Lower nasal distribution of cortical cataract: the Blue Mountains Eye Study

Calculation of solar ultraviolet influx in the eye considering the field of view and pupillary dilation due to sunglasses

The media and even the specialized literature report that the ultraviolet (UV) protection for sunglasses is critical, on the grounds that sunglasses can have a counter effect if the lenses do not provide adequate UV protection. They reason that the primary and natural mechanism is that the pupil of the eye contracts to attenuate radiation and protect the inner eye under sun exposure. Therefore, if dark lenses do not provide appropriate UV protection, there is an increased UV incidence in the inner eye due to pupil dilation, which enhances the adverse effects and impacts the ocular tissues more severely than in situations without UV protection. However, no existing literature properly quantified or supported this argument. In this work, the influx of solar UV throughout the pupil of the eye was calculated in two situations: when a person wear sunglasses and when he/she does not. In both situations, the pupil dilation and the field of view (squint) were considered with their dependence on the brightness of the ambient, calculated by modeling the solar irradiation. Finally, it was assessed whether sunglasses with poor UV protection actually increase the UV influx throughout the dilated pupil compared to the non-dilated pupil. A set of 214 sunglasses lenses were tested and the results show that pupil dilation does not play an important role in the UV influx throughout the pupil. It was observed that the FOV is the main player, surpassing the pupil size contribution by up to 314.3%, disproving the common explanation. Because of the major role of the FOV, our results show that sunglasses with UV-A protection below 86% may have a slight potential to increase hazards to the eye compared to not wearing sunglasses at all. These results can have direct impact on sunglasses standards regarding the UV protection linked to the category of the lenses.

Handing it to pterygium: Explaining pterygium laterality

PURPOSE

To evaluate any correlation between pterygium laterality and patient handedness.

METHODS

Our study represents a retrospective observational study of a series of consecutive pterygium patients recruited from two centres. Each patient was assessed for their handedness which was compared to the laterality of their presenting pterygium. Patients that possessed bilateral disease comparisons between pterygium size and handedness were made. Correlation statistics were performed to compare patient handedness and pterygium location (right or left). For patients possessing bilateral disease only, the pre-surgical differences between lengths and areas of pterygium were calculated and compared.

RESULTS

A total of 219 patients were recruited into our study. 172 patients possessed unilateral disease and in 47 patients, the disease was bilateral. A significant association was identified between handedness and pterygium laterality (p < 0.001). Patients with right-sided pterygia were more likely to be right-handed (OR 2.327) and left-sided presentations who were more likely to be left-handed (OR 5.717). For bilateral presentations, patients were found to have longer (mean increase 3.50 ± 0.47 mm) and larger (mean increase 4.38 ± 0.48 mm²) pterygia in the eye ipsilateral to their dominant hand.

CONCLUSIONS

A new insight of handedness as a contributing factor to pterygium laterality is consistent with evidence relating to the asymmetrical development of cortical cataract as well as to theories underlying the geometry of ocular UV exposure. A more complete understanding of factors contributing to ocular insolation may further inform as to improved protective measures and provides further evidence for the role of peripheral light focusing in pterygium pathogenesis.

UV-B-induced DNA damage and repair in the mouse lens

PURPOSE

Epidemiologic studies have linked UV-B exposure to development of cortical cataracts, but the underlying molecular mechanism(s) is unresolved. Here, we used a mouse model to examine the nature and distribution of DNA photolesions produced by ocular UV-B irradiation.

METHODS

Anesthetized mice, eye globes, or isolated lenses were exposed to UV-B. Antibodies specific for 6-4 photoproducts (6-4 PPs) or cyclobutane pyrimidine dimers (CPDs) were used to visualize DNA adducts.

RESULTS

Illumination of intact globes with UV-B-induced 6-4 PP and CPD formation in cells of the cornea, anterior iris, and central lens epithelium. Photolesions were not detected in retina or lens cells situated in the shadow of the iris. Photolesions in lens epithelial cells were produced with radiant exposures significantly below the minimal erythemal dose. Lens epithelial cells rapidly repaired 6-4 PPs, but CPD levels did not markedly diminish, even over extended postirradiation recovery periods in vitro or in vivo. The repair of 6-4 PPs did not depend on the proliferative activity of the epithelial cells, since the repair rate in the mitotically-active germinative zone (GZ) was indistinguishable from that of quiescent cells in the central epithelium.

CONCLUSIONS

Even relatively modest exposures to UV-B produced 6-4 PP and CPD photolesions in lens epithelial cells. Cyclobutane pyrimidine dimer lesions were particularly prevalent and were repaired slowly if at all. Studies on sun-exposed skin have established a causal connection between photolesions and so-called UV-signature mutations. If similar mechanisms apply in the lens, it suggests that somatic mutations in lens epithelial cells may contribute to the development of cortical cataracts.

Cx43, ZO-1, alpha-catenin and beta-catenin in cataractous lens epithelial cells

Specimens of the anterior lens capsule with an attached monolayer of lens epithelial cells (LECs) were obtained from patients (n=52) undergoing cataract surgery. Specimens were divided into three groups based on the type of cataract: nuclear cataract, cortical cataract and posterior subcapsular cataract (PSC). Clear lenses (n=11) obtained from donor eyes were used as controls. Expression was studied by immunofluorescence, real-time PCR and Western blot. Statistical analysis was done using the student's t-test. Immunofluorescence results showed punctate localization of Cx43 at the cell boundaries in controls, nuclear cataract and PSC groups. In the cortical cataract group, cytoplasmic pools of Cx43 without any localization at the cell boundaries were observed. Real-time PCR results showed significant up-regulation of Cx43 in nuclear and cortical cataract groups. Western blot results revealed significant increase in protein levels of Cx43 and significant decrease of ZO-1 in all three cataract groups. Protein levels of alpha-catenin were decreased significantly in nuclear and cortical cataract group. There was no significant change in expression of beta-catenin in the cataractous groups. Our findings suggest that ZO-1 and alpha-catenin are important for gap junctions containing Cx43 in the LECs. Alterations in cell junction proteins may play a role during formation of different types of cataract.

Ultraviolet radiation and the anterior eye

The eye is on the one hand dependent on visible light energy and on the other hand can be damaged by these and the contiguous ultraviolet (UV) and infrared wavelengths. Diseases of the eye in which sunlight has been implicated have been termed the ophthalmohelioses, and these conditions pose a significant problem to the eye health of many communities. The ophthalmohelioses have a tremendous impact on patients' quality of life and have significant implications on the cost of health care. Although cataract is not entirely caused by insolation, it now seems certain that sunlight plays a contributory role-cataract extraction is one of the, if not the most, commonly performed surgical procedures in many societies. Pterygium, typically afflicting a younger population, adds a tremendous burden, both human and financial, in many countries. We review evidence that peripheral light focusing by the anterior eye to the sites of usual locations of pterygium and cataract plays a role in the pathogenesis of these conditions. Recognition of the light pathways involved with foci at stem cell niches has directed our investigations into inflammatory and matrix metalloproteinase-related pathophysiologic mechanisms. An understanding of the intracellular mechanisms involved has provided some insight into how medical treatments have been developed for the effective management of ocular surface squamous neoplasia. The concept of peripheral light focusing has also provided direction in the prevention of these diseases. This has resulted in improved sunglass design and the further development of UV-blocking contact lenses. With the development of ocular UV fluorescence photographic techniques, we have been able to demonstrate preclinical ocular surface evidence of solar damage. Evidence that diet may play a role in the development of certain conditions is reviewed. The conundrum of the public health message about solar exposure is also reviewed, and in this context, the potential role of vitamin D deficiency is summarized. The eye may play a role in the development of individualized assessment techniques of solar damage, perhaps allowing us to provide better advice to both individuals and populations.

The differential effect of ultraviolet light exposure on cataract rate across regions of the lens

PURPOSE

In studies of cortical cataract, a severity score representing the area covered by cataract is often used as the primary outcome. However, additional disease information may exist in the spatial distribution of opacities. Further, it has been hypothesized that the lower nasal region of the lens is the most susceptible to damage by environmental ultraviolet light exposure.

METHODS

In a sample of 107 lens images from the Salisbury Eye Evaluation Study, a digital cortical cataract grading algorithm was used to capture the location of opacities in binary images. These images were used to estimate the severity of cataract in 16 regions around the lens. The effect of individual cumulative lifetime ocular exposure to ultraviolet B light on cortical cataract risk for each lens region was examined, as estimated by using an empiric model and baseline occupation and leisure activities data, in a linear mixed-effects model.

RESULTS

The lower nasal regions had the highest cortical cataract severity in both the right and left eyes. In the combined data, region 9 (the lower nasal corner of the lens) was estimated to have the highest severity. In an assessment of the high- and low-exposure ultraviolet light groups (dichotomized at the median exposure level), higher exposure had the most effect in the lower regions of the lens.

CONCLUSIONS

These results indicate that there are regional lens differences in the association between cataract and exposure to ultraviolet light but that ultraviolet light may not entirely explain the variations in cortical cataract severity across the lens.

Sex- and gender-based differences in healthy and diseased eyes

PURPOSE

The aim of this study was to identify sex- and gender-based differences in ocular anatomy, physiology, and disease susceptibility or manifestation.

METHODS

Review of current indexed literature was conducted.

RESULTS

Sex and sex hormones influence the lacrimal system, eyelids and blinking, corneal anatomy and disease, aqueous humor dynamics and glaucoma, crystalline lens and cataract, uveitis and retinal disease, ocular circulation, and optic nerve anatomy and disease. Systemic conditions, particularly autoimmune disease, and conditions that are unique to women, such as pregnancy and menopause, further illustrate the effects of sex hormones on the eye. Gender-based differences in ocular conditions and disease should be considered within the context of the underlying physical and social environment.

CONCLUSIONS

Many sex- and gender-based differences exist in healthy and diseased eyes.

External factors in the development of cataract

AIMS

To provide an update on the risk factors for cataract development.

METHODS

Review of the literature.

RESULTS

Age and heredity are the most important risk factors associated with the different types of cataract. While the hereditary component is self-explanatory, increasing age serves as a surrogate for a number of potential external risk factors, the effect of which is cumulative. Identification of the risk factors that have a causal effect on cataract development may provide means for cataract prevention. There are only a few risk factors that satisfy the criteria for causal effect: smoking, which results in the increased risk of nuclear cataract, excessive UV-B exposure and diabetes that increase the risk of cortical cataract, and steroidal treatment, diabetes and ionising radiation that lead to the formation of posterior subcapsular opacity. The effect of medications on cataract development requires further study, since the effect of the diseases should be distinguished from that of treatment. 'Stop Smoking' and 'UV-B protection' campaigns are gaining momentum as preventative measures, while the attempts to actively prevent cataract with antioxidants have not been successful. Cataract research has been facilitated lately by improvements of precision and standardisation in measuring lens opacities. However, measurement precision on its own cannot give us a solution to this problem.

CONCLUSION

The major studies repeatedly measure the exposure to the traditional health hazards, while the missing parts in the equation are those risk factors that we do not know about and therefore do not measure. New approaches and new hypotheses are needed.

Effects of cataract type and location on visual function: the Blue Mountains Eye Study

PURPOSE

To measure the effect of cataract type, severity and location on presenting, and best-corrected visual acuity, contrast sensitivity, and glare disability.

METHODS

In all, 3654 (82.4% participation rate) eligible noninstitutionalised residents aged 49 years or older, living in two postcode areas of the Blue Mountains, Australia, received detailed eye examinations including visual acuity, contrast sensitivity, and glare disability testing. Data from right eyes were analysed using multiple regression modelling.

RESULTS

The effect of age on visual acuity and contrast sensitivity only became evident in persons aged at least 60 years. Cataract severity was inversely related to visual acuity and contrast sensitivity. Late posterior subcapsular cataract caused the greatest reduction in visual acuity. Early grade cataract caused significant reduction in contrast sensitivity at intermediate and high spatial frequencies, but late grade cataract reduced contrast sensitivity across all spatial frequencies. There was insufficient study power to detect consistent significant effect of cataract on glare disability tests or cortical cataract location on visual function.

CONCLUSIONS

Age at least 60 years, cataract type, and cataract severity were principal determinants of visual acuity and contrast sensitivity in this study. Axial and superotemporally located cortical cataract had the greatest effect on visual function tests.

Peripheral light focusing as a potential mechanism for phakic dysphotopsia and lens phototoxicity

Our aim was to examine secondary image formation in the anterior segment caused by peripheral light focusing (PLF) in the human cornea, and in particular the crystalline lens. Non-sequential ray-tracing (OptiCAD) was applied to an anatomically based human eye model, which incorporates a gradient index crystalline lens. For analysis of the limbal effect, we varied the incident angle from 100 to 122 degrees, while for the crystalline lens effect, the incident angle was varied from 60 to 90 degrees. The corneal shapes studied included central radii from 7.4 to 8.2 mm with a range of shape factors. In each case, we computed the peak and average intensities, and the area of exposure at the limbus or lens periphery. The computation was repeated with a previous model eye for comparison. For the limbal effect, a peak intensity gain of x22.5 was found at an incident angle of 104 degrees which compares well with previous results. The average intensity gain at this angle was x7.5 over an area of 0.23 mm2. Steeper corneal curvature produced a greater PLF effect. For the crystalline lens effect, maximum UVA (365 nm) intensity gain peaked at x8.6 at 84 degrees with average intensity gain of x2.3. The area of UVA exposure peaked at 4.7 mm2 at 70 degrees. A relatively wide range (30 degrees ) of incident angles produced peak PLF gains of x3 or more in the lens. Significant focusing of light is directed to the nasal limbus, and to a lesser extent to the crystalline lens over a broad range of incident angles. PLF in the nasal cornea is reduced by an order of magnitude when a UV-blocking soft contact lens is used. The concentration levels and intraocular sites of PLF action on UV and visible light suggest a new mechanism of phakic dysphotopsia and lens phototoxicity.

Sunglasses and sunglass standards

Sunglasses and sunglass standards are reviewed from the point of view of the history of sunglasses and the development of sunglass standards. The need for eye protection from solar radiation is discussed and the provisions of the various national sunglass standards are discussed in relation to that need.