Monday, May 22, 2006

29

Aniseikonia

Aniseikonia…. “may be due to differences in the size of the optical images on the retina or may be anatomically determined by a different distribution in spacing of the retinal elements”. (Duke-Elder, 1963)

Aniseikonia is a term coined by Dr. Walter Lancaster in 1932. It means literally “not equal images (either size, shape, or both)” from the two eyes, as perceived by the patient and is one of the problems most frequently associated with the correction of anisometropia with spectacles. It is an anomaly of the binocular visual process that affects the patient’s perceptual judgment. The most common cause is the differential magnification inherent in the spectacle correction of Anisometropia. This difference in magnification produces different sized retinal images. Approximately 1/3 of the cases of aniseikonia are predicted from anisometropia. Aniseikonia is more commonly caused by unequal refractive errors common in conditions such as monocular aphakia or pseudophakic surprises. However, it is also found with retinal problems and occipital lobe lesions. Aniseikonia occurs in 5-10% of the population with only 1-3% having symptoms.

The perception of an image size disparity between the two eyes is due to the image on the retina not falling on corresponding retinal points. The ocular image is the final impression received in the higher cortical centers, involving the retinal image with modifications imposed by anatomical, physiologic, and perhaps psychological properties of the entire binocular visual apparatus. This is why there are cases of aniseikonia in individuals with emmetropia and isometropia (equal refractive errors).

In general aniseikonia is associated with a false stereoscopic localization and an apparent distortion of objects in space. Aniseikonia can be the cause of asthenopia, diplopia, suppression, poor fusion, headaches, vertigo, photophobia, amblyopia, and strabismus. The differences in size may be overall, that is, the same in all meridians, or meridional, in which the difference is greatest in one meridian and least in the meridian 90° away.

Clinically, aniseikonia usually occurs when the difference in image size between the two eyes approaches 0.75%. Individuals with greater than 4-5% image size difference, have such a large disparity in image size, that they generally do not have binocularity. It is usually assumed that patients can comfortably tolerate up to 1% of aniseikonia in non-astigmatic cases.

A change in refractive correction is always accompanied by some change in the retinal image size and in the conditions under which the patient sees. The magnitude of these changes and the patient’s tolerance determines whether these changes will produce symptoms of discomfort or inefficiency. Persons with normal binocular vision can readily discriminate differences in image size as low as 0.25 to 0.5 percent. For persons with normal binocular vision, a deviation of 4-5x the threshold of discrimination is usually considered significant.

Aniseikonia can be noted when a patient, for the sake of comfort, prefers to use one eye for reading or watching moving objects. If an individual can learn to rely on non-stereoscopic, rather than stereoscopic clues, they may be able to avoid irritation from aniseikonia, even when it is present.

Aniseikonic patients may see an apparent slant of level surfaces, such as tabletops and floors. The effect is more pronounced with objects on the surfaces, for instance, with an irregular pattern carpet on the floor. For high levels of cylinder correction, spherical equivalents may help reduce the aniseikonia.

  • The magnification for flat trial lens case cylinders is approximately 1.5% per diopter.
  • The uncorrected refractive myopic eye will have a larger image by 1.5% per diopter and the uncorrected refractive hyperopic eye will have a smaller image by 1.5% per diopter. This holds for anisometropia primarily of refractive origin.
  • The corrected refractive myopic eye will have a smaller image by 1.5% per diopter and the corrected refractive hyperopic eye will have a larger image by 1.5% per diopter. This holds for anisometropia primarily of refractive origin.
  • However, since anisometropia may be partially axial, an estimate of 1% per diopter is more clinically useful.

When considering axial versus refractive anisometropia:

If the amount of anisometropia is > 2D – assume it to be axial.

If the amount of anisometropia is < 2D or is in cylinder only – assume it to be refractive.

Spectacle correction of astigmatism produces meridional aniseikonia with accompanying distortion of the binocular spatial sense. Anisometropia is commonly stated to be present if the difference in the refractive correction is 2.00D or more either spherical or astigmatic. However, smaller differences than 2.00D may be significant.

When prescribing aniseikonic lenses, it is important to realize that the size and shape of the final image does not matter, it is only important that the images of each eye match each other. For this reason, instead of magnifying the image of one eye, it may be easier to minify the image of another. This may allow for a more cosmetically acceptable spectacles, or at least lenses that are easier to manufacture, and therefore, less costly.

a. Cylindrical Corrections

Cylindrical corrections in spectacle lenses produce distortion. This is a problem of aniseikonia, which may be solved by prescribing iseikonic spectacle corrections. Iseikonia is when perceived images are the same size. Iseikonic spectacle corrections may be complicated and expensive and the vast majority of practitioners prefer to prescribe cylinders according to cylinder judgment using guidelines that have evolved over the years. Remember the reason for intolerance of an astigmatic spectacle correction is distortion caused by meridional magnification which is more poorly tolerated. Unequal magnification of the retinal image in the various meridians produced monocular distortion manifested by tilted lines or altered shapes of objects. The monocular distortion by itself is rarely a problem. The effect is too small.

Oblique meridional aniseikonia causes a rotary deviation between fused images of vertical lines in the two eyes. The maximum tilting of vertical lines is called the Declination Error. The maximum declination error occurs when the corrected cylinder axis is at 45 or 135°, but even under these conditions, each diopter of correcting cylinder power produces only about 0.4° of tilt. This problem occurs more often with plus cylinder lenses which is why most spectacle lenses are now made in the minus cylinder form. Clinically significant problem begin to occur when the declination approaches 0.3%. Minor degrees of monocular distortion can produce major alterations in binocular spatial perception.

The Total Magnification of a Lens (M T) is found by adding the magnification from its power (M P) and the magnification from its shape (M S). Therefore, total magnification M T = M P + M S.

Magnification from Power (M P) is dependent on the dioptric power of the lens (D V) and its vertex distance (H). If H is measured in cm, the relationship is M P = D VH. From this formula, we see that moving a lens away from the eye increases the magnification of a plus lens and the minification of a minus lens. Moving a lens toward the eye (decreasing the vertex distance) decreases the magnification of a plus lens and the minification of a minus lens. These effects are especially notable with higher powered lenses.

Examples

+10.00D lens @ 10mm and 15mm vertex distance

@ 10mm, M P = D VH = +10.00 x 1.0 = +10

@ 15mm, M P = D VH = +10.00 x 1.5 = +15

-10.00D lens @ 10 and 15 mm vertex distance

@ 10mm, M P = D VH = -10.00 x 1.0 = -10

@ 15mm, M P = D VH = -10.00 x 1.5 = -15

For spectacle lenses remember, as you move a lens closer to the eye, you must add plus power to the lens. Therefore remember CAP = Closer Add Plus.

Magnification from Shape (M S) is dependent on the curvature of the front surface of the lens D 1 and the center thickness of the lens t. The 1.5 in the following equation is the index of refraction (approximately) of glass or plastic. M S = D 1 (t cm/1.5). Therefore, the more curved the lens, the larger the D 1 and the more magnification from shape the lens have. Also, the thicker the lens (t), the more magnification from shape.

Examples

Front curve of a +2.00D lens is +2.00D and +6.00D, Thickness is 2mm.

M S = D 1 (t cm/1.5) = +2.00(0.2/1.5) = +0.27

M S = D 1 (t cm/1.5). = +6.00(0.2/1.5) = +0.80

Front curve of a +2.00D lens is +2.00D and +6.00D, Thickness is 4mm.

M S = D 1 (t cm/1.5) = +2.00(0.4/1.5) = +0.53

M S = D 1 (t cm/1.5). = +6.00(0.4/1.5) = +1.60

Magnification may be reduced by making the front surface power of a lens less positive.

Decreasing center thickness also decreases magnification.

However, a change in either the front curve or the thickness of the lens will also cause the vertex distance (h) to be changed so that the magnification from the power factor (M P) is also affected. If the front curve is changed to give the magnification or minification needed, the back curve must also be changed to maintain the same power of the lens.

If, for example, the front curve is increased, the back curve must also be increased, which increases the vertex distance. If the front curve is flattened, the back curve must be flattened, which causes the vertex distance to decrease. If center thickness is increased to increase the magnification of the lens, but the front curve is left the same, the increase moves the back surface closer to the eye by the amount of the increase, therefore decreasing the vertex distance. On the other hand, it the center thickness is decreased, but the front curve is left the same, the decrease causes the vertex distance to be increased by that amount.

For further review on this subject, go to the THILL Aniseikonia Worksheet in Duane’s Clinical Ophthalmology (Lippincott Williams & Wilkins).

Contact lenses may provide a better solution than spectacles in most patients with anisometropia, particularly children, where fusion may be possible, because it gives the least change in image size from the uncorrected state in refractive ametropia.



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