If you look through a single lens reflex camera or camcorder, you
have to rotate the barrel of the lens to adjust its focus (unless
it's an autofocus camera, of course). If you focus at a distance,
things close up are blurry, and vice versa. The human eye works in
similar fashion. Without any effort, the optically ideal eye sees
distant objects easily. To look at something up close, focus must be
adjusted; this occurs by a process called accommodation.
A circular muscle behind the iris (the ciliary muscle or ciliary
body) contracts and moves forward slightly. This muscle is connected
to the lens by a network of tiny fibers called zonules. The change
in tone and position of the ciliary muscle changes the tension of
the zonules on the lens. This change in "tug" causes the lens to
change its' curvature, increasing its' focusing power.
In our youth, the lens is very flexible and accommodates easily.
As we mature, the lens grows in a fashion similar to the trunk of a
tree. Layers of protein are deposited over time like the rings on a
tree trunk, causing the lens to become thicker, denser, and less
flexible. After a certain point, close focusing becomes difficult,
and people need to use magnifying glasses to see near objects
without eyestrain. This condition, called presbyopia,
usually begins in the mid-forties and gets progressively worse for
about 15 years.
Presbyopia
and LASIK
If you are in your mid-forties and have mild myopia (up to
-3.00), you probably have discovered that it is more comfortable to
focus up close and read without your distance glasses. If you have
PRK or LASIK, your distance vision will be corrected, but you may
find it more comfortable, or necessary, to wear reading glasses for
comfort up close. This is because no type of vision correction
surgery can eliminate or reverse presbyopia, which is a part of the
natural aging process of the eye. Laser treatment can permanently
reduce or eliminate myopia, hyperopia and astigmatism; but it cannot
reverse, reduce, or eliminate presbyopia.
Monovision and other options for people over 45 who desire
vision correction
If both eyes are perfectly corrected for distance, one can use
"drug-store type" reading glasses for close viewing. Another option
is to adjust one eye to have better close focus than distance
clarity. This is referred to as monovision (mono for
one - one eye for distance, one for close), and is an acceptable
option for many (but not all!) individuals who desire to avoid
reading and distance glasses for as long as possible.
Monovision, like reading glasses and bifocals, is not a perfect
solution to the optical challenges of presbyopia. It is at best a
compromise. And it is not for everybody. If a group of people (all
appropriate candidates with equal prescriptions) are given a chance
to try monovision correction with soft contact lenses, roughly a
third will like it and desire to adopt monovision. About a third
will find it uncomfortable, unbalanced or otherwise unacceptable,
and will prefer having both eyes corrected equally (for distance).
The remaining third may grumble about monovision with the same
intensity that they would lament the shortcomings of bifocals, and
will be ambivalent about any long-term commitment to monovision.
The best way to know if you will like monovision-type correction
by laser treatment is to experience it first in contact lenses. If
you already wear monovision correction in soft lenses, you already
know the answer. At BEST LASIK Laser Eye center, we demonstrate
monovision correction (via soft lenses) to all individuals over 40
or so who come in to discuss laser care (if they have not tried this
previously). This is quite like trying on a pair of shoes before you
buy them. We find that satisfaction with laser treatment is
extremely high if people understand the limits of presbyopia and
participate in the process of choosing whether or not to have
monovision-style treatment.
Until very recently, artificial lenses implanted at the time of
cataract surgery were all single-focus or fixed-focus lenses. So,
even if distance vision could be rendered optimal with such care,
reading glasses would still be necessary. It is self-evident that
any patient who may contemplate cataract-style surgery would want to
enjoy clear distance vision, clear reading vision, and clarity
everywhere in between, without need to use glasses.
Significant challenges exist in designing a lens implant that can
accommodate, or adjust for close focus. Such a lens will (if and/or
when it can be developed) possibly revolutionize the treatment of
cataracts, and may also offer a surgical treatment for presbyopia.
The term Refractive Lens Exchange" refers to surgical procedures
that remove the natural lens of the eye, replacing it with different
optical material (plastic, silicone, or hydrogel). New designs of
lens implant hold the promise of possibly being able to adjust their
focus. If a lens could do this, it might be able to correct distance
vision and restore adjustability for close viewing in the human eye.
