The brain and eyes have to work together to produce clear vision. If the brain favors one eye — usually due to poor vision in the other eye — the weaker eye tends to wander inward or outward. Eventually, the brain may ignore the signals received from the weaker eye. This condition is sometimes referred to as lazy eye (amblyopia).

Lazy eye is the leading cause of decreased vision among children. And lazy eye is fairly common, affecting about two to three of every 100 children, according to the National Eye Institute.

Conservative treatments such as corrective eyewear or eye patches can usually correct lazy eye. Sometimes, lazy eye requires surgical treatment. Left untreated, lazy eye may lead to permanent vision impairment.

Signs and symptoms of lazy eye include:

• An eye that wanders inward or outward
• Eyes that may not appear to work together
• Poor depth perception

Although lazy eye usually affects just one eye, it's possible for both eyes to be affected.

When to see a doctor
Vision checks are often a routine part of well-child checkups — especially if there's a family history of crossed eyes, childhood cataracts or other eye conditions. For all children, a complete eye exam is usually recommended between ages 3 and 5. If you notice your child's eye wandering at any time beyond the first few weeks of life, consult your child's doctor for an evaluation. Depending on the circumstances, he or she may refer your child to a doctor who specializes in eye conditions (ophthalmologist).

Anything that blurs a child's vision or causes the eyes to cross or turn out may cause lazy eye.

The most common culprit is strabismus — an imbalance in the muscles responsible for positioning of the eyes, which can cause the eyes to cross in or turn out. The muscle imbalance prevents the eyes from tracking with each other.

Sometimes lazy eye is the result of an anatomic or structural abnormality, such as an abnormal central retina or a cloudy area in the lens of the eye (cataract). In other cases, an abnormal eye shape or a size difference between the eyes contributes to lazy eye.

Occasionally, a wandering eye is the first sign of an eye tumor.

Lazy eye tends to run in families. Lazy eye may be more likely among children who were born prematurely or those who have developmental delays as they get older.

Left untreated, lazy eye can cause permanent vision loss. In fact, lazy eye is the most common cause of single-eye vision impairment in young and middle-aged adults, according to the National Eye Institute.

Lazy eye is diagnosed with a thorough eye exam. The doctor will look for a wandering eye, as well as a difference in vision between the eyes or poor vision in both eyes. Special diagnostic tests aren't usually needed.

Ideally, lazy eye treatment begins in early childhood — when the complicated connections between the eye and the brain are forming. Depending on the cause and the degree to which your child's vision is affected, treatment options may include:

• Corrective eyewear. If a condition such as nearsightedness, farsightedness or astigmatism is contributing to lazy eye, corrective glasses or contact lenses will likely be prescribed. Sometimes corrective eyewear is all that's needed.
• Eye patches. To stimulate the weaker eye, your child may wear an eye patch over the stronger eye — possibly for two or more hours a day depending on the severity of the lazy eye. This helps the part of the brain that manages vision develop more completely.
• Eyedrops. A daily or twice weekly drop of a drug called atropine can temporarily blur vision in the stronger eye. This will encourage use of the weaker eye, and offers an alternative to wearing a patch.
• Surgery. If your child has crossed or outwardly deviating eyes, the eye muscles may benefit from surgical repair. Droopy eyelids or cataracts may also need surgical intervention.

For most children with lazy eye, proper treatment improves vision within weeks to several months — and the earlier treatment begins, the better. Although research suggests that the treatment window extends through at least age 17, results are better when treatment begins in early childhood.