What is amblyopia?
Amblyopia is a vision development disorder in which an eye has reduced visual acuity that does not improve with lenses. It is commonly known as lazy eye and develops in infancy or early childhood years. It usually occurs in one eye but could also occur in the two eyes.
How does amblyopia develop?
Vision is not fully developed at the time a child is born. After birth, it takes about 7 years for vision to become fully developed. For the visual system in the eye and the brain to develop properly, vision needs to be stimulated; the eyes need to see and form images that are transmitted to the brain, where they are fused and interpreted. The brain requires the images from the two eyes to be clear in order to fuse them into one. If a child has a condition that causes blurring of the image in an eye, the brain in order to avoid double vision may accept the clearer image and suppress the blurred image. Continuous suppression of image from an eye results in lack of stimulation of vision in that eye. When vision is not stimulated in an eye, vision development is impeded in that eye, resulting in amblyopia.
Types of amblyopia
There are 3 major types of amblyopia based on their underlying causes. These are strabismic, refractive and deprivation amblyopia.
Strabismus (a condition in which the eyes are misaligned) is the most common cause of amblyopia. In strabismus, when one eye focuses on an object, the other deviates away from the point of focus and either turns out, in, up or down. The quality of images from the misaligned eye is usually poor and the brain suppresses them to avoid double vision. If it is a particular eye that constantly deviates out of focus, over time, due to continued suppression of images from that eye, vision development gets impeded resulting in amblyopia.
This is amblyopia that occurs when there is a significant difference in the refractive errors in the two eyes. If the refractive errors in the two eyes are significantly different (a condition known as anisometropia), it becomes difficult for the brain to fuse the images from each of the eyes into one. The brain may develop a preference for the image from one eye and suppress the image from the other eye. Over time, this leads to amblyopia in the suppressed eye.
This is amblyopia that occurs when there is a condition that obstructs vision in an eye. It is the least common type of amblyopia. Usually, the obstruction prevents light from entering the eyes resulting in poor vision. Some causes of deprivation amblyopia are congenital cataracts, corneal opacity, drooping eyelid (ptosis) and eye tumour. For vision development to take place, these conditions require prompt treatment.
Signs and symptoms of amblyopia
Since amblyopia starts in infancy and early childhood, it may not be noticed on time because young children with this condition usually do not realize that they have a problem and others may not be able to communicate their problem effectively. In some cases, there may be no symptoms especially if the vision in one eye is very good.
However, some signs and symptoms that may indicate the need to exam the eye of a child for amblyopia include:
- An eye that deviates out, in, up or down
- Squinting or closing an eye
- Head tilt
- Bumping into objects on one side
You could also check for amblyopia by covering your child’s eyes one after the other while he performs a task. If the child resists covering a particular eye, it could be a sign that the eye is the better of the two eyes and that the second eye may have amblyopia.
Diagnosis of amblyopia
Amblyopia is diagnosed after a comprehensive eye examination. The eye doctor will be prompted to check for amblyopia during the examination if;
- There is poor vision or a significant difference in vision between the two eyes.
- There is a squint or the eyes do not align properly
- There is an obstruction of vision by either cataract, a tumour or ptosis
A check for amblyopia will include tests to measure refractive error, binocular vision tests and assessment of the health of the eye. Diagnosis of amblyopia is made if the vision of the child in the affected eye remains below the expected level after the refractive error or other underlying conditions have been treated.
Treatment of amblyopia
Treatment of amblyopia generally depends on the cause. The different treatment options are aimed at encouraging the use of the ‘lazy eye’. Some of them include:
Full lens correction for any refractive error
Providing full lens correction for the lazy eye can help improve images from that eye. To increase the chances of the brain using the image from the lazy eye, a lens that blurs vision in the good eye may be put in the child’s glasses.
Patching the good eye
When the good eye is patched, the brain is forced to use the image from the lazy eye, thereby stimulating vision in that eye. Patches are usually worn for a few hours every day over a period of months.
Patching is a very effective method of treating amblyopia but it can be difficult to get children to wear the patch as along as is required. Use of attractive eye patches and distraction of children with things they love (such as toys or outdoor play) could get them to keep the eye patches on at the onset. Over time, they get used to wearing it. For children who refuse to wear the eye patches despite all effort, the use of prosthetic contact lens to shut off light from the good eye can achieve the same effect as patching. Prosthetic contact lenses are however more expensive than eye patches.
Use of atropine drops
Atropine eye drop is used to dilate the pupil. It works by paralyzing the muscles responsible for the accommodative ability of the natural lens in the eye. This causes blurring of vision and so can be used in place of patching to treat amblyopia. When the vision in the good eye is blurred with atropine drops, the brain is forced to accept images form the lazy eye. The advantage of atropine is that there is no fear of noncompliance. However, atropine may cause side effects such as allergic reactions, flushing, light sensitivity and more.
This involves series of eye exercises aimed at aligning the misaligned eyes, improving depth perception and minimizing the suppression of the lazy eye.
In cases where amblyopia is due to cataract, surgery is recommended. If amblyopia is due to strabismus and does not respond to other treatment, surgery may be done align the misaligned eye.
Amblyopia, if not diagnosed and treated early can lead to permanent vision problems including reduced vision, limited peripheral vision and poor depth perception. Also, the chance of permanent visual disability is increased if vision is lost in the good eye due to an injury or disease.
For a better chance of success, treatment for amblyopia is better initiated on or before the age of 7. Commitment, patience and compliance with treatment regimen are essential for best visual outcomes.