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| Terminology:
What is the difference between the meridian and the axis? |
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The meridian is the line that
we are moving the streak along. In the example to the left,
we are streaking the 180 degree meridian. |
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| The axis depends
on whether we are using plus-cylinder or minus-cylinder. If we are
using plus-cylinder, the axis is 90 degrees from the meridian that
we are streaking. Another way to look at it is that the
plus-cylinder axis lines up with the streak, or the streak reflex.
In the above illustration, the plus-cylinder axis is 90 degrees.
If we are using minus-cylinder, the
axis is the same as the meridian that we are streaking. In the
above illustration, the minus cylinder axis is 180 degrees,
or perpendicular to the streak.
When performing retinoscopy, we may
say that the astigmatism (or the cylinder) is at 180. What we mean is
that we detect astigmatism when streaking the 180 degree meridian.
The actual axis designation (180 or 90) will depend on whether we are
using plus-cylinder or minus-cylinder. |
Recognizing the presence of
astigmatism
When you begin retinoscopy on an eye, you will know that there is
astigmatism present in the following situations:
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Streaking one
meridian gives you with-motion or against- motion, and streaking the
meridian 90 degrees away gives you a neutral reflex. |
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Streaking one meridian gives you against-motion, and streaking
the meridian 90 degrees away gives you with- motion. |
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Streaking one meridian gives you with-motion (or
against- motion) with a
wide streak reflex, and streaking the meridian 90 degrees away
gives you the same motion but with a narrower streak reflex. |
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The same basic rule applies as with neutralizing a spherical eye:
If
you do not see with-motion in every meridian, add minus sphere power until
you do.
The remainder of the discussion assumes that you have arrived at a with-motion streak reflex in every meridian.
Procedure
for neutralizing an astigmatic eye
1. The first step is to neutralize one of the meridians. You will
be adding plus sphere power and streaking each of the primary meridians after
each power change.
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The meridian with the narrow, fast reflex will neutralize
first. This meridian will be 90 degrees away from the meridian
with the widest, slowest streak reflex. In this
example, the 180 degree meridian will neutralize first. |
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As we add plus sphere power, the reflex at 90 narrows and the
reflex at 180 quickly widens and reaches neutrality. |
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2. The next step is to confirm/identify the axis of the
astigmatism. We have a good idea of what the axis is from the
neutralization process. When working in plus cylinder, the axis will
be 90 degrees from the meridian with the most defined with-motion streak
reflex. In the above example we know that the plus cylinder axis will be
very near, if not exactly, at 180 degrees.
There are several clues that we can use:
- The Thickness Phenomenon
- The Intensity Phenomenon
- The Break and Skew Phenomena
- Straddling the Axis
The Thickness Phenomenon:
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The streak reflex appears to be narrowest when
we are streaking the meridian of the correct axis. As you move away from the
correct axis, the streak reflex becomes wider. |
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The
Intensity Phenomenon
The streak reflex appears brightest when you are streaking the meridian of
the correct axis. As you move away from the correct axis, the streak
reflex becomes more dim.
The Break and Skew Phenomena
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The Break phenomenon: In higher amounts of
astigmatism, the streak reflex will tend to stay on-axis even if the
streak is rotated off-axis. This guides you back to the correct
axis. |
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The skew phenomenon: If we streak a meridian that is away from
the meridian of the correct axis, the streak reflex will tend to
travel along the correct meridian rather than follow the streak.
This guides us back to the correct meridian. |
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Straddling the Axis
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Assuming that there is regular astigmatism present,
when one meridian has been neutralized, the meridian exactly 90
degrees away will have the strongest, most defined with- motion
reflex. |
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The axis can be
confirmed by streaking the meridians 45 degrees to each side of what we
believe to be the meridian of the correct axis. In this case we
believe that streaking the 90 degree meridian gives the most defined
reflex. We streak the 45 degree meridian and the streak reflex
widens and degrades in sharpness. The same thing happens when we
streak the 135 degree meridian. This confirms 90 degrees as the
correct meridian. We are using plus cylinder, so the axis is
in line with our streak, which is 180 degrees (90 degrees from the
meridian).
If the reflex in one of the straddle
meridians is narrower than the reflex in the other straddle
meridian, then we would adjust our estimated axis in the direction
of the straddle meridian with the narrower reflex. We would retest
45 degrees to each side of the new axis to confirm that the reflex in
each straddle meridian is equally wide.
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3. The next step is to neutralize the astigmatism (with
plus-cylinder power).
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Remember that one meridian has already been neutralized. |
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The meridian 90 degrees away still has with-motion. We begin by
streaking this meridian that has the brightest, narrowest
with-reflex. |
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Since we are using a plus-cylinder phoropter, we will line up
our cylinder axis with the orientation of the streak. In other
words, at 180 degrees in this example. We are streaking the
90 degree meridian, but the axis of the correcting plus-cylinder
will be 180 degrees. |
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We streak 90 as we add plus cylinder power .50D at a time.
After adding cylinder power we again streak the 90 degree meridian
looking for with-motion. If with-motion is still present, we
repeat the process, adding cylinder power and streaking the
meridian. The
streak will eventually widen as neutrality is approached by adding the
cylinder power. |
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Once we have a neutral reflex, we have reached the endpoint.
Neutrality can be assumed when any with-motion just disappears.
This is preferable to relying on recognizing a neutral reflex,
because the reflex may appear neutral over a wide range of power
settings. |
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4. The final step is to subtract for our working
distance.
This
is usually 1.50 D and it is subtracted from the sphere power only. Suppose
our phoropter reads -1.00+150x180 when we have finished neutralizing the
astigmatic meridian. We then would subtract 1.50 D sphere power for a
final retinoscopic estimate of -2.50+1.50x180.
It is easiest to practice retinoscopy on younger
adults, ages 20 to 50. They usually have clear media,
relatively relaxed accommodation, and a definite refractometric endpoint
with which to compare your retinoscopy. As stated earlier, there is
more than one way to perform retinoscopy. If you get advice from different
sources and mix up your technique, you will become confused. The
technique described here is relatively simple and is very accurate. Once
you have mastered the routine, it will become second nature and you will
be able to perform retinoscopy very quickly. Practice, practice, practice.
You might also consider taking Module 6, Retinoscopy in Minus-Cylinder. You never know when you might be called upon to use a minus-cylinder
phoropter. If you take Module 6, you will notice that the
procedure is identical until the cylinder power neutralization step. This
is because, with this particular technique, you are really performing plus
cylinder retinoscopy using a minus-cylinder phoropter. When performing
retinoscopy with a minus-cylinder phoropter, we simply transpose during
the cylinder power neutralization step.
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