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When we examine your eyes, you may be interested in what
the different tests and measurements are all about. In case you have wondered,
this is an overview....
 | TONOMETRY: Measuring the fluid pressure inside your
eye. Your eye is filled with a liquid that is under pressure, and if the
pressure gets too high, it can damage your eye and cause a condition called
glaucoma. A tonometer measures the pressure of the fluid inside your
eye by measuring how hard we have to push on the surface of your cornea to
make it bend a little bit... sort of like squeezing a basketball and seeing
how easily it compresses, to determine whether it is fully inflated. An eye
with low fluid pressure bends easily, with gentle pressure. An eye
with high fluid pressure is stiff, and it takes a lot of pressure to bend
the surface. We can either use a computerized automatic tonometer to
determine how much air pressure it takes to bend the surface (the "air
puff" tonometer) or we can use an eye drop to numb the surface of the
eye, touch a special contact lens to the cornea, and measure how much
mechanical pressure it takes to slightly bend the cornea.
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| AUTOREFRACTION: You look into an instrument... at our
office, you see a picture of a house at the end of a path. While you
are looking at the picture, the autorefractor uses the same technology as an
auto focusing camera to repeatedly focus inside your eye. Instead of taking a
picture, however, it measures the optics required to focus on your retina,
and in a fraction of a second, measures the prescription of your eye. The
instrument does this repeatedly and averages the results, because your eyes
are constantly adjusting focus as it measures. If we were to take this
result and simply have it made into eyeglasses, you would see quite well
with the eyeglasses, but not quite as well as if we use "subjective
refraction" to further fine-tune the prescription. This result makes
the subjective refraction a lot faster, however, because we have a
"starting point" that is very close to your final prescription.
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| KERATOMETRY: We measure the curvature of your cornea...
the transparent surface of your eye... to determine its refractive power and
its shape. This helps to confirm the accuracy of your eyeglass prescription,
and it's very useful for determining the shape of your cornea in case we
plan to fit a contact lens. There is a manual device for making this
measurement, but we use an automatic instrument that is built into the
"same box" as the autorefractor mentioned above. At the same
time the autorefractor is measuring the prescription of your eye, the
autokeratometer inside the same instrument is measuring the shape of your
cornea.
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| CORNEAL TOPOGRAPHY: Using the same technology
that satellites use to map mountain ranges, it is possible to accurately
"map" the shape of your cornea, in detail. Corneal topography is
sort of a super-accurate, more-detailed form of keratometry... instead of
measuring an average value of the corneal curvature, topography measures
every tiny "lump and bump" on the surface of your cornea. This is
useful in planning refractive surgery, and in advanced contact lens fitting.
This is another function built into the same instrument that does
autorefraction and keratometry above... sort of like the office machines
that are a fax, copier and printer all in one.
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| VISUAL FIELD TESTING: At one point during your vision
exam, we ask you to look into a white, bowl-shaped device, to stare straight
ahead at a green light, and to push a button whenever you notice a flash of
light out of the corner of your eye. This is "visual field
screening", in which we make sure that your peripheral vision is OK.
This basic test of peripheral vision makes sure that you can see objects out
of the corner of your eye, so that you don't get blindsided by another car
when you pull out into traffic. There is a more advanced visual field
test that we perform if we are concerned that you might have glaucoma, but
that's beyond the scope of this brief overview.
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| CORNEAL REFLECTION PUPILLOMETRY: This is a fancy
name for a simple device that is important even though it is simple. You
look into a small hand-held device at a light, while a technician looks at
your pupils and adjusts crosshairs in the instrument so that they are
centered on points of light reflected from the exact center of the surface
of your eye. This measures the precise location f the center of each
eye. If you need eyeglasses, this lets us specify the location of the
"optical center" of your lenses so that when you look straight
ahead, you are looking through the portion of your lens that has the best
optics and gives you the sharpest vision.
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| EYE ALIGNMENT: You look into a device similar to the
one used to check your vision when you renew your driver's license. You see
an arrow or a line, and tell the technician where it seems to point.
This measures any tendency of your eyes to aim in, out, up, or down relative
to each other. If you eyes are "out of alignment", it can
sometimes help to either straighten the eyes surgically, or to "move
the image" using prism in your eyeglass lenses, to relieve double
vision or discomfort.
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| SUBJECTIVE REFRACTION: ("Which is better, #1 or
#2"). This is the part of the exam where you are asked to make repeated
judgments as to which lens gives you better vision. This is absolutely the
best way to arrive at a final prescription that gives sharp, comfortable
vision. Although a computerized autorefractor can accurately measure a
prescription that will give you sharp vision, your judgments will result in
a prescription that is more comfortable to wear. This has been repeatedly
determined by studies in which people have been given two pairs of
glasses... one from the autorefractor results and one from subjective
refraction. Although both pairs of glasses usually yield 20/20 acuity, most
people prefer the eyeglasses that were prescribed from subjective
refraction. This test may be easier for you if you realize that snap
judgments actually give better results than if you spend a lot of time
trying to study and analyze which lens gives better... and it may help if
you know that during some parts of this test, the goal is to determine which
two lenses are "equally bad", not which is better.
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| SLIT LAMP EXAMINATION: The "slit lamp" is a
special type of microscope that is used to examine the health of your eyes,
to observe the fit of contact lenses, and to perform any form of treatment
where a magnified view of the eye is useful. The microscope portion of this
instrument is simply a high-quality stereo microscope. The lighting system
is what makes this instrument so special. The lighting system is
designed to be in focus at exactly the same point as the microscope optics,
using a coaxial aiming system that allows the light to be moved so that it
is coming from any angle, but still focused at exactly the same point as the
optics of the microscope. Because many of the eye's tissues are
semi-transparent... the cornea, the lens, the anterior chamber, the
posterior chamber... a brightly focused narrow beam of light, projected
"at an angle" into the eye, can be used to optically view
cross-sections of the structures, and to see details that cannot be seen in
any other way. It almost seems magical that all the detailed layers of these
seemingly transparent structures can be seen, in detail, with very bight
light focused at the correct angle.
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| BINOCULAR INDIRECT OPHTHALMOSCOPY: Imagine being
able to make yourself very tiny, to get inside an eyeball, and to be able to
look around and see the structures inside... clearly, with depth perception,
and in great detail. By dilating your pupils and looking inside your eye
using this instrument, which looks sort of like an old-fashioned coal
miner's helmet, we get a view inside the eye that's almost like walking
around inside. I can see the health and condition of your blood vessels...
clearly on display, right on the surface of your retina, with nothing in the
way to block a perfectly clear view. Your optic nerve, which is
actually part of your brain tissue, is right there in plain view... and the
view is amazing. A few years ago I was fortunate enough to be able to
scuba dive the Great Barrier Reef off the shore of Australia, and my first
thought was that it was like going to Mars, seeing sights that are so
different and so special that it's difficult to describe. The view of the
inside of the eye through a binocular indirect ophthalmoscope is a lot like
that. |
I hope this gives you an idea of what it is that we do
when we examine your eyes. I would like your feedback... if there is anything I
can do to give you a better picture of what an eye exam is about, anything here
that is unclear and could perhaps be described in a way that makes more sense,
please go to the part of this site where you can send me an email and do
so. Thank you.
R. Michael Burley, O.D.
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