Aniseikonia refers to an unequal apparent size of images seen by the two eyes. It can result from differences in refractive errors between the eyes (refractive aniseikonia) or differences in the distribution of retinal elements (basic aniseikonia). Symptoms include headaches, asthenopia, and difficulties with mobility or fusion. Aniseikonia is usually caused by anisometropia above 1.50-2.00 diopters and analyzing ocular components can help determine if it is due to refractive or axial differences.
Detailed instumentaion and use of manual Lensometer and just a outline of automated lensometer.
I have used the picture of manual lensometer with out the parts describtion because i have explained orally by showing the picture..
Hope u all like it and may help you in learning better. :)
Detailed instumentaion and use of manual Lensometer and just a outline of automated lensometer.
I have used the picture of manual lensometer with out the parts describtion because i have explained orally by showing the picture..
Hope u all like it and may help you in learning better. :)
This presentation is prepared in the process of me teaching Visual Optics for graduate level optometry students. It narrates the type, clinical manifestations, clinical assessment and treatment of Aniseikonia.
Presbyopia ( Part 1 / lenticular approach )..Types of MFIOLDiyarAlzubaidy
Ophthalmology Lectures: Presbyopia Management can be done through the cornea or the lens or sclera ..in part 1 we discuss lenticular part & types of MFIOL
This is a slide show presentation I prepared for the Technical Support staff at Topcon Medical Systems to introduce and familiarize the art of refraction.
The term ‘‘aniseikonia” comes from the Greek words ‘‘an” (not) ‘‘is” (equal) & ‘‘eikon” (icon or image) so aniseikonia is a binocular condition in which the apparent sizes of the images seen with the two eyes are unequal.
Whenever refractive ametropias in the two eyes of a person are different (i.e., when there is an anisometropia), the corrected retinal images of the two eyes, and consequently the two visual images, differ in size.
This condition has been termed aniseikonia
Optical aniseikonia
Retinal aniseikonia
Cortical aniseikonia
2. Aniseikonia is a binocular condition in which
the apparent sizes of the images seen with the
two eyes are unequal (or)
Whenever refractive ametropias in the two
eyes of a person are different (i.e., when there is
an anisometropia), the corrected retinal images
of the two eyes, and consequently the two
visual images, differ in size.
This condition has been termed aniseikonia
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3. Aniseikonia literally means unequal imagery
Aniseikonia resulting from a corrected
refractive anisometropia may be termed
refractive aniseikonia
Anisiekonia may also exist in patients with an
equal ametropia in the two eyes or who may
have no ametropia at all
This type of image size difference may be
termed basic aniseikonia
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4. Basic anisiekonia is presumably result of a
difference in the distribution of the retinal
elements, or rather their spatial values, in the
two eyes
Examples of basic aniseikonia are patients with
epiretinal membranes and vitreomacular
compression that may cause aniseikonia from
separation or compression of photoreceptors
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5. The clinical significance of this condition arises
from the difficulty the visual system has in
combining these dissimilar images into a
unified single percept
The incongruities of the retinal images may be
of different types
The image size may differ or may be the same
in all meridians (overall size difference), or
One of the two images may be larger only in
the horizontal or vertical meridian (meridional
size difference)
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6. The images may differ in oblique meridians
(oblique meridional size difference), or
They may be asymmetrically different in the
two eyes (e.g., larger on the temporal side in
one eye than on the nasal side)
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7. Optical
Aniseikonia is frequently associated with
anisometropia
Isometropic aniseikonia may exist if one eye is
simply larger than the other, without a
refractive error difference and without a
compensating redistribution of neural elements
however such a condition is rare
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8. Clinically significant aniseikonia are the result
of differences in the optical components, in the
axial length of the two eyes, in the distribution
of neural elements, or a combination of those
factors
Anisometropia is usually divided into axial
and refractive forms
In uncorrected axial ametropia, the image
formed in the longer eye is larger, because the
retina is farther from the optical components
or, more specifically, from the secondary nodal
point
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9. Refractive anisometropia is difference in the
refractive error
Differences are attributed to differences in the
optical components of the eyes rather than to
axial length differences
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10. Spacing of retinal elements
The retinal elements that receive the optical
image carry image size information
The density and distribution of retinal
receptors would therefore be expected to
influence perceived image size
If the spacing or density of these retinal
elements differs between the two eyes, the
perceived image sizes may also differ as a
result of the differential spacing of these retinal
elements
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11. Distribution of cortical nerve fiber
Difference in the retinal image size may be
obtained if there is alteration in the visual
system after the retina
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12. Spatial Distortion
A person with anisiekonia may experience
Distortion
A person who has an interocular image size
difference and who also has well-developed
stereopsis may also perceive stereoscopic depth
distortions
This effect can be demonstrated with the use of
"size lenses"
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13. A size lens is a spectacle lens that alters the
magnification of the image but that does not
alter the vergence of the light passing through
the lens
If a size lens is held before the right eye, the
horizontal retinal image disparities generated
are consistent
Whether the observer appreciates such a
rotation depends on a number of factors,
including sensitivity to stereoscopic depth
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14. This phenomena is known as perceptual effect
Magnification in the vertical meridian also
produces a depth effect called the induced effect
This effect has less significance as because
differential magnification in the vertical
meridian has no effect
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15. Prismatic Effects, Induced
Anisophoria, Fusion, and Eye Movements
A phenomenon that is closely related to
aniseikonia is the differential prismatic effect
between the two eyes
This is produced by corrective lenses of
different powers
Anisometropia is a common antecedent to
aniseikonia, and it requires corrective lenses of
different powers.
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16. The fundamental problem is that lenses of
different powers induce differential prismatic
effects when fixation is directed through
different regions of the lenses
This differential prismatic effect is commonly
called induced anisophoria
Remole has also called this effect dynamic
aniseikonia
The term static aniseikonia to refer to
aniseikonia in the usual sense
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17. Dynamic aniseikonia refers to the differential
eye movements demanded by the prismatic
effect of the lenses
Angular eye movements are referenced to the
center of rotation of the eye (COR)
Consider a patient whose right eye is -1.00 DS
myopic and whose left eye is -5.00 DS myopic
as referenced to the spectacle plane
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18. The far points of each eye are 100 cm and 20 cm
from the spectacle plane, respectively
With spectacle lens correction correction of
these refractive errors with thin lenses the
virtual images of a distant object viewed by
this pair of eyes are at their far points
The right lens image is five times farther
away, the virtual image formed by this lens is
five times larger than the image formed by the
left lens
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19. Because of the difference in image
distance, these two images subtend the same
angle at the spectacle plane
If the patient described makes an eye
movement from one end of this image to the
other, a different excursion is demanded for
each eye
This is because the eye rotates about the
COR, behind the spectacle plane, and so the
angular rotation demanded of the more myopic
eye is less
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21. Assume that the COR is 2.8 cm behind the
spectacle plane
If the virtual image situated at the far point of
the right eye is 8.75 cm in height
For R.E
The angle formed can be found using formula
tan-1 (image size / distance)--------------------1
Therefore θ=tan-1 (8.75/102.8)
θ=tan-1(0.08511)
θ=4.86
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22. Headache and asthenopia
Photophobia or reading difficulty
Mobility difficulties as a result of diplopia or
another visual disturbance
Symptoms are more common among those
with meridional magnification differences
Meridional differences in magnification are
more likely among patients with astigmatic
anisometropia
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23. Spectacle Prescription
Aniseikonia is nearly always caused by
anisometropia
Aniseikonia is rarely clinically significant if the
image size difference is less than 2% (or)
Anisometropia of 1.50 to 2.00 D
High anisometropia causes clinically significant
Aniseikonia
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24. Ocular Component Analysis
Analysis of the ocular components helps in
estimating whether a person is anisometropic
because of refractive or axial differences
This in turn can suggest whether a contact lens
or spectacle lens is more likely to equalize
actual retinal image sizes
Keratometry may suggest the source of
anisometropia
For example, if corneal powers differ by 3.00D
in a 3.00D anisometrope,
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25. If the corneal powers are equal and there is still
anisometropia, it is more likely that an axial
length difference is responsible for the
refractive error difference
Axial Length can be found with the availability
of ultrasonography and other technologies in
clinical settings
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26. If this difference in corneal power is the
primary difference between the two eyes
contact lenses are more likely option to
minimize aniseikonia
In case of axial anisometropia spectacle are
more likely option than contact lenses
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27. The two primary means of optically treating
aniseikonia are spectacle correction and contact
lens correction
Refractive surgery may also be an option
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28. Regardless of what type of correction is used
and whether the ametropia is axial or
refractive, it is helpful to remember a couple of
principles
1. Decreasing the vertex distance always
increases magnification in hyperopia
Increasing the vertex distance always
increases minification in myopia
Hence vertex distance should be less
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29. The second principle is
Increased front surface curvature
Increased lens thickness
Decreased index of refraction
The above mentioned three factors causes an
increase in magnification
These effects do not depend on whether the
anisometropia is axial or refractive
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30. Space Eikonometer
The space eikonometer is an instrument
formerly produced by the American Optical
Company
It consist of size lens that could produce
magnification in two rotatable primary
meridians
The display consisted of four vertical rods
positioned at the four corners at the center of
the cube is another vertical rod, with a cross
consisting of two diagonal lines intersecting at
the middle
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33. If a size difference exists in the horizontal
dimension only cross appear to be rotated
about a vertical axis
With the closer rods being on the side having
the lower magnification
It is corrected with a meridional size lens
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34. A size difference in the vertical meridian only
has no effect on the vertical rods because there
is no horizontal disparity happening
The cross is rotated, with the closer side toward
the eye having the higher magnification
It is corrected in this instrument by applying
meridional magnification vertically
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35. If differential meridional magnification exists
in an oblique meridian, the cross appears to be
rotated about a horizontal axis
It is corrected by the size lens placing along the
axis of magnification
Limitation
Because it depends on stereopsis to generate
the perceptions of rotation and tilt, it is not
useful for patients who have poor stereopsis.
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37. The standard eikonometer is a projection
instrument that uses polarizing optics to
segregate right and left eye views
A fused cross and fixation target is viewed
with both eyes
Along the arms of the cross are Nonius lines
If the Nonius lines for the right and left eyes
coincide, there is no aniseikonia
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38. In the presence of aniseikonia, the Nonius lines
are mismatched along the horizontal or the
vertical axes of the cross
This mismatch can be nulled with the use of
size lenses
This instrument does not depend on the
stereoscopic effect
American Optical space eikonometers are no
longer manufactured and today are probably
mostly available in educational settings
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39. The New Aniseikonia Test (NAT) is a direct
comparison test of perceived image size
differences
It uses red/green filter to produce dissociation
The patient compares the size of adjacent right
and left images, and differences can be nulled
with size lenses
The images viewed by one eye can be altered in
size relative to the other
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40. One method for measuring aniseikonia
involves the use of a Maddox rod before one
eye
This method was first described by Brecher
As the patient views two penlights (or, better
yet, the face of the examiner who is holding the
penlights on either side)
One eye sees streaks of light, while the other
sees the penlights
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41. If there is a magnification difference between
the eyes, the streaks and the penlights are
separated by an amount proportional to the
aniseikonia
Size lenses are interposed to equalize the
distances, thereby directly estimating the image
size difference between the eyes
This procedure could be repeated for different
meridians by simply rotating the Maddox rod
and the penlights
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42. Red and green filter are used rather than
maddox rod to isolate the two eyes views
Comparisons between the separation between
the red and green lights are then the subjective
indication of size differences
Size lenses are placed in front of the eye and is
kept increasing till image separation is
overcome
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43. Bevel and Eyewire Distance
The parameter that may be the most important
for manipulating spectacle magnification is the
vertex distance
With high lens powers, changes in vertex
distance have a large effect on the power factor
Vertex distance changes can be made with
changes in the location of the bevel on the lens
and with adjustments to the eyeglass frame to
change the eyewire distance
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44. Frame Size and Type
The type and size of the eyeglass frame affect
the ability to adjust the frame as necessary to
achieve a given vertex distance
If the lens design calls for rather thick lenses
smaller eye sizes are obviously advantageous
Large eye sizes create greater difficulties from
differential prism, because they allow for a
greater range of eye movements.
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45. If long vertex distances are needed, adjustable
nosepads are useful
If unusual bevel positions are indicated, a wide
eyewire helps to conceal the lens edge and to
make the two lenses appear less different
cosmetically
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46. Cosmesis
Cosmesis and comfort are often limiting factors
for aniseikonic corrections
The lenses in aniseikonic spectacle corrections
are often heavy, have significant
distortions, result in induced prism, and make
one eye appear larger than the other
The selection of an appropriate frame-both to
achieve the required vertex distance and to
conceal edge thickness-can be critical for
achieving an acceptable cosmetic result
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47. Light tints conceal unusual lens designs
Smaller eye sizes improve edge thicknesses
contact lenses are usually a good solution
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48. Dispensing Considerations
In an aniseikonic correction, it is important to
ensure that the vertex distances are
accurate, consistent, and kept to a minimum
The optic center locations is important, because
of differential prism
Pantoscopic tilts that are different from glasses
used in testing may also affect the vertex
distance, the position of the optic centers, the
effective power of the lens
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49. Make sure that the patient understands in
layman's terms-the issues and problems
involved
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