Spectral transmission of the optical media of the human eye with respect to keratitis and cataract formation

UVR and RPE - The Good, the Bad and the degenerate Macula

Ultraviolet Radiation (UVR) has a well-established causative influence within the aetiology of conditions of the skin and the anterior segment of the eye. However, a grounded assessment of the role of UVR within conditions of the retina has been hampered by a historical lack of quantitative, and spectrally resolved, assessment of how UVR impacts upon the retina in terms congruent with contemporary theories of ageing. In this review, we sought to summarise the key findings of research investigating the connection between UVR exposure in retinal cytopathology while identifying necessary avenues for future research which can deliver a deeper understanding of UVR's place within the retinal risk landscape.

Natural Course of Solar and Laser-Associated Retinal and Macular Injuries at a Primary Care Hospital in Switzerland

BACKGROUND

Solar and laser-associated retinopathies are rare occurrences. The two retinopathies are both photo-induced but differ in the involved intensity and duration of exposure. The purpose of this study was to evaluate the clinical features and natural course of these two entities, with a focus on the changes in the outer retina over time.

PATIENTS AND METHODS

This retrospective analysis assessed patients with solar or laser maculopathy seen at the Department of Ophthalmology of the University Hospital Zurich in Switzerland over the last 10 years. Visual acuity (VA; Snellen) and optical coherence tomography (OCT) findings were reviewed and analyzed at baseline and last follow-up visit. Areas of damaged outer retina, identified on en face OCT images as hyporeflective areas, were tagged and compared between visits. Descriptive analysis was performed by calculating mean values ± standard deviation (SD). Statistical evaluation was done using the Wilcoxon signed rank test. A p value < 0.05 was considered statistically significant.

RESULTS

Five patients with solar retinopathy and six patients with laser-associated retinopathy were identified. In the solar retinopathy group, mean VA at baseline was 0.80 (SD ± 0.37) and improved to 0.90 (SD ± 0.36). This was not statistically significant (p = 0.066). In the laser-associated retinopathy group, mean VA at baseline was 0.89 (SD ± 0.18) and improved to 1.03 (SD ± 0.09), which was not statistically significant either (p = 0.063). At baseline, in OCT cross-sections, initial changes were observed in the interdigitation, myoid, and ellipsoid zone, as well as the outer nuclear layer and the Henle fiber layer. At follow-up, most cases presented an alteration in the residual ellipsoid zone, with the degree of the aforementioned alterations depending on the size of the initial defect. A decrease of the hyporeflective alterations measured in en face OCT scans was observed in both groups but was only statistically significant in the laser-associated retinopathy group (p = 0.018 versus p = 0.172).

CONCLUSIONS

OCT can help to detect and monitor solar and laser-associated retinal injuries. Most injuries are minor, with good functional restitution. Minor changes in the ellipsoid zone often persist, even in cases with full visual recovery.

Quantitative action spectroscopy reveals ARPE19 sensitivity to ultraviolet radiation at 350 nm and 380 nm

The role of ultraviolet radiation (UVR) exposure in the aetiology of retinal degeneration has been debated for decades with epidemiological evidence failing to find a clear consensus for or against it playing a role. A key reason for this is a lack of foundational research into the response of living retinal tissue to UVR in regard to modern ageing-specific parameters of tissue function. We therefore explored the response of cultured retinal pigmented epithelium (RPE), the loss of which heralds advanced visual decline, to specific wavelengths of UVR across the UV-B and UV-A bands found in natural sunlight. Using a bespoke in vitro UVR exposure apparatus coupled with bandpass filters we exposed the immortalised RPE cell line, ARPE-19, to 10 nm bands of UVR between 290 and 405 nm. Physical cell dynamics were assessed during exposure in cells cultured upon specialist electrode culture plates which allow for continuous, non-invasive electrostatic interrogation of key cell parameters during exposure such as monolayer coverage and tight-junction integrity. UVR exposures were also utilised to quantify wavelength-specific effects using a rapid cell viability assay and a phenotypic profiling assay which was leveraged to simultaneously quantify intracellular reactive oxygen species (ROS), nuclear morphology, mitochondrial stress, epithelial integrity and cell viability as part of a phenotypic profiling approach to quantifying the effects of UVR. Electrical impedance assessment revealed unforeseen detrimental effects of UV-A, beginning at 350 nm, alongside previously demonstrated UV-B impacts. Cell viability analysis also highlighted increased effects at 350 nm as well as 380 nm. Effects at 350 nm were further substantiated by high content image analysis which highlighted increased mitochondrial dysfunction and oxidative stress. We conclude that ARPE-19 cells exhibit a previously uncharacterised sensitivity to UV-A radiation, specifically at 350 nm and somewhat less at 380 nm. If upheld in vivo, such sensitivity will have impacts upon geoepidemiological risk scoring of macular sensitivity.

Mechanisms of corneal intrastromal laser dissection for refractive surgery: ultra-high-speed photographic investigation at up to 50 million frames per second

Every year, more than a million refractive eye surgeries using femtosecond lasers are performed but the intrastromal cutting process remains an area of development. We investigated the mechanisms of laser dissection in cornea by ultra-high-speed photography. We found that the intrastromal bubble forms multiple lobes along the elongated laser plasma and the overlying lobes expand along the corneal lamellae. Videography demonstrated that the cutting process relies on crack propagation in the stroma along the bubble lobes with the crack originating from the pre-existing bubble layer. These insights are important for further improvement of the cutting mechanisms in refractive surgery.

Monte-Carlo simulation and tissue-phantom model for validation of ocular oximetry

Ocular oximetry, in which blood oxygen saturation is evaluated in retinal tissues, is a promising technique for the prevention, diagnosis and management of many diseases and conditions. However, the development of new tools for evaluating oxygen saturation in the eye fundus has often been limited by the lack of reference tools or techniques for such measurements. In this study, we describe a two-step validation method. The impact of scattering, blood volume fraction and lens yellowing on the oximetry model is investigated using a tissue phantom, while a Monte Carlo model of the light propagation in the eye fundus is used to study the effect of the fundus layered-structure. With this method, we were able to assess the performance of an ocular oximetry technique in the presence of confounding factors and to quantify the impact of the choroidal circulation on the accuracy of the measurements. The presented strategy will be useful to anyone involved in studies based on the eye fundus diffuse reflectance.

Spectral Transmission of the Human Corneal Layers

We have assessed the spectral transmittance of the different layers of the human cornea in the ultraviolet (UV), visible, and near-infrared (IR) spectral ranges. Seventy-four corneal sample donors were included in the study. Firstly, the corneal transmittance was measured using a spectrophotometer. Then, all samples were fixed for histopathological analysis, which allowed us to measure the thickness of each corneal layer. Finally, the absorption coefficients of the corneal layers were computed by a linear model reproducing total transmittance. The results show that corneal transmission was almost in unity at the visible and IR ranges but not at the UV range, in which the layer with higher transmission is Descemet’s membrane, whereas the stroma showed the lowest transmittance. Regarding the absorption coefficient, the most absorptive tissue was Bowman’s layer, followed by the endothelium. Variations on transmittance due to changes in the stroma, Bowman’s layer, or Descemet layer were simulated, and important transmission increases were found due to stroma and Bowman changes. To conclude, we have developed a method to measure the transmittance and thickness for each corneal layer. All corneal layers absorb UV light to a greater or lesser extent. The absorption coefficient is higher for Bowman’s layer, while the stroma is the layer with the lowest transmittance due to its thickness. Variations in stroma thickness or changes in the corneal tissue of Bowman’s layer or the endothelium layer due to some pathologies or surgeries could affect, to a greater or lesser degree, the total transmission of the cornea. Thus, obtaining accurate absorption coefficients for different layers would help us to predict and compensate these changes.

A Purkinje image-based system for an assessment of the density and transmittance spectra of the human crystalline lens in vivo

A method for rapid and objective assessment of ocular lens density and transmittance is needed for research and clinical practice. The aim of this study was to determine whether the Purkinje image-based technique can be used for objective and accurate quantification of spectral density and transmittance of ocular media (the mainly crystalline lens) in visible light. Twenty-six individuals (10 young, 9 middle-aged and 7 older individuals) participated in this study. Spectral lens density was evaluated by detecting the intensity of the IVth Purkinje image for different wavelengths. Subsequently, optical density index (ODI), the area under the curve in the lens density spectrum, was calculated and ODIs were compared with clinical lens opacification scales assessed subjectively using a slit lamp. Spectral lens transmittance was estimated from the lens density spectrum. Lens densities were higher in the short wavelength region of the visible spectrum across all age groups. ODI was highly correlated with the clinical opacification scale, while lens transmittance decreased with aging. Our results showed that spectral transmittance of the human crystalline lens can be easily estimated from optical density spectra evaluated objectively and rapidly using the Purkinje image-based technique. Our results provide clinicians and scientists with an accurate, rapid and objective technique for quantification of lens transmittance.

Age-Related Absorption of the Human Lens in the Near-Ultraviolet Range

The purpose of the present study was to determine the age dependence of the ultraviolet (UV) absorption of the different parts of the human crystalline lens. Cryostat sections of human cadaveric lenses (60 μm) were cut. The UV absorbance of nine samples, derived from different parts of the lens, was determined using a Shimadzu scanning spectrophotometer. The absorbance of the anterior and posterior lens capsules was measured separately. The absorption coefficients were calculated from the measured absorbance and values taken at 280 as well as at 360 nm were compared statistically. ANCOVA analysis of the values taken at 280 and at 360 nm wavelengths shows that correlation between the absorption coefficients and age can be found only in the case of the posterior layers. These results suggest a differential age-dependent increase of the UV absorption of the posterior layers compared to the anterior ones and can be related to the differential protein expression in the anterior and posterior parts. Posterior crystalline lens capsules have higher absorption coefficients than the anterior ones regardless of age.

Spectral Evaluation of Eyeglass Blocking Efficiency of Ultraviolet/High-energy Visible Blue Light for Ocular Protection

Supplemental digital content is available in the text.

SIGNIFICANCE

We investigated, for safety and awareness, ultraviolet and high-energy violet light–blocking protection provided by assorted types of eyewear. Ultraviolet and high-energy violet light–filtering efficiency varied and did not correlate with price or advertised claims. Standardization of methods and specifications for lens spectral transmission evaluation is recommended.

PURPOSE

Studies have linked exposure of high-energy visible blue light to effect and damage on retinal epithelial cells, photoreceptors, and ganglion cells. “Blue light” is more accurately differentiated into “high-energy visible blue-violet light” and “circadian rhythm blue-turquoise light.” This study measured and compared spectral transmission of ultraviolet and high-energy violet light of low-, medium-, and high-priced sunglasses.

METHODS

Sunglasses and lens blanks were obtained from the University of Texas Medical Branch Optical Shop and vendors. Groups were based on promotional, retail, designer sunglasses, or “blue blocker” lenses. The percent transmittance of ultraviolet/visible spectral scans (800 to 350 nm) was measured using an Agilent Cary 50 spectrophotometer. High-energy violet/blue light was defined as 400 to 450 nm.

RESULTS

Promotional sunglasses (tinted polycarbonate) blocked 100% ultraviolet and 67 to 99.8% high-energy violet blue light. Retail sunglasses filtered out 95 to 100% ultraviolet A and 67% high-energy violet light. The tested designer sunglasses varied widely in their optical transmissibility with respect to their ultraviolet A and high-energy violet light–blocking properties, with some not blocking ultraviolet A. Clear and colorless Kodak Total Blue provided maximal high-energy violet protection, whereas clear Essilor Crizal Prevencia provided less high-energy violet blocking between 400 and 450 nm.

CONCLUSIONS

The ultraviolet and high-energy violet (400 to 450 nm) light–filtering efficiency varied between sunglasses and clear lenses and did not correlate with price or advertised claims. Standardization of methods and specifications for lens spectral transmission evaluation is recommended.

Rods contribute to the light-induced phase shift of the retinal clock in mammals

While rods, cones, and intrinsically photosensitive melanopsin-containing ganglion cells (ipRGCs) all drive light entrainment of the master circadian pacemaker of the suprachiasmatic nucleus, recent studies have proposed that entrainment of the mouse retinal clock is exclusively mediated by a UV-sensitive photopigment, neuropsin (OPN5). Here, we report that the retinal circadian clock can be phase shifted by short duration and relatively low-irradiance monochromatic light in the visible part of the spectrum, up to 520 nm. Phase shifts exhibit a classical photon dose-response curve. Comparing the response of mouse models that specifically lack middle-wavelength (MW) cones, melanopsin, and/or rods, we found that only the absence of rods prevented light-induced phase shifts of the retinal clock, whereas light-induced phase shifts of locomotor activity are normal. In a “rod-only” mouse model, phase shifting response of the retinal clock to light is conserved. At shorter UV wavelengths, our results also reveal additional recruitment of short-wavelength (SW) cones and/or OPN5. These findings suggest a primary role of rod photoreceptors in the light response of the retinal clock in mammals.

The mammalian retina contains a circadian clock that plays a crucial role in adapting retinal physiology and visual function to light/dark changes. In addition, the retina coordinates rhythmic behavior and physiology by providing visual input to the master hypothalamic clock in the suprachiasmatic nucleus through a network of retinal photoreceptor cells involving rods, cones, and intrinsically photosensitive melanopsin-containing ganglion cells (ipRGCs). In contrast, recent studies argue that none of these photoreceptors are involved in light responses of the retinal clock and propose that photoresponses are exclusively mediated by the UV-sensitive photopigment neuropsin (OPN5). Our study demonstrates that rods are required to phase shift the retinal clock, while melanopsin and middle-wavelength (MW) cones influence the intrinsic period of the clock.

Non-invasive tryptophan fluorescence measurements as a novel method of grading cataract

Development of non-invasive treatments for cataract calls for a sensitive diagnostic assay. We conducted a study to test whether the ratio of folded tryptophan to non-tryptophan fluorescence emission (F-factor) may be used for grading cataracts in human lenses. The F-factor was measured on aspirated lens material from eyes undergoing femtosecond laser assisted cataract surgery (FLACS) and was compared to a preoperative optical grading of cataract using Scheimpflug imaging. The preoperative optical grading allocated the cataracts to 1 of 4 categories according to the density of the cataract. All cataracts were age-related. Lens material from 16 eyes of 14 patients was included in the study. Cataracts were preoperatively graded in categories 1, 2 and 3. No lenses were category 4. For nuclear cataracts mean values of F-factor were 52.9 (SD 12.2), 61.7 (SD 5.3) and 75.7 (SD 8.9) for categories 1, 2 and 3 respectively. Linear regression on F-factor as a function of preoperative grading category showed increasing values of F-factor with increasing preoperative grading category, R² = 0.515. Our experiment showed that preoperative optical grading of cataracts by Scheimpflug imaging may correlate to measures of tryptophan and non-tryptophan fluorescence in human lenses. Based on our results we find that measuring the ratio between tryptophan- and non-tryptophan fluorescence may be a future tool for grading cataracts, but further research is needed.

UV-B induced fibrillization of crystallin protein mixtures

Environmental factors, mainly oxidative stress and exposure to sunlight, induce the oxidation, cross-linking, cleavage, and deamination of crystallin proteins, resulting in their aggregation and, ultimately, cataract formation. Various denaturants have been used to initiate the aggregation of crystallin proteins in vitro. All of these regimens, however, are obviously far from replicating conditions that exist in vivo that lead to cataract formation. In fact, it is our supposition that only UV-B radiation may mimic the observed in vivo cause of crystallin alteration leading to cataract formation. This means of inducing cataract formation may provide the most appropriate in vitro platform for in-depth study of the fundamental cataractous fibril properties and allow for testing of possible treatment strategies. Herein, we showed that cataractous fibrils can be formed using UV-B radiation from α:β:γ crystallin protein mixtures. Characterization of the properties of formed aggregates confirmed the development of amyloid-like fibrils, which are in cross-β-pattern and possibly in anti-parallel β-sheet arrangement. Furthermore, we were also able to confirm that the presence of the molecular chaperone, α-crystallin, was able to inhibit fibril formation, as observed for ‘naturally’ occurring fibrils. Finally, the time-dependent fibrillation profile was found to be similar to the gradual formation of age-related nuclear cataracts. This data provided evidence for the initiation of fibril formation from physiologically relevant crystallin mixtures using UV-B radiation, and that the formed fibrils had several traits similar to that expected from cataracts developing in vivo.

Invisible Shield: Review of the Corneal Epithelium as a Barrier to UV Radiation, Pathogens, and Other Environmental Stimuli

The ocular surface is comprised of the cornea and conjunctiva, which are structures that not only protect the eye but also enable vision. The corneal epithelium is the most superficial layer of the cornea, and therefore first line of defense against external assaults. Damage to this highly specialized structure could lead to vision loss, making it an important structure to investigate and understand. Here, we conducted a search of the current literature on the mechanisms the corneal epithelium has adapted against three frequent insults: UV-radiation, pathogens, and environmental assaults. This review systematically examines the corneal epithelium's response to each assault in order to maintain its role as an invisible shield. The goal of this review is to provide insight into some of the critical functions the corneal epithelium performs that may be valuable to current regenerative studies.

LED Light Characteristics for Surgical Shadowless Lamps and Surgical Loupes

Background:

Blue light has more energy than longer wavelength light and can penetrate the eye to reach the retina. When surgeons use magnifying loupes under intensive surgical shadowless lamps for better view of the surgical field, the total luminance is about 200 times brighter than that of typical office lighting. In this study, the effects of 2 types of shadowless lamps were compared. Moreover, the effect of various eyeglasses, which support magnifying loupes, on both the light energy and color rendering was considered.

Methods:

The light intensity and color rendering were measured on 3 variables: light transmittance, light intensity, and color rendering.

Results:

Under shadowless lamps, the light energy increased with low-magnification loupes and decreased with high-magnification loupes. Filtering eyeglasses reduced the energy, especially in conditions where the low-magnification loupe was used. The best color-rendering index values were obtained with computer eyeglasses under conventional light-emitting diode shadowless lamps and with no glass and with lightly yellow-tinted lenses under less-blue light-emitting diode.

Conclusions:

Microsurgeons are exposed to strong lighting throughout their career, and proper color rendering must be considered for easier recognition. Light toxicity and loss of color rendering can be reduced with an appropriate combination of shadowless lamps and colored eyeglasses.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals

Although ultraviolet (UV) sensitivity is widespread among animals it is considered rare in mammals, being restricted to the few species that have a visual pigment maximally sensitive (λmax) below 400 nm. However, even animals without such a pigment will be UV-sensitive if they have ocular media that transmit these wavelengths, as all visual pigments absorb significant amounts of UV if the energy level is sufficient. Although it is known that lenses of diurnal sciurid rodents, tree shrews and primates prevent UV from reaching the retina, the degree of UV transmission by ocular media of most other mammals without a visual pigment with λmax in the UV is unknown. We examined lenses of 38 mammalian species from 25 families in nine orders and observed large diversity in the degree of short-wavelength transmission. All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity. Other mammals, however, such as hedgehogs, dogs, cats, ferrets and okapis had lenses transmitting significant amounts of UVA (315-400 nm), suggesting that they will be UV-sensitive even without a specific UV visual pigment.

Optical effects of exposing intact human lenses to ultraviolet radiation and visible light

BACKGROUND

The human lens is continuously exposed to high levels of light. Ultraviolet radiation is believed to play a causative role in the development of cataract. In vivo, however, the lens is mainly exposed to visible light and the ageing lens absorbs a great part of the short wavelength region of incoming visible light. The aim of the present study was to examine the optical effects on human lenses of short wavelength visible light and ultraviolet radiation.

METHODS

Naturally aged human donor lenses were irradiated with UVA (355 nm), violet (400 and 405 nm) and green (532 nm) lasers. The effect of irradiation was evaluated qualitatively by photography and quantitatively by measuring the direct transmission before and after irradiation. Furthermore, the effect of pulsed and continuous laser systems was compared as was the effect of short, intermediate and prolonged exposures.

RESULTS

Irradiation with high intensity lasers caused scattering lesions in the human lenses. These effects were more likely to be seen when using pulsed lasers because of the high pulse intensity. Prolonged irradiation with UVA led to photodarkening whereas no detrimental effects were observed after irradiation with visible light.

CONCLUSIONS

Irradiation with visible light does not seem to be harmful to the human lens except if the lens is exposed to laser irradiances that are high enough to warrant thermal protein denaturation that is more readily seen using pulsed laser systems.

Exploring the possibility of early cataract diagnostics based on tryptophan fluorescence

A novel route for early cataract diagnostics is investigated based on the excitation of tryptophan fluorescence (TF) at the red edge of its absorption band at 317 nm. This allows penetration through the cornea and aqueous humour to provide excitation of the ocular lens. The steepness of the red edge gives the potential of depth control of the lens excitation. Such wavelength selection targets the population of tryptophan residues, side chains of which are exposed to the polar aqueous environment. The TF emissions around 350 nm of a series of UV-irradiated as well as control lenses were observed. TF spectra of the UV cases were red-shifted and the intensity decreased with the radiation dose. In contrast, intensity of non-tryptophan emission with maximum at 435 nm exhibited an increase suggesting photochemical conversion of the tryptophan population to 435 nm emitting molecules. We demonstrate that the ratio of intensities at 435 nm to that around 350 nm can be used as a measure of early structural changes caused by UV irradiation in the lens by comparison with images from a conventional slit-lamp, which can only detect defects of optical wavelength size. Such diagnostics at a molecular level could aid research on cataract risk investigation and possible pharmacological research as well as assisting surgical lens replacement decisions.

Synergistic effects of ultraviolet A/riboflavin and glucose on corneal collagen cross-linking

PURPOSE

To investigate the possible synergistic effect of ultraviolet A (UVA)/riboflavin and glucose on corneal collagen cross-linking (CXL) by means of the mechanical properties and thermal shrinkage temperatures.

METHODS

Forty rabbits were divided into four groups: control group; UVA/glucose group; UVA/riboflavin group; and UVA/riboflavin/glucose group. After the treatments, the mechanical properties and thermal shrinkage temperatures of the corneal strips were analyzed to determine the CXL effects of corneal collagen induced by different treatments.

RESULTS

Compared with the control group and other experimental groups, the UVA/riboflavin/glucose group showed significant alterations of the biomechanical properties. The ultimate stress value for the UVA/riboflavin/glucose group was increased by 120%, ultimate strain decreased by 80%, and Young's modulus increased by 70% relative to the control group. An increase in ultimate stress by 24.5%, a decrease in ultimate strain by 34%, and an increase in Young's modulus by 17.4% were found in the UVA/riboflavin/glucose group compared to the UVA/riboflavin group. A significant increase in thermal shrinkage temperatures was also noted after corneal collagen CXL induced by UVA/riboflavin/glucose relative to other groups.

CONCLUSIONS

The combination of glucose and UVA/riboflavin may synergistically induce corneal collagen CXL, resulting in increased mechanical strength of corneal collagen.

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.

Optical fiber-coupled ocular spectrometer for measurement of drug concentration in the anterior eye--applications in pharmaceuticals research

This paper describes in detail a novel optoelectronic system designed to measure drug absorption in the anterior segment of the eye following topical application of drug formulations. This minimally invasive measurement technique offers both a method for determining drug concentration in human eyes, and demonstrates an alternative to current testing processes in model animals, which require paracentesis of the anterior chamber of the eye. The optoelectronic technique can be used with formulations, which possess appropriate spectral characteristics, namely unique absorption or fluorescence spectra. Preliminary experiments using our measurement system have been performed in rabbit and man, where we have been successful in achieving the direct measurement of topically applied brimonidine, an alpha-2 agonist used in the treatment of glaucoma. This demonstrates the feasibility of performing real-time, in vivo testing of ophthalmic drug formulations in the eye of human test subjects. We further demonstrate the novel application of the optoelectronic system for detection of topically applied UV-absorbing compounds in rabbit cadaver eyes, with a view to evaluating potential ocular sunscreen formulations. In summary, this method can be applied for the rapid comparison of the penetration of different drug formulations into the anterior eye at greatly reduced cost and time.

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.

Optical density of the aging human ocular media in the visible and the UV

We analyzed the literature on the absorption in the young and aging human eye media. Five templates were derived to provide an adequate description of the spectra from 300 to 700 nm for the lens, cornea, aqueous, and vitreous. Two templates were found in all media. They stand for Rayleigh scatter and the absorbance of tryptophan. Three additional templates for the lens represent absorbance in kynurenine derivatives, such as 3-hydroxykynurenine glucoside (3HKG), and absorbance in two substances found at older age. Except for Rayleigh scatter, all templates have a Gaussian shape. Aging-trend functions were derived that show a linear slope on an age-squared scale. The result can be used to correct for media losses in visual perception tasks, in fundus reflectometry, and in studies on light damage.

Safety of UVA-Riboflavin Cross-Linking of the Cornea

PURPOSE

To study potential damage to ocular tissue during corneal collagen cross-linking (X-linking) by means of the riboflavin/UVA (370 nm) approach.

METHODS

Comparison of the currently used technique with officially accepted guidelines regarding direct UV damage and the damage created by the induced free radicals (photochemical damage).

RESULTS

The currently used UVA radiant exposure of 5.4 mJ/cm and the corresponding irradiance of 3 mW/cm2 is below the known damage thresholds of UVA for the corneal endothelium, lens, and retina. Regarding the photochemical damage caused by the free radicals, the damage thresholds for keratocytes and endothelial cells are 0.45 and 0.35 mW/cm, respectively. In a 400-microm-thick cornea saturated with riboflavin, the irradiance at the endothelial level was 0.18 mW/cm, which is a factor of 2 smaller than the damage threshold.

CONCLUSIONS

After corneal X-linking, the stroma is depopulated of keratocytes approximately 300 microm deep. Repopulation of this area takes up to 6 months. As long as the cornea treated has a minimum thickness of 400 microm (as recommended), the corneal endothelium will not experience damage, nor will deeper structures such as lens and retina. The light source should provide a homogenous irradiance, avoiding hot spots.

Minimally invasive, direct, real time measurement of drug concentration in the anterior eye

AIMS

To evaluate a corneal contact lens which effectively turns the anterior chamber of the eye into a cuvette, enabling the concentration of a drug to be measured using absorption spectroscopy.

METHODS

A hand held contact lens incorporating optical fibres connected to a spectrograph enabled a beam of light to be directed in, across, and out of the anterior chamber. The device was used to follow the time course of drug concentration in the anterior chamber of rabbit (sedated) and humans, using topical brimonidine or fluorescein (with or without local anaesthesia). Absorbance measurements were taken for a 5-25 second period, repeated every 30 minutes. Drug concentrations were compared using absorbance peak height.

RESULTS

Corneal absorption starts to rise rapidly at wavelengths shorter than 315 nm. The light path within the anterior chamber is 6.9 mm (rabbit) and 5.8 mm (human), the absorbance measured also includes a corneal component. Application of fluorescein (three drops of 2% solution) in rabbit allowed detection, 60 minutes later, of a large absorbance peak at 490 nm. In the human eye, the device could not measure fluorescein (applied as in rabbit), but clearly detected brimonidine for 3 hours following topical application of 0.6 mg. Modification of the device to measure fluorescence resulted in the detection of 5.3 nM fluorescein in the ex vivo rabbit eye, an increase in sensitivity of two orders of magnitude over the absorption measurements.

CONCLUSION

This device has the potential to allow repeated measurements of drug concentrations in the anterior eye provided the drug has suitable absorption or fluorescence characteristics.

Minimally invasive spectroscopic system for intraocular drug detection

A novel, minimally invasive measurement technique has been developed for the detection of drugs in the anterior chamber of the eye. Presently there is no satisfactory, real-time detection method available to the ophthalmic community. Accurate determination of drug concentrations in the eye would be of great value and assistance to researchers and manufacturers of ophthalmic drugs and ocular implants, to enable a better understanding of intraocular pharmacokinetics. At present researchers use techniques of direct sampling of the aqueous humor from laboratory animal eyes into which the drug has been introduced topically or systemically. Rabbit eyes are frequently used in this context. Sampling via paracentesis is invasive, and does not yield a continuous measurement. Our approach to addressing this measurement requirement is, in effect, to turn the eye into a cuvette and use optical absorbance spectroscopy measurements to detect drug concentrations. A novel contact lens has been designed using commercial, off-the-shelf, optical design software. The lenses have been optimized to direct light across the anterior chamber of a rabbit's eye. Practical demonstration and characterization of light propagation across the eye have been undertaken and are reported. Preliminary results of the identification of drug compounds introduced into model eyes are also reported.