INTRODUCTION

Aniseikonia occurs when images from the 2 eyes are perceived as being different in size or shape.1–7 The relationship between aniseikonia and anisometropia, known as optically induced aniseikonia, is well known and understood among eyecare professionals. However, when retinally induced, aniseikonia commonly goes underrecognized and underdiagnosed. Because of this, the true prevalence of optically and retinally induced aniseikonia remains unknown. Without appropriate diagnosis and management, patients with aniseikonia are left not fully understanding their condition and suffer from symptoms that negatively impact their quality of life every day.

Common symptoms associated with aniseikonia include double vision, nausea, distorted spatial perception, headache, dizziness, visual discomfort, and eyestrain. Individuals with either optically and/or retinally induced aniseikonia may experience these symptoms. Refractive lenses used to correct for anisometropia are the cause of optically induced aniseikonia, whereas mechanical forces at the level of the retina are the presumed cause of retinally induced aniseikonia. The most reported retinal conditions associated with aniseikonia include epiretinal membranes and reattached retinal detachments. An individual may also experience a combination of both optically and retinally induced aniseikonia. Regardless of the cause, proper evaluation is needed for the diagnosis and treatment of aniseikonia.

The purpose of this case series is to describe the assessment and management of 3 cases of optically and/or retinally induced aniseikonia. Important elements in each assessment include a thorough ocular history, refraction, binocular vision evaluation, image size testing, and afocal magnification trialing. Each patient in this case series perceived benefit from trialing afocal magnification lenses, and spectacles with an eikonic lens design were prescribed as treatment. No identifiable health information was included in this case report.

CASE REPORTS

Case 1

A 73-year-old White man presented for a new patient aniseikonia evaluation in 2023, referred by his comprehensive eyecare provider because of his significant anisometropia. His chief concern was that he did not see well out of his left eye. His symptoms started in 2017 after having cataract surgery with trabectome in the left eye that resulted in a postoperative large cyclodialysis cleft with hypotony maculopathy. In 2019, the cyclodialysis cleft was repaired and pressures were controlled with a trabeculectomy. The patient was phakic in the right eye, and intraocular pressures were controlled with 1 drop of COMBIGAN (brimonidine tartrate/timolol maleate ophthalmic solution) twice a day and 1 drop of LUMIGAN (bimatoprost ophthalmic solution) at bedtime in the right eye.

Best-corrected visual acuities were 20/20 in the right eye and 20/40+2 in the left eye with a high myopic refractive error of -7.00 -1.75 ×125 in the right eye and a moderate hyperopic refractive error in the left eye of +3.25 -2.25 ×075. The extraocular muscle movements were smooth, accurate, full, and extensive for both eyes. The cover test was orthophoria at distance and near. The patient had no global or local stereo acuity with Randot Stereotest. Using the Awaya New Aniseikonia Test, the patient reported both semicircles were equal when the image of the right semicircle was increased by 7% presented horizontally and vertically at 40 cm. The patient reported having improved comfort and vision with a 4% afocal magnification trial lens over the right eye.

This patient had significant anisometropia because of having 1 pseudophakic eye with complications and 1 phakic eye. After experiencing complications, the patient did not want to proceed with cataract surgery in the phakic eye. He was diagnosed with optically induced aniseikonia because of the difference in refractive error between the eyes; however, he may also have a retinally induced component due to his history of hypotony maculopathy. Benefit of treatment was confirmed using trial afocal magnification lenses, and the patient expressed interest in spectacles with 4% magnification in the right eye using an eikonic lens design. Prescribed parameters included an 8.0-mm center thickness and 8.50 D base curve for the right eye and a 3.5-mm center thickness and 5.00 D base curve for the left eye with polycarbonate lens material.

Case 2

A 71-year-old White man presented for a new patient aniseikonia evaluation in 2023 and complained that his right eye saw images larger than his left eye. He also complained of headache, eyestrain, and pain around the eyes. He reported that symptoms started in 2022 after having cataract surgery. The patient reported having a history of monovision LASIK in 2004; however, he stated that intraocular lenses used for cataract surgery were corrected for distance in both eyes.

Best-corrected visual acuities were 20/20 in the right eye and 20/20 in the left eye with mild hyperopic refractive errors of +1.00 -0.75 ×150 in the right eye and +0.25 -0.75 ×060 in the left eye. The extraocular muscle movements were smooth, accurate, full, and extensive for both eyes. The cover test was 4 prism diopters exophoria at distance and 12 prism diopters exophoria at near. The patient had 20 arc seconds local and 250 arc seconds global stereopsis with Randot Stereotest. Using the Awaya New Aniseikonia Test, the patient reported both semicircles were equal when the image of the left semicircle was increased by 4% presented horizontally and vertically at 40 cm. The patient reported having improved comfort and vision with a 3% to 4% afocal magnification trial lens over the left eye.

Although the patient’s refractive error appeared similar in both eyes, to correct the patient for distance viewing, intraocular lenses of different powers were used during cataract surgery because of the patient’s history of monovision LASIK. The patient was diagnosed with optically induced aniseikonia. Benefit of treatment was confirmed using trial afocal magnification lenses, and the patient expressed interest in spectacles with 3.5% magnification in the left eye using an eikonic lens design. Prescribed parameters included a 2.0-mm center thickness and 4.00 D base curve for the right eye and a 7.0-mm center thickness and 8.00 D base curve for the left eye with polycarbonate lens material.

Case 3

A 62-year-old White man presented for a new patient aniseikonia evaluation in 2024, referred by his comprehensive eyecare provider. The patient complained of discomfort and difficulty using his eyes together while viewing through his most recent subjective refraction in a trial frame at his last eye examination. He reported having a history of retinal detachment and repair with scleral buckle in his left eye 13 months before the examination. He also reported that he had never worn glasses for distance and only uses over-the-counter readers for near.

Best-corrected visual acuities were 20/20 in the right eye and 20/40+1 in the left eye. The patient was plano sphere in the right eye and had a moderate myopic refractive error in the left eye of -2.25 -0.50 ×007. The extraocular muscle movements were smooth, accurate, full, and extensive for both eyes. The cover test was orthophoria at distance and 2 prism diopters exophoria at near. The patient had no global or local stereo acuity with Randot Stereotest. Using the Awaya New Aniseikonia Test, the patient reported both semicircles were equal when the image of the left semicircle was increased by 12% presented horizontally and vertically at 40 cm. The patient reported having improved comfort and vision with a 5% afocal magnification trial lens over the left eye.

Historical records revealed that before the retinal detachment and repair, both eyes were emmetropic with uncorrected visual acuities of 20/20. The patient was diagnosed with both optically and retinally induced aniseikonia. Benefit of treatment was confirmed using trial afocal magnification lenses, and the patient expressed interest in spectacles with 5% magnification in the left eye using an eikonic lens design. Prescribed parameters included a 2.0-mm center thickness and 2.00 D base curve for the right eye and a 7.5-mm center thickness and 9.75 D base curve for the left eye with polycarbonate lens material.

DISCUSSION

Patients with aniseikonia can present with a variety of symptoms, and not all patients with the same degree of aniseikonia respond or experience symptoms in a similar way. The exact etiology is not well researched or well understood; however, aniseikonia has been associated with several ocular conditions over the years. It is also common for aniseikonia to be accompanied by other conditions that cause barriers to good clarity of vision and normal binocularity. All these complexities contribute to why aniseikonia often goes underdiagnosed and why the true prevalence of aniseikonia remains unknown.

There are 2 general types of aniseikonia: optically induced aniseikonia and retinally induced aniseikonia. Optically induced aniseikonia is the more commonly recognized type and is due to anisometropia. Corrective refractive lenses of different powers form images of different sizes on the 2 retinas. Retinally induced aniseikonia, the less commonly recognized type, is presumed to be due to mechanical forces at the level of the retina. If the retina is compressed, the image of an object can stimulate more photoreceptors, causing the image to be perceived as larger. If the retina is stretched, the image of an object can stimulate fewer photoreceptors, causing the image to be perceived as smaller.1,3,6,8

In this case series, the patient in case 1 had a high myopic refractive error in the right eye and a moderate hyperopic refractive error in the left eye after having cataract surgery with complications. His corrective lenses caused minification in the right eye and magnification in the left eye. In cases of optically induced aniseikonia, there is a general rule of thumb that 1.00 D of anisometropia roughly equals a 1.0 % magnification difference between the eyes, although this is thought to be a gross overestimation.8,9 Using this rule with the patient’s refractive error, it would estimate the patient experiencing a 10% magnification difference between the eyes. When measured in the office, the patient reported a 7% magnification difference. It is possible there is also a retinal component of the patient’s aniseikonia because of his history of hypotony maculopathy and potential displacement of photoreceptors.

Although the patient in case 2 presented with similar refractive error between the eyes after cataract surgery, he reported having a history of monovision LASIK. Before having monovision LASIK, the patient reported he had an equal moderate myopic prescription in both eyes of “around -2.75 D.” He stated the monovision LASIK corrected his right eye for distance and left eye for near. The patient obtained records from his cataract surgeon and reported his intraocular lenses were the RxSight Light Adjustable Lens with a power of +18.50 D in the right eye and +15.00 D in the left eye. The patient was very happy with his treatment of 3.5% magnification in the left eye and ordered multiple pairs of spectacles with an eikonic lens design.

An example of both optically and retinally induced aniseikonia is shown in case 3. It is well known and often seen clinically that scleral buckles cause myopic refractive shifts. This patient went from being emmetropic to having a refractive error of -2.25 D in the left eye. However, when measured in the office, the patient reported a 12% magnification difference. This is much greater than what would be estimated using the 1:1 ratio rule of thumb. It is presumed that this is because the repair of the retinal detachment using the scleral buckle resulted in stretching of the retina, causing micropsia, or the perception of minification.

Retinal conditions that have also been reported to cause minification include macular edema and macular holes. Traction of the retina such as with epiretinal membranes cause macropsia or perception of magnification. Retinal conditions that have been associated with aniseikonia include vitreoretinal traction, macular trauma, retinoschisis, and central serous chorioretinopathy.2,3,5,6 Other than differences in size perception, patients with aniseikonia may also experience symptoms of double vision, headache, visual discomfort, eye strain, nausea, dizziness, and distorted spatial perception.8 Aniseikonia is also commonly accompanied by conditions causing additional barriers to normal binocular vision such as reduced visual acuity, heterophorias, strabismus, and metamorphopsia.1,5

When assessing aniseikonia, it is important not only to measure the image size difference between the eyes but also to perform a thorough ocular history, refraction, and binocular vision evaluation. In all 3 cases in this case series, having a good understanding of the ocular and surgical history played an important role in diagnosis. All patients received a final spectacle prescription with an eikonic lens design and refraction was required. However, having a refraction also gave us a better understanding of the degree of aniseikonia caused by the patient’s refractive error versus disruption to the retina. Symptoms associated with aniseikonia are also very similar to those of common strabismic and nonstrabismic binocular vision disorders. A thorough binocular vision evaluation is needed to rule out any differential or concomitant diagnoses.

Because of the lack of awareness and demand for image size testing, there are very few products currently manufactured and available for purchase to evaluate aniseikonia. In this case series, image size differences were measured using the Awaya New Aniseikonia Test. This is a booklet that presents 2 adjacent red/green semicircles to the patient, with 1 side increasing in 1% size increments up to 24%. The patient performs the test dissociated while wearing red/green glasses and reports when the semicircles are perceived as being equal in size. Afocal magnification trial lenses were used to determine if symptoms could be improved by prescribing an eikonic lens design. Afocal magnification lenses are lenses with no dioptric power; however, they have differing base curves and center thicknesses to create magnification. All patients in this case series reported a visual benefit using the magnification lenses, and eikonic lens designs were prescribed.

Treatment of aniseikonia depends on the individual’s symptoms. For all patients in this case series, the goal was to decrease the perceived image size difference between the 2 eyes. This was achieved by using an eikonic lens design. An eikonic lens, also known as an iseikonic lens or a size lens, allows for an increase in magnification of the smaller perceived image without altering the power needed to correct the patient’s refractive error. This is accomplished by altering the shape factor or the afocal components of the lens. The shape factor includes center thickness, refractive index, and front curvature of the lens.7 The total amount of magnification achieved is the difference in magnification between the 2 eyes. A shape magnification nomograph, which is a visual representation of the relationship between front curve, center thickness, and magnification, can be used to help determine parameters. Calculators have also been developed and are available for use online. Unfortunately, because of the complexity of the lens design, excessive lens thickness, and base curve required, it is difficult to create a lens with greater than 5% magnification.1

CONCLUSION

Although diagnosing this condition has lost popularity over the years, aniseikonia has not disappeared and is likely more prevalent than previously reported. Additional literature, increased awareness, and appropriate diagnosis are needed for better understanding of the epidemiology and etiology of aniseikonia. It is important to treat patients with measurable image size differences as symptoms can cause a significant impact on vision and quality of life.

TAKE HOME POINTS

  • Aniseikonia can be optically induced due to anisometropia and/or retinally induced due to mechanical forces at the level of the retina.

  • Symptoms associated with aniseikonia include double vision, headache, eyestrain, eye pain, eye fatigue, distorted spatial perception, and dizziness.

  • Image size difference between the eyes can be measured using the Awaya New Aniseikonia Test, and benefit of treatment can be confirmed using afocal magnification trial lenses.

  • Spectacles with an eikonic lens design can be prescribed to correct for the patient’s refractive error and to decrease the magnification difference between the 2 eyes.

ACKNOWLEDGMENTS

Thank you to the Merton C. Flom Leadership Academy and especially Denise Goodwin, OD, FAAO, for her mentorship.