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Aniseikonia is a condition in which
the sizes of retinal images do not match one another. In most cases, it is
associated with anisometropia, but is sometimes caused by the layout of
the retina’s receptors. For the purposes of this article, we will assume
that aniseikonia is either refractive or axial.
Like most ametropias, there are two
main causes, these being axial and refractive. In the case of axial
aniseikonia, the refractive power of the eye is normal but it is the
length of the eye that causes the image to either form in the front or the
back of the retina. In the case of refractive aniseikonia, the length of
the eye is normal but its refractive power may either be too weak or too
strong, again placing the image in front or behind the retina.
Interestingly, it is suggested that both conditions, axial and refractive
aniseikonia, are better treated by different methods. In the case of
simple refractive errors like hyperopia and myopia, both refractive and
axial conditions can either be treated with the use of spectacles or
contact lenses. With aniseikonia it is suggested that refractive should be
treated with contact lenses, while axial should be treated with
spectacles. This is known as Knapp’s Law.
Knapp’s Law states that a lens
placed before an eye with axial ametropia will produce an image that is
the equivalent of an emetropic eye. A contact lens, on the other hand,
will create magnification, which would be desirable for the treatment of
refractive aniseikonia. However Knapp’s Law is imperfect and many feel
that it’s not quite appropriate for actual clinical use. It’s
difficult to tell whether the ametropia is completely axial or refractive,
or a combination of the two. Knapp’s Law does also not apply to
aniseikonia caused by the layout of the retina’s receptors.
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In an attempt to help equalize image
size, a style of lens fabrication is used to create an iseikonic lens. In
the design of an iseikonic lens, the magnification can be manipulated
without affecting the lens’ refractive power. The determining factors in
spectacle magnification are 1) base curve 2) vertex distance 3) thickness.
With these three factors, the designer can adjust each of these values to
determine the best design. But first we must determine the difference in
image size.
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“Knapp’s Law states
that
a lens placed before an eye with axial ametropia will produce an
image that is the equivalent of an emetropic eye. A contact lens,
on the other hand, will create magnification, which would be
desirable for the treatment of refractive aniseikonia."
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EXAMPLE:
A patient comes in with significant aniseikonia and requires an iseikonic
lens.
First we need to determine the size
difference caused by the lens.
This is done with the spectacle lens
magnification formula.
SM = 1/1-
(t/n) D1 x 1/ 1-hD
t - thickness
n - index of
refraction
D1 - base curve
D - actual power
h - vertex distance + 3mm
(must be converted to meters)
^ - change in value
Our above patient has the following
prescription:
OD + 4.00
sph
OS + .75 sph
And is wearing lenses made to the
following specifications
Lens index - 1.498 Vertex distance - 14m
Base curves - OD = 9.00 OS = 6.00
Thickness - OD = 5mm OS = 2mm
Using the above formula, we will
calculate the percentage of spectacle magnification.
1/1-(0.005/1.498)(9.00)
x 1/1-(.017)(.75)
1/1-.0300400
x 1/1-.01275
(1.03097)(1.01291)=1.044
(1.044-1) 100= 4.4%
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Now the same is done for the left
eye (for the sake of space we will not do the left eye here). Once the
left eye is done you subtract the percentage of the right eye from the
left and this gives you the magnification difference.
Now how do we reduce this
percentage? As mentioned earlier, we can control the amount of
magnification by changing the base curve, center thickness and vertex
distance. These are a few formulas that will help calculate which changes
are needed.
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^%SM = ^D1 t/h To calculate base curve
changes.
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^%SM = ^D1 t/h To calculate thickness
changes.
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^%SM = ^h D/10 To calculate vertex
distance.
In the case of a hyperope with
anisometropia, frame selection is very, very important. Remember that
thickness can increase magnification and in the case of a hyperope, the
larger the frame, the thicker the center of the lens, creating more
magnification! A case in point may be an anisometropic hyperope who wants
a small fashionable frame for work, but desires a large coverage sunglass
to protect his sensitive eyes while enjoying the outdoors. He may not feel
too comfortable about the vision in his sunglasses compared to his dress
glasses. This may be due to the increased magnification created by the
extra thickness required in a larger frame.
In reality people have a very large
tolerance of both anisometropia and aniseikonia and do not always require
slab off or iseikonic lenses. However, it is something that can be a
useful tool for EyeCare professionals. By trying to understand these
conditions, it helps make the EyeCare professional a little more
knowledgeable about concepts such as prismatic effect, image size,
spectacle magnification, axial and refractive ametropia, and the
importance of base curves, thickness, vertex distance and proper frame
selection.
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