Aniseikonia
There are few subjects that have been developed with greater care or are supported by more extensive research than that of aniseikonia. Although it is relatively common in clinical practice to encounter patients with symptoms of aniseikonia, iseikonic lens designs are seldom prescribed. As a result, it often becomes difficult to recall the procedures needed for diagnosis and treatment. This chapter briefly describes the condition of aniseikonia and presents a simplified method for the design of iseikonic corrections.
Definitions of Aniseikonia
Aniseikonia, which means “not-equal images,”1 is defined as a condition of binocular vision in which there is a relative difference in the size or shape, or both, of the ocular image of the two eyes.2 Aniseikonia is expressed as a perceived image size difference relative to the right eye. Thus, when aniseikonia measures affirm 2.5% aniseikonia, the image of the right eye must be enlarged 2.5% (or the left image minified 2.5%, or a combination thereof) to “correct” the aniseikonia. A size difference that causes symptoms (generally 0.75% or more) is defined as clinically significant aniseikonia.3 Smaller amounts of image size difference are usually not clinically significant, although they are relatively common. Even large amounts of image size difference do not cause aniseikonic symptoms for some patients.
The size of each ocular image depends on the retinal image formed by the dioptric systems of the eye, the distribution of retinal receptive elements, and the physiologic and cortical processes involved in vision. As a result, the two ocular images are seldom, if ever, exactly equal. There are also normal differences in image size when looking at objects in left or right gaze and when objects are located at different distances from the eyes.4 These normal image size disparities form the basis of stereopsis and provide a signal representing where one object is with respect to another.
STATIC VERSUS DYNAMIC ANISEIKONIA
Aniseikonia can be considered to be composed of two different, but related, magnification-induced problems—static and dynamic aniseikonia.5 Measures of static aniseikonia assess the actual difference in image size between the eyes; it is these measures, rather than the normal or physiologic differences in image size, that we are typically concerned with in clinical determinations of aniseikonia. The amount of dynamic aniseikonia is determined by analyzing differences in induced phoria that occur when a patient looks in various fields of gaze through an anisometropic correction.6 A patient can have either static aniseikonia or dynamic aniseikonia, or both problems at the same time. For example, a patient with emmetropia or ametropia (with no difference in the refractive correction of each eye) could have measured aniseikonia. This would be static aniseikonia. Another patient, corrected with spectacle lenses for a large myopic anisometropia, would be expected to have dynamic aniseikonia because of the difference in spectacle lens powers. Obviously, contact lens correction is the preferred method of minimizing dynamic aniseikonia.
Historical Perspective
Prior to 1945, theoretical and clinical courses in aniseikonia were given at the Dartmouth Eye Institute, and a clinician had to be certified by that institute in order to obtain an eikonometer.7 As instrumentation was simplified and techniques for measuring aniseikonia were improved, the obligatory Dartmouth courses were discontinued. However, the initial investigations of the Dartmouth group provided the technical and clinical papers that underlie instruction in professional schools and discussion of aniseikonia in textbooks.
Differences in the retinal image size may result from correction of refractive errors, including antimetropia and anisometropia. Donders8 described the difference in the relative size of the images of the two eyes because of correction of anisometropia and suggested that these differences may interfere with binocular vision. Lippincott,9 Green,10 Friedenwald,11 and Koller12 also discussed changes in the retinal images resulting from correction of ametropia. Hess13 believed that symptoms that occur with lens correction of anisometropia are caused by prismatic effects in the lens periphery. Von Rohr14 calculated image size differences occurring in unilateral aphakia and high anisometropia. Erggelet15 pointed out that astigmatic corrections also introduce size differences between the retinal images. He considered these size differences unimportant, because they rarely exceed 4% or 5%. Earlier, Erggelet16 had considered the possibility that a physiologic image size difference might result from unequal distribution of the retinal elements in the two eyes. The correctness of this conjecture is illustrated by the statistical analysis of Carleton and Madigan,17 which showed that aniseikonia occurs in bilateral emmetropia and isoametropia as well as anisometropia.
KNAPP’S LAW
For some clinicians, confusion results from too liberal an application of Knapp’s law, which states that the corrected eye with axial ametropia has a retinal image equal in size to that of an emmetropic eye of equal power, provided the lens is placed at the anterior focal point of the eye.2 However, there are a substantial number of patients with axial anisometropia who cannot comfortably wear spectacle lens corrections because of dynamic aniseikonia (differences in induced phoria that occur when a patient looks in various fields of gaze through an anisometropic correction). The fact that a large proportion of patients with anisometropia are free of aniseikonia symptoms suggests that Knapp’s law is more useful as a guideline than a “law.” The ultimate determinant of the retinal image size is based on the separation of retinal photoreceptors and on the registration of these in the visual cortex, not solely on the power or form of the refractive correction. As a result, Knapp’s law fails in many cases, because simply correcting the anisometropia and providing clear retinal images often has a more beneficial effect on binocular fusion than the detrimental effect of the potentially unequal image sizes.
Diagnosis
It is not generally difficult to decide whether a patient has aniseikonia. Careful review of the case history and a few basic clinical tests should give sufficient information to make a tentative diagnosis on nearly all of the patients suspected of having aniseikonia. After reviewing the patient’s symptoms, consider the refractive state and corneal curvature. If these do not allow accurate diagnosis, then a period of diagnostic occlusion or an iseikonic clip-on (or both) should lend further diagnostic support.
Definitive diagnosis of aniseikonia is done by measuring the image sizes with an instrument using either space perception eikonometry (e.g., space eikonometer) or direct comparison eikonometry (e.g., Aniseikonia Inspector) if one of these is available in the office or on referral. The objective of space perception eikonometry is to neutralize space distortions arising from the aniseikonia. The principle of direct comparison eikonometry is to equalize perception of different size targets presented to each eye, typically by physically changing the size of one of the targets (but occasionally by placing size lenses in front of one eye). When measurement devices are not available, the aniseikonic correction may be estimated from the refractive correction required or from comparison of the images seen by the two eyes.
HISTORY
The symptoms that the patient experiences are important in the diagnosis of aniseikonia. Symptoms of aniseikonic patients are similar to those of patients with uncorrected ametropia and heterophoria. The incidence of symptoms reported by 500 aniseikonic patients is listed next. Although local eye discomfort (asthenopia) and headaches are the most frequent symptoms, there are a variety of conditions that may produce symptoms similar to aniseikonia.18 As a result, we recommend that other possible causes for a patient’s complaints be investigated and treated before considering iseikonic correction.
Symptoms of aniseikonia patients include the following:
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To conduct the examination, it is important to determine the patient’s primary symptom, along with its duration and frequency. Begin by having the patient describe the current symptoms using questions such as “In what way do your eyes bother you most?” Directing the patient to give an accurate description of his or her difficulties requires tact and judgment, because some patients are anxious to report any visual phenomenon or relate their numerous visits to various specialists and it is easy to lose track of the problem that brought them.
The most important symptoms are those present when the refractive correction is worn. Ocular symptoms can conveniently be grouped under the term asthenopia. Typical ocular symptoms include those directly associated with the eyes, such as aching, burning, eye pain, itching, pulling sensations, and tiring. These symptoms may also include subjective observations, such as blurred or doubled vision and even slanting or tipping of level surfaces. Visual experiences such as these may or may not be accompanied by any ache, pain, or other discomfort, but they often greatly concern the patient. Ocular symptoms are usually related to the use of the eyes and frequently result from uncorrected ametropia, heterophoria, or aniseikonia. As such, they are often readily relieved by properly prescribed visual correction.
Referred symptoms include dizziness, headaches, nausea, and nervousness. Such symptoms are seldom definitively related to the use of the eyes and, when they exist, a refractive or iseikonic correction is less likely to bring relief. These symptoms, which frequently seem to be of ocular origin, prompt patients to consult with an eye specialist in the hope that the headaches may be due to the eyes and that the eye doctor can “cure” them. However, careful questioning by the clinician may reveal that the headaches are due to other causes, such as allergies or sinus conditions. It is the clinician’s task to determine whether referred symptoms have an ocular etiology or whether the patient should be advised to consult other specialists.
The length of time that symptoms have been present should also be considered. Long-standing symptoms that have been previously investigated without success tend to strongly suggest aniseikonia. This is because a patient with long-standing symptoms has usually sought a variety of treatments. If all previous treatments have been unsuccessful, it tends to rule out other possible causes for the symptoms and increase the chances for aniseikonia.
REFRACTIVE CONDITION
A few patients with equal or no refractive error have aniseikonia and symptoms.19,20 Many of these patients are those who have had unilateral cataract extraction and are pseudophakic in one eye after cataract surgery.21 However, in patients who have not had refractive or cataract surgery, the likelihood of aniseikonia is generally not high unless the patient is anisometropic. Once the anisometropia is corrected, aniseikonia becomes a definite possibility. Aniseikonic symptoms occur infrequently in the presence of uncorrected anisometropia because one of the ocular images is usually so blurred that the patient uses only one eye and aniseikonic symptoms are not present.
CORNEAL CURVATURE
A difference in the corneal power of each eye indicates that at least a portion of the anisometropia is refractive. Generally, corneal shapes are equal and differences are uncommon. However, unequal corneal shapes frequently happen after refractive surgeries, including laser-assisted in situ keratomileusis (LASIK) or photorefractive keratectomy. Postsurgically induced unequal corneal shapes often cause aniseikonia. Thus, it is important during history to question the patient regarding prior refractive surgery treatments. When correcting such patients with spectacles, a difference in image size will result. Astigmatism is virtually always of corneal or, occasionally, lenticular origin. Either type of astigmatism can be considered a refractive problem, in which spectacle correction will result in aniseikonia. In refractive anisometropia, correction of refractive errors with contact lenses rather than spectacle lenses will minimize these differences in image size, especially when the anisometropia is not very high (<6.00 diopters [D]) and the difference in corneal power matches the amount of anisometropia. The other advantage of contact lens correction for anisometropic patients is the reduction in the amount of variable prism power induced by versional eye movements (reduced dynamic aniseikonia).
When prescribing spectacle correction for a patient with anisometropia and equal corneal curvatures, there should be minimal problems with static aniseikonia, provided the lenses do not produce undesired shape magnification. This can be avoided by prescribing equal front curves and center thickness. However, the theory behind equalizing the lens front curves and center thickness assumes that the anisometropia must result from a difference in axial length when the corneal curvatures are equal. Unfortunately, this is not always the case, because the anisometropia may be due to a refractive difference associated with the lens or the back surface of the cornea. Aniseikonia cannot be definitely ruled out simply because the corneal curvatures are equal. Further, the problem of dynamic aniseikonia induced by the spectacle lenses is often a significant deterrent to binocular visual function. In general, these factors suggest that contact lenses should be the initial prescription consideration when there is significant anisometropia.
RETINAL CONDITIONS (FIELD-DEPENDENT ANISEIKONIA)
There are retinal conditions currently being successfully treated (e.g., epiretinal membrane) that could not be effectively treated in the past. These treatments often restore vision, but sometimes either the treatment or the underlying condition causes the photoreceptors of the eye to assume different relative locations than they initially had. This monocular change in retinal photoreceptor location can cause a rather debilitating asymmetric aniseikonia that can also be very large (many times >15%). The patient complains of one image being significantly larger than the other, and the image size difference may vary in different portions of the retina and at different distances. When these patients report an image size difference, there is often not “an aniseikonia” because there is simply no single value for each distance or direction as the problem is field dependent.
When testing patients with field-dependent aniseikonia, it is important to use a test that only measures static aniseikonia. When there are large image size differences (>5% to 7%), this must be done using a direct comparison test (Aniseikonia Inspector) with a short viewing time. An important clinical problem with field-dependent aniseikonia is that it cannot be corrected fully with conventional optics, which exhibit a relatively constant magnification as a function of visual field angle.22 Further, the overall image size is often too great to correct (6% to 7% is about the maximum that can be made in glasses) and there is not an available lens that can provide variable correction across the field.23
Fortunately, optically correcting the aniseikonia for part of the visual field often improves the vision comfort considerably24 (see Case 19.1). In cases where iseikonic correction is not successful, other “solutions” might be helpful; these generally involve making the patient increasingly monocular in successive steps until the symptoms are tolerable. These steps include:
1. Focusing one eye for distance and the other for near. This can be done by simply holding up a plus trial lens over one eye of the best distance correction and seeing if the patient has reduced symptoms. If so, single-vision lenses that focus one eye for distance and the other for near (spectacle monovision) can be considered. If this is not successful, try:
2. Defocusing the poorer-seeing eye for both distance and near. Again, using a trial lens to blur the vision in the poorer-seeing eye, add to the distance correction about -1.50 (for these typically presbyopic patients). This will often blur the central vision enough that the image size symptoms are minimized and, if successful, can be prescribed in bifocal form. If this is unsuccessful, consider:
3. Blocking central vision of one eye (typically the most distorted one). This can be done with a small circle (about 25 mm diameter) of clear contact paper. This type of partial monocular occlusion is often cosmetically and visually acceptable. If not, use:
4. Occlusion of the full field (clear contact paper)
Case 19.1 Use of the Aniseikonia Inspector
A 67-year-old man had a right macular epiretinal membrane 2 years previously. After removal of the membrane, he subsequently had a retinal detachment. After repair of the detachment and cataract surgery with an intraocular lens (IOL) implant in that eye, his primary complaint was blurred vision at distance and near in the right eye, vertical diplopia, and an image size difference, with the right image being about 20% smaller than the left. In addition to the right image being smaller, he also reported that it varied in size from the top of the field to the bottom, presumably a result of asymmetric stretching of retinal elements from the membrane or its removal. Vision with the spectacles was not satisfactory because of vertical diplopia and retinal image size difference. Refractive error was as follows:
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Keratometric readings were as follows:
OD: 43.25 at 173; 43.00 at 083
OS: 44.00 at 001; 43.87 at 091
Measures of eye alignment were 6.5 base-down on the right (a moderately comitant right hypertropia that did not increase significantly on right and left gaze or on right or left head tilt). There was fair sensory fusion with intermittent right suppression (determined using the American Optical [AO] vectographic adult slide).
There was a clear, well-centered posterior chamber IOL in the right eye and early nuclear sclerotic lens changes in the left eye. Interocular pressures were 18 (right eye) and 16 (left eye) as determined by Goldmann applanation tonometry. There was a mottled macula on the right, with a retinal detachment repair and a normal fundus appearance on the left.
Because of the large reported image size difference, the patient was assessed using the Aniseikonia Inspector. The right image was found to be about 16% smaller than the left, with significant variation in response, presumably caused by the variation in retinal elements and instability of the initial correction of the vertical deviation. The problem with his aniseikonia is that it cannot be corrected fully with conventional optics because the total image size is too great and there is variable image size across the vertical meridian. Iseikonic spectacle correction was ordered that maximally increased the right image (only about 7% could be achieved in spectacle correction) and incorporated a total of 6.5 base-down before the right eye (OD, 3.5 base-down; OS, 3.0 base-up).
The patient returned after adapting to the lenses for 3 weeks. He had noticed reduced symptoms during the adaptation period. His acuities were as follows:
OD: 6/7.5 -1
OS: 6/6 -1
At 6 m, there was no suppression. He was fusing well with the prism, and stereopsis was 3 minutes of arc (AO vectographic slide). At 40 cm, he was fusing well and stereopsis was 80 seconds of arc (AO near card series). Repeat assessment with the Aniseikonia Inspector with his new glasses on revealed about 4% residual aniseikonia. In spite of the residual aniseikonia, the patient was happy with his vision, had improved visual comfort for reading, and generally had fewer binocular problems than might be expected with residual aniseikonia of this amount.
When iseikonic correction is not possible and you consider the steps earlier, it is clinically important to involve the patient in the decision making. The patient comes with the intention of getting his or her problem solved (and solved often means back-to-normal as it was before the condition and surgical treatment) and this is often not possible. If necessary, demonstrate to the patient why you cannot make an iseikonic correction (your vision has different image sizes in different places in your vision) and describe the reasons “regular” glasses will not work (there is not a lens available that can correct your full image size difference/there is not a lens that can provide variable magnification across the visual field). Show how “monocular” vision can reduce symptoms (notice how focusing one eye for near/blurring one eye reduces your symptoms) and then work together to determine which type of monocular correction works best. Failure to include the patient in these decisions may result in both the patient and the doctor being unnecessarily frustrated.
OCCLUSION
Occlusion may be useful as an aid in the diagnosis of aniseikonia. If the patient’s symptoms are eliminated by wearing a patch, they are probably due to a binocular problem. Once all other binocular problems have been treated or ruled out, aniseikonia is left as the probable cause of the symptoms.
CLIP-ON ANISEIKONIC CORRECTION—VERIFICATION OF THE RESULTS WITH “SIZE LENSES”
Before designing an iseikonic lens, we recommend verifying, using a clip-on iseikonic correction, whether the patient would actually be helped with aniseikonia correction. The clip-on lenses, which have plano power with magnification based on the combination of front curve and center thickness used in their manufacture, are known as size lenses. When a size lens that equals the aniseikonia measurements reduces the symptoms, aniseikonia is very likely to be the problem. To further test the assumption, the size lens can be placed in front of the other eye. If symptoms are exacerbated, diagnosis is complete. Unfortunately, when a size lens does not help, aniseikonia may still be the problem, because symptoms may result from the weight and additional reflections caused by the clipon lens. Table 19.1 shows the lens thickness and curve combinations for various iseikonic clip-on size lenses.25
MINIMUM AMOUNT OF ANISEIKONIA CORRECTION
Size lens testing is also important when there is retinally induced (field-dependent) aniseikonia because for these patients it may not be clear what amount of aniseikonia correction will be needed to recover visual comfort. Also, when there is a very large aniseikonia (>5%), it is advisable to assess the possible impact of correction on the ultimate lens cosmesis. In these two cases, subjective testing with size lenses can help determine the minimum amount of aniseikonia that needs to be corrected to reduce symptoms. Start with a size lens that will provide a cosmetically acceptable correction and gradually increase the amount until the patient reports a satisfactory reduction in symptoms.
Table 19.1 SPECIFICATIONS FOR PLASTIC SIZE LENSES | ||||||||||||||||||||||||||||||||||||||||||||||||
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Determination of the Presence of Aniseikonia
Several methods can be used to determine whether aniseikonia is present. These include estimation of the image size between the two eyes simultaneously using diplopic images or sequentially using the alternate cover test, Turville testing, or the Maddox rod and double-light technique. Assessment can also be made using tests such as the New Aniseikonia Test, the Aniseikonia Inspector, or space eikonometry. Although each of these techniques has clinical use, the space eikonometer is probably the most accurate and practical. However, few space eikonometers are now available; as a result, the Aniseikonia Inspector is the current test of choice.
SIZE COMPARISON OF DIPLOPIC IMAGES
Comparison of double images is a simple, albeit relatively insensitive, test of the image sizes between the two eyes that can be used to estimate horizontal, vertical, or overall aniseikonia.
1. The patient wears the appropriate spectacle correction and views a square target that is doubled, using vertical prism of about 5 Δ. The target will be seen horizontally displaced if any horizontal heterophoria is present.
2. The patient compares the perceived horizontal extent of the top target with the perceived horizontal extent of the bottom target. A difference suggests horizontal aniseikonia.
3. To estimate the horizontal aniseikonia, a size lens (Table 19.1) is placed in front of the eye with the smallest image. The percentage magnification of the size lens is changed until the two targets appear to be of equal horizontal lengths.
4. The process is repeated for the vertical dimensions.
5. The results are recorded, indicating the magnification needed to equalize the perceived images (e.g., 2.0% on OD horizontal, 1.0% on OS vertical; 1.5% on OD overall).
ALTERNATE COVER TEST
Brecher26 proposed using the alternate cover test to detect and estimate the magnitude of aniseikonia.
1. Have the patient wear the appropriate spectacle correction and fixate a distance square target that is alone in the visual field.
2. Occlude each eye alternately and ask the patient to compare the horizontal size of the target seen with each eye. The cover paddle should be moved quickly between eyes and held for about 1 second in front of each eye to facilitate comparison of the two images.
3. If there is a difference in perceived size, repeat the test with a size lens in front of the eye with the smaller perceived image. Change the size lens until the image seen with each eye appears to be the same size as the cover paddle is alternated.
4. The process is repeated for the vertical dimensions.
5. The results are recorded, indicating the magnification needed in each meridian to equalize the perceived images of the two eyes.
TURVILLE TEST
The Turville test can be used for detecting and measuring aniseikonia in the vertical meridian using the slide with two horizontal lines that Morgan developed27 (Fig. 19.1).
1. Position the septum so that the patient sees the right half of the target with the right eye and the left half of the target with the left while wearing the appropriate spectacle correction.
2. Have the patient compare the vertical separation of the two lines on the right target with the separation of the two lines on the left target. A difference in the perceived vertical separation of the lines on the right side suggests vertical aniseikonia.
3. Aniseikonia can be measured using a size lens in front of the eye with the smallest separation and changing the size lens to equalize the perceived vertical separation of the lines on both sides.
4. This measure of the vertical aniseikonia should be recorded.
MADDOX ROD AND TWO-POINT LIGHT SOURCES
Testing with a Maddox rod and two light sources is performed as follows:
1. Two small light sources are placed about 60 cm away from the patient, with a horizontal separation of about 20 cm. The patient wears the appropriate spectacle correction and views the lights through a Maddox rod in front of only one eye, with the axis at 180 degrees. One eye sees the two light sources, and the other (behind the Maddox rod) sees two vertical luminous lines.
2. Have the patient compare the relative separation of the lights with the relative separation of the luminous streaks. A difference in the separation suggests aniseikonia. Prism can be used to align the light and line on one side if a lateral heterophoria makes the judgment difficult by causing a displacement of the streaks from the light sources.
3. A size lens in front of the eye that perceives the smallest separation (of the lights or streaks) can be used to measure aniseikonia. Change the power of the size lens to equalize the separation between the lights and the streaks.
4. The test can be repeated with the light sources separated vertically and the Maddox rod at axis 90 degrees placed in front of only one eye to determine the presence and measurement of vertical aniseikonia.
5. The size lens that produces the same distance between the lights and the luminous streaks is recorded as a measure of aniseikonia.
THE NEW ANISEIKONIA TEST
The procedure for the New Aniseikonia Test28 is as follows:
1. The patient wears red and green filters over the appropriate spectacle correction.
2. Have the patient compare the red and green half-moons in the booklet (Fig. 19.2) to determine the half-moons that seem to have identical vertical diameters.
3. Rotate the booklet to a horizontal position and repeat the test.
4. The percentage of aniseikonia that is present in each meridian is recorded. This test tends to provide a smaller estimate of the aniseikonia present than the amount measured using a space eikonometer.
 ▪ Figure 19.1 The slide developed by Morgan for Turville testing appears as two parallel horizontal lines emanating from a central vertical line. The patient’s task is to report the relative separation of the horizontal lines on either side of the vertical line. Report of an unequal separation of the lines can be neutralized with a size lens, providing an estimated amount of magnification to prescribe. |
 ▪ Figure 19.2 The New Aniseikonia Test consists of a book with a number of pairs of red/green half-moons. When viewed with red/green glasses, one of the half-moons is seen by the right eye and the other is seen by the left eye. The patient’s task is to determine which pair of targets contains half-moons with the same vertical diameter on each side. This gives an estimate of the amount of magnification to prescribe. |
THE ANISEIKONIA INSPECTOR*
The aniseikonia test of the Aniseikonia Inspector program is based on direct comparison eikonometry2 which, although it tends to underestimate aniseikonia,29,30 can have a sensitivity of less than 0.5%.31 Although underestimation could be a concern, the large range (up to 25%) of the Aniseikonia Inspector31 is often a substantial advantage when testing patients with new-onset aniseikonia caused by conditions such as unilateral aphakia, retinal detachment, and epiretinal membrane (e.g., Case 19.1). Many of these patients have developed significant aniseikonia as a result of treatment to preserve their vision. The aniseikonia is often so large in magnitude that it cannot easily be assessed by other means, including the space eikonometer (which will only accurately measure up to 5% image size difference). This large measurement range and clinical availability (compared with the space eikonometer) makes the Aniseikonia Inspector a very useful clinical test.
When assessing aniseikonia using the Aniseikonia Inspector:
1. The test is performed in dimmed room illumination with the patient sitting 16 in to 2 ft from the computer monitor.
2. The patient looks at the computer monitor screen through red/green glasses so that each eye sees a separate portion of the screen (typically the red lens is in front of the right eye, although the program allows the examiner to determine which eye will have the red lens).
3. The patient’s task is to identify for each presentation which comparison target is perceived as larger. An aniseikonia setting is made by changing the size of one of two half-circles (Version 1) or bars (Version 2 or 3) with keys on the keyboard or with the mouse. Presentations continue until the two targets appear equal in size (Fig. 19.3).
4. The aniseikonia is measured in the vertical, horizontal, and diagonal directions, compensating for fixation disparity if needed. In each direction, the measurement is done twice, once starting with -25% preset aniseikonia and once starting with +25% preset aniseikonia.
5. The average of the two measurements is taken as the aniseikonia value and the total amount of aniseikonia is automatically determined by computer calculations of the results.
6. The potential aniseikonia correction can be simulated by the computer to verify that the patient’s symptoms are reduced. The aniseikonia correction can also be manipulated to refine the correction for maximal patient satisfaction.
This test tends to provide a smaller estimate of the aniseikonia present than the amount measured using a space eikonometer.30
 ▪ Figure 19.3 The Aniseikonia Inspector is a computer program with either pairs of red/green halfmoons (Version 1) or red/green bars (Version 2). When viewed with red/green glasses, one of the half-moons (bars) is seen by the right eye and the other is seen by the left eye. The patient uses the keyboard or mouse to make the targets have the same dimension on each side. The task is done twice in each of three meridians (vertical, horizontal, and oblique), and the average of each set of measures gives the aniseikonia in that direction. The computer program can then be used to calculate the overall amount of magnification to prescribe. |
THE SPACE EIKONOMETER
The most accurate prescribing for aniseikonia is based on measurement of the image size differences, rather than guessing at the amount of aniseikonia present. From a clinical standpoint, measurement of aniseikonia provides the best means of determining whether a patient’s symptoms are related to aniseikonia. Very accurate measurements of up to 5% image size difference can be made using the space eikonometer. Using an eikonometer also facilitates prescription of noniseikonic corrections. For example, practitioners are sometimes reluctant to prescribe an anisometropic correction that might induce aniseikonia. However, the full correction of significant spherical anisometropia does not always result in significant aniseikonia. A solution to this dilemma is to use an eikonometer more frequently when aniseikonia is suspected. When the patient’s response to the refractive correction is measured with the eikonometer, it is often a pleasant surprise to find little or no aniseikonia. Evaluating the patient’s responses to a tentative refractive correction with the eikonometer also allows the clinician to modify the refractive correction to minimize induced aniseikonia when the visual needs of the patient permit the sacrifice of optimum acuity and binocular function for the sake of comfort or expense.
The space eikonometer is extremely accurate—perhaps the most accurate clinical measurement of a binocular function. The test is also soundly based in physiologic optics research on single binocular vision and stereopsis. The space eikonometer is no longer currently available for purchase as a new instrument. However, a substantial number of used eikonometers are available, with an increase in the numbers as practitioners retire. Because this instrument is still available at times, and many clinicians are not familiar with its use, we have included this short section on use of the eikonometer so that the practitioner who acquires one can more easily become acquainted with the idiosyncrasies of using it to measure aniseikonia.
Target
The target of the space eikonometer appears as two bright white (or yellowish) vertical lines behind a red cross with two dull green vertical lines in front of the cross (Fig. 19.4). The appearance of this target is varied by altering the positions of the controlling levers. The ×90 lever at the top of the instrument (Fig. 19.5) moves the position of the left side of the outside lines closer for right eye magnification or farther for left eye magnification. The ×180 lever moves the relative position of the right sides of the red cross closer for right eye magnification or farther for left eye magnification. The declination wheel rotates the top of the red cross toward the observer for plus settings and away for minus settings. The clinician should familiarize himself or herself with the phenomena described in this section by looking into the instrument both monocularly and binocularly.
 ▪ Figure 19.4 The space eikonometer target appears as two bright white (or yellowish) vertical lines behind a red cross with two dull green vertical lines in front of the cross. The patient’s task during measurement of aniseikonia with the eikonometer is to report the relative positions of the line targets as changes in the magnifications of each are achieved by moving the controlling levers. The test is complete when the patient reports that all portions of the target are equidistant. |
The patient’s task during measurement of aniseikonia with the eikonometer is to report the relative positions of the line targets as changes in the relative magnifications of each are made by moving the controlling levers. The test is complete when the patient reports that all portions of the target are equidistant.
Making Eikonometer Settings
Position the patient comfortably in front of the eikonometer, with the refractive correction in place and the interpupillary distance set on the instrument (Fig. 19.5). Have the patient observe the target and report the position of the lines when all settings are on zero. Call attention to the outer lines first. By using a bracketing technique, the ×90 wheel can be moved until the outer lines are seen equidistantly. Extinguishing visibility of the targets with the micro switch (Fig. 19.5) between changes of the lever setting allows for more accurate findings. The measurement procedure is then repeated for the sides of the red cross, using the ×180 lever, and for the tilt of the red cross, using the declination wheel. The method of limits is applied for determining the final location of the levers and declination wheel. The sensitivity of the patient to the three parts of the test is estimated by one-half the range within which alignment of the targets is reported. The manual provided with the instrument provides more detail concerning the testing procedure for routine cases.
 ▪ Figure 19.5 The appearance of the space eikonometer target is determined by altering the positions of the controlling levers. The ×90 lever at the top of the instrument moves the position of the left side of the outside lines closer for right eye magnification or farther for left eye magnification. The ×180 lever moves the relative position of the right side of the red cross closer for right eye magnification or farther for left eye magnification. The declination wheel rotates the top of the red cross toward the observer for plus settings and away for minus settings. Between changes in the controlling levers, visibility of the target is extinguished by using the switch at the bottom of the eikonometer. |
Determining Image Size
After the settings have been determined, the aniseikonic correction is obtained from the three measurements (×90, ×180, and declination) by using magnification tables. The decision to prescribe a full, partial, or no aniseikonic correction is based on the measurements and professional judgment regarding the availability of the correction, the cost, and the likelihood that the patient will have successful relief of symptoms. These points will be discussed further in a later section. Next, however, we discuss some of the difficulties experienced during aniseikonic examination with the eikonometer.
Aniseikonic Examination Difficulties
Monocular Suppression
Patients who lack sufficient stereopsis to respond during eikonometry report that the space eikonometer target appears flat. Other indications of monocular suppression are patient reports that the bright white (yellowish) vertical lines appear in front of the red cross or that the dull green vertical lines appear behind the red cross. These reports indicate that the patient is using the monocular clue of brightness, rather than stereopsis, to evaluate the appearance of the target.
If there is a question as to whether an eye is being suppressed, have the patient observe the target with one eye and then the other. With the right eye, the two right vertical lines are closer together than the two left vertical lines. With the left eye, the left vertical lines appear closer together than the right lines. Thus, with both eyes open, the suppressing eye can be determined by asking the patient whether the right or left vertical lines appear closer together.
Heterophoria
For some patients with good fusion and stereopsis, the space eikonometer test indicates the presence of small amounts of heterophoria, most frequently hyperphoria. When there is even as little as 0.5 Δ uncorrected hyperphoria, one of the oblique lines of the cross in the target often appears in front of the other (Fig. 19.6). If this observation is reported, fixation disparity testing should be done to determine the vertical prism required to reduce the fixation disparity to zero (Chapter 15). Then, determine (by the method of bracketing) that the prism ensures exact coincidence of the oblique lines. This is done by placing a 0.5 Δ prism alternately base-down and then base-up before one eye. If there is no remaining hyperphoria, one oblique line will appear in front of the other (and vice versa) as the prism is flipped from base-down to base-up. The proper power prism (it may require more than 0.5 Δ) is then placed in a trial frame and the aniseikonic test is continued.
Management
Prescribing iseikonic corrections requires clinical judgment similar to that used when prescribing refractive and prismatic corrections. Factors to be considered include the age of the patient, the nature of the previous corrections and the patient’s reaction to them, the type of work and hobbies done by the patient, the patient’s
temperament and concern about the appearance and expense of the correction, and, above all, the nature of the symptoms and the likelihood of their elimination or reduction by iseikonic correction.
temperament and concern about the appearance and expense of the correction, and, above all, the nature of the symptoms and the likelihood of their elimination or reduction by iseikonic correction.
 ▪ Figure 19.6 When there is an uncorrected hyperphoria, the space eikonometer target may appear to be distorted, and one limb of the cross will tilt toward the observer. Reports of this phenomenon should alert the examiner to place small amounts of vertical prism in front of the hyperphoric eye to restore the perception of a symmetrical cross so that more accurate judgments of the position of the cross can be made. |
PRACTICAL CONSIDERATIONS
In designing iseikonic prescriptions, it is easy to allow the desire to solve the optical problem to overshadow the problems that the patient may have with the correction being prescribed. Remember the primary complaint of the patient and attempt to solve that problem without creating a new one. A prescription that is optically correct might be considered unwearable by the patient. In certain instances, it is preferable not to prescribe the full refractive findings, but rather to modify them instead of ordering bitoric lenses. A slight change in cylinder axis or power may only reduce the acuity slightly and be preferable to an expensive prescription with unacceptable appearance or weight. In almost all instances, common sense dictates use of the simplest solution and determines the difference between success and failure of management.
LENS PRESCRIPTION
Although there are no hard and fast rules for prescribing aniseikonic corrections, we recommend considering the following factors when deciding whether or not to recommend an iseikonic correction.
Factors that suggest not prescribing include the following:
Inconsistent or variable measurements of the size difference on repeated trials
Poor depth perception
Aniseikonia in reverse to that expected from the anisometropia
Symptoms that are not related to use of the eyes or have not been improved by refractive or heterophoric corrections
A patient who is comfortable, even with a significant aniseikonia. This can occur if a partial refractive correction for one eye has been worn for several years
Factors that suggest prescribing include the following:
Aniseikonia that can be measured with a sensitivity smaller than the size difference measured (e.g., 1.0% ± 0.50% rather than 0.75% ± 1.5%)
Definite symptoms related to the use of the eyes
Relief of symptoms with monocular occlusion when there is no significant lateral or vertical heterophoria
Improvement in symptoms while wearing a temporary size lens clip-on for 1 to 2 days
Anisometropia, where the full refractive correction causes (or is likely to cause) discomfort
Failure of other corrections to provide relief of the symptoms
PRESCRIPTION DECISIONS REGARDING PATIENTS WITH ANISEIKONIA
The issues in designing aniseikonic corrections include two slightly different points of view—eliminating all estimated magnification difference between the two eyes or eliminating all measured magnification difference between the two eyes. Both of these philosophies have merit, and the fact that each works clinically indicates that iseikonic lens design is often not an exact science and is frequently more of an art of patient management.
Estimated Magnification Prescriptions
When prescribing to eliminate all estimated magnification difference between the two eyes,32 the actual aniseikonia is generally not measured, but it is assumed to be related to the difference in the spectacle magnification of the lenses. In practice, this technique is often used when instruments, such as the Aniseikonia Inspector or space eikonometer, are not available. Clinically, practitioners who prescribe using this philosophy generally choose to prescribe less magnification than that which would be expected to reduce the magnification difference between the lenses to zero. Typically, around 1.0% per diopter of anisometropia is used to estimate the amount of magnification to prescribe. Unfortunately, this technique tends to be less accurate when prescribing for patients with anisometropic myopia. In general, however, prescriptions designed from this philosophy contain slightly more magnification than those prescribed based on actual measures of the aniseikonia.
Measured Aniseikonia Prescriptions
When the aniseikonia present is measured with, for example, the Aniseikonia Inspector or a space eikonometer, a prescription can be designed that eliminates the measured magnification difference between the two eyes. In this technique, measurements are usually taken through a patient’s best spectacle correction. From knowledge of the parameters of the old spectacle correction (eyewire [vertex] distance, front curve, thickness, and index of
refraction), an iseikonic correction is designed by altering these parameters to reduce the measured magnification difference to zero. As long as aniseikonia can be measured, this technique is equally satisfactory for both anisometropic hyperopia and anisometropic myopia. Prescriptions that are designed based on this philosophy usually have slightly less magnification than those that are based on estimates derived from the calculation of possible aniseikonia from differences in spectacle lens power. In most cases, as a rule of thumb, aniseikonia remaining after correction should be kept smaller than 1% to 3%, if possible.
refraction), an iseikonic correction is designed by altering these parameters to reduce the measured magnification difference to zero. As long as aniseikonia can be measured, this technique is equally satisfactory for both anisometropic hyperopia and anisometropic myopia. Prescriptions that are designed based on this philosophy usually have slightly less magnification than those that are based on estimates derived from the calculation of possible aniseikonia from differences in spectacle lens power. In most cases, as a rule of thumb, aniseikonia remaining after correction should be kept smaller than 1% to 3%, if possible.
Iseikonic Lens Design
Regardless of the philosophy of prescribing for patients with aniseikonia, the iseikonic correction must be correctly designed. Iseikonic lens design can be accomplished in one of two ways.
ANISEIKONIA INSPECTOR
Most preferable by far is to use the Aniseikonia Inspector where calculations for designing the iseikonic prescription are done interactively by computer.33 Calculations use all lens parameters that affect the magnification of spectacle lenses (e.g., eyewire [vertex] distance, front curve, thickness, and index of refraction) and each variable can be freely adjusted using slider bars. The resultant aniseikonia (as well as optically induced anisophoria) is shown in real time when changing the lens parameters. Additionally, images of the lenses and graphs of the edge thickness that are provided help determine the cosmesis of the proposed correction and whether the lenses design will fit in the frame.
TRANSLATION
The process of designing lenses to correct aniseikonia, combined with refractive correction, is known as translation. When prescribing to eliminate an estimated difference in magnification, simply decide how much residual magnification you wish to leave and design a lens that achieves this requirement. If aniseikonia is measured, it is time consuming, but not difficult, using the tables, to design an iseikonic correction that reduces the measured difference to zero. The issue of dynamic and static aniseikonia is also not a problem when the aniseikonia is measured and the measurement includes both static and dynamic components of aniseikonia.
When designing lenses to correct aniseikonia, changes in the dimensions of the patient’s current spectacle lenses (front curve, thickness, and position from the eye) can be made to introduce the desired iseikonic correction. This procedure makes it unnecessary to consider the magnification properties of trial lenses and simplifies the design of iseikonic lenses. When patients are already wearing spectacle lenses, the only thing that has to be determined is the amount of magnification needed.
After iseikonic lens design, but before lens fabrication, two steps should be tried. Each of these has merit and together they eliminate the need for many iseikonic corrections. First, prescribe contact lenses whenever possible. Often a contact lens correction that eliminates the problem of dynamic aniseikonia will allow comfortable binocular vision in the presence of a moderate to large amount of static aniseikonia. The success of this premise is illustrated by Case 19.2.
Case 19.2 Contact Lensesto Treat Aniseikonia
A 39-year-old woman had radial keratotomy (RK) 2 years previously on her right eye. She did not have the left eye done because of displeasure with the visual results of the first procedure. Her primary complaint was blurred vision at distance and near. She had been about 1 D myopic before RK and had worn spectacles occasionally. She had never worn contact lenses. She had post-RK spectacles, but had broken them. Vision with the spectacles was not satisfactory, because of frequent fluctuations of vision. Refractive error was as follows:
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Keratometric readings were as follows:
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Because of the large anisometropia, contact lenses were advised. The contact lens for the right eye was fitted to vault the RK-flattened (30.00 D) central cornea, using a front curve of 9.0 mm (38.50 D). This resulted in a high plus refractive tear film, which, combined with the contact lens, corrected the refractive error. The power ordered was determined by trial fitting and overrefraction, as is frequently necessary because of the unreliability of post-RK central keratometry readings.
The patient returned after adapting to the lenses for 3 weeks. She had no trouble during the adaptation period. With the contact lenses, her acuities were as follows:
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At 6 m, there was no suppression. She was orthophoric, and stereopsis was 2 minutes of arc. At 40 cm, she was orthophoric and stereopsis was 80 seconds of arc (with the AO vectographic slide and near card series). Space eikonometry with her contact lenses in place revealed little or no aniseikonia, in spite of the anisometropia. She was happy with her vision and had none of the binocular problems that might be expected with anisometropia of this amount. She was advised to continue to increase her wearing time and return in 1 month or as needed.
When a patient either does not want to, or cannot, wear contact lenses, judicious changes in the prescription may reduce the potential problem. Thus, our second recommendation is to consider small axis or power alterations for older patients who might be expected to have difficulties with space perception or the moderate changes in astigmatism axis or power that frequently occur. Modification of the correction, when needed, will minimize patient dissatisfaction with a new correction that causes perceptual distortion. Such modifications often alleviate the need to prescribe bitoric iseikonic bifocal lens. Case 19.3 illustrates these tenets.
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