Clinical

39 | December 12 | 2003 OT

Clinical management of aniseikoniaAn overview

Clinical management of aniseikonia has long been neglected by alarge part of the optometric community. One of the mainreasons is the lack of simple and accurate instrumentation fordiagnosing and measuring aniseikonia. With the knowledge that thenumber of aniseikonia patients is significant, and that aniseikoniarules of thumb often do not predict the actual situation well, newproducts have recently emerged on the market to manageaniseikonia clinically.

Gerard C. de Wit PhD and Arnulf Remole BFA, OD, MS, PhD

Aniseikonia is a binocular condition inwhich left and right images differ in size orshape. There are two types of aniseikonia static and dynamic aniseikonia1. The firsttype is the classical aniseikonia, denoting aperceived image size difference with afixed gaze direction. The second type ofaniseikonia is also called inducedanisophoria2 and denotes a perceivedimage size difference due to unequal prismeffects when looking through differentparts of the two (anisometropic) spectaclelenses. For clinical purposes, the two typesof aniseikonia are often related. The staticaniseikonia is typically (but not always)two thirds of the dynamic aniseikonia1.

SymptomsTable 1 classifies the symptoms ofaniseikonia. Because most of these arerather general, it is sometimes difficult forthe optometrist to recognise the condition.However, recognising and treating thesymptomatic aniseikonia will usuallyresult in very grateful patients and mayalso be financially rewarding for theoptometrist.

Alternatively, the aniseikonia can also besimulated by presenting images ofdifferent size to the two eyes. This isshown in Figure 1, where binocularseparation should be achieved by usingred-green anaglyph glasses (see later).

IncidenceThe incidence of aniseikonia is oftenunderestimated. The most well knownpatient group at risk is theanisometropes. The prevalence ofanisometropia (>1D difference) above theage of 20 is 5-10%4. A second large groupof patients at risk for aniseikonia is thepeople who have had cataract orrefractive surgery. For example, Kramer etal5 found that 40% of all pseudophakeshad ophthalmic complaints referable toaniseikonia. In England alone, there areapproximately 250,000 cataractoperations annually6. Because thesenumbers are significant and becauseaniseikonia rules of thumb have beenproven unreliable7,8, testing for, andmanaging, aniseikonia is important.

ManagementThe three basic steps of aniseikoniamanagement are: Objectively measuring the aniseikonia Subjectively verifying that the patient

would be helped by prescribingiseikonic lenses

Determining a new spectacleprescription to correct for theaniseikonia

Measurement (objective eikonometry)There are two methods of measuringaniseikonia: space perception eikonometryand direct comparison eikonometry. Theobjective in a space perceptioneikonometric measurement is to neutralisespace distortions induced by theaniseikonia. Although this method can bequite accurate in a laboratory setting, it isless suited for clinical use. To ourknowledge, there are also no commerciallyavailable instruments based on thismethod.

Regarding direct comparisoneikonometric tests, there are at least twotests commercially available. One is theNAT (New Aniseikonia Test, Handaya,Tokyo, Japan). The other is the aniseikoniatest of the aniseikonia managementsoftware called the Aniseikonia Inspector(Optical Diagnostics, Culemborg, theNetherlands).

The principle of direct comparisoneikonometry is that a different size target ispresented to each eye and that those twosize targets have to be made equal in size

SSyymmppttoomm PPeerrcceennttaaggee ooff ppaattiieennttss

Headaches 67%Astenopia (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%

For someone to experience thediscomfort of aniseikonia, he/she couldput an afocal size lens in front of one eye.This type of lens induces a magnification,but does not have an optical power.

Table 1Characteristic symptoms reported by 500patients referred for aniseikonia examination3

Figure 1When using red-green anaglyph spectacles, this image shows the discomfort produced by3% of aniseikonia (assuming the viewer does not have inherent aniseikonia)

Clinical Gerard C. de Wit PhD and Arnulf Remole BFA, OD, MS, PhD

40 | December 12 | 2003 OT

by either holding size lenses in front ofone eye, or by physically changing the sizeof one of the size targets.

Figure 2 shows the half-circle sizetargets of the Aniseikonia Inspector test.The layout of the test, in particular, the(in)visibility of binocularly fuseableobjects around the size targets, isimportant in comparison witheikonometry9. Due to binocularly visibleobjects around the size targets, the NATtest seems to underestimate aniseikonia10,while the Aniseikonia Inspector testmeasures aniseikonia more correctly10,11.

Verification (subjective eikonometry)The second step in aniseikoniamanagement is often to verify if thepatient would be helped by iseikoniclenses. The reason is that the sensitivity toaniseikonia can vary a lot from patient topatient. Some patients are very grateful if1% of aniseikonia is corrected, whileothers might not be bothered by as muchas 3% of aniseikonia. Subjectiveeikonometry can be done by simulation,as shown in Figure 3, but a better way maybe to use size lenses.

CorrectionEquivalent to a sphere and cylinderrefractive error, there is an overall and ameridional aniseikonia. For clinicalpurposes, correcting the overallaniseikonia is usually most important andsufficient1. That is, overall aniseikonia givesrise to headache and asthenopia.Meridional aniseikonia, on the otherhand, gives rise to distorted spaceperception.

The most effective way to reduce oreliminate aniseikonia is to provide aniseikonic prescription. One cannot changethe effective power at the cornea, becausethis would reduce the patients visualacuity. However, one can change theaccompanying spectacle magnifications ofthe corrective lenses by manipulating the

base curve, centre thickness, index ofrefraction, and back vertex distance.

Besides the lack of instrumentation,determining an iseikonic prescription wastoo big a hurdle for some optometrists toactually manage aniseikonia. However,with the advent of computers,determining such a prescription hasbecome much easier (Figure 3).

ConclusionClinical management of aniseikonia usedto be done only by a few specialists. Themain reasons for this lack of skill andknowledge among optometrists arebelieved to be outdated and insufficientinstruction, the lack of simple andaccurate instrumentation, and therelatively complicated or time-consumingdetermination of iseikonic spectacles. Onthe other hand, the number ofaniseikonia patients is substantial andgrowing, due to the ageing populationand the increase in cataract and refractivesurgery operations.

Another reason, heard sometimes, fornot managing aniseikonia, is thatiseikonic prescription spectacles can becosmetically unattractive. Of course, thisdepends a lot on the amount ofaniseikonia to be corrected and the framesize. There might also be a trade-off toundercorrect aniseikonia to keep thespectacles attractive. The trade-off betweenappearance and correction will depend alot on the patient and on the severity ofthe symptoms. Many aniseikonia patientswould prefer to trade a reduction in goodappearance for more visual comfort. Also,the patient might purchase two pair ofspectacles one for optimum visualcomfort for daily routine and one foroptimum appearance during social events.

A product like the AniseikoniaInspector now gives the optometrist theopportunity to manage aniseikonia.Potential rewards will be some verygrateful patients, a larger patient base, anda possible increase in revenue.

About the authorsDr Gerard C. de Wit is involved withresearch at Optical Diagnostics in theNetherlands. Dr Arnulf Remole is on thefaculty of the School of Optometry at theUniversity of Waterloo in Canada.

Special offerFor viewing Figure 1, Dr de Wit is offeringto send a simple pair of red-greenspectacles to the first OT readers to emailhim at dewit@opticaldiagnostics.com.

References1. Remole A, Robertson KM (1996)

Aniseikonia and Anisophoria: CurrentConcepts and Clinical Applications.Runestone Publishing, Waterloo, Ontario,Canada.

2. Friedenwald JS (1936) Diagnosis andtreatment of anisophoria. Arch.Ophthalmol. 15: 283-307.

3. Bannon RE, Triller W (1944) Aniseikonia a clinical report covering a ten-yearperiod. Am. J. Optom. 21: 171-182.

4. Weale RA (2002) On the age-relatedprevalence of anisometropia. OphthalmicResearch 34: 389-392.

5. Kramer PW, Lubkin V, Pavlica M, Covin R(1999) Symptomatic aniseikonia inunilateral and bilateral pseudophakia. A projection space eikonometer study.Binoc. Vis. Strabis. Q. 14: 183-190.

6. NHS Executive (2000) Action on Cataracts:Good practice guidance. Department ofHealth, London(www.doh.gov.uk/cataracts).

7. Lubkin V, Shippman S, Bennett G. et al(1999) Aniseikonia quantification: errorrate of rule of thumb estimation.Binoc. Vis. Strabis. Q. 14: 191-196.

8. Kramer P, Shippman S, Bennett G et al(1999) A study of aniseikonia and KnappsLaw using a projection space eikonometer.Binoc. Vis. Strabis. Q. 14: 197-201.

9. Ogle KN (1950) Researches in BinocularVision. WB Saunders, Philadelphia, USA.

10. McCormack G, Peli E, Stone P (1992)Differences in tests of aniseikonia. Invest.Ophthalmol. Vis. Sci. 33: 2063-2067.

10. De Wit GC (2003) Evaluation of a newdirect-comparison aniseikonia test. Binoc. Vis. Strabis. Q. 18: 87-94.

Figure 3Determining an aniseikonia corrected prescription with the AniseikoniaInspector software is fast and easy

Figure 2Layout of the aniseikonia test of the Aniseikonia Inspector. The patientuses red-green spectacles to separate the two half-circle size targetsbinocularly. The objective of the test is to make the two half-circlesvisually equal in size