Every morning about 67 million Americans, including 4.6 million school-age children, start the day by groping for a pair of glasses. This daily patting down of the night table is essential for those who are nearsighted. Glasses bring the world into focus, let nearsighted people see the clock, find the bedroom door and assess the weariness on the face in the bathroom mirror.
Nearsighted people wonder what it would be like to open their eyes in the morning and see clearly, even if it’s just the cracks in the ceiling. Most nearsighted adults can’t remember what it was like to view the world any other way; they have needed assistance to see since they were in elementary or junior high school. Nearsighted parents watch their children carefully, hoping they will somehow be blessed with perfect vision. By the time their children hit high school, that dream usually has been dashed.
We tend to think of myopia – the technical term for nearsightedness, or the inability to see objects clearly at a distance – primarily as an inconvenience. But it’s a serious health problem, affecting 1 of every 4 Americans.
Myopia is responsible for half of all cases of retinal detachment, the seventh leading cause of blindness. And myopes (people who are nearsighted) have a fourfold to fivefold risk of developing glaucoma, an eye disease that can cause gradual loss of vision. The more nearsighted you are, the greater your risk. Nearsighted people may also have an increased risk of developing cataracts.
That’s why researchers have increasingly turned their attention to myopia, searching for its cause and, ultimately, its treatment and even prevention. They are closer to understanding some of the most basic questions about myopia: how it develops, why it develops, who is most at risk, and what can be done about it.
The National Eye Institute in Bethesda, Md., a division of the National Institutes of Health, invested $10 million beginning last year to fund research to determine if nearsightedness can be prevented. One of the projects being funded, the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study, is examining 3,500 children in grades 1 through 8 in California, Alabama, Ohio and Texas to understand how the eye grows and how ethnic background affects the development of myopia. The goal is to predict which children are at risk for developing myopia, then find treatments to slow its progression or prevent onset altogether.
Almost no one is born nearsighted. In fact, babies are born slightly farsighted. The eyeball grows knowing where perfect vision is, and by 1st grade, vision usually is perfect. “It’s as if all kids were to measure the same height in 1st grade,” says Karla Zadnik, associate professor at the Ohio State University College of Optometry and principal investigator of the CLEERE study. “In 1st grade, kids’ eyes are remarkably similar.”
For some kids – 2 percent of elementary school students and 15 percent of junior high school students are nearsighted – the eyeball continues to grow. Myopia is characterized by elongation of the eyeball, which causes light rays that should meet at the retina to fall short and produce a blurry image on the retina. How and why this happens is what the CLEERE study is attempting to find out. “It’s hard to know what’s wrong until we know how it works,” Zadnik says.
The CLEERE study is looking at Caucasians, African-Americans, Hispanics and Asians to determine if there are ethnic differences in the prevalence of nearsightedness. Data from the first year of the study indicates that there is. Asian children have the highest prevalence and severity of myopia, Zadnik says. “Eighty percent of the population in Taiwan has significant myopia,” Zadnik explains. “We thought Asian students simply studied more. But we’re finding that Asian-Americans, too, have a higher prevalence of myopia, and that seems to suggest some kind of genetic basis for it.”
The preliminary CLEERE data indicate that after Asian-Americans, African-Americans have the highest prevalence of myopia, followed by Hispanics and then Caucasians.
The idea that there might be a hereditary or genetic basis to the development of nearsightedness is being investigated from another angle by Dr. Jeffrey Walline, professor of pediatrics at the University of Iowa. Using DNA from cheek swabs submitted by participants in the CLEERE study, Walline is looking for mutations in genes related to eye development. Walline is also looking at the DNA of families with two or more myopic children for patterns of inheritance and comparing them to normal DNA sequence variation.
The analysis is in its very early stages, and Walline doesn’t expect any exciting discoveries for a couple of years. “But the approach is potentially very powerful,” Walline says, “and if we can better understand the genetic causes of myopia, we can devise more effective treatments and even learn about more serious eye disease.”
If researchers discover a gene that controls eyeball growth, there might be a pharmaceutical product that could turn that gene off in nearsighted people and on in farsighted people (those who see clearly at a distance but not up close).
There is no greater debate among the myopia research community than whether nearsightedness is caused by genetics or environmental influences — or, more precisely, which has a more profound effect. Both certainly have been found to play a role.
Giving credence to the “nature” theory of myopia development are some findings from Zadnik’s study. A few years ago, when none of the children in the study were nearsighted yet, she separated them into groups according to whether they had two myopic parents, one myopic parent, or none. She found that the eyes of the children with two myopic parents were already longer, assuming a myopic shape even though the children had not yet become nearsighted.
“We’re finding that the association for inheritance as a cause of myopia is far more important in terms of the strength of the relationship (than environmental causes),” Zadnik says.
Zadnik believes that the interplay of genetics and environment in the development of myopia is similar to that in other diseases. “My guess is that it’s much like what we’ve learned about heart disease or colon cancer: There’s a genetic component, but there are ways to increase or decrease the risk.”
Diet, for instance, plays an important role in reducing the risk of heart disease and colon cancer. There may be environmental factors that can reduce the risk of developing myopia as well.
What these environmental factors might be is also the subject of much research. Scientists know that certain types of visual experience can cause nearsightedness. Animal studies show that if you prevent a clear image from being formed on the retina, the eye almost always becomes myopic, says Earl Smith, professor of optometry at the University of Houston College of Optometry. Disorders such as lid droop, in which the eyelid covers the pupil, are known to cause the eye to become severely myopic.
The association between near work (reading, for instance) and the development of myopia has long been recognized. But it may be that difficulty with adjusting focus on objects at various distances can lead to myopia, or at least cause myopia to progress more rapidly.
This piece of the puzzle is being worked on in several studies across the country. Two are attempting to determine the potential of bifocal glasses to slow the progression of myopia. The Myopia Progression Study is testing the use of regular bifocal glasses (the kind with a correction at the top for distance and a correction at the bottom for near work) in elementary and junior high school children with a particular type of myopia called “near point esophoria,” which affects about 30 percent of all myopes.
Previous retrospective studies have shown that bifocals were able to slow the progression of myopia by almost 50 percent in children with this kind of myopia, in which the eyes move in too much to focus on a near object. If the results of the study, due a year from now, show that bifocal lenses have a positive effect, “the next step would be to see if we can prevent myopia in the first place by having kids wear reading glasses before they become myopic,” says George Fulk, principal investigator of the study and professor of optometry at the Northeastern State University School of Optometry in Oklahoma.
Another type of bifocal, the progressive addition lens (PAL), also shows promise in slowing the progression of myopia. Progressive lenses gradually change from distance to reading correction so there isn’t the dramatic leap that there is in the standard bifocal lens. The advantage is that, depending on how you hold your head or move your eyes, objects at any distance would come into focus, such as computer screens, which are neither at reading distance nor far distance. This could further alleviate stress on the eye and reduce myopia progression.
Contact lenses, too, may be able to slow the progression of myopia. The three-year Contact Lens and Myopia Progression study is comparing the effects of rigid gas-permeable contact lenses to soft contact lenses in the control of nearsightedness in children.
Whether researchers will ever sort out genetic causes from environmental causes of myopia remains to be seen. But it does appear that the pieces of the puzzle are being assembled more quickly than ever before. And even having parts of the picture will help doctors recognize, treat and prevent nearsightedness.
IS OUR COUNTRY BECOMING INCREASINGLY MYOPIC?
Some data at first seemed to suggest so. Comparing the incidence of myopia in a major study in California in the 1950s with the incidence today showed that a higher percentage of the population is nearsighted today.
However, when demographics are factored in, the prevalence of myopia appears to be relatively stable. That’s because the increase in myopia mirrors the increase in the Asian-American population, which is known to have a dramatically higher incidence of nearsightedness.
Many eye practitioners, however, are noticing an increasing prevalence of myopia of adulthood, in which adults are either first developing nearsightedness or suddenly becoming more nearsighted after their eyes had stabilized. This is largely seen among people who do a lot of near work, says Karla Zadnik, associate professor at Ohio State University College of Optometry.
The question is, what is triggering elongation of the eyeball in adulthood? It’s possible that near work, including the increasing use of computers for work and leisure activities, may play a role in the development of myopia in adulthood.
