Retinal phototoxicity

Deficiency of the RNA-binding protein ELAVL1/HuR leads to the failure of endogenous and exogenous neuroprotection of retinal ganglion cells

Introduction

ELAVL1/HuR is a keystone regulator of gene expression at the posttranscriptional level, including stress response and homeostasis maintenance. The aim of this study was to evaluate the impact of hur silencing on the age-related degeneration of retinal ganglion cells (RGC), which potentially describes the efficiency of endogenous neuroprotection mechanisms, as well as to assess the exogenous neuroprotection capacity of hur-silenced RGC in the rat glaucoma model.

Methods

The study consisted of in vitro and in vivo approaches. In vitro, we used rat B-35 cells to investigate, whether AAV-shRNA-HuR delivery affects survival and oxidative stress markers under temperature and excitotoxic insults. In vivo approach consisted of two different settings. In first one, 35 eight-week-old rats received intravitreal injection of AAV-shRNA-HuR or AAV-shRNA scramble control. Animals underwent electroretinography tests and were sacrificed 2, 4 or 6 months after injection. Retinas and optic nerves were collected and processed for immunostainings, electron microscopy and stereology. For the second approach, animals received similar gene constructs. To induce chronic glaucoma, 8 weeks after AAV injection, unilateral episcleral vein cauterization was performed. Animals from each group received intravitreal injection of metallothionein II. Animals underwent electroretinography tests and were sacrificed 8 weeks later. Retinas and optic nerves were collected and processed for immunostainings, electron microscopy and stereology.

Results

Silencing of hur induced apoptosis and increased oxidative stress markers in B-35 cells. Additionally, shRNA treatment impaired the cellular stress response to temperature and excitotoxic insults. In vivo, RGC count was decreased by 39% in shRNA-HuR group 6 months after injection, when compared to shRNA scramble control group. In neuroprotection study, the average loss of RGCs was 35% in animals with glaucoma treated with metallothionein and shRNA-HuR and 11.4% in animals with glaucoma treated with metallothionein and the scramble control shRNA. An alteration in HuR cellular content resulted in diminished photopic negative responses in the electroretinogram.

Conclusions

Based on our findings, we conclude that HuR is essential for the survival and efficient neuroprotection of RGC and that the induced alteration in HuR content accelerates both the age-related and glaucoma-induced decline in RGC number and function, further confirming HuR's key role in maintaining cell homeostasis and its possible involvement in the pathogenesis of glaucoma.

Macular phototoxicity after corneal cross-linking

Purpose

To assess potential vascular, structural, and functional changes to the macula in patients with keratoconus that underwent ultraviolet A (UVA)–riboflavin-mediated corneal collagen cross-linking (CXL) therapy.

Patients and methods

Seventeen eyes from 17 patients of age 16 years or older with keratoconus undergoing CXL treatment were studied. The same eye served as its own control (before CXL vs after CXL). Eyes were evaluated in terms of best-corrected visual acuity (BCVA), refractive error, intraocular pressure, Amsler grid, retinography, fluorescein angiography, autofluorescence, and spectral domain optical coherence tomography (SD-OCT) prior to CXL and 7 and 30 days after treatment. Multifocal electroretinography (mfERG) was recorded prior to and 7 days after CXL.

Results

Mean (SD) BCVA by logMAR chart was 0.47 (±0.12) pre-CXL, 0.55 (±0.15) 7 days post-CXL (P=0.57), and 0.46 (±0.10) 30 days post-CXL (P=0.87). Mean (SD) SD-OCT central macular thickness (µm) was 253.62 (±20.9) pre-CXL, 260.5 (±18.7) 7 days post-CXL (P=0.48), and 256.44 (±21.6) 30 days post-CXL (P=0.69). In 12 eyes, mfERG revealed a statistically significant increase (P=0.0353) in P1 latency (ms) of ring four from the pre-CXL period (39.45±2.05) to 7 days post-CXL (41.04±1.28) period. Regression analysis showed that the increase in P1 latency was correlated with the increase in central macular thickness (P=0.027). Furthermore, nine patients experienced a significant decrease in P1 amplitudes of rings 1 (P=0.0014), 2 (P=0.0029), 3 (P=0.0037), 4 (P=0.0014), and 5 (P=0.0012) from pre-CXL to 7 days post-CXL. Conclusion: In this pilot study, most of the patients exhibited slight changes in their mfERG parameters and OCT thickness, despite a lack of vascular abnormalities observed on fluorescein angiography/autofluorescence imaging, no alteration in BCVA, and no reports of symptoms. These changes could, therefore, be categorized as a mild subclinical effect of the corneal cross-linking procedure.

Evaluation of 3D heads-up vitrectomy: outcomes of psychometric skills testing and surgeon satisfaction

Objectives

To evaluate the use of a three-dimensional heads-up microscope (3DM) during 25-gauge pars plana vitrectomy (PPV) compared with a traditional ophthalmic microscope (TM) in terms of efficacy, safety, and teaching and learning satisfaction.

Methods

Prospective comparative interventional study. Fifty eyes affected by one of the following diseases: rhegmatogenous or tractional retinal detachment, epiretinal membrane, full-thickness macular hole, vitreous hemorrhage, or dropped lens. The 50 eyes were randomly assigned to one of two groups: group A (25 eyes) underwent 25-gauge PPV with 3DM, and group B (25 eyes) underwent 25-gauge PPV with TM. The main outcome measures were the duration of the operation, intraoperative complications, and surgeon and observer satisfaction. A questionnaire was used to assess surgeon satisfaction according to the following parameters: comfort, visibility, image quality, depth perception, simplicity of use, maneuverability, and teaching. A questionnaire to assess observer satisfaction was completed by 20 observers (surgical residents or ophthalmic surgeons).

Results

The degree of satisfaction was higher using 3DM for both surgeons and observers (P < 0.001). The average duration of the operation did not differ significantly between the two methods. No major complications occurred for either method.

Conclusions

PPV with 3DM is more comfortable for the surgeon and poses no substantially greater risk of complications for the patient. The high-definition screen delivers excellent depth perception and better screen parameter control, which results in high-quality surgical performance. 3DM surgery helps to significantly improve teaching and learning intra-operative surgical procedures.

Wide-field Digital Ophthalmic Imaging in Infants using Nasal Endoscopic System

OBJECTIVE

To report the utility of nasal endoscopic system in viewing the iridocorneal angle and fundus using various nasal endoscopes and light sources and to determine which type suits best to image a given location in the eye.

METHODS

This is a prospective observational study of 20 eyes of 10 consecutive infants, who were scheduled for irrigation and probing for congenital nasolacrimal duct obstruction under general anesthesia. The pupils were dilated, viscoelastic gel was applied on the cornea and imaging was performed using nasal endoscopes. The endoscopes used were of 2.7 mm and 4 mm diameter, each with various angulations of 0(0), 30(0) and 70(0). Two different cold light illuminators (Xenon and Halogen) were used uniformly in all cases. Images were analyzed for their quality, clarity and extent of the fields captured.

RESULTS

In all the 20 eyes, nasal endoscope provided a clear circular view of the iridocorneal angle and wide-field fundus view in a single glimpse. The 4 mm endoscope offered a wider view while in-air disc analysis was better with the 2.7 mm endoscope. Angulated endoscopes were more useful in imaging the iridocorneal angle, peripheral retina up to the ora serrata and ciliary body. The image quality with the xenon and halogen light sources were found to be comparable. The time taken for a single eye examination did not exceed 5 min.

CONCLUSIONS

With good techniques and appropriate selection of nasal endoscopes, viewing of the iridocorneal angle and fundus is easy, possibly less expensive and very useful in infants.

Development of Animal Models of Local Retinal Degeneration

PURPOSE

Development of nongenetic animal models of local retinal degeneration is essential for studies of retinal pathologies, such as chronic retinal detachment or age-related macular degeneration. We present two different methods to induce a highly localized retinal degeneration with precise onset time, that can be applied to a broad range of species in laboratory use.

METHODS

A 30-μm thin polymer sheet was implanted subretinally in wild-type (WT) rats. The effects of chronic retinal separation from the RPE were studied using histology and immunohistochemistry. Another approach is applicable to species with avascular retina, such as rabbits, where the photoreceptors and RPE were thermally ablated over large areas, using a high power scanning laser.

RESULTS

Photoreceptors above the subretinal implant in rats degenerated over time, with 80% of the outer nuclear layer disappearing within a month, and the rest by 3 months. Similar loss was obtained by selective photocoagulation with a scanning laser. Cells in the inner nuclear layer and ganglion cell layer were preserved in both cases. However, there were signs of rewiring and decrease in the size of the bipolar cell terminals in the damaged areas.

CONCLUSIONS

Both methods induce highly reproducible degeneration of photoreceptors over a defined area, with complete preservation of the inner retinal neurons during the 3-month follow-up. They provide a reliable platform for studies of local retinal degeneration and development of therapeutic strategies in a wide variety of species.

Ultraviolet phototoxicity to the retina

OBJECTIVE

This overview of ultraviolet (UV) phototoxicity considers the interaction of UVA and short-wavelength VIS light with the retina and retinal pigment epithelium.

METHODS

The damage mechanisms underlying UV retinal phototoxicity are illustrated with a literature survey and presentation of experimental results.

RESULTS

Depending on the wavelength and exposure duration, light interacts with tissue by three general mechanisms: thermal, mechanical, or photochemical. Although the anterior structures of the eye absorb much of the UV component of the optical radiation spectrum, a portion of the UVA band (315-400 nm) penetrates into the retina. Natural sources, such as the sun, emit energetic UV photons in relatively long durations, which typically do not result in energy confinement in the retina, and thus do not produce thermal or mechanical damage but are capable of inducing photochemical damage. Photochemical damage in the retina proceeds through Type 1 (direct reactions involving proton or electron transfers) and Type 2 (reactions involving reactive oxygen species) mechanisms. Commonly used drugs, such as certain antibiotics, nonsteroidal anti-inflammatory drugs, psychotherapeutic agents, and even herbal medicines, may act as photosensitizers that promote retinal UV damage, if they are excited by UVA or visible light and have sufficient retinal penetration.

CONCLUSIONS

Although the anterior portion of the eye is the most susceptible to UV damage, the retina is at risk to the longer UV wavelengths that propagate through the ocular media. Some phototoxicity may be counteracted or reduced by dietary intake of antioxidants and protective phytonutrients.

Retinal light toxicity

The ability of light to enact damage on the neurosensory retina and underlying structures has been well understood for hundreds of years. While the eye has adapted several mechanisms to protect itself from such damage, certain exposures to light can still result in temporal or permanent damage. Both clinical observations and laboratory studies have enabled us to understand the various ways by which the eye can protect itself from such damage. Light or electromagnetic radiation can result in damage through photothermal, photomechanical, and photochemical mechanisms. The following review seeks to describe these various processes of injury and many of the variables, which can mitigate these modes of injury.

[Light exposition in vitreoretinal surgery. I. Basics]

Due to its function of light perception, the eye is exposed to high levels of radiation of the optical spectrum. Most of the ultraviolet and infrared radiation is absorbed in the cornea and lens, and mostly only radiation of the visible spectrum can reach the retina. Visible light can cause retinal damage by photomechanical, photothermal, and photochemical mechanisms. The most important mechanism of light damage to the retina under daily conditions or when using ophthalmologic light sources is the photochemical light toxicity caused by light-induced chemical reactions. The extent of damage depends on several factors, such as wavelength, exposure time, and irradiance. Particularly the shorter portion of the visible light spectrum (blue light) is responsible for photochemical damage to the retina.

Functional implications of short-term retinal detachment in porcine eyes: study by multifocal electroretinography

PURPOSE

The aim of the study was to determine the type and magnitude of detectable changes in pig multifocal electroretinography (mfERG) induced by the vitreoretinal surgical procedures necessary to gain access to the subretinal space.

METHODS

Twenty pigs underwent posterior segment surgery. Six animals had a vitrectomy (V), six had in addition a retinal bleb detachment (V + B); five had in addition a retinal diathermia on the bleb (V + B + D) and three received a retinotomy in the diathermized retinal area (V + B + D + R). mfERG evaluation was performed at baseline and 1 and 6 weeks postoperatively. Selected eyes were enucleated for histological evaluation.

RESULTS

The retinal detachments blebs all reattached spontaneously. All four surgical sequences resulted in slight, non-significant changes in the mfERG peaks. A trend towards an amplitude reduction of the mfERG peaks N1, P1 and N2 were observed within the first postoperative week. After 6 weeks, all amplitudes had normalized. Of the implicit times only that of peak N1 (after retinal diathermia) was prolonged significantly at 1 week (P = 0.037). However, it returned to the preoperative level after 6 weeks. Histologically, the retinal detachment bleb was characterized by transient double layering of the retinal pigment epithelium (RPE) and loss photoreceptor outer segments.

CONCLUSION

Access to the subretinal space in pigs can be gained without permanent detectable changes in the mfERG. A short-term retinal detachment was found to cause only reversible electrophysiological and histological changes in the outer retina, which suggests that this procedure is tolerated well in the porcine retina. The size of the known destructive lesion (retinotomy) was too small to be detected, given the spatial resolution of the mfERG method applied. In the future, the presented protocol can be used to assess the functional outcome of surgery and transplantation in the subretinal space in pigs.

PHOTIC RETINOPATHY AFTER CATARACT SURGERY IN DIABETIC PATIENTS

PURPOSE

To report and emphasize photic retinopathy (PR) after cataract surgery in diabetic patients.

METHODS

The charts of 2,573 patients who underwent cataract surgery at Baskent University Hospital (Ankara, Turkey) between January 2001 and December 2003 were retrospectively analyzed for PR and diabetes mellitus (DM). All surgeries were performed using the same coaxial illuminated microscope. The diagnosis of PR was confirmed by fluorescein angiography, showing typical mottled fluorescein transmission in a well circumscribed area in each case. Statistical analyses were performed with chi2 and Mann-Whitney U tests.

RESULTS

Diagnoses of PR and DM were established in 16 cases (0.62%) and 382 cases (14.85%), respectively. DM was present in 7 (43.75%) of the PR cases. PR occurred more frequently in DM patients than in non-DM patients (7/382 vs. 9/2,191, respectively; P = 0.001; chi2= 10.64). All 16 patients with PR had received retrobulbar anesthesia (RA). RA was used in 61.21% of all surgeries and in 78.01% of the surgeries for DM patients. Occurrence of PR during RA was higher for DM patients than for non-DM patients (7/298 vs. 9/1,277, respectively; P = 0.011; chi2= 6.50). Mean operation times +/- SD for PR cases were 29.9 +/- 9.9 minutes and 38.2 +/- 5.3 minutes in DM and non-DM cases, respectively (P = 0.091). None of the photic lesions were sight threatening.

CONCLUSIONS

Diabetic patients may be more vulnerable to photic injury. Surgeons should take precautions to prevent PR when performing cataract surgery on diabetic patients.

Phototoxicity to the retina: mechanisms of damage

Light damage to the retina occurs through three general mechanisms involving thermal, mechanical, or photochemical effects. The particular mechanism activated depends on the wavelength and exposure duration of the injuring light. The transitions between the various light damage mechanism may overlap to some extent. Energy confinement is a key concept in understanding or predicting the type of damage mechanism produced by a given light exposure. As light energy (either from a laser or an incoherent source) is deposited in the retina, its penetration through, and its absorption in, various tissue compartments is determined by its wavelength. Strongly absorbing tissue components will tend to "concentrate" the light energy. The effect of absorbed light energy largely depends on the rate of energy deposition, which is correlated with the exposure duration. If the rate of energy deposition is too low to produce an appreciable temperature increase in the tissue, then any resulting tissue damage necessarily occurs because of chemical (oxidative) reactions induced by absorption of energetic photons (photochemical damage). If the rate of energy deposition is faster than the rate of thermal diffusion (thermal confinement), then the temperature of the exposed tissue rises. If a critical temperature is reached (typically about 10 degrees C above basal), then thermal damage occurs. If the light energy is deposited faster than mechanical relaxation can occur (stress confinement), then a thermoelastic pressure wave is produced, and tissue is disrupted by shear forces or by cavitation-nonlinear effects. Very recent evidence suggests that ultrashort laser pulses can produce tissue damage through nonlinear and photochemical mechanisms; the latter because of two-photon excitation of cellular chromophores. In addition to tissue damage caused directly by light absorption, light toxicity can be produced by the presence of photosensitizing agents. Drugs excited to reactive states by ultraviolet (UV) or visible light produce damage by type I (free radical) and type II (oxygen dependent) mechanisms. Some commonly used drugs, such as certain antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and psychotherapeutic agents, as well as some popular herbal medicines, can produce ocular phototoxicity. Specific cellular effects and damage end points characteristic of light damage mechanisms are described.