Clinical Management of
Aniseikonia
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Gerard C. de Witt PhD
Arnulf Remole, BFA, OD, MS, PhD
Abstract
Clinical management of Aniseikonia has long been neglected by a large
part of the optometric community.
One of the main reasons is the lack of simple and accurate instrumentation
for diagnosing and measuring Aniseikonia. With the knowledge that
the number of Aniseikonia patients is significant and that Aniseikonia rules of thumb often do not predict
the actual situation well, new products
have recently emerged on the market to manage Aniseikonia clinically.
Introduction
Definition
Aniseikonia is a binocular condition
in which left and right images differ in size or
shape. There are two types of Aniseikonia: static
and dynamic Aniseikonia1. The first type is the classical Aniseikonia, denoting a
perceived image size difference with
a fixed gaze direction. The second type of Aniseikonia is also called induced anisophoria2 and
denotes a perceived image size difference due to unequal prism
effects when looking through different parts of
the two (anisometropic) spectacle lenses.
For clinical purposes, the two types of Aniseikonia are often related.
The static Aniseikonia is typically
(but not always) two thirds of the dynamic Aniseikonia1.
Aniseikonia symptoms
The symptoms an Aniseikonia patient
experiences have been classified in Table
1. Because
most of these symptoms are rather general, it is
sometimes difficult for the optometrist to recognize the
condition. However, recognizing and treating
the symptomatic Aniseikonia will usually result in
very grateful patients and may also be financially
rewarding for the optometrist. For someone to experience the
discomfort of Aniseikonia, he/she could put an a focal
size lens (e.g. from Optima Low Vision Services Ltd)
in front of one eye. This type of lens induces a
magnification, but does not have an optical power.
Alternatively, the Aniseikonia can also be simulated by
presenting images of different size to the two eyes.
This is shown in figure 1, where binocular separation
should be achieved by using red-green anaglyph glasses.
Table 1: Characteristic
symptoms reported by 500 patients referred for Aniseikonia examination3.
Symptom Percentage of patients
Headaches 67%
Asthenopia (fatigue, burning,
tearing, ache, pain, pulling, etc.) 67%
Photophobia 27%
Reading difficulty 23%
Nausea 15%
Motility (diplopia) 11%
Nervousness 11%
Vertigo and dizziness 7%
General fatigue 7%
Distorted space perception 6%
Figure
1: When
using red-green anaglyph glasses, this image will demonstrate to the viewer the
discomfort produced
by 3% of Aniseikonia (assuming the
viewer does not have inherent Aniseikonia).
Incidence of Aniseikonia
The incidence of Aniseikonia is
often underestimated. The most well known patient group at
risk are the anisometropes. The prevalence of
anisometropia (>1D difference) above the age of
20 is 5-10% 4. A second large group of patients at risk for Aniseikonia
are the people who have had cataract or
refractive surgery. For example, Kramer et al.5 found that 40% of all
pseudophakes had ophthalmic
complaints referable to Aniseikonia. In England alone, there
are approximately 250,000 cataract operations
annually6. Because these numbers are significant and because Aniseikonia
rules of thumb have been proven
unreliable7,8, testing for and managing of Aniseikonia is important.
Management of Aniseikonia
The three basic steps of Aniseikonia management are:
1) objectively measure the Aniseikonia
2) subjectively verify that the
patient would be helped by prescribing iseikonic
lenses
3) determine a new spectacle
prescription to correct for the Aniseikonia.
Measurement of Aniseikonia (objective eikonometry)
There are basically two methods of
measuring Aniseikonia: space perception
eikonometry and direct comparison eikonometry. The
objective in a space perception eikonometric measurement
is to neutralize space distortions induced by the Aniseikonia. Although this method can be quite accurate in
a laboratory setting, it is less suited for
clinical use. To our knowledge there are also no
commercially available instruments based on this method.
Regarding direct comparison
eikonometric tests, there are at least two tests
commercially available. One is the NAT (New Aniseikonia
Test) This is available from Richmond Products at www.RichmondProducts.com. The other is the
Aniseikonia test of the Aniseikonia management software
called the
Aniseikonia Inspector This is available from Richmond
Products at www.RichmondProducts.com.
The principle of direct comparison
eikonometry is that a different size target is
presented to each eye and that those two size targets have to
be made equal in size by either holding size lenses
in front of one eye, or by physically changing the size of
one of the size targets. Figure 2
shows the
half-circle size targets of the Aniseikonia Inspector test. The
layout of the test, in particular the (in)visibility of
binocularly fuseable objects around the size targets, is
important in comparison eikonometry9.
Due to binocularly visible objects around the size targets, the
NAT test seems to underestimate Aniseikonia10,
while the Aniseikonia Inspector test measures Aniseikonia more correctly10,11.
Figure 2:
Layout of the Aniseikonia test of the Aniseikonia Inspector.
The patient uses red-green glasses to separate the two half-circle size targets
binocularly. The objective of the test is to make the two half-circles
visually equal in size.
Verification (subjective eikonometry)
The second step in Aniseikonia management is often to
verify if the patient would be
helped by iseikonic lenses. The reason is that the
sensitivity to Aniseikonia can vary a lot from person to
person. Some patients are very grateful if 1% of Aniseikonia is corrected, while others might not be bothered
by as much as 3% of Aniseikonia. Subjective
eikonometry can be done by simulation, as was shown in Fig. 3.
But a better way may be to use size lenses.
Correcting Aniseikonia
Equivalent to a sphere and cylinder
refractive error, there is an overall and a meridional Aniseikonia. For
clinical purposes, correcting the
overall Aniseikonia is usually most important and
sufficient1. That is, overall Aniseikonia gives rise to headache
and asthenopia. Meridional Aniseikonia, on the other
hand, gives rise to distorted space perception.
The most effective way to reduce or
eliminate Aniseikonia is to provide an
iseikonic prescription. One cannot change the effective power at
the cornea, because this would reduce the
patient's visual acuity. However, one can change the
accompanying spectacle magnifications of the corrective
lenses by manipulating the base curve, center thickness,
index of refraction, and back vertex distance.
Besides the lack of instrumentation,
determining an iseikonic prescription was too
big a hurdle for some optometrists to actually manage Aniseikonia. However, with the advent of computers,
determining such a prescription has become much easier
(see Figure 3).
Figure 3:
Determining
an Aniseikonia corrected prescription with the Aniseikonia
Inspector software is fast and easy.
Discussion and conclusion
Clinical management of Aniseikonia used to be done
only by a few specialists. The main
reasons for this lack of skill and knowledge among
optometrists are believed to be: outdated and
insufficient instruction, the lack of simple and accurate
instrumentation, and the relatively complicated or time
consuming determination of iseikonic
spectacles. On the other hand, the number of Aniseikonia patients is substantial
and growing, due to the aging
population and the increase in cataract and refractive
surgery operations. Another reason, heard sometimes, for
not managing Aniseikonia is that
iseikonic prescription glasses can be cosmetically
relatively unattractive. Of course, this depends a lot on the amount of Aniseikonia
to be corrected and the frame size.
There might also be a trade-off to under correct Aniseikonia to keep the glasses attractive. The trade-off
between appearance and correction will depend a lot on
the patient him or herself and on the severity of the
symptoms. Many Aniseikonia patients would prefer to
trade a reduction in good appearance for more visual
comfort. Also, the patient might purchase two pair of
glasses: one for optimum visual comfort for his or
her daily routine and one for optimum appearance during
social events. A product like the Aniseikonia
Inspector now gives the optometrist the
opportunity to manage Aniseikonia. His/her potential
rewards will be some very grateful patients, a larger
patient base, and a possible increase in revenue.
References
1. Remole A, Robertson KM (1996)
Aniseikonia
and Anisophoria: Current concepts and
clinical applications.
Runestone Publishing, Waterloo, Ontario, Canada
2. Friedenwald JS (1936) Diagnosis
and treatment of anisophoria. Arch.
Of Ophthalmology 15: 283-307
3. Bannon RE, Triller W (1944)
Aniseikonia – a clinical report covering a ten year
period. Am. J. of Optometry & Arch. of Am. Ac. Of
Optometry 21: 171-182
4. Weale RA (2002) On the
Age-Related Prevalence of Anisometropia. Ophthalmic
Research 34: 389-392
5. Kramer PW, Lubkin V, Pavlica M,
Covin R (1999) Symptomatic Aniseikonia in
Unilateral and Bilateral Pseudophakia. A Projection
Space Eikonometer Study. Binocul
Vis Strabismus Q 14: 183-190
6. NHS Executive (2000),
Action
on Cataracts: Good practice guidance.
Department
of Health, London (http://www.doh.gov.uk/cataracts)
7. Lubkin V, Shippman S, Bennett G.
et. al. (1999) Aniseikonia Quantification: Error
Rate of Rule of Thumb Estimation. Binocul
Vis Strabismus Q 14: 191-196
8. Kramer P. Shippman S. Bennett G.
et. al. (1999) A study of Aniseikonia and Knapp's
Law Using a Projection Space Eikonometer.
Binocul
Vis Strabismus Q
14:
197-201
9. Ogle KN (1950)
Researches
in Binocular Vision. W.B. Saunders Company, Philadelphia,
USA 10. McCormack G, Peli E, Stone P
(1992) Differences in Tests of Aniseikonia.
Invest
Ophthalmol Vis Sci
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11. De Wit GC (2003) Evaluation of a
new direct comparison Aniseikonia test.
Binocul
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18:
87-94
Aniseikonia Evaluation Software
Aniseikonia And Cyclodeviation Index
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