Long-term effects of a single dose of ultraviolet-B on albino rabbit cornea--II. Deturgescence and fluid pump assessed in vitro

Temporal evolution of the biological response to laser-induced refractive index change (LIRIC) in rabbit corneas

Laser-induced refractive index change (LIRIC) is a new, non-incisional, non-ablative, femtosecond photo-modification technique being developed for vision correction in humans. Prior, exvivo studies showed intra-tissue refractive index change to induce minimal cell death, restricted to the laser focal zone in the corneal stroma, and with no observable damage to the epithelium or endothelium. Here, we used live rabbits to ascertain longer-term consequences of LIRIC in vivo. Specifically, we assessed cell death, fibrosis, corneal nerve distribution, endothelial cell density, and corneal structure for up to 3 months after LIRIC. A +2.5 D gradient-index LIRIC Fresnel lens was inscribed inside 20 applanated corneas of Dutch Belted rabbits, over a circular region of the mid-stroma measuring 4.5 mm in diameter. Twelve additional rabbit eyes were used as applanation-only controls to differentiate the effects of laser treatment and suction applanation on biological and structural parameters. In vivo optical measurements were performed pre-operatively, then immediately, 2, 4, and 12 weeks after the procedure, to measure endothelial cell density and changes in corneal structure. Groups of four rabbits were sacrificed at 4 hours, 2, 4, and 12 weeks after LIRIC for histological determinations; the TUNEL assay was used to evaluate cell death, H&E staining was used to assess inflammatory infiltration, and immunostaining for α-smooth muscle actin (α-SMA) and βIII tubulin (Tuj-1) was performed to assess myofibroblast differentiation and corneal nerve distribution, respectively. Consistent with prior ex vivo data, only minimal cell death was observed in the laser focal zone, with TUNEL-positive cells restricted to the stromal region of refractive index change 4 h after LIRIC. No TUNEL-positive cells were evident anywhere in the cornea 2, 4, or 12 weeks after LIRIC. Applanation-only corneas were completely TUNEL-negative. Neither LIRIC-treated nor applanation-only eyes exhibited α-SMA-positive staining or altered corneal nerve distributions at any of the time points examined. In vivo confocal imaging revealed normal endothelial cell densities in all eyes (whether LIRIC-treated or applanation-only) at all time points. Optical coherence tomography showed suction applanation to cause a temporary decrease in central corneal thickness, which returned to normal within 4 h. Corneas into which LIRIC Fresnel lenses were written while applanated did not undergo major structural or shape changes beyond the temporary thinning already described for suction applanation. The present findings suggest that LIRIC patterns, which generated a clinically-relevant refractive correction in the mid-stromal region of live rabbit corneas, induced little-to-no disruption to corneal structure and biology for 3 months after the procedure. This affirms the relative safety of LIRIC and predicts that compared to traditional laser vision correction surgeries, common post-operative complications such as dry eye, haze, or patient discomfort may be entirely avoided.

Oxidative stress to the cornea, changes in corneal optical properties, and advances in treatment of corneal oxidative injuries

Oxidative stress is involved in many ocular diseases and injuries. The imbalance between oxidants and antioxidants in favour of oxidants (oxidative stress) leads to the damage and may be highly involved in ocular aging processes. The anterior eye segment and mainly the cornea are directly exposed to noxae of external environment, such as air pollution, radiation, cigarette smoke, vapors or gases from household cleaning products, chemical burns from splashes of industrial chemicals, and danger from potential oxidative damage evoked by them. Oxidative stress may initiate or develop ocular injury resulting in decreased visual acuity or even vision loss. The role of oxidative stress in the pathogenesis of ocular diseases with particular attention to oxidative stress in the cornea and changes in corneal optical properties are discussed. Advances in the treatment of corneal oxidative injuries or diseases are shown.

Ultraviolet radiation transmittance of the mouse eye and its individual media components

Recently, the mouse has become the preferred animal model in ophthalmic research. Therefore, there is a need for enhanced understanding of the mouse eye to validate its use in different experimental setting. The purpose of this study was to determine the ocular transmittance of the whole mouse eye, the cornea and the crystalline lens, particularly in the ultraviolet radiation (UVR) wavebands. This was carried out using a non-cuvette based fiber optic spectrometer system and the resulting transmittance curves were compared with published cone spectral response curves and mouse ocular transmittance data. First, transmittance curves of the whole mouse eye were measured by removing a small disc of sclera from the posterior pole to provide an anterior to posterior optical path. No statistical difference was found between left and right eye in each of the four mice sampled, therefore, all eight eyes were included in the final statistical analysis. The average of five test measurements from each left and right eye for the four test mice showed a transmittance cut off at approximately 310 nm. Secondly, the cornea with a scleral rim was excised and transmittance curves obtained for all eight eyes. This data showed an average transmittance cut off at 280 nm for the cornea. Similarly measured data for the excised crystalline lens showed UVR transmittance down to 310 nm. The good correlation between total ocular UVR transmittance and the sum of the individually measured components (i.e. the cornea and the crystalline lens) supported the validity of our method and its findings. This experiment demonstrated that the mouse cornea transmits more UV-B (280-315 nm) than the rabbit and the human corneal transmittance. The mouse crystalline lens on the other hand showed a cut off in the UV-B at 310 nm, which is at a much lower UV-B wavelength than the approximate UV-A (315-400 nm) cut off for the human crystalline lens at around 390 nm. The increased transmittance of UVR in the mouse eye serves its vision, since the mouse has a cone photopigment peaking at approximately 350 nm. Due to the above stated differences between the mouse and the human it is concluded that the mouse is not an ideal model for the human eye in experiments involving UVR.

Protective effects of melatonin in experimental free radical-related ocular diseases

Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine with a range of antioxidative properties. Melatonin is endogenously produced in the eye and in other organs. Current evidence suggests that melatonin may act as a protective agent in ocular conditions such as photo-keratitis, cataract, glaucoma, retinopathy of prematurity and ischemia/reperfusion injury. These diseases are sight-threatening and they currently remain, for the most part, untreatable. The pathogenesis of these conditions is not entirely clear but oxidative stress has been proposed as one of the causative factors. Elevated levels of various reactive oxygen and nitrogen species have been identified in diseased ocular structures. These reactants damage the structure and deplete the eye of natural defense systems, such as the antioxidant, reduced glutathione, and the antioxidant enzyme superoxide dismutase. Oxidative damage in the eye leads to apoptotic degeneration of retinal neurons and fluid accumulation. Retinal degeneration decreases visual sensitivity and even a small change in the fluid content of the cornea and crystalline lens is sufficient to disrupt ocular transparency. In the eye, melatonin is produced in the retina and in the ciliary body. Continuous regeneration of melatonin in the eye offers a frontier antioxidative defense for both the anterior and posterior eye. However, melatonin production is minimal in newborns and its production gradually wanes in aging individuals as indicated by the large drop in circulating blood concentrations of the indoleamine. These individuals are possibly at risk of contracting degenerative eye diseases that are free radical-based. Supplementation with melatonin, a potent antioxidant, in especially the aged population should be considered as a prophylaxis to preserve visual functions. It may benefit many individuals worldwide, especially in countries where access to medical facilities is limited.

Photokeratitis and other phototoxic effects on the cornea and conjunctiva

Except when sleeping, the cornea and interpalpebral conjunctiva are exposed to the ambient environment, both natural and man-made. Levels of solar ultraviolet irradiance reaching the eye may exceed the damage threshold under a number of circumstances. The consequences of overexposure may be acute after a latent period, sequelae to an acute exposure, or long-term chronic effects. Previously derived action spectra for photokeratitis and photoconjunctivitis due to incoherent ultraviolet are presented. These reveal interspecies similarities for the levels of radiant energy reaching each tissue. The initial in vivo (clinical) signs of photokeratitis are due to lost or damaged epithelial cells with other signs produced by this primary response. The conjunctival signs include injection and chemosis. Chronic exposure to solar ultraviolet is a factor in climatic droplet keratopathy and pterygium. Phototoxic compounds or their by-products potentially can reach the cornea from the air, via the tears or aqueous humor, or from the limbal capillaries. However, the human cornea appears to be much less susceptible to the influence of phototoxic agents than the skin.

The effect of ultraviolet-B irradiation on the cell shedding rate of the corneal epithelium

In this study cell shedding rates of the corneal epithelium were determined in vitro following a single suprathreshold dose of ultraviolet-B irradiation. Rabbit corneas were excised and superfused in pairs. The epithelial and endothelial surfaces were bathed with solutions containing essential ions and adjusted to appropriate pH and osmolality. One cornea in each pair was irradiated, while the other cornea served as a control. Shed cells were periodically collected from the corneal surface and the shedding rate determined by counting the number of cells in a 50 min time period. Following a latent period of about 3 h, the epithelial shedding rate accelerated, and remained elevated for at least 8 h after irradiation. This result suggests that suprathreshold doses of ultraviolet-B irradiation disrupt the normal orderly cell shedding process and homeostatic equilibrium of the corneal epithelium. The elevated cell shedding rate exposes subsurface nerve endings and causes the characteristic pain of photokeratitis.

ArF-excimer laser-induced secondary radiation in photoablation of biological tissue

Secondary radiation, emitted during and after the irradiation of corneal, dermal, and dental tissue by an ArF-excimer laser (193 nm), was qualitatively and quantitatively characterized. Emission of secondary radiation was found in the range of 200-800 nm. The intensity of secondary radiation in the range of 200-315 nm (UVC and UVB) is approximately 20% of the total intensity at high laser fluences (> 2 J/cm2), and approximately 50% at moderate laser fluences (< 500 mJ/cm2); 10 muJ/cm2 in the UVC and UVB were measured at the sample surface, at fluences (< 1J/cm2) which are of relevance for clinical procedures on soft tissues. In dental tissue processing, very high fluences (> 5 J/cm2) are required. As a consequence, laser-induced plasma formation can be observed. Secondary radiation can be used as a visible guide for selective removal of carious altered tissue. The data we have found might be of assistance in estimating potential hazards for future mutagenic studies in the field.

Acute effects of ultraviolet-B irradiation on the corneal surface of the pigmented rabbit studied by quantitative scanning electron microscopy

The eyes of female pigmented rabbits were exposed to a single dose of UV-B (300 +/- 9 nm, 0.05 J/cm2 total dose) between 13.30 and 15.00 h. The average irradiance was 225 +/- 36 microW/cm2 delivered over 191 to 264 s. At various time periods thereafter (24, 48, 72 and 96 h post-irradiation), the animals were euthanized by pentobarbital overdose and the eyes fixed with 2% glutaraldehyde in 80 mM cacodylate buffer (pH 7.4, total osmolarity of 330-340 mOsm/L). Corneal quadrants were examined by high resolution scanning electron microscopy at 100 X and 500 X at-stage magnification at central, mid-peripheral and peripheral sites. The micrographs from the central cornea were subjected to a quantitative analysis using a computer based digitization system. A peak effect was observed at 48 h at which cellular exfoliation was noted at the corneal apex. In the region immediately adjacent to the exfoliating cells, the number of light and dark electron reflex cells decreased to 48 h while the numbers of medium-reflex cells decreased after 48 h. The relative surface area of all cells was also decreased at 48 h compared to unirradiated controls. Significant recovery was observed by 96 h. Mid-peripheral and peripheral sites were largely unaffected by this just supra-threshold irradiation.