Aniseikonia

What is Aniseikonia?

Aniseikonia is a term derived from Greek words meaning “not,” “equal,” and “image.” It happens when there’s a difference in the size or shape of an image as the person perceives it. This often occurs due to a condition called anisometropia, a difference in vision between the two eyes. If anisometropia causes aniseikonia, it’s referred to as optical aniseikonia. Changes in the structure and location of the macula in the eye can also cause aniseikonia, which is then called retinal aniseikonia.

Aniseikonia in young people can result in a condition called amblyopia, or lazy eye. In older age, it may cause a variety of symptoms including headaches, double vision, dizziness, nervousness, imbalance, nausea, difficulty tolerating glasses, eye suppression, and distorted perception of space. Usually, symptoms start to appear when aniseikonia is more than 0.75%, and when it’s more than 5%, it can disrupt binocular vision, the ability to merge the two images from each eye into one image.

In many cases, the existence of optical aniseikonia is predicted by determining the difference in the magnification of the lenses in glasses for each eye. A general rule assumes that every 0.25 diopter of difference in vision between eyes causes 0.25% to 0.5% of aniseikonia. But, studies have shown that actual measurements are more accurate than this rule.

Aniseikonia can be classified into symmetrical or asymmetrical forms. Symmetrical aniseikonia is further divided into spherical (a difference in overall magnification) and cylindrical (a difference in magnification in one direction). Asymmetrical aniseikonia, also referred to as distortion, happens when changes in perceived image size are uneven across different parts of the visual field.

Different types of distortion include prismatic, pincushion, barrel, and oblique distortion. These refer to variations in the perception of image size in one direction, stretching of corners of an image, squeezing corners inward, and rotation of one image relative to the other, respectively. Aniseikonia is most accurately evaluated using a device called an eikonometer, but such equipment is not available in most clinics. Other methods using minimal equipment can be done in regular eye clinics. These tests make use of various techniques to separate the two images perceived by the eyes and compare their shape and size difference.

Aniseikonia only occurs when both eyes are working together and doesn’t happen if vision is suppressed or impaired. This can be the case with certain eye conditions like strabismus (cross-eyed) and amblyopia (lazy eye), or certain eye diseases. According to a principle known as Knapp’s law, anisometropia doesn’t cause aniseikonia in specific situations. However, the applications of this law are limited in practical terms.

The best way to treat aniseikonia is to address its root cause. This could include surgical or medical treatment, corrective lens surgery, contact lenses, or specialized glasses with iseikonic lenses. If these options are not feasible, the only solution may be to block the vision in one eye.

What Causes Aniseikonia?

Optical Aniseikonia is a condition where there’s a difference in the size or shape of the images seen by each eye. This difference can be caused by different refractive errors in each eye. For example, when one eye sees a different size of the object vs. the other, it’s called anisometropia. When the shape of the object seems different to each eye due to irregular focus, it’s called aniso-astigmatism.

Eyesight issues, like astigmatism, can make objects appear squished or stretched out. Different aspects of the eye or glasses, like the lens thickness, curve, or power, can also affect the size and shape of the images seen, causing Optical Aniseikonia. Changes in the eye’s cornea, lens, and length, as well as what fills the eyeball, can also cause these visual discrepancies.

Prescription glasses can also change the shape of objects. For instance, very strong glasses can make objects appear squeezed or stretched at the edges. This effect can also lead to Optical Aniseikonia if the distortion is different in each eye.

There are several possible causes for Optical Aniseikonia, including:

1. Conditions of the cornea like Keratoconus, edema, or various diseases caused by injury, inflammation, or infection. Various corneal surgeries also might leave patients with anisometropia.
2. Issues related to the lens, like cataracts, blood sugar fluctuations, and certain medications, may cause an increase in refractive error and hence aniseikonia. Unilateral lens removal and lens dislocation due to diseases like Marfan syndrome can also cause anileikonia.
3. Changes in the eyeball’s length due to birth differences, genetics, and strabismus (crossed eyes) can also cause aniseikonia, as can surgeries that change the eyeball’s length.
4. Retinal Aniseikonia, caused by distortion of the back of the eye (the macula). This can be due to swelling and shrinkage of the macula, caused by conditions like macular edema, retinal dragging, scarring, inflammation, and infection.
5. Changes in brain functions can also lead to distortions in vision, as seen in Alice in Wonderland syndrome and Charles Bonnet syndrome, though these are not due to aniseikonia as such conditions generally affect both eyes.

It’s important to diagnose and treat Aniseikonia, especially in children, to prevent it from causing amblyopia, also known as lazy eye.

Risk Factors and Frequency for Aniseikonia

Aniseikonia, a condition related to vision, isn’t typically measured in the everyday population. A study conducted in a hospital showed that 7.8% of patients with complaints about their binocular vision had some measure of aniseikonia, with 4% having vision disruption. Orthoptists (eye professionals in the U.S.) usually evaluate aniseikonia, but treatments are usually restricted to prescribing contact lenses or blocking vision in one eye.

Specific research has been conducted on patients after they’ve had a scleral buckle procedure (an operation to fix a detached retina). This showed that aniseikonia appeared in 6% of such patients, with persistent aniseikonia being experienced by only 1%.

Aniseikonia is also often observed after someone has had cataract surgery in one eye. Second-eye surgery is performed in roughly 70% of cases, leaving 30% of such patients potentially at risk of developing aniseikonia. In fact, one study found that 40% of all patients with cataract surgery in one or both eyes showed signs of aniseikonia.

• Anisometropia, a principal cause of aniseikonia, happens when there is a difference of more than 1 diopter in the refractive error of the eyes.
• One research showed prevalences of 1.96% at six months, 1.27% at five years, and 5.77% for individuals aged 12 to 15 years.

Aniso-astigmatism, where the astigmatism amount is over 1 diopter, can also occur even when there are relatively low amounts of astigmatism in each eye, but one eye follows a certain rule and the other eye doesn’t.

• A study found aniso-astigmatism was 7.7% in children aged 6 to 7 years and 5.6% in children aged 12 to 13 years.

According to the Sydney Myopia Study, among 6-year-olds, anisometropia and aniso-astigmatism were more prevalent in nearsighted children and generally associated with other conditions such as amblyopia, exotropia, low birth weight, and an older mother’s age. Anisometropia was directly related to increased axial length, and aniso-astigmatism was linked to higher corneal astigmatism. These conditions don’t have a specific racial or sex preference.

Further conditions including congenital glaucoma and keratoconus were also found to have strong ties to anisometropia.

• 31% of patients with congenital glaucoma had two or more diopters of anisometropia.
• Keratoconus was strongly associated with anisometropia, with a study citing a significantly increased risk (5.5-fold) of having 1 diopter or more of anisometropia in keratoconus patients.

Signs and Symptoms of Aniseikonia

To identify aniseikonia, a condition that affects vision, doctors will ask the patient about several symptoms. These may include eye strain, headaches, double vision, dizziness, nervousness, imbalance, nausea, discomfort with glasses, or issues with how they perceive space. Any history of eye surgery or disease is also important. The doctor can evaluate these symptoms and the patient’s glasses prescription to figure out if the person has anisometropia, a condition where each eye has a different visual power.

Several tests will also be carried out. These include:

• Binocular testing to check eye movement, alignment, and depth perception. This can identify issues like strabismus (crossed eyes), suppression of one eye, and lazy eye.
• A dilated eye exam, which can help rule out potential causes of aniseikonia such as keratoconus (thin, cone-shaped cornea) and cataracts, as well as issues on the retina, like macular edema (swelling in the retina) and an epiretinal membrane (a film over the retina).

Testing for Aniseikonia

There are a few ways to test for retinal aniseikonia, a condition where your two eyes see the same object as different sizes or shapes. One is called the Amsler Grid test. It’s a simple test you can even do at home. The most common Amsler Grid test is a white sheet of paper with black lines forming a checkerboard pattern and a black dot right in the center. Some other versions of this test might have white lines on a black background, lines coming from the corners to the center, or just dots instead of lines. The test focuses on any distortion in the central view.

Other ways to test for this condition include using a 3D digital device or testing the very center of your vision. Doctors also have a special camera to take pictures of the back of your eye to confirm any distortions found in these tests.

To test for optical aniseikonia, a different condition that causes distorted vision, doctors often calculate the difference in how much each of your eyes magnifies objects. This is done using a formula and measuring the distance from your glasses to your pupil, which is usually around 15mm. Doctors can then use those values to determine the required magnification for correcting the problem.

To test for both kinds of aniseikonia, doctors will often use a variety of methods, each examining how each eye takes in images differently. One common method involves placing a rod over one eye to change the size of what you’re seeing, and then checking how different that looks compared to your other eye. The tests can be somewhat complex and involve lenses and colorful images in a special booklet.

For a more detailed assessment, doctors may use a computer program that allows you to adjust the size of lines on a screen to match up – the program then uses that information to figure out how much distortion your eyes are causing. The most precise test, called the space eikonometer, is unfortunately no longer made, but it had been considered the gold standard. It used four lines that you had to align correctly.

Treatment Options for Aniseikonia

Retinal aniseikonia, a condition where the visual images perceived by each eye differ in size or shape, is often treated by addressing its root cause, typically a disorder of the retina and choroid.

The most popular treatment, contact lenses, are not very effective for this type of aniseikonia. While these lenses can generate similar images on the macula, the central area of the retina, if the retina is swollen or stretched, the image will appear smaller due to the spread-out photoreceptors. Moreover, alternatives like specially designed lenses that create different-sized images on the retina tend to be scarce, expensive, and technically challenging to produce.

Retinal aniseikonia often shows significant asymmetry, which can’t be treated with standard lenses. Many causes of this condition, like macular edema or epiretinal membranes, are volatile, making the use of traditional optical solutions impractical.

A potential solution to retinal aniseikonia includes matching a potent plus spectacle lens with a powerful minus contact lens, which might rebalance the perception of size in individuals with a specific form of aniseikonia called micropsia. However, specially designed spectacle lenses can also ascertain a good solution when available.

On the other hand, optical aniseikonia, which is due to a difference of refractive power between the eyes, has various solutions. One of them includes reducing the vertex distance, meaning the distance between the back surface of a spectacle lens and the front of the eye, which reduces spectacle magnification and can minimize aniseikonia to acceptable levels.

Contact lenses, due to their proximity to the entrance pupil of the eye, can effectively reduce aniseikonia even when used with widely varying lens strengths. Another strategy involves using a contact lens in one eye to correct the anisometropia, the difference in refractive power between the eyes, whilst prescribing glasses for additional vision correction.

When surgical interventions, contact lenses, or specially designed glasses are not viable, one of the spectacle lenses may be deliberately undercorrected or overcorrected to reduce aniseikonia, even though it might cause blurred vision in one eye. In some cases, patients may adjust to aniseikonia over time, which is known as neuroadaptation. Refractive surgery, in which the cornea’s shape is altered to adjust the refractive power of the eye, is also a common solution.

More invasive procedures include replacing the eye’s natural lens with an artificial one, either by removing the natural lens or by inserting the artificial one alongside the natural lens. Although less common due to risks such as infection and retinal detachment, these procedures are often considered worth the potential complications due to the significant improvements they can bring about.

Lastly, selectively blocking the vision in one eye can be an effective temporary measure to prevent dangerous consequences of distorted vision, such as during driving, or while waiting for other solutions to take effect.

In a nutshell, contact lenses are a top choice for treating optical aniseikonia due to their effectiveness and convenience. At the same time, iseikonic lenses, while potentially effective for both optical and retinal aniseikonia, can be hard to obtain and may result in visually unattractive spectacles.

What else can Aniseikonia be?

Anisometropia is a condition where the two eyes have different refractive strength, and it can result in something called induced prism. When someone with anisometropia experiences symptoms such as eye strain, double vision, nausea, imbalance, and headaches, it’s often assumed these are caused by aniseikonia, another condition that creates a difference in image size between the two eyes. Yet, glasses for people with anisometropia can cause similar issues. This happens due to either the glasses causing a difference in magnification between the eyes, or a refractive effect that happens when looking away from the center of the lenses. Contact lenses can manage both of these issues.

To figure out if the symptoms come from the glasses or aniseikonia, there are certain tests that can be done, discussed under treatment methods. Another way is to check if the corrective glasses that do not require looking away from the center help in reducing the symptoms.

Alterations in brain functions can cause issues like micropsia (objects appearing smaller) or macropsia (objects appearing bigger) along with other distortions. Conditions like Alice in Wonderland syndrome, Charles Bonnet syndrome, migraines, and certain types of hallucinations can produce similar symptoms to aniseikonia. However, these conditions can be eliminated from consideration because they usually temporarily affect both eyes.

Retinal diseases often show up with aniseikonia-like symptoms, but they do not cause retinal aniseikonia, which is usually medically treated rather than with glasses or lenses. Testing for aniseikonia is often not done when retinal disease is present, and glass or lens solutions are not usually considered. This means in many cases, it is not clear whether the blurring caused by the retinal disease is the cause of the symptoms or if the presence of aniseikonia is rather a factor.

What to expect with Aniseikonia

Optical aniseikonia, which affects how differently each eye perceives the size of objects, typically has a very good outlook. This is because most cases can be corrected through surgery or wearing contact lenses. When these are not options, glasses specialized for aniseikonia can help.

Retinal aniseikonia, where the disorder originates in the retina, also generally has a good prognosis. If it is caused by retinal edema or inflammation due to central serous chorioretinopathy, it usually resolves on its own without any treatment. In other situations, swelling can be reduced with medication or surgery. If the retina’s center, or macula, shrinks due to conditions like staphyloma, there’s currently no medical treatment available. If macropsia, seeing objects larger than they are, arises as a symptom, aniseikonic glasses can help, but any resulting barrel distortion cannot be treated.

Amblyopia, a vision development disorder also known as “lazy eye”, can result from aniseikonia. If detected in a patient younger than nine years old, following treatment guidelines from the Pediatric Eye Disease Investigator Group can greatly improve their vision. However, if aniseikonia leads to amblyopia and suppression of one eye in a patient older than nine years, the prognosis becomes far less favorable.

Possible Complications When Diagnosed with Aniseikonia

If aniseikonia, a vision condition, isn’t caught in children during a crucial developmental period (age 8 to 10), it can cause permanent vision loss due to a condition called amblyopia. Moreover, if this vision condition isn’t detected or treated properly, it can have a significant impact on visual ability leading to continuous symptoms such as eye strain, dizziness, double vision, difficulty performing visual tasks, and headaches.

If aniseikonia can’t be treated, the person might end up suppressing their vision in one eye, misalignment of eyes, or even go as far as obscuring the vision of one eye if there is ongoing double vision. This results in a loss of ability to use both eyes together and perceive depth.

Medical eye surgeries, like scleral buckle procedures, monocular cataract extraction, or corneal transplant procedures, generally offer excellent visual outcomes for one eye. If aniseikonia hampers the use of both eyes together, patients may feel that their vision function is actually worse than before surgery.

There are different forms of treatment, such as contact lenses and iseikonic lenses, but each carries its own set of complications:

• Contact lenses increase the risk of contact lens-related dry eye, corneal inflammation and infection, allergies and additional stress due to the techniques and potential complications of inserting and removing contact lenses.
• Iseikonic lenses can cause prism and distortion. Also, they may not be problem-free for the user. Their use can alter how an individual looks because they enlarge one eye.

Preventing Aniseikonia

Taking preventive measures can come in different ways to manage eye conditions like anisometropia (different degrees of nearsightedness or farsightedness in each eye) and aniseikonia (an imbalance in the size of the images each eye sends to the brain). One example is conducting eye exams at an early age, which allows for early detection and treatment to avoid lazy eye, or amblyopia.

Referring patients for medical or surgical treatments quickly for diseases that can cause aniseikonia is also crucial. Some of these conditions are:

• Cataracts: Prompt referral for surgery is beneficial to prevent imbalance in vision and the size of the image each eye sees.
• Keratoconus: This condition often appears in a person’s second decade of life. Effective ways to treat it include contact lenses or surgery. Slowing down its progression with corneal cross-linking procedures is possible, but they need to be started early on.
• Epiretinal membrane: It causes varying types of aniseikonia or image size imbalance.

We can also avoid image size imbalance by meticulously planning ocular surgeries. For instance, surgeries that involve correcting one eye for nearsightedness and another for farsightedness can lead to aniseikonia if the patient can’t adapt. Sometimes, a second surgery is needed after cataract surgery in one eye, which affects about 30% of patients. Here, detailed planning of the refractive outcome is vital to avoid aniseikonia while the patient waits for the second cataract to mature. Procedures for retinal detachment like scleral buckle and insertion of silicone oil, can lead to enough visual imbalance to produce symptoms of aniseikonia.

Patient education is just as important for managing these eye conditions:

• An evaluation and plan for surgical options may be needed. In some instances, fitting patients with contact lenses can simulate vision correction to ensure they can tolerate the planned change in their vision.
• Reviewing and planning optical solutions, which include different eyeglass corrections, contact lenses, or a combination of contact lenses and eyeglasses, is also important.
• It may also be necessary to review medical treatments, especially for patients who have developed retinal aniseikonia. The same applies to patients who require vision shaping treatment or treatment for nearsightedness in the case of developmental visual imbalance.