Normally, our brain processes the images sent by both eyes equally. This is needed for the best possible vision.
In some children, though, one eye is favored by the brain because it provides a sharper image. As a result, the brain neglects the images from the other eye, which affects the child's visual development. This is known as amblyopia, or “lazy eye.”
If your child has a squint or you think he or she may have an eye problem, it is important to have it checked out by an eye doctor. Difficulties recognizing objects may also be a sign of lazy eye. They may, for instance, have to hold toys or other things very close to their eye to see them properly, or always turn their head to one side.
Amblyopia develops when the two eyes keep sending such different images to the brain that the brain can’t merge them into one. The most common cause of this is a squint (strabismus). If a child has a squint, one eye will look straight ahead while the other looks up, down or to the side, so it is taking in a completely different image. In an effort to avoid seeing double, the brain often blocks out the images from the eye that doesn’t see as well, and relies on just one eye.
About 60 to 70% of children who have a squint have a lazy eye, compared to only 2% of children who don’t have a squint.
Refractive errors are another common cause of amblyopia. They make the image that appears on the retina blurry. There are three different kinds of refractive error:
- Near-sightedness (short-sightedness or myopia): The eye can only focus clearly on objects that are close by.
- Far-sightedness (hyperopia): The eye can only focus clearly on objects that are far away.
- Astigmatism: Everything appears blurry because the lens or the cornea is deformed.
Refractive errors are more likely to lead to a lazy eye if each eye is affected in different ways: for example if one eye has a refractive error and the other one doesn’t, or if one eye is near-sighted and the other is far-sighted.
Lazy eye is only rarely caused by an eye disorder. Eye disorders that can cause lazy eye include cataracts, a droopy eyelid (called ptosis) or an eye that does not have a lens (aphakia).
Lazy eye is a common vision problem in children. It is estimated that about 4 to 6% of children in Germany are affected. This problem usually develops before the age of 7.
There are several tests that eye doctors can do to find out whether a child has amblyopia:
- They can use eye charts to see how good the child’s vision is. The exact kind of test will depend on several factors, including the child’s age. There are special tests for babies and toddlers.
- A slight squint isn’t always visible to the naked eye. To tell whether a child has a squint, the position of their eyes can be measured – for instance, with a test in which the eyes are covered one after the other. The doctor then checks whether the other eye moves.
- A physical check-up can help to see whether the poor eyesight is caused by something else, like a cataract.
- This involves shining light into the eye with a special instrument called a retinoscope, and seeing how the light reflects off the retina at the back of the eye. By holding different corrective lenses in front of the light, it is possible to determine exactly how well the eye can focus.
These tests are generally safe. Eye drops are typically used to dilate (open) the pupils before examining the eyes with a retinoscope. The eye drops may cause skin irritations or burning in the eyes.
Specialists believe that eyesight mainly develops in our first 5 years of life. They think that lazy eye should be detected and treated as early as possible in order to avoid lifelong vision problems, as well as problems at school and in the child's social development.
For this reason, an extra screening test was introduced in Germany in 2008 for all preschool children who have statutory health insurance. This test is called “U7a” in the German health care system, and its main aim is to detect vision problems. It is offered in addition to the existing screening examinations for children in Germany (the so-called “U-Untersuchungen”), and is aimed at toddlers who are nearly three years old (between 33 and 35 months old).
Lazy eye can’t be fixed instantly by putting on glasses that correct the problem. But there are a number of different ways to improve vision in the weaker eye, or at least to help make sure that the problem doesn't get worse over time. Treatment might only be needed for a few weeks, but sometimes it has to continue for quite a long time to get good results.
If another eye problem (e.g. a droopy eyelid) is causing the lazy eye, that problem is treated first.
When people are ill or need medical advice, they usually go to see their family doctor or pediatrician first. Read about how to find the right doctor, how to prepare for the appointment and what to remember.
Institute for Quality and Efficiency in Health Care (IQWiG, Germany). Screening for visual impairment in children: Final report; Commission S05-02. April 01, 2008. (IQWiG reports; Volume 32).
Jefferis JM, Connor AJ, Clarke MP. Amblyopia. BMJ 2015; 351: h5811.
West S, Williams C. Amblyopia in children (aged 7 years or less). BMJ Clin Evid 2016: pii: 0709.
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