Diagnosing Lazy Eye

Ophthalmologists at Hassenfeld Children’s Hospital at NYU Langone are experts at diagnosing amblyopia, commonly known as lazy eye, a condition in which vision does not develop properly in one eye—even though the structure of the eye may be normal.

A number of eye disorders can cause the brain to see a clear image in one eye and a blurry image in the other. The brain ignores the blurry image and favors the clear one coming from the healthy eye.

If this continues for months or years, nerve pathways from the brain to the eye producing blurry images begin to atrophy, or waste away, causing lazy eye. Left untreated, permanent vision loss may occur, as well as a partial loss of depth perception.

Lazy eye develops in early childhood and can worsen, if not treated, until the age of 10 or 12. When a child reaches adolescence, the condition stabilizes. At this point, however, damage to the eye may be irreversible.

Symptoms in a young child include bumping into objects, because depth perception is affected, or a wandering eye. An older child may have difficulty drawing, reading, or writing.

Adults with lazy eye may have a wandering eye, blurred vision, double vision, poor depth perception, and headache.

Lazy eye tends to run in families. Children who are born prematurely are at risk, as are those with neurological conditions, including autism, Down syndrome, and cerebral palsy.

Causes of Amblyopia

Three eye disorders can cause amblyopia.

Strabismus

Strabismus, a condition in which one or both of the eyes are misaligned, causes the eyes to look in different directions. It is the most common cause of amblyopia.

With strabismus, also known as crossed eyes, a child’s eye can turn inward or outward, preventing the eyes from focusing on the same image. This can result in double vision.

A child’s developing brain compensates by “turning off” the images in the misaligned eye—or the eye with the greater misalignment if strabismus occurs in both. This causes the brain to favor the vision in one eye. Left untreated, strabismus can lead to vision loss.

Anisometropia

Anisometropia is a condition in which the eyes have different degrees of refractive error. Refractive error is a common vision disorder in which people can be either nearsighted or farsighted.

In people who are nearsighted, the eyes focus light at a point in front of the retina, the layer of tissue at the back of the eye that receives light and transmits nerve signals to the brain. As a result, people can see close-up objects clearly, but not those that are far away.

In people who are farsighted, the eyes focus light in the back of the retina. They see distant objects more clearly than those that are close-up. Refractive error can also result in astigmatism, in which the eye does not focus light evenly onto the retina, causing images to appear blurry or stretched out.

Normally, people have refractive error equally in both eyes. If your child has a refractive error in one eye that differs greatly from the other eye, a condition known as anisometropia, the brain favors the eye with the lower error. The vision in the eye with the greater refractive error remains blurry, which may cause lazy eye and subsequent vision loss.

Visual Occlusion

If your child is born with a condition that obstructs or clouds vision, called a visual occlusion, the brain favors the eye without the obstruction, leading to lazy eye.

Conditions that may cause a visual occlusion include congenital cataract, a clouding of the lens, the part of the eye that focuses light onto the retina; or ptosis, in which the eyelid droops over the eye, obstructing vision.

Diagnostic Tests

Your child’s ophthalmologist at Hassenfeld Children’s Hospital detects amblyopia by performing a series of tests in his or her office. If your child is very young and can’t yet communicate well, the doctor must rely on cues given during an eye exam.

Differential Occlusion Test

During this test, the doctor alternates between covering the right and left eyes with a patch to observe your child’s response. Children who are unable to see well out of one eye become irritable if the healthy eye is covered.

Retinoscopy

Retinoscopy allows doctors to accurately assess vision in children, despite their level of cooperation or ability to speak. The goal is to determine your child’s refractive error—farsightedness, nearsightedness, or astigmatism—and whether he or she needs glasses. During this test, the ophthalmologist uses a retinoscope, an instrument that projects a beam of light onto your child’s eyes.

The doctor then examines how the retina reflects this light while moving the beam across the eye. The reflection from the retina tells the doctor where in the eye light is being focused and what type of refractive error is present.

Your doctor then places lenses with different powers in front of the eye until the light is properly focused and reflected and the refractive error is corrected. The power of the lens required to focus light in the eye enables the doctor to determine what type of eyeglass prescription your child may need.

Visual Acuity Testing

If your child is able to speak or read, your doctor may perform visual testing with an alphabet chart to determine if your child’s distance vision is normal. Many people recognize this as the “20/20” eye exam.

Prism Test

Your child’s ophthalmologist may use a prism-shaped tool to measure the amount of misalignment of the eyes, or strabismus. The prism bends the path of light that the doctor shines into the eye, enabling him or her to measure the degree of misalignment. The test can also help doctors determine if your child has a preference for one eye over another when asked to look at an object.