the enzyme responding to DNA damage and secures substantial energy pools in the cells allowing the arranged cascade activities to occur in this type of apoptotic event 58, 93. Aydin and his colleagues also reported the retinal endoilluminator toxicity of xenon and LED light source in a rabbit model 83. Although their study did not directly match the experimental setting of this study, it explained the potential effect of LED retinal light injury in different operation conditions.
4.3.6 Iron-related RPI oxidative pathway
As the retina absorbed the light under a high oxygen (O2) condition, O2-.
was initially generated and converted by anti-oxidant enzymes to H2O2 and then ultimately to H2O 78. However, excessive short-wavelength light causes an imbalance in this conversion. Blue light irradiation accelerates the mitochondrial superoxide radical formation 38. The O2-. radicals can be easily converted to toxic hydroperoxyl radicals (HO2.). Under the high iron condition, the abnormal accumulation of H2O2 can lead to a Fe2+ oxidation to Fe3+ 60, 97. Although the Fe2+-melanin complex is readily oxidized by H2O2 and O2, few highly noxious hydroxyl radicals (OH.) may escape from the melanin polymer via the quick interaction of melanin and OH.,causing further oxidative injuries. The above-mentioned process is illustrated in Figure 42E.
4.3.7 Wavelength (hue) discrimination and specie differences
The permanent and serious injury from the blue LED exposure but the mild injury from the green and red LED exposure at the same irradiance level could possibly be explained by photoreceptor wavelength discrimination 98. It is
which only affects the blue cones, green light may induce reversible RPI over both the M (green) and L (red) cones. Sperling and his colleagues reported that, in a rhesus monkey study, green light exposure primarily affected post-receptoral processes, not the receptors themselves 98. Nevertheless, Kokkinopoulos reported that short period of 670 nm LED exposure may regulate innate immunity and alleviate RPI inflammation in a mouse model 99.
However, the photoreceptor structure and functions are different between pigmented and albino species 100, 101. It’s been reported that albino SD rat has L-cones with a mean density of 2000 cells/mm2 in their retina 102. The S-cones are about 133 cells/mm2 with a ratio of S- to L-cones of 1:15 103. Despite the population of S-cones are much less than M- or L-cones, S-cones are extremely sensitive to photons compare to the other two cones. Therefore, S-cones can be triggered instantly and easily result in exhaustion from over-excitement.
4.3.8 Environmental health perspectives
Based on the findings, we understand that the albino strain is approximately twice as susceptible to RPI compared with the pigmented strain 17, and the experimental results from this study cannot fully describe the injury mechanism and the theories apply to human from a rod dominated albino rats.
However, from the environmental health perspective, it is necessary to urge manufactures to disclose the detail product specifications. Government agencies should further arrange that information available to the general public (as shown in Figure 43). Continuous investigations on potential light injuries to animal or human are also required to clarify the exposure risk and determinate the exact threshold.
5 CONCLUSION 5.1 LED lighting induces retinal light injury
LEDs are expected to become the primary domestic light sources in the near future. Certain amounts of LED light exposure may induce retinal injury, and this animal model provides comparative measures of injury from different commercial light sources. Albino rats are commonly used for retinal light injury experiments . It has been reported that retinas from rats maintained in the dark for 14 days are more susceptible to light- induced injury than normal pigmented retinas . As a comparison, the study results show that the SPDs of bluish- white (high CCT) LEDs contain a major fraction of short-wavelength light that causes irreversible retinal neuronal cell death in rats. Furthermore, this model shows that the SPD of white LEDs now being introduced for domestic lighting pose a theoretical risk compared to CFLs (or incandescent lamps that have little blue light). When analyzing blue-light hazards, the risk of chronic effects from daily exposure cannot excluded considering photochemical injury may not induce an acute syndrome; instead, blue light exposure may cumulatively induce photoreceptor loss.
Regardless of whether the initial injury is caused by a photochemical effect, LED light injury is dependent on wavelength and duration. The entire retinal neuronal cell is affected, regardless of whether the injury is localized in the outer segment, mitochondria, or other subcellular organelles. Because illuminance levels of LED domestic light sources may induce retinal degeneration in experimental albino rats, the exact risks for the pigmented human retina require further investigation.
5.2 Blue light makes the most contribution to retinal light injury
There have been several investigations into the formation of ROS, lipid peroxidation, and the weakening of phagocytosis and POS digestion by RPE cells are thought to be the mechanism of iron-induced retinal toxicity. The increased Ft, Fpn, and CP (Figure 41) suggest that the retinal cells detected an increase in intracellular iron, which could be a sign for iron accumulation in all exposure groups. The strong expression of HO-1 in the blue light exposure group indicates its anti-oxidative function as reported by several publications previously. In contrast, HO-1 could also catalyze Fe2+ and carbon monoxide production, which may potentially exacerbate oxidative stress by generating free radicals. Moreover, SOD2 encoded by distinctive nuclear gene is localized in the mitochondrial matrix and converts the O2-. generated by aerobic respiration to H2O2. This is the critical cell defense mechanism against the oxidative stress as reported previously. The activation of cytosolic glutathione peroxidase (GPx1) in the outer retina may also be an important factor in the response to photo-oxidative stress mitigating retinal lipid peroxidation, and CP acts as an antioxidant by oxidizing iron from its Fe2+ to Fe3+ form. The concomitant activation of several antioxidants in the same detoxification pathway could possibly be an indicator of short wavelength cytotoxicity superinduction. These defense gene expressions support that light-induced retinal degeneration involves oxidative stress. This study thus proposes that an iron-related RPI pathway, as shown in Figure 42E, based on these findings.
5.3 The way ahead
experimental results have confirmed that the general finding of the increased RPI from blue light found from in vitro and anesthetized-animal studies applies to a free-running animal model. It also showed a greater risk of LED blue-light injury in awake, task-oriented rod-dominant animals. Four dependent effects are considered with light-induced retinal injury, including wavelength-, oxygen-, iron- (site-specific), and time/dose-dependent effects, which indicate a cumulative effect and a possible association with chronic ocular diseases. The associated oxidatively damaged biomolecules, cell destruction, and chronic inflammation should be carefully considered when switching to LED lighting.
However, the exact mechanism underlying these effects will be the subject of ongoing investigation with more analytical methods. The interpretation from the animal study to human applications should also be carefully considered based on the risk assessment perspective.
Base on the study findings, this study proposes the following suggestions for LED domestic lighting application.
1. Manufacturers are advised to design LED lamps with less portion of short wavelengths.
2. Consumers should be educated to select yellow LED lamps (CCT <
4000) and reduce the brightness by applying dimmable fixtures for domestic lighting.
3. Reduce the total exposure time or allow a short break in between.
4. Consult with ophthalmologists before switching to LED lamps for domestic lighting, if a person has any pre-existing retinal diseases or special ocular conditions.
conforming the rapid improvement of the LED lighting technology.
Proper certification and labeling of cautions are required before LED lamps being launched to the market.
6. Detail product specifications of all LED lamps should be disclosed by manufacturers which are approved by the government agencies.
This product information can be accessed by the general public.
7. Future studies are suggested to focus on the pigmented species or occupational exposures to determine the exact risk and injury threshold to human.
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