By Omondi Nyong’o, M.D.

"Lazy eye" is the number one cause of vision loss in children and a problem I often see in my pediatric ophthalmology patients. When most of us think of "lazy eye," we think of people with wandering eyes, but that is only one form of the disorder. There are other types that can be just as damaging to a child’s vision and that may have no easily visible outward signs that parents can detect.

What is "lazy eye"?

Doctors call "lazy eye" by its technical name, amblyopia, and it means vision loss resulting from the brain tuning out information sent by one eye. It affects between 3 and 5 percent of children in the United States and over 40 million children are at risk for permanent vision loss from it.

Amblyopia occurs because of the unique way vision develops in children. When babies are born, their vision is very blurry. To establish "20/20" vision by early childhood, they must be exposed to clear and steady light stimulation of their eyes.

However, if the image from either eye is not appropriately sharp when it reaches the brain, or if the image from one eye is diminished while the other eye continues to see normally, the brain will concentrate on the eye that is sending a clearer picture. The brain will slow down development of vision for the eye that is sending the fuzzy images, and over time, it will "learn" to only see blurry with the eye sending the fuzzy picture even after the vision in that eye is corrected.

Amblyopia is not the fault of parents, guardians or of children themselves. It is simply the brain trying to adapt by developing a permanent preference for the eye that is seeing most clearly.

How can I tell if my child is at risk?

Because children do not complain of poor eyesight when young, it can be hard to detect the conditions that cause amblyopia before permanent vision loss begins. Therefore, screening for amblyopia, and for physical problems that can cause amblyopia, is important.

Newborn babies receive a first assessment of their eyes in the newborn nursery. During well-child visits, the primary care physician will include a screening physical examination of the eyes and vision. Sometimes automated machines are also used to measure the eyes.

When children are old enough to read the eye chart with pictures or letters (ages 3 to 5), they are tested at school and by their primary care physician. If they do not pass the eye chart test, they are then re-tested at the next visit, or referred to an eye doctor for a more detailed examination.

What conditions can lead to amblyopia?

Generally, there are three types of situations in which amblyopia can occur. The easiest to spot is a misalignment of the eyes called "strabismus." In strabismus, both eyes do not look at the same point in space simultaneously. For instance, in crossed-eyed strabismus, one eye looks to the left while the other eye looks to the right. When children have strabismus, they frequently adapt by preferring to look at the world with only one eye while ignoring the "doubled" image from their other eye. This ignored eye gives rise to amblyopia.

Near-sightedness or far-sightedness in one or both eyes can also lead to amblyopia. This is called "optical defocus amblyopia" and it occurs when the eye is shaped in such a way that the light entering the eye doesn't focus properly at the back of the eye where the nerves are.

Finally, the most serious situation in which amblyopia occurs is when there is a physical blockage to light entering the eye, such as from a cataract or a droopy eyelid. In a cataract, the lens inside the eye is cloudy and light is blocked on its way into the eye. In the case of a droopy eyelid, light may not enter the front of the eye at all. This causes "deprivation amblyopia," meaning that the brain is deprived of signals from the blocked eye and will not develop the structures necessary to receive signals from that eye even after the condition that blocked the light is corrected.

How is amblyopia treated?

The good news is that with timely and proper medical intervention, amblyopia can be successfully treated to maximize the visual potential of every child. Treatment typically begins with first fixing the underlying cause of the blurry vision. This usually involves glasses or contact lenses to correct the focusing ability of the eye. However, if a cataract or droopy eye lid is blocking vision, surgery may be necessary to provide clear vision.

Once the eye has been treated, the child's vision is retested to see if the brain is now correctly picking up on the signals from that eye. If it is, no other treatment is necessary. However, if the brain has "learned" to only see fuzzy with that eye, further treatment is necessary to make the brain form the neural connections necessary to receive signals from that eye. Doctors do this by using an eye patch to block vision from the eye the brain has been relying on to for vision. Without signals from the eye the brain has favored, the brain is forced to use the other eye and this restarts the process of forming the neural connections in the brain necessary for clear vision.

Once treatment is completed, the benefits can last a lifetime. To find out more about amblyopia and other childhood eye conditions, visit the American Association of Pediatric Ophthalmology and Strabismus and the Amblyopia Foundation of America Web sites.

Dr. Nyong'o is an ophthalmologist at the Fremont Center and the Palo Alto Center of the Palo Alto Medical Foundation. He has a special interest in pediatric ophthalmology and adult strabismus.