Amblyopia (“lazy eye”) is the condition wherein the brain does not fully acknowledge the signals coming in from that eye, so it relies on the other eye, which becomes dominant. When looking at the non-dominant eye, it may look normal to the outside viewer. In some cases, that eye may be misaligned with the dominant eye such as turning inward (endotropia) or outward (esotropia).
Insufficient development of the vision system in early childhood is the cause of amblyopia; it is a significant cause of lifelong vision difficulty. Strabismus is a common cause of amblyopia and can have important effects on social integration. Amblyopia is considered to be a cause of blindness that is preventable through early treatment and other treatment options that have been evolving over the years.
Amblyopia Begins in Early Childhood
Amblyopia usually begins in early childhood before the visual system has matured (ages eight to nine). In a study of young Baltimore children (30 months to 71 months) the prevalence of amblyopia was found to be a little less than 2%. Amblyopia also occurs in adults; an Australian study found the condition in about 2.5% of the adult participants. A German study found that 5.6% of adults suffer from the condition.
In the first three years of life, the human eye increases two to three times in size. Eye movement in the newborn develops in steps (saccades) rather than smooth movements. Divergence and convergence movements are not reliable until about three months of age. The curvature of the lens is still nearly spherical, and focusing is limited to about 20 centimeters. At this age, the retina is not fully developed. The rods that perceive shades, peripheral, and night vision are almost as developed as those of adults, but the cones that perceive color and that are responsible for detailed vision are not nearly mature yet. In addition, the visual cortex, which interprets what we are seeing for us, is not mature at birth, though it develops quickly over the first six months of life.
If light is blocked from reaching the retina (form-vision deprivation) as in a congenital cataract, or if there is a neurological dysfunction in the pathways to the retina and to the visual cortex, then the visual system cannot develop properly and amblyopia can result.
Types of Amblyopia
There are three main categories of amblyopia, which are based on the cause of the impairment.
Strabismic. The brain's visual cortex mis-reads or ignores the sensory signal from the retina. In the case where the amblyopia is due to strabismus, then one will notice one of the child’s eyes is misaligned with the other (one eye turns in) or both eyes turn inward (crossed eyes). When the two eyes are not able to align on a target, the child will initially experience double vision. Double vision is intolerable for the brain; one adaptation reduces the turned eye visual acuity so that the brain learns to ignore it.
Refractive is due to unequal focusing (refractive) errors that are significant enough to cause the eyes to not work together. This could include significant differences in near- or far-sightedness or astigmatism.
This inequality, due to discrepancies in image sizes being sent to the brain, makes it difficult for the visual cortex to sort and integrate the two received images. The amblyopic eye is never blind in the sense of being entirely without sight, and the patient typically retains peripheral vision in that eye; but, because the two images the brain "sees" are incompatible, the brain cannot easily blend them into one image (fusion).
When a child's brain cannot blend the images, the brain begins to ignore the blurry image. If this goes on for months or years in a young child, the vision in the eye that sees the blurry image will deteriorate.
Deprivative is due to mechanical obstruction of light, such as a congenital cataract.
Signs and Symptoms
Lazy eye usually does not cause any obvious symptoms. Parents can suspect vision problems in an older baby or toddler if a child squints, cocks her head at an angle to see something, or has obvious crossed eyes. But in the very young children, when one eye focuses more than another, and the brain learns to favor the stronger eye, initially these signs may not be noticeable.
For this reason, vision testing of very young children is recommended, since amblyopia may not be detected without an eye exam. Amblyopia develops starting at birth and get worse until age 10, but mostly by age 5. Here are signs you might see along the way:
- An eye that wanders toward, or away from, the nose, independently of the other eye, or eyes that don't work in sync with each other
- Poor depth perception, signaled by stumbling or the inability to keep balanced
- Squinting or shutting one eye, especially with double vision experienced
- Tilting of the head
Causes and Risk Factors
- Anisometropia (refractive error) causes 50% of amblyopia instances. Strabismus causes 25% of amblyopia cases. A combination of anisometropia and strabismus causes 16% of amblyopia.
- Genetics may be a risk where lazy eye runs in the family, which may be due to a genetic propensity for cataracts, for significant differences in refraction, or for heredity-related reasons. Researchers are beginning to identify mutations in genes that are needed for brain stream, ocular motor-neuron development, and connection capacity.
- Premature birth or birth with low body weight is a risk.
- Cataracts upon birth are the most common cause of deprivative amblyopia and are relatively rare.
- Ocular pathologies include the following:
- Posterior keratoconus (bulging cornea) is a potential cause.
- Uveitis, such as pars plantis (a kind of chronic intermediate uveitis), increases the risk.
- Loeys-Dietz syndrome (genetic) puts children at risk for amblyopia.
- Duane syndrome is a strabismus syndrome.
- Leber's is a genetically based retinal malfunction.
- Saethre-Chotzen syndrome is premature fusion of a baby's head bones.
- X-linked retinoschisis is a cause of macular degeneration in boys.
Glasses may improve visual acuity to some degree but usually not completely. With amblyopia, the brain is "used to" seeing a blurry image, and it needs to learn how to better see with the lazy eye. Over time the brain may learn to “see" again with the lazy eye.
When a patch is prescribed for the normal eye, it requires the lazy eye to do more work. The extent and duration of the patching depends on the age of the child, the severity of the lazy eye, and the response to treatment. Treatment time can take weeks to months before results are seen. If treatment starts before the age of seven years old, most children will gain back vision in the lazy eye. Amblyopia becomes much more difficult to treat after about 7–9 years of age.
A 2003 NEI-funded study1 found that patching the unaffected eye of children with moderate amblyopia for two hours daily works as well as patching for six hours daily. Shorter patching time can lead to better compliance with treatment and an improved quality of life for children with amblyopia. However, a more recent study2 found that with children whose amblyopia persists, despite two hours of daily patching, they might improve if daily patching is extended to 6 hours.
A nationwide clinical trial3 showed that many children from ages 7 to 17 years benefited from treatment for amblyopia. This is in contrast to what was previously thought possible by professionals, confirming that age alone should not be used as a factor to decide whether or not to treat a child for amblyopia.2
In the occasional case where patching cannot be done, a prescribed eyedrop called atropine can be prescribed that temporarily blurs vision in the dominant eye, so that the child will use the eye with amblyopia, especially when focusing on near objects.
Other methods or procedures may be recommended to correct a lazy eye, including eye muscle surgery, vision therapy, or prescription glasses. The surgery, however, only corrects the misalignment of the eye; vision therapy will still be needed afterwards to train the brain to learn to see through that eye again.
Psychological Aspects of Amblyopia
The loss of visual skills related to amblyopia is expanded and reinforced by a person’s belief that they are unable to "trust" the information that is coming through the affected eye. When patients are placed in a position where they must use their amblyopic eye, they usually respond by insisting, "I just don't use that eye. It feels old and lazy." When they are shown that the eye has many more abilities than they might perceive, or were led to believe, a "miraculous" increase in vision occurs in that eye.
We see a clear relationship between the eyes and the mind when working with a patient with amblyopia. Getting them to develop trust that their eye is seeing, and breaking down the belief system that it can't see, are powerful tools to improving vision.
The complementary approach to all binocular vision problems is to make sure that the basic protocols of good vision are met: good nutrition through diet (also juicing and supplements if needed), drinking pure water, exercise, eye exercises, etc.
Dichoptic treatment. By thinking about the development of amblyopia as primarily due to the loss of binocular vision, researchers are testing a way of artificially presenting different images to each eye. The adult patients are able to render the resulting image correctly. The more time spent successfully perceiving the correct image, the more binocular capacity is strengthened and the more the amblyopia resolved.4
Binocular zone. In the visual cortex, a region called the binocular zone normally receives input from both eyes, but if one eye is stronger, it takes over the binocular zone, blocking it from the other eye. Researchers have been able to manipulate a specific brain circuit in mice to prevent dominance.5
Video game. In a pilot study of children with amblyopia, a custom-made video game allowed children to play either monocularly with the strong eye patched or with both eyes but reduced contrast in the strong eye. Improvements resulted, and most were retained after treatment.6
- Key self help tips Maintain good vision with eye disease prevention pointers
- If you are a computer user, please review our section on computer vision syndrome
1. National Eye Institute. (2003). Reduced Daily Eye Patching Effectively Treats Childhood's Most Common Eye Disorder. Retrieved May 1 2018 from https://nei.nih.gov/news/pressreleases/051203.
2. National Eye Institute. (2013). Extended daily eye patching effective at treating stubborn amblyopia in children. Retrieved May 1 2018 from https://nei.nih.gov/news/briefs/eye_patching.
3. National Eye Institute. (2005). Older Children Can Benefit From Treatment For Childhood's Most Common Eye Disorder. Retrieved May 1 from https://nei.nih.gov/news/pressreleases/041105.
4. Hess, R.F. News Medical Life Sciences. (2018). New hope for adults with amblyopia (lazy eye). Retrieved May 8 2018 from https://www.news-medical.net/health/New-hope-for-adults-with-amblyopia.aspx.
5. National Eye Institute. (2013). NIH-funded study could lead to new treatments for amblyopia. Retrieved May 8 2018 from https://nei.nih.gov/news/pressreleases/082613.
6. Gambacorta, C., Nahum, M., Vedamurthy, I., Bayliss, J., Jordan, J., et al. (2018). An action video game for the treatment of amblyopia in children: A feasibility study. Vision Res, Apr 27.