 |
 |
 |
 |
 |
THE PHYSICIAN'S PERSPECTIVE
David L. Guyton, MD
Krieger Professor of Pediatric Ophthalmology
Johns Hopkins' Wilmer Eye Institute,
Baltimore, Maryland, USA
Health News, September 19, 1995
According to old-wives' tales, wearing glasses makes the eyes
worse. Generations of ophthalmologists and optometrists have told their
patients just the opposite, that the eye's development is predetermined by
genetics and cannot be affected by glasses. A growing body of animal and
human research, however, suggests that the old wives were right after all.
The ability of young rhesus monkeys' eyes to gradually change shape in
response to what they see comes as no surprise to vision scientists. Over the
past two decades, their studies have demonstrated that the eyes of young
birds, tree shrews, guinea pigs, and marmosets react to unfocused images by
altering their growth to correct the problem.
It is highly likely that the eyes of infants and young children also
adapt to what they see. This adaptation occurs by a relative change in eye
length that works something like this:
As the front of the eye grows and becomes less curved, images focus
deeper and deeper within the eye. If the lengthwise growth perfectly matches
the change in the eye's other dimensions, then
images continue to focus on the retina. If there is a mismatch and the focus is off by even the thickness of this paper, then vision will be blurred. Remarkably, the eye apparently senses where images focus and compensates when needed. If light focuses in front of the retina, the eye will stop lengthening until the images catch up. If the focus is behind the retina, the eye grows in length at an accelerated rate until the retina is pushed back to the correct spot relative to the eye's other dimensions.
images continue to focus on the retina. If there is a mismatch and the focus is off by even the thickness of this paper, then vision will be blurred. Remarkably, the eye apparently senses where images focus and compensates when needed. If light focuses in front of the retina, the eye will stop lengthening until the images catch up. If the focus is behind the retina, the eye grows in length at an accelerated rate until the retina is pushed back to the correct spot relative to the eye's other dimensions.
Thanks to this feedback mechanism, the eyes generally maintain clearly
focused images throughout early life despite dramatic changes in size.
In addition to eye size and shape, the distance between the eye and the
objects it is viewing also determines where images focus. Near objects come to
focus behind the retina, but the lens changes shape and pulls the images
forward until they are clear enough to recognize. However, they often remain
slightly behind the retina. This slight mismatch may be the mechanism by
which prolonged close work such as reading can signal the eye to grow longer.
If such a signal occurs frequently and strongly enough in early life, the
human eye may gradually lengthen and become permanently focused for near
objects. This produces nearsightedness.
Most of the adaptive changes in eye length occur during infancy and
youth, while the eye is still growing in its socket. When the front of the eye
stops growing, around age nine or ten, any further adaptive change can occur
only in the myopic direction the eye can grow longer, but not shorter.
Activities such as prolonged reading at close distances may cause the eyes to
continue lengthening well into one's 20s.
If this cycle of incomplete focus and eye lengthening is the primary
cause of myopia, how can we intervene in this process? Some practitioners
believe that limiting the amount of close-up reading or television watching a
child or young adult does each day may prevent myopia. These days that is a
difficult task. So I advise parents to encourage children to hold objects and
reading materials as far away from their faces as comfortable, and to sit at
least three feet away from the television screen.
(Those who insist on holding books close to their eyes, or sitting a foot from the television or computer, may already have developed significant myopia or some other problem that warrants a professional eye examination.)
For my young patients with simple myopia, I suggest they leave their
distance glasses off while reading, something I have always done myself. A
child who cannot see the board at school, for example, should wear glasses to
see the board, but remove them when reading a book or writing.
Prolonged reading without glasses shouldn't stimulate the eye to lengthen
any farther than what is needed to comfortably focus the eye at rest at the
customary reading distance. By comparison, when one reads through glasses or
contact lenses designed to bring the distant world into sharp focus, the page
is focused behind the retina. This may prompt another round of eye
lengthening with worsening of the myopia.
For someone who is quite myopic or has astigmatism, the glasses-off
technique is not really feasible. In such cases often prescribe glasses that
correct only part of the myopia, or correct only the astigmatism. This leaves
the patient exactly focused for his or her customary reading distance.
Since contact lenses cannot be removed as easily as glasses for prolonged
reading, wearing full-power (plus lens) reading glasses in addition to
contacts may help reduce further increases in eye length. Surgical procedures
that correct myopia by reshaping the curvature of the eye, if performed too
early in life, will likely have the same effect as wearing glasses that
correct for distance only, and the myopia may simply reappear.
By understanding exactly what stimulates the eyeball to lengthen, we hope to learn how to prevent myopia from developing in the first place.
FOR MORE INFORMATION: National Eye Institute, Bethesda, MD, 301-496-5248