Introduction
Anisometropia is a condition where there is an unequal focus between the two eyes due to a difference in refractive power. Studies have shown that very few people have the same optical power in both eyes. However, for anisometropia, the difference in vision is significant and interferes with normal binocular vision.
The intraocular difference is of 2 Diopters (D) or more. In most cases, anisometropia results in amblyopia, which is then referred to as anisometropic amblyopia.
Types
Anisometropia has two broad categories, and these are; absolute anisometropia and relative anisometropia.
Relative anisometropia is when the refractive power of the eyes is similar, but the eye sizes are different. Images from both eyes are of different sizes but have equal clarity.
Absolute anisometropia is where the refractive power of both eyes are different enough to be a problem and can be further categorized into:
Simple anisometropia
Only one eye has a refractive error. It can either be hyperopic, that is farsighted or myopic (nearsighted). One of the eyes usually has a blurry vision, while the other is clear.
Compound anisometropia
An instance where both eyes are hyperopic and myopic. The difference in refractive error for both eyes is significant, and they both have blurry vision. However, one eye is more blurry than the other.
Antimetropia (also mixed anisometropia)
A disorder where both eyes have refractive errors but of different types. One eye can be myopic, while the other is hyperopic.
Compound astigmatic anisometropia
In this case, both eyes are astigmatic but to an unequal extent. Astigmatism is a disorder that causes light to not focus directly on the retina hence making the image blurry.
Simple astigmatic anisometropia
One eye is either myopic or hyperopic, and the other eye is astigmatic.
Causes & Risk Factors
Anisometropia is caused by defects in the eyes during childbirth and varying sizes of the eyes.
A 5-20% difference in refractive power of each eye is more likely to result in uneven vision. The condition is prevalent among children, with an estimated 6% of children aged between 6-18 known to suffer from it.
Cataract has also been shown to cause anisometropia. Intraocular gas often used to repair retinal detachment increases cataract formation, leading to myopia.
Anisometropia can also appear when the eye's lens is removed and not replaced by an artificial lens, a condition known as aphakia. This can be done intentionally.
Signs & Symptoms
A child is less likely to show any immediate signs of a problem with their vision. Anisometropia is mostly present at birth, but in most cases, it does not become apparent until later years in life.
More severe conditions will show:
Amblyopia (lazy eye)
A condition that occurs where the brain cannot use both eyes together. A child will show crossing or wandering eyes, squinting or tilting the head while trying to better look at things.
Strabismus (crossed eye)
The eyes are unable to align, making it impossible for both eyes to focus on the same point.
Diplopia (double vision)
With this presentation, one experiences visual fatigue, headaches, sensitivity to light, dizziness, and nausea.
Diagnosis
An eye exam that involves the use of a visual screen is used to diagnose anisometropia. For anisometropic amblyopia, contrast sensitivity tests, grating acuity and Vernier acuity tests may be employed.
Treatment
It is vital to treat anisometropia before it becomes acute. If not treated, the brain can select the eye giving the most precise image and ignore the other eye. Treatment depends on how severe the condition is.
Medical Therapy
Treatment involves correction of the refractive difference and includes corrective lenses, which is the most common treatment. Corrective lenses work best when the difference is less than 4D. Contact lenses are recommended if a child is less than 12 years. If older, the recommended option to correct vision is either glasses or contacts.
Surgery
An ophthalmologist may also employ corrective surgery. Tropical surgeries include:
- Refractive corneal surgery which improves the cornea's refraction of light.
- Removal of the crystalline lens, which improves overall vision for some patients. Sight is then adjusted using further surgeries or with special glasses.
- Intraocular lens implantation. In this case, the doctor will cut the eye precisely to break up the natural lens then put a plastic lens in its place. The surgery is mainly for severe cases of astigmatism and cataract.
- Phakic IOL - The doctor does not remove the crystalline lens in this case, but the intraocular lens is put in the eye to work together with the natural lens.
Prognosis & Long-Term Outlook
Treatment at a younger age is more successful than when the child is older. Corrective surgeries are mostly not risky, but just like any other surgery, some complications may arise after the surgery. They may include blindness, infection, reduced visual acuity or swelling or redness.
It is advisable for patients to immediately seek medical attention once they experience any of the problems.
Prevention & Follow Up
The doctor should prescribe eye drops that facilitate healing. Patients shouldn't touch their eyes as they heal after corrective surgery.
Follow up in accordance with the doctor's recovery plan is essential. Patients should have regular eye exams by ophthalmologists who can handle ocular misalignments and identify signs of visual disability before things get worse.
