Anisometropia

What is Anisometropia?

Anisometropia is a condition in which there’s an uneven focus between the right and left eye. It’s commonly defined by a difference of one or more “diopters,” which is a unit to measure the focusing power of the eyes. Studies have shown that this condition affects anywhere from about 4% to nearly 22% of people worldwide.

Why is this significant? Well, for some people, this unequal focus, or anisometropia, isn’t corrected and can result in having consistently poor eyesight, or low visual acuity. In cases where there are no other abnormalities present, this usually leads to a condition known as anisometropic amblyopia. In fact, this condition accounts for 24-37% of all cases of amblyopia, a condition where vision in one or both eyes does not fully develop during childhood.

What Causes Anisometropia?

Anisometropia refers to a condition where the two eyes have different refractive power, which can lead to vision problems. In older people, this condition often occurs due to changes in the lens of the eye or the development of cataracts. However, among infants, the condition is usually linked to differences in eye length.

Eye length can change if one eye is compromised in some way. Unilateral damage to any part of the eye, such as the eyelids, cornea, lens, or retina, can cause this. Examples of such damage include eye traumas, cataracts, and diseases that affect the retina.

These eye problems are often easily detected and recognized as the cause of vision loss. However, it is not always clear how these conditions lead to changes in refractive power and vision.

Research shows that anisometropia is more severe when vision disruption occurs during periods of rapid eye growth, such as the first three years of life. Babies born prematurely or with lower birth weight, particularly, have a higher risk for developing this condition.

For toddlers and children who do not have any obvious structural abnormalities in their eyes, other changes in eye power may be the cause of anisometropia. Sometimes, the condition resolves naturally as the eyes normalise. However, if one eye has significantly more refractive error than the other, this can lead to persistent anisometropia and even lead to amblyopia, or lazy eye.

Studies show that anisometropia occurs more often in people who are nearsighted. The risk of developing this condition also increases with higher levels of nearsightedness and astigmatism. It has been observed more frequently in young children with astigmatism.

Similar findings have been reported in adults, and higher degrees of astigmatism are often associated with more cases of anisometropia. An anisometropia of 1.5 or more increases the risk for vision deterioration over time. Early uncorrected farsightedness can lead to cross-eyes with anisometropia and amblyopia.

In these cases, one eye, typically the dominant eye, fails to align properly with the other, and this misalignment continues as the individual grows. This can also disrupt vision in the affected eye and have long-term effects on vision. Studies suggest that anisometropia is a risk factor for developing cross-eyes.

In cases where cross-eyes occur alongside anisometropia, they usually turn inward rather than outward. There’s an additional issue seen in those with lazy eye caused by farsightedness: the eye’s ability to normalize is hindered, preventing any forward shifting of focus. However, in situations where a lazy eye is caused by early vision deprivation, there is often a noticeable shift toward nearsightedness.

Risk Factors and Frequency for Anisometropia

Anisometropia, or having different vision powers in each eye, is more common in adults than in young children. Interestingly, many newborns show this condition, but it usually gets better on its own within the first year. After that, about 5% of people have anisometropia until they reach their teenage years. The rate jumps to around 10% in early adulthood, particularly when nearsightedness starts to develop. The percentage increases again in older adults.

People who have a larger difference in vision power between their eyes, especially if they’re nearsighted, and those with other eye diseases, are more likely to have anisometropia. It doesn’t matter whether you’re a man or a woman, but your race might play a role. For example, about 4.2% of African-Americans and 4.3% of Hispanics aged 6 to 72 months in the United States have this condition. The rates are a little lower for African-American and Caucasian babies between 6-71 months, at 1% and 1.5% respectively.

• Anisometropia is more common in adults than in children.
• Many newborns have this condition, but it usually corrects itself within the first year.
• From ages 2 to puberty, about 5% of people have anisometropia.
• In early adulthood, this rate increases to about 10%, especially among those who are nearsighted.
• Older adults have even higher rates.
• People with larger vision differences between their eyes and those with other eye conditions are more likely to have anisometropia.
• Men and women are equally likely to have this condition. However, race might play a role. About 4.2% of African-Americans and 4.3% of Hispanics aged 6 to 72 months in the U.S. have anisometropia.
• Rates are slightly lower for African-American and Caucasian babies between 6-71 months, at 1% and 1.5% respectively.

Signs and Symptoms of Anisometropia

Anisometropia is a condition where the two eyes have different visual powers. This condition may be associated with symptoms or signs of early eye issues in one eye. These problems can be varied, like defects in eyelid closure, cataracts that are present at birth or develop after an injury, cloudy areas in the cornea, and different forms of retinal diseases. Anisometropia often happens together with strabismus, which is a condition where the eyes do not line up in the same direction when focusing on an object. If someone has strabismus, they may also have visibly deviated eyes, an abnormal head position, and other related signs. These should all be checked and evaluated.

Patients with Retinopathy of Prematurity (ROP), a potentially blinding eye disorder that primarily affects premature infants, exhibit different signs related to this condition. These include irregular and incomplete blood vessel growth in the retina, scarring in the retina and vitreous (gel-like substance that fills the eye), up to cases where the retina detaches in severe scenarios. This condition is especially common amongst babies born before 31 weeks of gestation and/or with a birth weight of less than 1250 grams.

• Anisometropia is likely to be more common, severe, and difficult to manage when it happens together with ROP. About 13.8 to 15.4% of ROP patients develop anisometropia.
• In comparison, the occurrence of anisometropia in children born early, but without ROP is very less. It ranges between 0 to 4.5% only when they are one year old.
• Infants with severe ROP and low birth weight are more likely to have two or more diopters (a measurement unit of refractive error) of anisometropia.
• On a positive note, research indicates that without ROP, anisometropia often resolves itself within the first 12 months of life. Factors like premature birth, low birth weight, maternal smoking, or breastfeeding have no impact on this outcome.
• However, the likelihood of developing anisometropia is higher in babies treated in a neonatal intensive care unit and in those with amblyopia or exotropia, irrespective of the presence of ROP.

Testing for Anisometropia

Amblyopia (lazy eye) and anisometropia (having two eyes with significantly different prescriptions) are both conditions that are commonly caught during routine school vision screenings. They’re often detected at this time because there typically aren’t clear symptoms present before then. Amblyopia, known as “lazy eye,” is usually found in the eye with a higher prescription and is more common in those with anisohyperopes (different levels of long-sightedness) than in anisomyopes (different levels of short-sightedness).

An imbalance of 1-2 diopters (a unit measuring the optical power of a lens) in long-sightedness can lead to amblyopia, while a similar imbalance in short-sightedness doesn’t typically cause the condition. This could be because long-sighted imbalances produce blurry vision earlier in life, affecting the development of the visual cortex more than short-sighted imbalances. It might also be because short-sightedness is less common in young children.

How do doctors detect amblyopia? They can measure the quality of your vision both uncorrected and corrected using various types of eye charts, like Landolt C charts, or “E” charts or even picture charts, depending on what’s suitable for the patient’s age, behavior, and intellectual capability. As part of the assessment, your eye doctor will measure your refraction (how light waves are bent when they enter your eye) under conditions where your eyes are temporarily unable to focus.

Specific tests like Lang’s or Titmus’, can identify defects in stereopsis – the perception of depth and 3-dimensional structure obtained on the basis of visual information deriving from two eyes. Clinicians also need to check for the presence of phorias and tropias (types of eye misalignments) with a thorough orthoptic (eye muscle) evaluation. They can use a cover test, Maddox Rods, or Worth’s Four Dot tests to see if eye deviation is establishing along with amblyopia and anisometropia.

Your practitioner can measure eye deviation precisely by conducting retinoscopy through prism bars or other techniques like the synoptophore. It’s important to note that anisometropia can occur without any obvious cause of amblyopia, so more hidden factors may also be involved. This includes conditions like microstrabismus with fixation instability. Some experts have recommended a specific device known as the Pediatric Vision Screener (PVS) to assess eye fixation (holding the eyes steady) which could help identify this condition that would otherwise remain unnoticed with standard testing methods.

Treatment Options for Anisometropia

Currently, amblyopia (lazy eye) and anisometropia (unequal vision in two eyes) are primarily treated through glasses or contact lenses, and the use of eye patches or atropine eye drops to enforce the use of the weaker eye. While some degree of natural balance between the eyes can occur to prevent amblyopia, it’s generally agreed upon that these conditions do not go away without any treatment.

Glasses and contact lenses work if they’re used when the connections in the vision-part of the brain are still malleable. If the difference in vision between the eyes is smaller and the lazy eye isn’t too weak, these treatments can be quite successful. Patches and atropine drops help by encouraging the brain to use the lazy eye more, promoting its development. With these treatments, an improvement is typically seen after 150 to 250 cumulative hours of therapy over the course of 3 to 5 months.

Adherence to treatment is vital for it to be effective. Regular check-ups with a doctor and close monitoring at home are essential for tracking progress. Please note that age plays a significant role: Treatment is more effective between ages 3 to 7, and may become less effective as children get older.

In some cases, glasses and contact lenses may be uncomfortable or cause problems like reduced vision clarity, smaller visual field, difficulty with both eyes working together, and feeling self-conscious. Where this is the case, surgical treatment options can be explored. There are several procedures like radial keratotomy, photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK), and laser-assisted subepithelial keratectomy (LASEK) that have been found safe and effective in improving vision and preventing amblyopia in children with anisometropia.

However, corneal surgeries may not be effective in all. They can correct vision to a certain degree, after which the treatment may face issues like corneal haziness and a decrease in vision correction.

There are also intraocular (inside the eye) surgeries like replacing the person’s clear lens with an artificial lens, similar to cataract surgery. While it is a minimal invasive and a relatively predictable procedure, it can cause complications like retinal detachment and glaucoma, especially in nearsighted eyes.

Another alternative is the implantation of an artificial lens in front of the clear lens without removing it. Although it retains the benefits of an artificial lens, in rare cases, it can lead to loss over time of the cells that line the back of the cornea or increased eye pressure.

What else can Anisometropia be?

Issues with the power of the eye lens can be detected in adults and older people during regular eye check-ups. These issues are often related to conditions like cataracts. On the other hand, babies can sometimes have temporary vision differences between their eyes, but this usually goes away on its own.

What to expect with Anisometropia

All variations of anisometropia – which is a condition where the two eyes have different degrees of refraction – can lead to amblyopia or ‘lazy eye’. Specifically, every case of uncorrected anisometropia greater than 6 D (a measure of lens power), or anisohyperopia greater than 4 D will result in amblyopia.

However, conventional treatments like the use of glasses, patching the better eye, or using atropine drops in the unaffected eye can prevent most cases of amblyopia caused by anisometropia. Those with anisometropia ranging between -3 to 3 D, have a 75 to 94% chance of achieving a corrected vision of 20/40 or better. On the other hand, patients with a greater level of anisometropia, exceeding -6 D, only have a 25% of achieving this level of vision correction.

Possible Complications When Diagnosed with Anisometropia

As mentioned earlier, the main risk linked to untreated anisometropia (unequal vision in both eyes) is developing amblyopia. Amblyopia, often referred to as “lazy eye,” results from the brain favoring one eye over the other.

Preventing Anisometropia

Treating eyes affected by anisometropic amblyopia – a condition where the two eyes have different visual strengths – requires wearing corrective eyewear and patches. For these treatments to work at their best, patients need to use them regularly, as confirmed by studies using a device to track how consistently patients wore their patches.

Other researchers have found that the more consistently the treatment is followed, the better the vision improvement. They reported that the best vision improvement is seen after 150 to 250 hours of treatment in total.

Therefore, doctors need to thoroughly explain the treatment to the patient’s caregivers or parents and motivate them to help the patient stick to the treatment plan for best results.