Clinical Management of Aniseikonia                                                     View Shopping Cart

Gerard C. de Wit1, Ph.D.

Arnulf Remole2, B.F.A., O.D., M.S., Ph.D.

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 33:2063-2067

11. De Wit GC (2003) Evaluation of a new direct comparison Aniseikonia test. Binocul Vis Strabismus Q 18: 87-94
Aniseikonia Evaluation Software

 

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