Aniseikonia

7/8/2012 1Fakhruddin Aliasger

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

7/8/2012 Fakhruddin Aliasger 2

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

Documents

Transcript of Aniseikonia