快猫短视频

IMAGINE if half the children and adults in your city wore hearing aids. No
doubt everyone would be appalled that so many people had defects in such an
important sense, and would probably be clamouring for scientists to find out
what had gone wrong and if anything could be done about it. But look around you
right now. Most people will probably be wearing glasses or contact lenses
because of defects in an even more important sense, vision.

Why so many otherwise healthy people can鈥檛 see much beyond the end of their
nose has been a mystery for centuries. But now, at last, scientists are
beginning to understand what causes nearsightedness, or myopia. They鈥檙e even
testing a drug that they hope will keep children鈥檚 vision from deteriorating,
and an anti-myopia pill for children could be just a few years away.

The progress comes not a moment too soon, because myopia rates are rising
alarmingly. A large 1996 study in the US found that about 60 per cent of
23-to-34-year-olds had myopia, compared with only about 20 per cent of people
over the age of 65. The problem is especially severe in parts of Asia. The
Singapore National Eye Center, for example, estimates that myopia afflicts more
than 80 per cent of the country鈥檚 18-year-old men.

All of this could mean real trouble. Eye specialists know that myopia can be
more than just a nuisance that keeps people hunting for their lost glasses.
People with severe myopia鈥攚ho need lenses with a focusing power of more
than 6 dioptres鈥攔un a greater risk of glaucoma and retinal detachment
later in life. Degenerative myopia is one of the leading causes of blindness in
the US, Britain and Canada.

As most short-sighted people know, myopia usually starts in childhood and
gets progressively worse through adolescence. A treatment that could stop the
deterioration could help prevent severe myopia and its complications, even
though it probably would not let people throw away their glasses altogether. But
you can鈥檛 stop myopia from getting worse without understanding how it develops
in the first place.

Clearly, genes play a part. According to one study, children with two
near-sighted parents are more than six times as likely to become myopic as
children with one myopic parent or none at all. Identical twins are more likely
to have similar eyesight than fraternal twins. However, geneticists have yet to
find the genes involved.

In any case, many researchers think genes govern only a susceptibility to
myopia, which some environmental factor must trigger. The biggest contender is
near work such as reading, or sitting at the computer or too close to a TV
screen. Many epidemiologists blame the rising tide of myopia on the increased
amount of time children spend on these activities rather than playing
outside.

鈥淲e always see the prevalence of myopia is greater in the near-work
population,鈥� says Neville McBrian of the University of Melbourne. Studies have
found that children鈥檚 myopia progresses more rapidly during term time than the
summer vacation (Journal of the American Optometric Association, vol
69, p 262). In places where people do less close work, such as rural Nepal and
the upper Amazon regions of Brazil, only about 1 per cent of the population has
myopia.

It鈥檚 still not clear why close work has such a profound effect on eyesight,
but the answer probably involves the way the eye copes with poorly focused
images. As astronomer Johann Kepler first explained almost 400 years ago, in
myopia light rays don鈥檛 focus on the retina at the back of the eye. Instead,
they meet at a point farther forward and continue past each other to create a
blurred, or defocused, image on the retina
(see Diagram, p 35). Corrective
lenses bend the light rays to push the focal point back onto the retina,
creating a clear image. But a child鈥檚 eye can also compensate for its defocus by
changing shape as it grows.

Twelve years ago, Frank Schaeffel of the University Eye Hospital in
T眉bingen covered chicks鈥� eyes with 鈥渕inus鈥� lenses, which shift the focal
point farther back in the eye and thus are used to correct for myopia. The
chicks, which weren鈥檛 myopic to begin with, compensated by growing their eyes
longer so they could focus properly, making them myopic when Schaeffel took the
lenses off. Chicks that wore 鈥減lus鈥� lenses, which pull the focal point forward,
ended up with eyes shorter than normal, so they were far-sighted, or hyperopic,
when the lenses came off. These changes happened even if the optic nerve was
cut, which suggests that the eye itself鈥攏ot the brain鈥攚as detecting
the defocus and changing its growth to correct the problem.

Schaeffel鈥檚 experiment turned people鈥檚 thinking on myopia upside down and
made it possible for later researchers to try to tease out the effect of
reading. Today鈥檚 leading theory is that reading may cause a slight defocus. When
people shift their gaze from far to near, their eye muscles change the shape of
the eye鈥檚 lens so that it focuses on the new place and 鈥渁ccommodates鈥� for the
change. But some children may fail to accommodate completely, perhaps because
they don鈥檛 notice the difference between a crisp image and a slightly blurred
one.

This would create a bit of defocus on the eye, similar to what Schaeffel saw
in his chicks wearing minus lenses, and the eye would elongate to compensate,
causing myopia. And indeed, when Mark Rosenfield of the State University of New
York in New York City asked adult volunteers to pick the clearer of two images,
he found that myopic people couldn鈥檛 discriminate as well as those with normal
vision. Whether this difference in sensitivity caused the myopia or had happened
as a consequence still isn鈥檛 clear.

More recent variations on Schaeffel鈥檚 lens experiments have complicated the
picture still further. Earlier this year, Josh Wallman and Jonathan Winawer of
the City University of New York alternated plus and minus lenses several times
each day in chicks to see if the two lenses would cancel each other out and let
chicks鈥� eyes grow normally. To their surprise, the effect of the plus lens
dominated, even if chicks wore it only one-fifth as often as the minus lens.

In another experiment, the researchers raised chicks without lenses, except
that every two hours they put plus lenses on for two minutes. The chicks鈥� eyes
still became far-sighted, even though they鈥檇 been exposed to the lenses for only
a few minutes a day. Somehow, even that brief period with the lens pulling the
focal point forward of the retina was enough to trigger the eyes to compensate
by growing into a less elongated shape. 鈥淭here鈥檚 something special about plus
lenses,鈥� says Wallman, who described his results at the Eighth International
Conference on Myopia held last month in Boston. 鈥淓ven very small amounts of plus
lenses can cancel out the whole day of negative lenses.鈥� Wallman plans to try
the same lens experiment in monkeys next.

These findings may revive the old belief鈥攄ismissed years ago without
being seriously tested鈥攖hat giving children glasses might make their
myopia worse. A child who reads all day long is the equivalent of a chick
wearing minus lenses, and the blurriness a myopic child sees when looking at a
distant object without glasses is the equivalent of a chick wearing plus lenses.
Giving children glasses may remove this powerful 鈥減lus lens鈥� effect. 鈥淭his
raises the idea that a brief dose of plus-lens wear in a child might prevent
myopia,鈥� says Tom Norton of the University of Alabama at Birmingham. 鈥淚t
certainly merits further exploration.鈥� However, he notes, most children already
experience many brief periods of defocus when they take their glasses off, yet
their myopia still gets worse over time.

Even though scientists still don鈥檛 know exactly what causes myopia or what
effect corrective lenses have on the process, they are already testing a drug
that promises to moderate or even prevent the condition. The drug is a new and
improved version of that old standby atropine鈥攖he belladonna that
Renaissance women used to make their pupils dilate to darken their eyes. Among
other actions, atropine relaxes the muscle that focuses the eye. For many years,
people thought myopia resulted from strain to this muscle during close focusing,
so atropine was suggested as a treatment for myopia as early as 1811.

鈥淭hey were using the right drug for the wrong reason,鈥� says Ian Flitcroft at
the Royal Victoria Eye and Ear Hospital in Dublin. Few now believe that this
muscle has anything to do with myopia鈥攂ut still, experiments show that
atropine stops myopia in its tracks. Yung-Feng Shih of the National Taiwan
University Hospital in Taipei compared atropine eyedrops to a placebo in 186
Taiwanese children aged 6 to 13 years. Only 39 to 58 per cent of the children
who got various doses of atropine saw their myopia worsen over a two year
period. In contrast, 92 per cent of the control group got worse, said Shih last
year in the Journal of Ocular Pharmacology and Therapeutics (vol 15, p
85). A more recent Taiwanese study found much the same.

Atropine has serious drawbacks that make it unsuitable for everyday use.
Because it relaxes the muscle that focuses the eye, children who take atropine
eyedrops have to wear reading glasses for close work. Atropine also dilates the
pupil, making the children extremely sensitive to light. So researchers decided
to look for other drugs that might offer the same benefits without as many side
effects.

They turned to a drug called pirenzepine that has been used for years to
block the production of gastric juices as a treatment for ulcers. Like atropine,
pirenzepine blocks receptors for the nerve-signalling chemical acetylcholine,
but it does so more selectively. Back in 1991, Richard Stone of the Scheie Eye
Institute at the University of Pennsylvania School of Medicine in Philadelphia
and his team showed that pirenzepine could prevent myopia in chicks. Stone
believes that the drug may block signals from the retina that tell the eye to
grow, though other scientists, such as McBrian, aren鈥檛 so sure.

Nevertheless, pirenzepine eye gel is now undergoing clinical trials in
children aged 8 to 12 in the US, the first drug to be tested there against
myopia. There are 24 children in the trial so far, which is being run by Valley
Forge Pharmaceuticals in Irvine, California. Another 150 children will be
included in 10 additional centres. Starting in August, the drug will also get
tested in 250 children in Singapore, Hong Kong, Thailand and Taiwan. The
researchers will follow the children for a year, and move into larger trials if
this trial shows it works as well in humans as it does in animals. The drug
could reach the market in four years.

Because pirenzepine has been used as a drug before, it has an excellent
safety record. 鈥淢illions of people have taken billions of doses of it,鈥� says
John Parrish of Valley Forge Pharmaceuticals. The eye gel should have no major
side effects in children, since the dose of pirenzepine will be lower than in a
pill.

Still, in preliminary tests, Flitcroft found that oral pirenzepine worked
even better than the eye gel at blocking myopia in chicks. This first indication
that a pirenzepine pill might work for myopia is exciting, says McBrian. But he
asks: 鈥淚s that what we really want in terms of a drug treatment for myopia?鈥�
Whether pill or gel, parents would have to treat their children every day for a
decade or more. And even then, the drug won鈥檛 reverse any myopia that has
already developed.

Also, since scientists don鈥檛 know exactly how the drug works, we don鈥檛 yet
understand what it will do to visual performance in the long run, he warns. But
for parents, he acknowledges, 鈥渋t鈥檚 really difficult for them not to jump on
that bandwagon鈥�. It may be especially appealing to parents with severe myopia
who are worried about their young children whose eyesight seems to be
deteriorating rapidly.

Part of pirenzepine鈥檚 appeal is that parents don鈥檛 have a lot of other
options for preventing myopia in their children. (Laser surgery can usually
correct myopia permanently, but it sometimes leads to complications and its
long-term effects are still unknown.) Taking breaks from near work every 30
minutes or so might help, as might holding the book well away from the
eyes鈥攁t least as far away as your elbow when you make a fist and hold it
to your nose, eye specialists say. A good light source may also help, because
the eye can accommodate to near work better in bright light. On the other hand,
there鈥檚 little evidence that eye exercises, pinhole glasses, and other gimmicks
advertised on the Internet will do much good. 鈥淐ertainly no controlled studies
have been done on any of that stuff,鈥� says Karla Zadnik of Ohio State University
in Columbus, who calls it 鈥渏ust plain out-there, loony stuff鈥�.

Drugs like pirenzepine seem like the best bet for the future, even though
we鈥檙e not quite sure how they work. But as time goes on and scientists unravel
the chemical signalling between different parts of the eye that influences eye
growth, they might be able to design drugs that interfere with those signals to
block or perhaps even reverse myopia, ensuring perfect vision for life. If that
happens, future generations will go to museums and marvel at the glasses and
contact lenses locked up in a display case. They won鈥檛 be able to understand how
so many of us lived so casually with defective sight.