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By Daniel M. Keller

WASHINGTON, DC — Clinical data behind claims that night lights, schoolwork and genes are causes of childhood myopia was reviewed by researchers at a seminar sponsored by the Research to Prevent Blindness Foundation.
Karla Zadnik OD, PhD analysed the pathogenesis of myopia and methods of predicting its occurrence and ultimately preventing its progression.
She noted that the search for the cause of myopia is "a classic nature versus nurture argument". If two myopes have children together, each child has a 30% to 40% chance of being myopic, she said.

That figure is 25% if only one parent is myopic and the child has less than a 10% risk if neither parent is myopic. The primary risk factor is whether or not a child has myopic parents, she said.
On the "nurture" side of the argument, myopia has some association with reading. For example, increased rates of myopia among Asians have been anecdotally attributed to more rigorous schoolwork in some of those countries.

Session moderator Harold Spalter MD remarked that near distance work, requiring increased effort for accommodation, may weaken the ciliary muscles and have some influence on the need for glasses in later life.
Increased amounts of paper work over the past 100 years, and more recently computer work, require more frequent near accommodation. Poor lighting may also contribute to the need for glasses.

Myopia is fairly easy to detect using a distance visual acuity chart by a paediatrician, eye care professional or in vision screenings at school.
But since the age of onset is typically between eight and 12 years, it would be useless to screen six year olds in school. Only 2% of this age group has myopia. Once myopia begins, it often progresses until about age 16 because the eye continues to grow and elongate.

Dr Zadnik noted that a couple of years ago, the media reported on the use of night time nursery lighting and the possible onset of childhood myopia if the lights were used before the age of two years.
She was not able to replicate the original researchers’ findings. The more likely explanation, she said, is that myopic parents lit the baby’s room at night so the parents could see what they needed to do and the lighting was just a surrogate for the child’s genetic predisposition to myopia.

"My take on the issue is that it is not an important environmental risk factor for the development of myopia, even though it received an inordinate amount of press coverage," she said.
Dr Spalter also discounted the influence of any nursery lighting on the development of myopia while a child is asleep.
"When the eye is closed, the eyeball rotates upward. It’s really no longer facing whatever is out there. The eye is out of range of really receiving much light," he said.
Myopia is now treated optically, with spectacles for younger children and possibly contact lenses for older ones once they can take care of the lenses.
After age 18, refractive surgery is a possibility. But Dr Zadnik noted these approaches just "manage the distance blur".

A treatment or "cure" for myopia would be to slow down the elongation of the eye so that by age 16 the child would not be as near-sighted as they might otherwise have been. Even better would be to stop the abnormal elongation before it started.
One approach has been to use bifocal optical lenses to try to relieve the stress of near work, the rationale being that the child may not become myopic.
If it worked, it would lend credence to the "nurture" etiology of myopia. Attempts in this regard have been made for about 50 years.
Dr Zadnik cited a recent paper from Hong Kong that showed no benefit of bifocal lenses in children there. A randomised clinical trial of bifocal spectacles in school-age children in Oklahoma, US published in 2000 showed progression of myopia was slowed by only 20% compared to children wearing regular glasses.

The results of the larger COMET study (Correction of Myopia Evaluation Trial) should appear within about a year. More information on that trial is available online http://www.nei.nih.gov/neitrials/static/study9.htm.
Another randomised clinical trial is under way at Ohio State University to test rigid contact lenses to slow myopia progression. The trial will compare outcomes in patients wearing rigid or soft lenses. See www.nei.nih.gov/neitrials/ static/study81.htm.
Another approach is eye drops that actually would slow down the abnormal growth of the eye during these crucial years. Topical drops containing anti-muscarinic agents such as atropine have been shown to react with retinal receptors that influence eye growth in animals. But because these drugs dilate the pupil and inhibit close focusing, they are not practical.

A new eye drop developed by Valley Forge Pharmaceuticals, Irvine, California, US is currently undergoing clinical trials in the US and Asia. It is said to slow elongation of the eye without producing unwanted side effects.
The active compound, pirenzepine, is applied as a 2% gel to the eye twice a day, and results of clinical trials in children are pending.
Besides preventing refractive error, such treatments should also address the risks of the severe problems of retinal detachment and glaucoma later in life, she noted.

Up until now the proposed treatments for myopia aim to slow down the progression of the condition in a child in whom it has already been diagnosed.
But if any treatments prove effective, then it would be logical to extend them to preventing myopia from occurring in a child at risk. Therefore, developing a method of detecting at-risk children is required.

Between 1989 and 2000, Dr Zadnik conducted a longitudinal study of children aged six to 14 years in Orinda, California, US (the Orinda Longitudinal Study of Myopia).
Among this predominantly Caucasian, higher socio-economic population, she found that the best prediction of future myopia was visual acuity approaching emmetropia at age eight.

She believes this may be an indication that the eye will grow too fast and the child would be likely to develop myopia. The overall length of the eye was also predictive and this finding was most prevalent in children of myopic parents.
To investigate a more ethnically diverse population, the multi-centre Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) in children study enrolled 2,523 African American, Asian, Hispanic, Caucasian and Native American children to look at the prevalence of refractive error as a function of ethnicity.
In the study, 9.2% of the children were myopic, 12.8% were hyperopic and 28.4% were astigmatic.

Refractive error was strongly correlated with ethnicity (p<0.0001) even after controlling for age and gender. The highest prevalence of myopia was among Asians (18.5%), followed by Hispanics (13.2%), African Americans (6.5%), and Caucasians (4.4%).
Conversely, Caucasians had the highest prevalence of hyperopia (19.3%), followed by Hispanics (12.7%), African Americans (6.4%) and Asians (6.3%).

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