Does season of birth play a role in the development of nearsightedness? Do corneas from older donors meet quality standards for transplant surgery? These are among the topics explored in the April 2008 issue of Ophthalmology, the journal of the American Academy of Ophthalmology.
Do natural light levels make a baby born in June more likely to be nearsighted, or myopic, than if he or she had been born in December? While scientists think genetic factors play the strongest role in nearsightedness, a number of studies show that light exposure before and just after birth generates biological signals that influence the development of the eye's ability to focus and refract light properly. Research had suggested that the influence of light on vision development in this perinatal period might occur through mother-baby biological signals before birth, or through the baby's direct exposure after birth, or both. Because the effect of light levels on myopia, if any, was likely to be slight, a large population study was needed to further explore the question.
Yossi Mandel, MD, and his research team found a suitable study group in candidates for Israeli military service who were medically evaluated between 2000 and 2004. All 276,911 participants (157,663 male, 119,248 female) were born in Israel, ensuring their exposure to the same seasonal light variations. Myopia prevalence rates, determined by visual examination and defined at three levels of severity, were: mild, 18.8 percent; moderate, 8.7 percent; and severe, 2.4 percent. The risk of moderate and severe myopia varied with seasonal levels of light, or photoperiods, with highest rates in babies born in June/July and lowest in December/January. These correlations were considered highly statistically significant. The findings were adjusted for other known myopia risk factors, such as gender, education level, and father's country of origin. Mild myopia was not associated with season of birth or perinatal light exposure.
Myopia allows people to see close-up objects clearly while distant objects appear blurred. The condition is on the rise worldwide, but causes remain uncertain. Treating myopia in the United States costs more than $4.6 billion per year, and people with the condition are at higher risk for retinal detachment and other complications. Since the link to seasonal birth date was most pronounced for severe myopia, Dr. Mandel concludes: “It seems reasonable to assume that only a fraction of the population might be genetically prone to develop myopia if exposed to environmental risk factors such as a long perinatal photoperiod. Further exploration of the mechanisms underlying the effect of light on the development and progression of myopia in humans is needed to devise effective preventive measures.”
Meeting the Demand for Cornea Transplants
More than 33,000 transplants are done in the United States each year to treat diseases that affect the cornea, the transparent front part of the eye that covers the iris, pupil and anterior chamber and that provides much of the eye's visual power. Although the supply of donor corneas has been adequate, new Food and Drug Administration regulations could restrict the donor pool, and already the international demand exceeds the supply. Raising the acceptable donor age limit above age 65 could expand the supply by 20 to 35 percent, especially as the U.S. population ages. But use of older donor corneas has been controversial among surgeons. The Cornea Donor Study, principally funded by the National Eye Institute of the National Institutes of Health, compared five-year outcomes of corneal grafts from older and younger donors.
Sixty-five percent of the study patients received corneas from eye banks with donors aged 12 to 65, and 35 percent from donors aged 66 to 75. Patients had conditions defined as “moderate risk,” such as Fuch's dystrophy or a serious form of edema. The donor source (including age) was unknown to the surgeons and recipients. All donor tissue met standards, such as endothelial cell density, known to be important to corneal transplant success. In the five year follow-up evaluation, both groups attained an 86 percent transplant success rate, and no significant differences in outcomes were found. The authors conclude: “Our results indicate that the donor age pool should be expanded to 75 years. Surgeons and patients now have evidence that older donor corneas comparable in quality to those used in this study are suitable for transplantation.”