Aniseikonia

7/8/2012 Fakhruddin Aliasger 1

 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


 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


 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


 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)

 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)


 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


 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

 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


 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

 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


 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"


 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

 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


 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.

 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

 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


 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

 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


 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


 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


 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


 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,

 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


 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


 The two primary means of optically treating
aniseikonia are spectacle correction and contact
lens correction
 Refractive surgery may also be an option


 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


 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


 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


 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


 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


 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.


 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


 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

 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


 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


 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

 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


 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

 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.


 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


 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

 Light tints conceal unusual lens designs
 Smaller eye sizes improve edge thicknesses
 contact lenses are usually a good solution


 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

 Make sure that the patient understands in
layman's terms-the issues and problems
involved


Aniseikonia

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