The quick fix of laser surgery may be freeing more and more people from
eyeglasses and contact lenses, but no matter how successful the surgery, their
liberation is only temporary.
Sooner or later the need for reading glasses descends on all adults -- and
from those devices there is still no quick escape.
Human bodies age at enormously different rates with one single exception: the
fine-focus mechanism of the human eye. The lens of the eye and the muscle that
surrounds it tick away the passage of time like tiny atomic clocks, slowly
deteriorating with a precision so immutable that ophthalmologists say they can
accurately estimate patients' ages to within a few years by measuring their
ability to focus close up.
The loss of close-up focus is known as presbyopia, from the Greek for
"elderly vision," but the process actually begins in very young adulthood.
Children with normal vision can generally focus on objects an inch or so from
their eyes, but they lose that ability as teenagers, and by the age of 50 or so,
when the eyes can focus no closer than a yard or two away, almost all adults in
the world need glasses to read newspapers or thread needles.
Those who claim they do not, researchers say, are simply unusually skilled at
"blur interpretation," fooled by good guesswork into thinking they are
exceptions to a universal biological rule.
Presbyopia is thought to represent not so much a sign of approaching
decrepitude as a leftover feature of primate life, when scanning for food or
predators on the horizon was a more important survival task for adults than was
any close-up work.
Remarkably, though, for such a longstanding and uniform feature of human
biology, scientists are still not sure exactly what causes presbyopia or what
might reverse it.
And for the last five years, they have been arguing heatedly over the risks
and the merits of an experimental surgical procedure, scleral expansion surgery,
that, despite a host of scientific objections, appears to fix it.
"It's extremely difficult to study the internal structures of the eye," said
Dr. Adrian Glasser, an expert in the physiology of the eye at the University of
Houston College of Optometry in Texas.
"It's such a finely regulated organ that many of the technologies that have
been available to study it have in one way or other disrupted the very thing
we're trying to study."
Dissections of human specimens have provided only partial snapshots of the
focusing process, while animal research is of limited help because few animals
focus the way humans do.
The lens of the eye is suspended in the eyeball right behind the iris,
connected by a halo of fibers to a ring of circular muscle. The lens, the fibers
and the muscle itself all show changes with the passing years, but exactly which
of these changes drives the loss of focusing ability is disputed.
The lens is the best-studied part of the system. A protein-filled disk, it
derives from the same type of cells as do the skin and the hair and, like them,
continues to grow throughout life. Unlike other continuously growing structures,
though, the lens cannot shed its older cells, or blindness would result.
Instead, it slowly gets bigger and thicker, bulging forward into the
fluid-filled space that separates it from the iris.
According to the principles of optics, the bulging curvature should make the
lens focus better and better with advancing years.
The fact that exactly the opposite occurs means that other powerful factors
are operating too, and the clue to presbyopia lies with them.
Many scientists think that the most important factors are the age-related
changes that take place within the lens proteins, making the lens harder and
less elastic with the years.
"The proteins in the lens of an adult are decades old," said Dr. Jane F.
Koretz, a professor of biophysics at Rensselaer Polytechnic Institute in Troy,
N.Y.
"They are even older than we are, because the first ones were laid down
before we were born. In no other part of the body is that the case.
Everything else gets replaced."
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