information provided here is not a substitute for professional care.
If you are having any problems with your eyes, you should see your
ophthalmologist or optometrist for diagnosis and treatment.
It is estimated that 25% of the world population is myopic.The following
information may help you to understand some of the vision problems.
The Eye is a magnificent
piece of engineering: it takes external light rays through the lens
system and focuses them on the Retina, where the optical
nerve connects to the brain where the vision is processed.
simplified Eye Cross Section
parts of the eye lens system are the Cornea
and the Lens. The cornea, optically, is a fixed
PLANO POWER bent lens. The Lens
itself, optically, is a variable radii BI-CONVEX flat
For distance vision,
the eye is in a fixed focus mode: the Lens is at a
rest position, that is, is in a fixed position with maximum
radii. The light rays coming through the cornea, are bent
toward the Lens, where they are bent (or refracted) again
toward the back of the eye and focused on the Retina
For close vision, the
eye is in an auto focus mode. The change in the Lens
radii is called Accommodation. If the Lens
would not accommodate, the light rays would be focused behind the
retina and the vision would be blurred, as shown in below.
Lens accommodates, that is, it changes its
spherical radii, the focal point moves to the correct point on the
retina. Muscles in the Lens change its shape, making it of
stronger power by decreasing the radii of both curvatures.
The figure below shows the accommodated Lens
for close vision. Notice the difference in the Lens shape
before and after accommodation.
As we get
older, the Lens of the eye loses its flexibility and the
muscles that control its shape become weaker: the eye auto-focus
system for close distance vision is degraded. Objects seen at short
distance focus behind the retina, causing blurred vision. Figure
2 above shows this condition.
This condition is called
PRESBYOPIA (Greek for old eyes), and refers to the eye's loss of
ability to focus on close objects by changing the Lens shape.
Presbyopia is not an eye disease, it is caused by the natural process
of aging, and as we all know, it can't be prevented. The first sign
of Presbyopia is difficulty reading fine print at normal reading
distance, especially in the absence of good reading light, the vision
is blurred. That is why presbyopic persons tend to hold reading
material at arm's length.
Presbyopia may seem to
occur suddenly, but it actually progresses gradually from a young
age and it begins to be noticeable when one reaches their forties.
Unfortunately, it does not stop at the forties, it keeps on progressing.
To correct this condition,
your optometrist or ophthalmologist can prescribe reading glasses,
bifocals, trifocals or contact lenses. Since presbyopia may be more
advanced in one eye than the other, the reading glasses may have
lenses of different power.
Reading Glasses are simple
"Plus Lenses", which move the focal point of light from
the back of the retina forward directly to the retina. Mild presbyopia
is corrected with low power lenses (1.0 or 1.5 diopter). Stronger
presbyopia needs higher power lenses, 3.0 or higher. Since presbyopia
progresses with time, stronger lenses are required as we age. At
45 we may need 1.5 diopter lenses, but at 60, we will need stronger
If your eye doctor determines
that your presbyopia is the same for both eyes and you do not have
any other vision problems, he/she may suggest that off the shelf
reading glasses may be OK for you.
in the above figure, the Lens has not changed its shape (radii).
Your eyes are precious.
Periodic check-ups with an optometrist or ophthalmologist will ensure
your vision health. Below there is information about other eye problems.
Hyperopia or Farsightedness, like Presbyopia, also moves
the focal point of light behind the retina for close vision. This
is not an age-induced condition, but it is caused by an abnormality
on the eye shape (cornea, lens or eyeball). People with this condition
have trouble with close vision, but may also have problems with
distance vision. This condition is also corrected with Plus Lenses.
Figure 4 above also applies to the Farsightedness condition.
Myopia or Nearsightedness,
unlike Farsightedness, moves the focal point of light in front of
the retina instead of directly on it. Like Hyperopia, this is not
an age-induced condition, but it is also caused by an abnormality
on the eye shape (cornea, lens or eyeball). People with this condition
may have good close vision, but have difficulty for distance vision.
This condition may be corrected with Minus Lenses: surgery and other
procedures are available for adults who desire to see without glasses.
Consult with your eye doctor on how to best treat this condition.
is a condition that causes light to focus on more than one point
on the retina, resulting in blurred, close and distance vision and
often occurs along with Hyperopia or Myopia. This is not an age-induced
condition, but it is also caused by an abnormality on the eye shape
(cornea, lens or eyeball): a normal eye Lens-System is spherical,
but an Astigmatic one is not, having a more elliptical shape. You
may remember from your high school geometry that an ellipsis has
two focal points: the light rays entering the eye are focused in
more than one point on the retina causing the blurred vision. This
condition may be corrected with Toroidal lenses or surgery. Consult
with your eye doctor on how to best treat this condition.
often occurs along with Hyperopia or Myopia, the lenses to correct
the vision may be a combination of Toroidal, Minus or Plus lenses.
Procedures for vision correction.
As we discussed before,
the mayor vision problem can be corrected with optical lenses. In
the XX century, great advances have been made in surgery for vision
correction. The following is a brief informational non-technical
description of some of the procedures. Although there are other
surgical procedures, here we describe some of them. For more details,
consult with your eye doctor.
RK (Radial Keratotomy).
This procedure was discovered accidentally in Russia in the early
1970's. One young myopic boy got in an accident while wearing his
eyeglasses. A minute sliver of glass cut the boy's cornea and a
few days later, his myopic vision improved. Dr Svyatoslav Fyodorov,
his doctor, realized that the cuts had flattened the cornea, somehow
correcting the boy's myopia. He developed the technology to correct
myopia by making very small precise and small radial cuts in the
cornea. Radial Keratotomy (RK) became successful in Russia and came
to the US in the late 1970s. With the advent of Laser surgery, it
is now seldom performed. If you remember, myopic vision is caused
mainly because the cornea, (and maybe the lens), reflects the light
rays in front of the retina.
The incisions are made in a radiating pattern, like the spokes on
a bicycle wheel. By varying the number, length, depth, and location
of these incisions, different amounts of nearsightedness can be
corrected. By flattening the cornea with the radial cuts, the cornea's
radius is increased.
Astigmatic keratotomy is a variation of RK, used to treat astigmatism.
AK uses arc-shaped incisions in the cornea, whereas RK uses radial
incisions, like the spokes of a wheel.
The four procedures described below use the computer controlled
Excimer Laser to reshape the front of the cornea for Myopia, Farsightedness
In general, for Myopia, the cornea is reshaped to have a larger
radius: for Farsightedness, the cornea is reshaped to have a smaller
radius an for Astigmatism, the cornea is reshaped with an appropriate
PRK (PhotoRefractive Keratectomy)
The top thin membrane on the surface of the cornea is removed. Laser
energy is then applied to reshape the cornea with the appropriate
contour. A contact lens is worn for 5-7 days while the soft material
regenerates. Only one eye is usually treated at a time; the other
eye is usually treated in 2-4 weeks.
LASIK (laser in-situ
This is the most common type of treatment. A flap is made in the
cornea using an instrument known as a microkeratome. The flap is
folded back 180 degrees on one side ("hinge"). The laser
energy is then applied under the flap to the cornea to reshape it
with the appropriate contour. After the cornea has been re-shaped,
the hinged Flap is returned to its original position on the cornea.
LASIK allows for the most rapid visual recovery. Both eyes can be
treated at the same time.
This procedure is similar to LASIK, the only difference is how the
Flap is made. Here, the Flap is created with the Intralase laser.
The Intralase laser beam, computer controlled, passes through the
cornea and creates a very precise cut, similar to the one created
by the microkeratome. The vision is then corrected using the Excimer
laser as in the LASIK procedure. The visual recovery is slower than
with standard LASIK, though both eyes can be treated at the same
time. People with thin corneas are good candidates for this procedure.
LASEK (laser epithelial keratomileusis).
This procedure is similar to the LASIK. The major difference is
that the Flap is done on almost exclusively on the top material
covering the cornea.
don't forget, periodic check-ups with an eye doctor will ensure
your vision health.