It’s important to first understand how normal vision works before moving on to the definition of anisometropia. Light from the outside world passes through the various parts of the eye, namely the cornea, lens and vitreous, to finally strike the retina at the back of the eye.
The retina converts the light into an electrochemical signal, which is sent to the brain. The brain then interprets this signal as a clear image.If the light does not focus on the retina precisely, the message that the brain translates, creates a blurred picture. This is known as a refractive error.
This error, or degree of blurriness, can be measured in each eye. Glasses can then be prescribed to counteract this error.The refractive error in each eye can be different, in other words, the degree of blurriness in each eye can actually differ! This is known as “anisometropia”.
Did you know that treatment and diagnosis can lead to the prevention of permanent vision loss?
WHAT CAUSES ANISOMETROPIA?
The condition is usually congenital (one is born with it). To have perfectly symmetrical eyeballs is as rare as having a perfectly symmetrical face. So most of us are born with some degree, albeit tiny, of anisometropia.
A second cause is that the eyes in an infant or child grow at different rates to one another. If one eye grows a little faster than the other, it will be slightly larger and therefore light will need to travel further to hit the retina. Thus, the refraction measurement in each eye will be different.
The third cause is due to trauma or surgery. Either can change the shape of the eye and therefore, change the refractive error in that eye.
HOW WOULD I KNOW IF MY CHILD HAS ANISOMETROPIA OR NOT?
Remember that your child should be screened at: birth, 6 months and just before attending school.
This protocol is designed to identify problems with vision, especially in infants and in children who are still too young to describe their symptoms clearly.
If your child is at an age where they can verbalize their symptoms, they may complain of double vision (diplopia), headaches, nausea, tired eyes, dizziness and sensitivity to light.
One of the most common complications of anisometropia in infants is amblyopia. When the eyes refract unequally, a fuzzy image is created in the brain. To limit the confusion, the brain starts to favour the stronger eye by gradually “switching off” the signals from the weaker eye. If not identified and treated promptly, this eye could be permanently “turned off” (amblyopia) by the brain, resulting in blindness.
Amblyopia can be difficult to identify in your child because the condition doesn’t have any external signs. It is also more common in pre-verbal children making the process even trickier. Some clues that amblyopia may be present include, watching your baby’s behavior when one eye is covered, noticing that your baby cries continuously and observing that your child constantly bumps into objects.
The final sign of anisometropia that you might notice is a squint, either intermittent or permanent.
The treatment has a high success rate, especially if treated early enough.
WHO DO I GO TO FOR HELP?
Prompt and accurate diagnosis is essential, and in the case of amblyopia, will aim to prevent permanent vision loss.
An ophthalmologist will conduct a thorough examination to make a diagnosis. From there, a treatment plan will be created to address your child’s specific needs.
HOW IS ANISOMETROPIA TREATED?
The condition can be treated with glasses or contact lenses. If your child has amblyopia or a squint, your ophthalmologist will need to treat these conditions as well.
The use of spectacles is the most convenient for parents, however, their use may have some problems, in which case contact lenses are ideal.
Contact lens insertion and removal can be traumatic for both the parents and the child, so the type of lens is chosen carefully to limit this distress. For example, using a monthly prescription rather than having to insert and remove them daily.
The use of laser surgery to correct anisometropia has been studied in certain university settings and this may be used in very specific cases.
The outcome, especially if the condition is identified early, is usually very good.
DID YOU KNOW BABIES CAN WEAR CONTACT LENSES?
Download the article here:
- Oxford Handbook of Ophthalmology, Second Edition. By Alastair Denniston and Philip Murray. P. 650
- Ophthalmology Secrets, Third Edition. By James Vander and Janice Gault. P. 23
- Duke-Elder’s Practice of Refraction, Third Edition. By David Abrams. P. 37, 76, 105, 106, 107, 219, 234.
- “Your Eyesight.com”
- “Review of Optometry.com”