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Presenter: Decreasing incidence of myopia would be significant public health benefit
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SEATTLE – “The first lens you prescribe to a child should be a lens that does something to slow the progression of myopia,” Earl L. Smith, OD, PhD, told attendees here at the American Academy of Optometry annual meeting.
Smith, a researcher, dean of the University of Houston College of Optometry and interim chief health officer for the University of Houston, shared his perspectives on the past, present and future of myopia research.
Earl L. Smith
“Anything we can do to decrease the amount of myopia would have a big significant public health benefit,” he said. However, identifying the cause has been a challenge.
“The genetic house held sway for so long, and we have 300 years of research in humans and we weren’t able to identify why near work causes myopia,” Smith said. “Our treatment strategies didn’t work. In the 1970s it began to change; lab work with animal models blossomed. And what we see in animals occurs in humans.”
Local ocular growth was altered in monkeys when half the nasal field was restricted, he said.
“Exaggerated growth corresponds to the area of the retina that’s treated,” Smith said. “Local retinal mechanisms control eyeball growth. Most of us assumed that the fovea would; it turns out that’s not true. It makes sense if you think about it from an evolutionary point of view.
“In every animal that’s been studied, these phenomena are quantitatively similar,” he continued. “The periphery is a large part of the eye. The fovea makes up less than 1% of the area of the retina. If you put conflicting images between the periphery and the fovea, the periphery wins.”
A host of lens designs, including multifocal contacts, orthokeratology lenses and executive bifocals, have been shown to reduce the degree of peripheral hyperopia and reduce myopia progression by 40% to 50%, Smith said.
“We need to be a bit more aggressive about how we deal with myopia,” he said. “It’s a big deal.
“There’s a lot we don’t know,” he continued. “What is the optimum focus? A lot of us talk about the periphery, but we don’t define what we mean by the periphery. We don’t know the ideal hot spot, what hemi-meridian. That’s a big deal. Peripheral refraction and eye shape vary from one person to the next. We’ve only been using a one-shoe-fits-all strategy. It’s no doubt the optimum optical treatment will have to take these into effect.”
A consistent relationship is not seen between peripheral hyperopia and axial (myopia) progression in humans, Smith said.
“There are many patient factors,” he continued. “Who can benefit the most? Is it possible to identify, prior to the onset of myopia, who will have high myopia, and develop a prophylactic lens to prevent it? Are partial effects acceptable? How long do you need to treat?”
Smith also discussed recent research in the strong effects of outdoor activities against myopia in children.
“It’s not just a matter of them not doing near work,” he said. “It’s not related to exercise. This shows up 3 or 4 years prior to the onset of myopia. It could be what causes the eye to become myopic. What is it about outdoor activity?”
Smith said perhaps it has to do with lighting levels. Indoor lighting is 60 lux to 70 lux; bright sunlight is 130,000 lux, he said.
Smith said research has shown that chickens and tree shrews reared in high light levels experienced a reduction in myopia of 65%. In his lab, three-fourths of monkeys exposed to high light levels developed hyperopic errors.
“This is likely to be activated by a dopaminergic reaction,” he said. “It’s likely that the effects come about due to influences on diurnal rhythms.
“Melanopsin-containing retinal ganglion cells play a key role in regulating circadian rhythms and things associated with lights,” Smith continued. “These mechanisms evolve early on. This indicates the more primitive systems in the eye may be running the game. It’s likely that the melanopsin-containing retinal ganglion cells are the most prominent, probably evolved from light spots on the skin.”